When you have completed this work, you will be able to:

  1. Describe the selection and installation procedures associated with air-conditioning systems.
  2. Describe the selection and installation procedures associated with refrigeration systems.
  3. Identify the different special types of refrigeration systems.
  4. Describe the process of mechanical component selection.
  5. Describe the purpose and function of single phase hermetic motors.
  6. Describe the purpose and function of single phase hermetic motor windings and terminals.
  7. Describe troubleshooting methods associated with electrical systems.
  8. Describe testing procedures associated with motor windings.
  9. Identify the different types of electrical circuit components.
  10. Identify the equipment utilized in testing electrical circuit components.
  11. Interpret hermetic electrical schematic wiring diagrams.


1.0.0 Selection and Installation of Air-Conditioning Systems

2.0.0 Selection and Installation of Refrigeration Systems

3.0.0 Special Types of Refrigeration Systems

4.0.0 Mechanical Component Selection

5.0.0 Single Phase Hermetic Motors

6.0.0 Split Phase Hermetic Windings and Terminals

7.0.0 Troubleshooting Electrical Systems

8.0.0 Testing Motor Windings

9.0.0 Electrical Circuit Components

10.0.0 Equipment and Test Procedures for Electrical Circuit Components

11.0.0 Hermetic Electrical Schematic Wiring Diagrams

Review Questions


There are two types of air-conditioning systems that you must consider before selecting and installing equipment. These two systems consist of the forced air system and the hot and chilled water system. This handbook discusses only the forced-air system.

1.1.0 Forced Air

Forced air units are used when the areas to be air-conditioned are close to each other, are being used for similar purposes, or have the same humidity and comfort zone requirements. A few examples are office spaces, single-dwelling homes, and single-purpose shops. Some characteristics of the system that you must take into consideration during the planning phase are the following:

See Figure 1 for two examples of a forced air unit with accompanying ductwork.

Figure 1 — Arrangements for package-type air-conditioning units and air ducts.

1.2.0 Heat Load Calculations and Air Movement

Once the type of air-conditioning system has been chosen, the next step is to figure out its appropriate size. There are two primary factors that must be considered. The first factor is heat load calculation. Humidity comfort temperature and psychometrics are the three primary considerations necessary for calculating heat load. The second factor is air movement. Velocity, pressure, and drafts are the three main factors that are important when you are designing and planning the size of an installation. Figure 2 shows the relationship between humidity, temperature and air movement.

Figure 2 — Humidity, temperature and air movement relationship.

Test Your Knowledge

1.  Once the type of air-conditioning system has been chosen, the next step is to figure out its appropriate size.

A. True
B. False


- To Table of Contents -


In a refrigeration system, the major consideration is the heat load calculation. The following five general factors affect the refrigeration heat-load estimate required for a particular application:

There are several charts and graphs available that depict the relationship between the factors listed above. To do the job right, you must take into consideration the total effect of the factors before selecting a particular refrigeration system.


- To Table of Contents -


There are certain applications where an electrically powered refrigeration system cannot be used. This requires knowledge of special applications and selection of a refrigeration system that will work effective. The absorption and the expendable refrigeration systems are discussed in this section.

3.1.0 Absorption Refrigeration System

An absorption system uses either, water, ammonia or lithium bromide as the refrigerant. The system can range from very simple (small refrigerator) to complex (commercial freezer). This type of system is used in domestic and industrial refrigeration and air-conditioning applications. The absorption system is also used in recreational vehicles. Normally, these systems are identified by the type of heat source being used to power them, such as kerosene, natural gas, steam, or electricity.

Because of the high pressure (400 psi), you should remember that welded steel tube construction must be used throughout the system. Also, because of the reaction between ammonia and copper or brass, you need a set of steel manifold gauges. Figure 3 shows an absorption refrigeration cycle using ammonia as the refrigerant.

 Figure 3 — Absorption refrigeration cycle.

3.2.0 Expendable Refrigeration System

An expendable refrigeration system is for used in trucks, railroad cars, and shipping containers that transport perishable items. The three types of refrigerants presently being used in an expendable system are liquid nitrogen, carbon dioxide, and liquid helium. The evaporator system and the spray system are two types of expendable systems commonly used.

3.2.1 Evaporator Systems

In the expendable evaporator system, liquid refrigerant is stored in large metal insulated cylinders. These cylinders are normally located in the front of the cargo vehicle. Each cylinder is equipped with a temperature control to provide a temperature range of –20°F to 60°F. The temperature control is connected to a temperature sensor. As the temperature rises, the switch operating the control valve opens and liquid refrigerant flows into the evaporator. The evaporator can be blower coils, plates, or eutectic plates. As it passes through the evaporator, the refrigerant vaporizes. The vapor is pushed through the evaporator by the pressure difference between the cylinder and the vent. When the selected temperature is attained, the refrigerant valve closes. The vapor that has been used is then discharged from the evaporator at about the same temperature as the air in the cargo vehicle. With this system, the refrigerant does not mix with air in the vehicle space.

An example of the expendable evaporator system is shown in Figure 4. This example shows two nitrogen cylinders located inside a truck body connected by a manifold to regulators and to temperature control solenoid valves. The vaporizing liquid nitrogen flows into the vaporizers or cold plates to refrigerate the true box.

Figure 4 — Expendable evaporation refrigeration system.

3.2.2 Spray Systems

In the expendable refrigerant spray system, liquid nitrogen or carbon dioxide is sprayed directly into the vehicle space that is to be cooled. This system uses liquid containers, a control box, a fill box, spray headers, emergency switches, and safety vents. The fill box is normally located on the front of the vehicle. It contains the valves, gauges, and connections that allow the liquid containers to be filled. The liquid containers are insulated cylinders similar to thermos bottles. The control box contains the valves, gauges, and thermostats that are necessary for safe release of the liquid to the spray headers. Once the liquid is received at the spray headers, the nozzles spray it into the vehicle.

The remaining two components are primarily safety devices. These emergency interlock switches are attached to each door. That means, whenever a door is opened, the system shuts down. The safety vent is a small trapdoor that vents air directly to the atmosphere whenever the air inside the truck box exceeds atmospheric pressure.

