SPLIT-PHASE MOTORS
Two general classifications of split-phase motors are used in the industry. The resistance-start—induction-run motor and the capacitor-start— induction-run motor are types of split-phase motors in common use today. Each of these motors has different operating characteristics while being similar in construction. Split-phase motors use some method of splitting the phase of incoming power to produce a second phase of power, giving the motor enough displacement to start. The split-phase motor uses two windings to displace the phase and create the needed displacement between the run and start windings to produce rotation.
Resistance-Start—Induction-Run Motor
The resistance-start—induction-run motor shown in Figure 9.25 has both a starting winding to assist the motor in starting and a running winding to continue rotation after the motor has reached a certain speed. Most single- phase motors have some method of beginning the rotation; in a split-phase motor, rotation is started by splitting the phase to make a two-phase cur rent. The single-phase current is split between the running and the starting winding, which puts one of the windings out of phase by 45 to 90 degrees. The starting windings are used to assist the split-phase motor in starting. They are also used until the motor has reached a speed that is about 75% of its full-capacity speed. The starting windings then drop out of the circuit by the use of a centrifugal switch. After that occurs, the motor operates at full speed on the main or running winding alone.
Fig. 9.25 Split-phase motor
Fig. 9.26 Cutaway view of an electric motor: Bearings; Stator; Squirrel cage rotor; Bearings; Motor windings
A cutaway view of a split-phase motor is shown in Figure 9.26.
The split-phase motor can be operated on 120 volts—single phase—60 hertz or 208/240 volts—single phase—60 hertz. Some split-phase motors can operate on either voltage by making simple changes in their wiring if so desired. Thus, they are dual-voltage motors. Split-phase motors can be reversed by reversing the leads of the starting winding at the terminals in the motor.
Split-phase motors are used when a high starting torque is not required. They are used in such equipment as belt-driven evaporator fan motors, hot- water pumps, small hermetic compressors, grinders, washing machines, dryers, and exhaust fans.
Operation
The phases in a split-phase motor are split by the makeup of the starting windings. The starting winding is designed with smaller wire and more turns than is the running winding, which has a greater inductance. Therefore, the running winding is displaced from the starting winding because of its greater inductance. This displacement causes a resistance to current flow to build up in the running winding. The phase displacement means the current reaches the two windings at separate times, allowing one winding to lead, in this case the starting winding. However, some manufacturers allow current to reach the running winding first by designing an increased resistance into the starting winding and a decreased induction into the running winding. Whatever method is used, the motor basically operates on the same principle: splitting the phase.
Fig. 9.27 Layout of a split-phase motor: Running windings, Starting windings
Legend:
A: Running windings
B: Starting windings
C: Rotor
The operation of a split-phase motor, referring to Figure 9.27, is as follows:
1. Power is applied to the running and starting windings in parallel. The motor itself splits the phase by the counter electromotive force (emf) in the running winding, which acts as a resistance to hold back the cur rent flow to the running winding. On the alternation of power, the starting winding creates a higher magnetic field than the running winding.
2. In half of a cycle the alternations are changed. The running winding has the stronger magnetic field, moving the rotor a certain distance depending on the number of poles in the motor. For the motor in Figure 9.27, the distance is one-fourth of a rotation.
3. As the alternations continue at the rate of 60 cycles per second, the motor continues to rotate with the magnetic field of the stator. Therefore, the rotor, with its magnetic field permanent, attempts to keep up with the rotating magnetic field of the stator.
4. The motor is equipped with a centrifugal switch that drops the starting winding out of the circuit when the motor has reached 75% of its full speed.
Troubleshooting
Split-phase motors are one of the most reliable types of motors used in the heating, cooling, and refrigeration industry. They are used on most types of single-phase equipment. The split-phase motor is easy to troubleshoot if the service technician has a good understanding of its operation. The three probable areas of trouble are the bearings, the windings, and the centrifugal switch.
The bearings of any motor often give trouble because of wear and improper maintenance. Identification of a motor with bad bearings is simple. The motor will have trouble turning and in some cases may be locked down completely.
The windings of a split-phase motor can be shorted, open, or grounded. This is easily diagnosed with an ohmmeter.
Fig. 9.28 Centrifugal switch used in a split-phase motor: Wiring terminals
The centrifugal switch, shown in Figure 9.28, is the hardest section to diagnose for troubles because it stays in the circuit only a short time. The centrifugal switch has a tendency to stick in an open or closed position because of wear and often must be replaced. The centrifugal switch can usually be heard when it drops in after the motor is cut off. Hence, it can be checked effectively in this manner. If the centrifugal switch does not drop out of the circuit, the motor will pull an excessively high ampere draw and cut off on overload. One sure method of checking the centrifugal switch is by disassembling the motor and making a visual inspection.