A benefit of this system is that liquid nitrogen or carbon dioxide replaces the oxygen inside the space being refrigerated. Therefore when fruits, vegetables, meats, and fish are being refrigerated, they are also preserved by the inert atmosphere.

A vehicle equipped with this type of system must display the following safety sign:

Liquid nitrogen will instantly freeze any part of the human body that it touches. Since liquid nitrogen can be dangerous, you should always inspect the refrigerated space before closing the doors. An expendable spray system for a refrigeration truck is shown in Figure 5. In this system, the liquid nitrogen is in an insulated container that is installed vertically inside the truck body. Another similar type of spray system with the refrigerant container mounted horizontal under the truck body is shown in Figure 6.

Figure 5 — Vertically installed expandable spray system.

Figure 6 — Horizontally installed expandable spray system.

3.3.0 Thermoelectric Refrigeration Systems

This type of system is used to move heat from one area to another by use of electrical energy. The electrical energy, rather than the refrigerant, serves as an “earner.” The primary use of thermoelectric systems has been in portable refrigerators, water coolers, cooling of scientific apparatus used in space exploration, and in aircraft. The main advantage of this system is there are no moving parts. The system is compact, quiet, and requires little service.

3.4.0 Multistage Refrigeration System

Multistage refrigeration systems are used where ultralow temperatures are required but cannot be obtained economical through the use of a single-stage system. The reason for this is the compression ratios are too high to attain the temperatures required to evaporate and condense the vapor. The following two general types of systems are presently in use:

3.4.1 Cascade System

The cascade system has two separate refrigerant systems interconnected in such a way that the evaporator from the first unit cools the condenser of the second unit. This allows one of the units to be operated at a lower temperature and pressure than would otherwise be possible with the same type and size of single-stage system. It also allows two different refrigerants to be used, and it can produce temperatures as low as −50°F.

In this typical cascade system (Figure 7), condenser B of system 1 is being cooled by evaporator C of system 2. This arrangement enables ultralow temperatures in evaporator A of system 1. The condenser of system 2 is shown at point D in the figure. Two thermostatic expansion valve (TEV) refrigerant controls are also indicated in the figure. Notice the use of oil separators to minimize the circulation of oil. 

Figure 7 — Cascade refrigeration system.

3.4.2 Compound System

The compound system uses two or more compressors connected in series in the same refrigeration system. In this type of system the first stage compressor is the largest and for each succeeding stage the compressor gets smaller. This is because as the refrigerant passes through each compressor, it becomes a denser vapor. A two-stage compound system can attain a temperature of approximately –80°F. A three-stage system (Figure 8) can attain a temperature of –135°F efficiently. Compressor 1 pumps vapor into the intercooler and then into the intake of compressor 2. This operation is repeated between the second and third stages. In the third stage, the refrigerant vapor is further cooled and travels to the evaporator for specific cooling use.

 Figure 8 — Compound refrigeration system (three-stage).


Test Your Knowledge

2. What is the temperature range of the cylinders that store the liquid refrigerant in an expandable evaporator system?

A. −5°F to −10°F
B. −10°F to −20°F
C. −20°F to −60°F
D. −60°F to −80°F

3.  The compound refrigeration system allows two different refrigerants to be used, and can produce temperatures as low as −50°F.

A. True
B. False


- To Table of Contents -


There are several mechanical components required in a refrigeration system. This section will discuss the four major components of a system and some equipment associated with the major components. These components include the following:

4.1.0 Condensers

There are several condensers to be considered when making a selection for installation. They are air-cooled, water-cooled, shell and tube, shell and coil, tube within a tube, and evaporative condensers.

Each type of condenser has its own unique application. Some determining factors include the size and the weight of the unit, weather conditions, location (city or rural), availability of electricity, and availability of water. For example, is single phase or three phase electricity available? Is electricity economical of prohibitive? Water in some locations may be scarce, expensive, or contain chemicals that make it unsuitable for use. Local zoning laws should also be checked to ensure there are no restrictions as to use of electricity, water, or location of the unit. If you installed a unit on a roof, the roof load strength is very important. In some locations, the noise factor of an operating unit is an important consideration.

With the rapid advances in technology, you should contact a manufacturer whenever possible to get the latest condenser design features available for a special-purpose installation.

4.2.0 Evaporators

There are almost as many different types of evaporators as there are applications. However, evaporators are divided into two general groups. The first group has evaporators that cool air that, in turn, cools the product. The second group has evaporators that cool a liquid such as brine solution that, in turn, cools the product. Normally, the proper evaporator comes with the unit (system) that you will be installing. However, there may be an occasion when you are designing a system. At this time, you will need to determine the requirements and select the proper evaporator from a manufacturer’s catalog or manual.

Figure 9 is a blower-type evaporator shown in a small space. The air enters the bottom of the evaporator, is cooled, and exits at the front of the unit. In Figure 10, view A, a forced circulation evaporator is shown partially installed; view B shows the unit with the fan removed.

Figure 9 — Blower-type evaporator.

Figure 10 — (A) Forced circulation evaporator partially installed; (B) fan unit removed.

A compact blower evaporator for use in low headroom fixtures is shown in Figure 11. A vertical, flat-type blower evaporator designed for mounting behind either a window or a door frame is shown in Figure 12.

Figure 11 — Compact blower evaporator.

Figure 12 — Vertical flat-type blower evaporator.

In the dual fan evaporator unit shown in Figure 13, the motor drives two propellertype fans and the cool air exits at both ends of the evaporator. Figure 14 shows a low-velocity blower evaporator. In this type of evaporator, the air enters at the two fan grills and exits on both sides. Figure 15 shows a low-temperature blower evaporator. The low-temperature evaporator has two axial-flow fans and an electric defrost.

Figure 13 — Dual fan evaporator.

Figure 14 — Low-velocity blower evaporator.

Figure 15 — Low-temperature blower evaporator.

4.3.0 Compressors

With present technology, the newer air-conditioning and refrigeration systems use reciprocating, rotary, screw, centrifugal, swash plate, and scroll compressors. There are many designs and models available for all types of applications. A typical hermetic compressor is shown in Figure 16. For more in-depth information about compressors, you can refer to sources, such as Modern Refrigeration and Air Conditioning by Althouse/ Turnquist/Bracciano.

Figure 16 — Hermetic compressor.

4.4.0 Thermostats

The thermostat is a control that responds to changes in temperature and directly or indirectly controls the temperature. There are many different designs of thermostats. Figure 17 shows a few of the common thermostats used in modem heating and cooling systems. 

 Figure 17 — Thermostats.

Thermostats are of three types: heating, cooling, and dual (combined heating and cooling thermostat in one).

The common sensing element of a thermostat is bimetal. A bimetal sensing element simply uses two different types of metal, brass and invar, which have different expansion rates. Figure 18 depicts three common profiles of bimetals used in thermostats.

Figure 18 — Bimetal profiles.

The bimetal element in Figure 19 has a set of contacts on one end. The top contact is fixed. The two contacts open or close when the temperature changes around the bimetal.

Figure 19 — Bimetal element contacts.

When the contacts close, a path is created for current to flow. The snap action in the magnetic type makes the contacts close or open quickly. This eliminates any spark and extends the life of the contacts. Figure  20 shows enclosed contacts that use a bimetal element for movement and contacts or mercury for making contact between two electrodes.

Figure 20 — Enclosed contacts.

The manufacturing engineers determine what type and design of thermostat should be installed in a particular system. Knowing and understanding the advantages and disadvantages of different types of thermostats will help you identify the type of thermostat being used in a system and enable you to troubleshoot an inoperative system efficiently

Electrical room thermostats are in three categories: line voltage, low voltage, and millivoltage. Line-voltage thermostats are usually 115 volts. When line-voltage thermostats are installed, there is no need for lowering voltage with a transformer. However, line-voltage thermostats are dangerous for the users and the cost is higher. Normally line-voltage thermostats are located only in industrial commercial applications.

Low-voltage thermostats (24 volts) are not dangerous to the user. They are also more cost efficient than line-voltage models. The disadvantage of low-voltage thermostats is the extra requirements of wiring and additional components; they are less rugged than line-voltage thermostats.

The millivoltage thermostat operates at 750, 500, or 250 millivolts. This thermostat uses its own power source for operation and is not affected by power interruptions. The system requires only a small amount of wiring compared to other systems. However, this system is limited for use only in heating applications. The temperature control is less precise than other systems, wire length and size are critical, and the system requires a separate device to power a 24-volt control, or you must use a millivoltage control.

4.4.1 Anticipators

One component that enhances the operation of a thermostat is an anticipator. The two types of anticipators are heating and cooling.

The heating anticipator produces false heat in a thermostat to prevent extreme temperature changes within a space. The false heat created by resistance increases the thermostat rate of response. Basically, the thermostat receives false heat which shuts down the heating source before the thermostat reaches the desired temperature. This action reduces overshooting and is economically efficient. The heating source shuts off and the blower continues to run using the heat transferred from the surface of the furnace and ductwork. When adjusting a heating anticipator, you must set the anticipator resistance to match the current rating of the primary control.

The cooling anticipator adds false heat to the thermostat bimetal element the same way as a heating anticipator. Unlike the heating anticipator, cooling anticipators are not adjustable; they are sized by the manufacturer of the thermostat. The cooling anticipator is placed in parallel with the cooling contacts. By studying Figure 21, you can see that the cooling anticipator is energized when the unit is in the OFF cycle (thermostat contacts open). The small amount of heat produced by the resistance heat closes the TC before the actual temperature in the space reaches the thermostat cut-in setting. This action allows the unit to start removing heat before the temperature in the space climbs above the desired temperature. When the cooling thermostat contacts close, the current flow through the anticipator is insignificant because the contacts of the thermostat offer less resistance to current flow than the anticipator resistance, so the anticipator is de-energized.

Figure 21 — Cooling anticipator.

4.5.0 Refrigerant Lines and Piping

Because of the progress made in this field, construction has become much simpler. Since pre-charged lines are in everyday use, the problems of installation are being eliminated. However, pay particular attention to neatness and cleanliness when you are installing support brackets (hangers) and insulation. Figure 22 shows a schematic piping diagram of a typical commercial refrigeration system. This system has a roofmounted air-cooled condenser and two motor compressors. Each motor compressor has a suction and liquid header and is connected to six refrigerant lines. A detailed view of an oil separator installation is shown in Figure 23.

Figure 22 — Schematic piping diagram for a commercial refrigeration system.

Figure 23 — Installation of an oil separator.

4.6.0 Refrigerant Capacity Controls

In a single-stage installation, one evaporator, one condensing unit, and any one of the five types of refrigerant controls will work. However, in a compound (multistage) installation (Figure 8) with one condenser, only two types of controls can be used. There are very few low-side float systems in actual operation, but you should be aware that there are some units that still use this control. Thermostatic expansion valves are the most commonly used, and on large capacity units, they usually operate a pilot valve that, in turn, operates a larger valve.

The biggest problem associated with capacity controls is using the wrong size. When ordering replacements and when making repairs, you should always ensure that the control markings are appropriate for the intended system. Also, ensure the replacement part being ordered is compatible with the type of refrigerant being used in the system.

4.6.1 Automatic Expansion Valve (AEV)

The AEV maintains a constant pressure in the evaporator (Figure 24). There are five pressures that affect the operation of the AEV. The pressures are P1 (atmospheric pressure), P2 (evaporator pressure), P3 (liquid line pressure), S1 (adjustable spring pressure), and S2 (fixed spring pressure). To adjust the valve, turn the adjusting screw until the desired pressure is obtained in the evaporator. Automatic expansion valves are installed on systems that have a relatively constant load. Primarily, the AEV is used on domestic refrigerators and small water coolers.

 Figure 24 — Automatic expansion valve.

4.6.2 TEV Adjustment

The thermal expansion valve (TEV) is the most widely used expansion device. The TEV controls the flow of refrigerant to maintain a constant superheat in the tail coil of an evaporator. Figure 25 shows the three pressures that affect the operation of this valve. They are P1 (bulb pressure), P2 (evaporator pressure), and P3 (spring pressure).

Figure 25 — Thermal Expansion Valve.