Capacitor-Start-Induction-Run Motor
The capacitor-start motor, shown in Figure 9.29, produces a high starting torque, which is needed for many applications in the industry. The open capacitor-start motor operates like a split-phase motor except that a capacitor is inserted in series with the centrifugal switch and the starting winding. The centrifugal switch breaks the flow of current to the starting capacitor and starting winding. The centrifugal switch opens when the motor has reached a speed that is 75% of its full speed. Figure 9.30 shows a schematic diagram of the motor.
Capacitor-start motors are used on pumps, small hermetic compressors, washing machines, and some types of heavy-duty fans.
Fig. 9.29 Capacitor-start motor; Fig. 9.30 Schematic diagram of an open capacitor-start motor with a centrifugal switch. Relays may be used instead of centrifugal switch.
Open Type
As we have said, the open capacitor-start motor is similar in design to the split-phase motor with the exception of the capacitor. Therefore, troubleshooting the open capacitor-start motor is similar to checking the split- phase motor except for checking the capacitor. There are four possible areas of trouble in the open capacitor-start motor: windings, bearings, centrifugal switch, and capacitor.
The windings can be, easily checked with the use of an ohmmeter by checking for shorts, opens, and grounds.
The bearings of a motor usually fail because of lack of maintenance or wear. Motor bearings will usually become tight or lock down completely. This condition can be determined by trying to turn the motor. If the motor has a tight place in its rotation or will not turn at all, the bearings are faulty in the motor.
Due to the constant opening and closing of the centrifugal switch, it is often the culprit in motor problems. The centrifugal switch may stick in an open or closed position or its contacts may be defective. A centrifugal switch, in some cases, can be checked with an ohmmeter to determine its position, open or closed. In other cases, the motor will have to be disassembled to check the switch.
The capacitor is easy to check with an ohmmeter. The capacitor is often mounted in one end bell of the motor rather than on top of the stator.
Enclosed Type
When capacitor-start motors are used in small hermetic compressors, a centrifugal switch cannot be used because of the oil used to lubricate the compressor. Instead, an external relay is used to break the power going to the starting winding and the starting capacitor. In this case, the capacitor- start motor is an enclosed motor with a starting relay. By inserting a capacitor in the starting winding, a phase displacement is created between the running and starting windings, causing the motor to rotate.
The enclosed capacitor-start motor has an external relay to drop the starting winding and starting capacitor out of the circuit. This capacitor should be checked to determine its condition.
The condition of the windings of an enclosed motor can easily be checked with an ohmmeter. The windings have a set of terminals on the outside of the casing that lead to the windings. Use an ohmmeter to check across these terminals to determine if the windings are shorted, open, or grounded.
The enclosed motor can also be locked down due to worn bearings or to internal failure of some component of the motor. This condition can be detected with an ammeter or by the humming sound of the motor on an attempted start.
PERMANENT SPLIT-CAPACITOR MOTORS
Permanent split-capacitor motors, also known as PSC motors, are simple in design and have a moderate starting torque and a good running efficiency, which makes them a popular motor in the industry. Figure 9.31 shows a PSC motor used as a fan motor. Figure 9.32 shows a hermetic compressor that uses a PSC motor to power the compressor.
The starting winding and the running capacitor of the PSC motor are connected in series, as shown in Figure 9.33(a). The schematic diagram of the hookup is shown in Figure 9.33(b). The running and starting windings are in parallel, but the capacitor causes a phase displacement.
Permanent split-capacitor motors are used on compressors, where the refrigerant equalizes on the “off” cycle, on direct-drive fan motors, and in other applications in the industry. It has a relatively low cost in comparison with other motors because it does not have a switch to drop the starting winding out of the circuit. The PSC motor can be used only when a moderate starting torque is required to begin rotation.
Fig. 9.31 Permanent split-capacitor motor used as a fan motor; Fig. 9.32 Hermetic compressor utilizing a PSC motor
Fig. 9.33 Diagrams of permanent split-capacitor motor: (a) Hookup; (b) Schematic. Running capacitor; Hermetic compressor,
Legend:
C: Common terminal
R: Running winding terminal
S: Starting winding terminal
RC: Running capacitor
Operation
The permanent split-capacitor motor has two windings: a running (main) winding and a starting (phase) winding. Both windings are wound with almost the same size and length of wire. A running capacitor is put in series with the starting winding. The capacitor causes the electron flow through the starting winding to shift it out of phase with the running winding. Therefore, a rotating magnetic field is set up, causing the rotor to turn.