When the pressure of p1 is higher than the combined pressure of p2 and p3, the valve opens. This valve is equalized internally because the evaporator pressure is sensed through an internal port in the valve. Figure 26 provides another view showing how a TEV is equalized externally.

Figure 26 — Thermostatic expansion valve,

When a TEV is used on a large evaporator or an evaporator with a pressure drop of 6 to 7 pounds across the evaporator, the valve will prematurely cause hunting (valve fluctuates toward opening and closing). In the case of valve hunting, install a TEV equipped with an external equalizer line. Figure 27 shows the TEV installed with an external equalizer line. The external equalizer line compensates for a pressure drop from the inlet of the evaporator to the end of the tail coil and eliminates valve hunting. During installation of an equalizer line, ensure that it is located downstream from the sensing bulb. The bulb is filled with a volatile fluid that reacts to changes in temperature which in turn equalizes pressure within the expansion valve.  externally equalized.

Figure 27 — Externally equalized TEV.

Air-conditioning refrigeration units come equipped with a metering device that the manufacturer has engineered for the system. You should never change the recommended type of metering device for a system without consulting the manufacturer.

Most TEVs are adjusted at a predetermined superheat setting and tested at the factory prior to being shipped. If you need a different superheat setting, the steps in Table 10-1 may be useful, be sure to follow manufacturers recommendations.

Table 10-1 — Determining Superheat

1. Obtain the temperature of the suction line at the point where the TEV sensing bulb is attached.
  • Take the temperature reading with a dial thermometer similar to the one shown in Figure 28, or use some other temperature measuring device that senses surface temperatures accurately.

Figure 28 — Dial thermometer.

2. Obtain the suction pressure inside the piping at the location of the remote sensing bulb.
  • If the valve is externally equalized, you can place a gauge in the external equalizer line. This is the most accurate method.
  • The alternate method is to read the manifold pressure gauges at the compressor and add the estimated pressure drop through the suction line between the bulb and compressor. The sum of the two pressures provides approximate pressure at the location of the remote bulb.
3. Convert the pressure you received in step 2 into saturated evaporator pressure.
  • Use a pressure temperature chart. When using the chart ensure that you are looking at the proper refrigerant.
 4. Simply subtract the temperature in step 3 from the temperature in step 1. This will give you the superheat.



When adjusting the expansion valve, turn the adjusting stem no more than one full turn and wait approximately 15-30 minutes for the system to balance out. Once the system is balanced, recheck the superheat setting by following the steps in Table 10 above.

4.7.0 Receivers and Accumulators

The receiver is a storage tank for liquid refrigerant. When a refrigeration system is equipped with a receiver, you can close the outlet valve (king valve) and pump refrigerant into the receiver. This enables you to store the refrigerant while you work on the unit. Additionally, when a unit is equipped with a receiver, the quantity of refrigerant in the system is less critical than a unit not equipped with a receiver. Figure 29 shows the location of a receiver installed in a system. This is a commercial system with an air-cooled condenser, a thermostatic expansion valve, and a V type reciprocating compressor.

Figure 29 — Receiver location in a refrigeration cycle.

The accumulator is located inside the refrigeration cabinet and acts as a safety device. As a safety device it prevents the flow of liquid refrigerant into the suction line and the compressor. This is because liquid refrigerant causes considerable knocking and damage to the compressor. Figure 30 shows the location of an accumulator in a system.

Figure 30 — Accumulator location in a refrigeration system.

Test Your Knowledge

4. What type of blower evaporator is designed for mounting behind windows and door frames?

A. Low-temperature
B. Vertical flat-type
C. Low-velocity
D. Compact

5. What electrical room thermostats are usually 115 volts, are dangerous to the user, and are more expensive?

A. High-voltage
B. Low-voltage
C. Millivoltage
D. Line-voltage

6.  The three pressures that affect the operation of the TEV are; bulb, evaporator, and spring.

A. True
B. False


- To Table of Contents -


As an HVAC-R supervisor/manager, it important that you have an understanding of the four types of single-phase motors used in hermetic assemblies. These types of motors include the following:

5.1.0 Split-Phase

The split-phase motor is used generally on condensing units of 1/10-, 1/6-, and 1/4- horsepower capillary tube systems. It has a low starting torque and contains both a starting winding and a running winding. A disconnect device is required for the starting winding when the motor reaches sufficient speed to operate on the running winding. Figure 31 is a schematic wiring diagram of a split-phase motor circuit.

Figure 31 — Schematic wiring diagram of a split-phase motor circuit.

5.2.0 Capacitor-Start, Induction-Run

This motor is similar to the split-phase type except that a starting capacitor is connected in series with the starting winding to provide higher starting torque. Figure 32 is a schematic diagram illustrating this type of motor. A device is also required to disconnect the starting winding when the motor reaches rated speed. This motor is commonly used on commercial systems up to three-fourths horsepower.

Figure 32 — Schematic wiring diagram of a capacitor-start induction-run motor.

5.3.0 Capacitor-Start, Capacitor-Run

Two capacitors are used with the capacitor-start, capacitor-run motor: a starting capacitor and a running capacitor. The capacitors are in parallel with each other and in series with the starting winding. Figure 33 is a schematic diagram illustrating this type of motor circuit. The two capacitors turn the motor power surges into two-phase power when the motor is started. At approximately two-thirds rated speed, the starting capacitor part of the circuit is disconnected by a start relay device. Only the running capacitor remains in the circuit. This type of motor has a high starting torque and is used with hermetic systems up to 5 horsepower. 

Figure 33 — Schematic wiring diagram of a capacitor-start capacitor-run motor.

5.4.0 Permanent Split-Phase

The permanent split-phase motor has limited starting torque and is used basically with capillary tube air-conditioning equipment, such as window units. Capillary systems permit high-side and low-side pressure equalization when the compressor is not operating. A run capacitor is wired in series with the starting winding. Both the starting winding and the run capacitor remain in the circuit during operation. No start relay or start capacitor is needed. Figure 34 is a schematic wiring diagram of the circuits used in this type of motor.

Figure 34 — Schematic wiring diagram of a permanent split-phase motor.

Test Your Knowledge

7. What type of single-phase motor has limited starting torque and is usually used with window air conditioners?

A. Capacitor-start, induction-run
B. Permanent split-phase
C. Split-phase
D. Capacitor-start, capacitor-run


- To Table of Contents -


Split-phase motors used in hermetic refrigeration and air-conditioning applications are designed to start under load. These split-phase and capacitor motors use two sets of spiral windings: a starting winding and a running winding. The two sets of windings differ in their impedance and in their positions in the stator slots.

The starting winding has high resistance and small reactance, whereas the running winding has low resistance and high reactance. Reactance is the opposition to the flow of alternating current by inductance and capacitance.

The running winding has many turns of large wire and is placed in the bottom of the stator slots. The starting winding is wound of small, high resistance wire and is placed on top of the running winding.

Both windings are connected internally at one end to provide a common lead, and when starting, both are energized in parallel. The currents are out of phase with each other and their combined efforts produce a rotating field that starts the motor. Figure 35 shows the starting and running windings of a two-pole motor in their 90-degree out-ofphase positions.

Figure 35 — Two-pole motor with starting and running windings.

Hermetic motors have three electrical terminals connected through an insulated seal to the motor windings inside the dome. (Figure 36).

Figure 36 — Identifying motor terminals using an ohmmeter.

Troubleshooting procedures require that these terminals be identified with respect to the winding connected to each. The terminals must be identified as the START TERMINAL, the RUN TERMINAL, and the COMMON TERMINAL. Some manufacturers mark the S-, R-, and C-terminals for start, run, and common, respectively; other manufacturers use different designations.

The terminals can always be identified by using a low-range ohmmeter following the procedure below:

Test Your Knowledge

8. How many electrical terminals do hermetic motors have that are connected through an insulated seal to the motor windings inside the dome?

A. 1
B. 2
C. 3
D. 4


- To Table of Contents -


Electrical troubleshooting techniques are used on refrigeration and air-conditioning equipment. Electrical troubleshooting is done by a process of elimination. You should begin by checking the most obvious trouble and gradually progress to the more remote possibilities.

As an HCAC-R tech,  you cannot troubleshoot an electrical system for an air conditioner or refrigeration unit unless you understand the function of each component in a system.  When you can observe a unit operating and detect what is not functioning properly, you can identify the circuit or circuits that are having trouble.

At this point you must be able to test each of the components within a circuit that is not functioning properly. Of course to do all of this, you must also be able to do the following:

7.1.0 Circuits

The two basic types of circuits are load and control. A load circuit consists of a circuit that contains a load and all of the wiring that provides line voltage directly to the load, such as compressor motor, fan motor, solenoid valves, lights, or any device that consumes current (other than the movement of an electrical switch).

The control circuit contains controls that either open or close a path that operates a load device. Each load has a control circuit. Control circuits consist of thermostats, pressure switches, overload protectors, and all of the wiring in the control circuit.

7.2.0 Loads and Control Circuits

Air-conditioning and refrigeration units normally have two fan motors and a compressor. These components are considered load. A load is any device that consumes electrical energy. Most loads convert electrical energy into another type of energy to create some type of work. For example, electrical energy is converted to magnetism within a motor to make the motor run.

All loads have some type of control so they can be turned on, off, or regulated. These controls are located in a control circuit. The circuit is made up of controls and paths wiring. Controls and control circuits consume no electrical energy; they simply provide a path for electrical energy to flow, thus indirectly controlling the operation of various types of loads. Figure 37 shows an electrical schematic wiring diagram of a heat pump.

Figure 37 — Heat-pump schematic.

At first glance the diagram appears complex, but after studying the diagram briefly and looking at one circuit at a time, the diagram becomes easy to follow and understand. An example is as follows: Look at the first circuit in Figure 37. The circuit contains a control relay contact (CR), high-pressure switch (HP), liquid line pressure switch (LLP), compressor contactor (C), and an internal thermostat (IT). This is a complete circuit. The CR is simply a set of contacts and falls in the category of a path; the contacts are either open or closed. The HP and LLP are both pressure switches and are controls in this circuit; the pressure switches are either open or closed. The C is the compressor contactor. Actually this is a magnetic coil located within a contactor that simply closes all of the contacts in the diagram that are labeled C.

The IT is located inside the compressor and opens when there is a temperature rise. The only load in this circuit is the compressor contactor because it is a current consuming device. Now, look at the figure again and see if you can find the load in the second circuit. The load is the indoor fan motor (IFM) because it is a current consuming device. The indoor fan relay (IFR) contact only provides a path for the current to energize the IFM.

7.3.0 Testing Circuits

To troubleshoot an inoperative or improperly operating unit electrically, you must be able to use a process of elimination systematically and use a multimeter effectively. Remembering and understanding a few simple rules will enable you to use a multimeter to locate a faulty electrical component or control. The first circuit in Figure 37 is used as an insert to illustrate the different meter readings you will encounter when troubleshooting an electrical system. Refer to the insert next to the applicable troubleshooting procedure.

7.3.1 Voltage Readings

To begin, set your multimeter to voltage; ensure the power is on and all wires are connected to the component being tested. The four important troubleshooting procedures that apply to reading voltage are as follows:

  1. Meter probes on a path. Place one meter probe on the left side of the CR contact and the other probe on the left side of the HP switch (Figure 38). If you obtain a voltage reading, this indicates that either the path is open or the contacts are open.

Figure 38 — Voltage reading (path open).

  1. Meter probes on a path. Place one meter probe on the left side of the CR contact and the other probe on the left side of the HP switch (Figure 39). If you obtain no voltage, the path is closed. 

Figure 39 — Voltage reading (path closed).

  1. Meter probes across a load. Place one meter probe on the right side of the LLP switch and the other probe on the left side of IT (Figure 40). If you obtain a voltage reading, the compressor contactor is energized and the compressor should be running. If the load is NOT operating, you should check for a grounded winding and for winding resistance.

Figure 40 —Voltage reading (compressor running).

  1. Meter probes across a load. Place one meter probe on the right side of the LLP switch and the other probe on the left side of the IT (Figure  41). If there is not voltage reading, replace the load. 

Figure 41 — Voltage reading (no reading replace load).

7.3.2 Continuity Readings

To perform an ohmmeter continuity test, set your multimeter to resistance; disconnect the power and remove the wires from the component being tested. Perform a continuity test as follows:

  1. Meter probes across a path. Place one meter probe on the left side of the CR contact and the other probe on the left side of the HP switch (Figure 42). If you obtain a reading, the path is open.

Figure 42 — Continuity reading (path open).

  1. Meter probes across a path. Place on meter probe on the left side of the CR contact and the other probe on the left side of the HP switch. With the CR contact closed, you should obtain a zero reading, indicating the path is closed.

Figure 43 — Continuity reading (path closed).

  1. Meter probes across a load. Place one meter probe on the right side of the LP and the other probe on the left side of the IT (Figure 44). You should obtain a reading. If you obtain no reading, replace the load.

Figure 44 — Continuity reading (no reading replace load).

To further increase your understanding of electrical troubleshooting, review the rules you have just read using a different method. The flow charts in Figures 10-45 and 10-46 provide you with a review of electrical troubleshooting with a digital multimeter. To use a multimeter effectively and troubleshoot air-conditioning and refrigeration units electrically, you must not only know the information provided here but also practice by testing circuits. Always remember to respect electricity. Whenever possible, perform your electrical troubleshooting with the power OFF using continuity checks.


The flow charts in Figures 10-45 and 10-46 do not cover every electrical troubleshooting procedure you will incur. The charts are presented to help you understand electrical troubleshooting.

Figure 45 — Electrical troubleshooting loads.

Figure 46 — Electrical troubleshooting testing controls and paths.


Test Your Knowledge

9. What are the two basic types of circuits?

A. Load and control
B. Control and open
C. Load and direct
D. Closed and current


- To Table of Contents -


If, during the procedure for identifying motor terminals, the ohmmeter displays a blank readout during any test, there is probably a defective winding. A defective winding may be classified as open or shorted. The display will be zero if the winding is grounded.

8.1.0 Open Windings

Open windings can occur in the starting winding, the running winding, or both. An open winding is the result of a burned-out or grounded fault or simply a break somewhere in the lead or winding that prevents the current from completing the circuit. A motor with an open winding does not start. If only one winding is open, the motor hums, but if both windings are open, no sound is emitted nor current consumed. Open windings can be checked by an ohmmeter, a voltmeter, or a test light.

8.1.1 Ohmmeter Continuity Test Procedure

The procedure for making an ohmmeter continuity test is shown in Figure 47 and outlined below.

8.1.2 Voltmeter Test Procedure

8.1.3 Test Lamp Continuity Check Procedure

The procedure for conducting a test lamp continuity check is as follows:

Figure 48 — Testing for an open winding with a test lamp.

8.2.0 Shorted Windings

In an electric motor the winding turns lie side by side with only the insulating varnish separating one loop from another. When one loop of the copper wire contacts another, the winding is short. The pulling effect of the shorted portion of the winding is lost. This, in turn, places more load on the active winding, causing the motor to draw higher voltage and amperage with a concurrent increase in winding temperature. In this condition the motor fails to start, or it starts and continues to run causing the overload protector to open. The fuses may also blow. The result is likely to be a burnout where the insulating varnish deteriorates from excessive heat. Ultimately a ground or short occurs.

An ohmmeter can be used to check windings for shorts. For most applications a low-range meter with a scale graduated in tenths of ohms between 0 and 2 ohms is best. However, to check motors throughout the sizes normally encountered in hermetic motor-compressor units, a range of 0 to 25 ohms is necessary. The meter is used to measure resistance of the windings. The readings are compared with design resistances. A short is shown when measured resistance is less than design resistance. The ohmmeter connections are the same as those shown in Figure 36.

Often manufacturer’s data is not available and the design resistances are not known. Table 10-2 lists the approximate resistances for fractional horsepower single-phase motors. The following guidelines may also be helpful:

  1. The starting winding of low-starting torque motors usually has a resistance of about seven to eight times that of the running winding.
  2. The starting winding resistance of high- starting torque motors is usually three to four times that of the running winding.

Table 10-2 — Approximate Resistances for Fractional Horsepower Motor Windings

Horsepower (HP) Running Winding Starting Winding
1/8 4.7Ω 18Ω
1/6 2.7Ω 17Ω
1/5 2.3Ω 14Ω
1/4 1.7Ω 17Ω

8.3.0 Ground Windings

A ground is the result of an electrical conductor in contact, either directly or indirectly, with the motor frame or the metal shell of the unit. Either the starting winding, the running winding, or both can be affected. The ground is either one of low resistance or one of high resistance. A low-resistance ground is indicated when fuses blow repeatedly and the motor fails to start. A high-resistance ground is shown by an occasional blown fuse, but more often, by the opening of the overload protector.

Three methods of testing windings for grounds are the ohmmeter continuity test, the test lamp continuity check, and the resistance measurement with a megohmmeter (megger). The procedure to follow in making each of these tests is provided below.

8.3.1 Ohmmeter Continuity Test (Low-Resistance) Procedure

To perform an ohmmeter continuity test, complete the following steps:

  1. Disconnect the power and remove the wires from the motor terminals.
  2. Scrape off paint and clean a spot on the motor-compressor shell for testing.
  3. With the ohmmeter set on its highest scale, test for continuity between the terminals and the shell. This procedure is shown in Figure 49. There is a ground if continuity exists between the terminals and the shell.

Figure 49 — Testing windings for ground.

8.3.2 Test Lamp Continuity Check (Low-Resistance) Procedures

Complete the following steps for conducting a test lamp continuity check:

  1. Disconnect the power and remove the wires from the motor terminals.
  2. Ensure the lamp is connected in the hot side of the line. Plug the test lamp into a receptacle.
  3. Connect the hot-line probe to a motor winding terminal.
  4. Touch the free probe to the cleaned spot on the shell. Ensure that a good connection is made. There is a grounded winding if the light illuminates.

8.3.3 Megohmmeter (High-Resistance) Test Procedure

The megohmmeter consists of an indicating movement for which current is supplied by a small hand-driven generator. Figure  50 illustrates a typical megger. Two leads are supplied, one is marked Earth or Ground and the other is the free probe.

Figure 50 — Typical megohmmeter (megger).

The procedure for making the megohmmeter (high-resistance) test is as follows:

If any reading of low resistance is obtained, the motor is grounded.


You should always refer to the manufacturer’s instructions when using a megger.


Test Your Knowledge

10. Which method is used to test windings for grounds?

A. Ohmmeter continuity test
B. Test lamp continuity check
C. Resistance measurement with a megohmmeter
D. All of the above

11.  The first step in performing an ohmmeter continuity test is to disconnect the power and remove the wires from the motor terminals.

A. True
B. False


- To Table of Contents -


Starting relays, overload protectors, and capacitors are electrical components that can cause trouble in hermetic motor compressor circuits. It is essential that the individual servicing refrigeration and air-conditioning units be able to identify these components and test them using the proper equipment and procedures.

9.1.0 Starting Relays

The three basic types of starting relays are as follows:

In the hermetic motor control circuit, a starting relay allows electricity to flow through the starting winding until the motor reaches two-thirds to three-fourths of its rated speed. At this time, about 3 to 4 seconds after starting, it disconnects the starting circuit.

9.1.1 Current Relay

A current relay is an electromagnet, similar to a solenoid valve that employs a weight and spring to hold the contacts open when the circuit is idle. In operation the instantaneous surge of starting current actuates the magnetic coil, causing the start winding contacts to close. This closure allows starting current to the winding; rated speed and the current decreases, causing the relay contact to open and disconnect the winding. Current relays are ideal for use with split-phase, induction-run motors. Figure 51 is a schematic diagram of a current relay motor starting circuit.

Figure 51 — Schematic diagram of a current relay motor starting circuit.

9.1.2 Voltage Relay

A voltage relay looks much like a current relay; but differs in operation. It operates on increased voltage as the motor reaches rated speed, and unlike the current relay, the contacts remain closed during the off cycle. When the motor is first turned on, it draws heavy current and the voltage drop across the starting winding is low. As the motor picks up speed, there is less and less load; therefore, more and more voltage is induced into the winding. At about three-fourths rated speed the voltage is high enough to cause the relay coil to pull the contacts open and disconnect the winding. Voltage relays are used with capacitor-start motors. Figure 52 is a schematic diagram of voltage relay motor starting circuit.

Figure 52 — Schematic diagram of a voltage relay motor starting circuit.

9.1.3 Thermal Relay

A thermal relay is commonly known as a hotwire relay. It is available in at least two different basic designs and is supplied by several manufacturers. All thermal relays operate on the theory that electrical energy can be turned into heat energy and that, when the temperature of a metal is increased, the metal expands. Thermal relays, like current and voltage relays, operate the starting winding circuit. In addition, the thermal   relay controls the running winding circuit, if for any reason the circuit draws excessive current.

The device consists of a specially calibrated wire made from a material with high oxidation resistance and two sets of contacts, all of which are integrally attached to form the relay. Figure 53 illustrates a typical thermal relay motor starting circuit. The contacts are controlled by the hot wire, either through the use of heat-absorbing bimetallic metal strips, or by its expansion of the hot wire, depending on the design of the relay.

Figure 53 — Typical thermal relay motor starting circuit.

9.2.0 Overload Protectors

Essentially, an overload protector is a heat sensitive device much like a circuit breaker. When current in the circuit increases above normal, the added current heats a bimetallic strip that bends and opens a pair of contacts. The opening of the contacts disconnects the motor-running circuit and the motor stops. This prevents damage to the compressor motor when troubles occur, such as a defective starting relay, an open starting capacitor, or high-head pressure. Figure 54 shows a typical bimetallic disk-type overload protector. This overload protector is connected in the common line and mounted on the compressor motor shell.

Figure 54 — Bimetal disk-type overload.

9.3.0 Capacitors

In hermetic refrigeration and air-conditioning work, capacitors are identified in the following two groups:

These may be identified further as dry capacitors (start) that are used for intermittent operations and electrolytic capacitors (run) that are used for continuous operations.

9.4.0 Start Capacitors

Start capacitors are connected in series with starting windings. Figure 55 shows the location of the start capacitor in a circuit. Because a start capacitor is placed in series with one of the two stator windings, the current will lead, as compared to the current going directly to the connected stator winding. This, in turn raises the attraction of one stator winding over the other, allowing the motor to begin turning. Figure 56 shows that stator winding 2 is stronger than stator winding 1. This causes the motor to begin turning in the direction of the stronger winding. Once the initial starting of the motor is completed, the start capacitor is removed from the circuit. 

Figure 55 — Start capacitor location in a circuit.

Figure 56 — Motor starting.

9.5.0 Run Capacitors

These types of capacitors are connected in the circuit between the line side of the starting and running windings. A run type capacitor (Figure 57) serves to provide a smoother and quieter operating motor.

Figure 57 — Run capacitor.

Test Your Knowledge

12. What type of starting relay is electromagnetic and is used with split-phase induction-run motors?

A. Current
B. Thermal
C. Voltage
D. Diaphragm


- To Table of Contents -


As an HVAC-R supervisor/manager you should have a thorough understanding of the equipment and procedures for testing circuit components. These components include starting relays, overload protectors, and capacitors.

10.1.0 Starting Relays

Starting relays can be tested two ways with an ohmmeter. The meter can be used to check across the relay coil, or it can be used to check across the relay contacts. This does not apply to thermal relays. Figure 58 illustrates the procedures for these tests. 

Figure 58 — Testing a starting relay with an ohmmeter.

When you check the relay contacts, you must know if the contacts are normally open or normally closed, refer to the schematic. Voltage relay contacts and thermal relay contacts are normally closed, whereas current relay contacts are normally open. The meter reading should indicate continuity through voltage and thermal relays since the contacts are normally closed. On the other hand, if the meter indicates continuity through the normally open contacts of a current relay, the contacts are probably fused together.

Another method of checking starting relays is by using a test line cord and fuse combination to isolate the relay. Figure 59 illustrates the procedure used in making this test. Obtain a capacitor of the approximate size used with the compressor motor. Connect it from the hot side of the running winding to the hot side of the starting winding. Connect the test line to the motor terminals as illustrated in the figure and plug it in. If the compressor is good it should start running. After a short time, disconnect the capacitor. The compressor should continue to speed up and run normally. This procedure has accomplished manually what a properly functioning starting relay is supposed to accomplish. If the motor failed to start normally before the check, the relay is bad.

Figure 59 — Checking a starting relay with a test line.

Voltage and current relay coils can also be tested for resistance with an ohmmeter. When the coil is burned out, the meter indicates no resistance or an open coil. Commercial starting relay testers are available from several manufacturers.

10.2.0 Overload Protectors

Questionable Klixon external overload protectors (Figure 60) should be replaced with new ones. If the motor then operates properly, the old Klixon (protector) should be destroyed. Klixons can also be checked with an ohmmeter. Since the contacts are closed at ambient temperature, the meter should show continuity. When the meter shows an open, the Klixon should be replaced and destroyed.

Figure 60 — Klixon external overload protector.

Internal current temperature overloads can be tested by making continuity checks. Continuity checks must be made across terminals C and S, C and R, and S and R. When both C and S and C and R are open and continuity is indicated across S and R, the protector is open. When the temperature is normal and the continuity test indicates the overload contacts are open, the motor compressor assembly must be replaced. When the operating temperature is normal, the internal current temperature overload contacts should be closed.

10.3.0 Capacitor Test

The best test for a questionable motor capacitor is to try a new one of the correct size. If the motor operates properly, the old capacitor is defective and should be destroyed. Capacitors can also be tested with ohmmeter. First the power must be turned OFF and the capacitor disconnected and discharged with a 2 watt 20,000 ohm resister. Set the meter on the 0 to 10,000 ohm scale and touch the meter probes to the capacitor terminals.

If the digital display indicates 0 or low resistance and then climbs towards high resistance, the capacitor is good. If the display indicates 0 or low resistance and stays there, the capacitor is shorted. If the display stays blank, the capacitor is open. Figure 61 shows these procedures.

Figure 61 — Testing capacitors with an ohmmeter.

Test Your Knowledge

13. When a starting relay fails, which of the following devices can be used to start the compressor motor by bypassing the relay manually?

A. A test lamp and scale
B. An ohmmeter and four lead wires
C. A test line cord, fuse, and capacitor
D. A jumper placed across terminals C and R and a test lamp


- To Table of Contents -


It is important for you to understand how and why air-conditioning and refrigeration units work like they do. Schematic wiring diagrams provide the type of detail you need to meet this requirement. All wiring circuits are built around the following four requirements:

The schematic wiring diagram puts the symbol and line representation on paper in a manner that allows instant identification of all four requirements.

In the schematic wiring diagram, the source of electrons is a line drawn on one side of the diagram and it is usually designated as L1. Any and all points on this line have a surplus of electrons. On the opposite side, a line is drawn representing a shortage of electrons and it is usually designated as L2. There is a potential for electron flow between the two wires represented by L1 and L2. If a load is inserted between L1 and L2 the current flows and the load functions.

Figure 62 is a typical schematic diagram for a hermetic electrical system. Figure  63 is a wiring detail for a typical room air-conditioner.

Figure 62 — Typical hermetic system schematic wiring diagram.

Figure 63 — Wiring detail for a typical room air-conditioner.


- To Table of Contents -


In this study you were provided with technical information required for selecting and installing air-conditioning and refrigeration equipment. Also discussed were the individual components required in air-conditioning and refrigeration systems, along with the fundamental electrical knowledge needed to install, maintain, and repair the equipment for those systems.


- To Table of Contents -

Review Questions

1. What type of air-conditioning system should be used when the areas to be air-conditioned are in close proximity to each other?

A. Chilled water only
B. Hat and chilled water
C. Forced air
D. Natural draft

2. What type of manifold gauges are needed when working on an ammonia-absorption refrigeration system?

A. Brass
B. Cooper
C. Bronze
D. Steel

3. What type of evaporator system is used to preserve the freshness of fruits, vegetables, meats, and fish?

A. Spray
B. Thermoelectric
C. Eutectic
D. Evaporator

4. What type of refrigeration system has no moving parts? A. Evaporator B. Thermoelectric C. Spray D. Eutectic

5. What maximum temperature can be maintained in a cascade refrigeration system?

A. −50°F
B. −100°F
C. −150°F
D. −250°F

6. What maximum temperature can be attained in a three-stage compound refrigeration system?

A. −100°F
B. −135°F
C. −150°F
D. −250°F

7. When mounting a condenser on a roof, what consideration is considered the most important?

A. Noise level
B. Availability of water
C. Availability of electricity
D. Roof load strength

8. What types of metal are used in a bimetal thermostat?

A. Tin and antimony
B. Cooper and steel
C. Brass and invar
D. Tin and steel

9. What type of thermostat uses 24 volts?

A. Line-voltage
B. Low-voltage
C. Millivoltage
D. High-voltage

10. What is the most commonly used metering device?

B. Capillary tube
D. Low-side float

11. Which of the following types of motors should be used for 5-horsepower, high-starting torque requirement?

A. Split-phase
B. Capacitor-start, capacitor-run
C. Permanent split-phase
D. Capacitor-start, induction run

12. Which of the following components is considered a load?

A. Thermostat
B. High-pressure switch
C. Set of contacts
D. Compressor contactor

13. Which of the following conditions exists in the case of a shorted-winding?

A. A wire is burned in half
B. The winding has a high resistance
C. A loop of cooper wire is in contact with another wire
D. A wire is touching the hermetic shell

14. Which of the following devices can be used to test a hermetic motor for grounds?

A. Ohmmeter
B. Test lamp
C. Megger
D. All of the above

15. Which of the following starting relays is capable of de-energizing the running winding circuit when the circuit draws excessive current?

A. Hot wire
B. Voltage
C. Current
D. None of the above

16.  When checking relay contacts, voltage and thermal relay contacts are normally open.

A. True
B. False

17. When testing a capacitor with an ohmmeter, what general reading on the meter indicates the capacitor is good?

A. High resistance
B. Low resistance then climbs to high resistance
C. Medium resistance
D. Low resistance


- To Table of Contents -

Copyright © David L. Heiserman
All Rights Reserved