Multispeed PSC motors contain additional running windings. The starting winding is in series with the running capacitor and in parallel with the running winding. Figure 9.34 shows a schematic diagram of a three-speed PSC motor. For high-speed operation, the starting and main windings are energized. Medium-speed operation is accomplished by energizing the starting winding with the main and medium-speed windings connected in series. For low-speed operation, the main, medium, and low windings are connected in series with each other, and all are connected in parallel to the starting winding.
Fig. 9.34 Schematic diagram of a three-speed PSC motor
Troubleshooting
The PSC motor usually gives trouble-free operation for long periods. The three most common failures in a PSC motor are in the bearings, windings, or capacitor.
The bearings of a PSC motor often become faulty because of wear or lack of proper maintenance. Bearings in any motor can be diagnosed with little trouble by rotating the motor by hand and noticing rough places in the movement or the shaft being locked in one position.
The windings of a motor become faulty because of overheating, over loading, or a faulty winding. A bad motor winding can easily be checked with an ohmmeter. The windings could be shorted, open, or grounded. The service technician should use care in diagnosing problems with the windings of PSC motors because they are often built with several speeds.
A bad capacitor can keep a PSC motor from starting or can pull a high ampere draw when running. Capacitors can be checked by one of the methods covered in Section 9.5. In most cases, faulty PSC motors will be replaced with new motors rather than repaired. The PSC motor is easy to troubleshoot with the right tools and knowledge.
Probably the most difficult aspect of PSC motors is their design. PSC motors are often built with several speeds. Service technicians must pay careful attention when replacing a faulty PSC motor because if the motor is connected incorrectly, permanent damage can occur. Most PSC motors are furnished with a wiring diagram to ensure correct installation. Motor manufacturers, however, make only a limited number of motors to replace the many different motors in the field. Thus, a service technician may have to adapt the replacement motor to a specific application.
CAPACITOR-START-CAPACITOR-RUN MOTORS
The capacitor-start—capacitor-run motor, or CSR motor produces a high starting torque and increases the running efficiency. It is actually a capacitor- start motor with a running capacitor added permanently to the starting winding. The starting winding is energized all the time the motor is running. The capacitor-start—capacitor-run motor takes the good running characteristics of a permanent split-capacitor motor (see Section 9.7) and adds the capacitor-start feature. This produces one of the best all-around motors used in the industry.
Capacitor-start—capacitor-run motors are used almost exclusively on hermetic or semi-hermetic compressors. Rarely will this type of motor be used as an open-type motor because of the cost of the components necessary to produce it. Most open-type motors do not use a starting relay but use the centrifugal switch instead. Open-type motors are usually built as permanent split-capacitor motors or capacitor-start motors. Occasionally, a CSR motor will be used in an open-type motor when an extremely high starting torque is required.
Operation
The CSR motor begins operation on a phase displacement between the starting and running windings, which allows rotation to begin. The running capacitor lends a small amount of assistance to the starting of the motor, but its main function is to increase the running efficiency of the motor. Figure 9.35 shows a schematic diagram of this motor with its starting components.
Troubleshooting
The capacitor-start—capacitor-run motor is sometimes difficult to troubleshoot because of the number of components that must be added to a regular motor to produce it. The windings, bearings, potential relays, starting capacitor, and running capacitor must all be checked.
The windings of a CSR motor can be easily checked with an ohmmeter to determine if the windings are shorted, open, or grounded. In most cases, the windings will be enclosed in a hermetic casing and the terminals will be on the outside of the casing. However, the type of motor makes little difference in checking the winding as long as the technician uses the correct terminals.
Fig. 9.35 Schematic diagram of a capacitor-start-capacitor-run motor.
Legend:
C: Common terminal
R: Running winding terminal
S: Starting winding terminal
RC: Running capacitor
SC: Starting capacitor
SR: Starting relay (potential)
The bearings of a CSR motor can be worn so badly that the motor will not turn or will turn only with a great deal of difficulty. The bearings of hermetically sealed motors are enclosed and therefore harder to check, but the condition of the bearings can be determined by a whining sound, or by the motor pulling a larger-than-normal ampere draw. Care should be taken not to condemn the bearings of a motor because of a high ampere draw unless you are sure this is the problem.
The starting relay can be checked by diagnosing the condition of the contacts and the coil. The contacts can be checked with an ohmmeter or by visual inspection. On an ohmmeter the contacts should show zero resistance. The visual inspection is easy once the relay is disassembled. Then the condition of the contacts can be determined: sticking, pitting, or mis alignment. The coil is checked like the windings of a motor.
The starting and running capacitors are easily checked with an ohmmeter to determine their condition.
Troubleshooting a CSR motor is done by checking all components of the motor. These motors must be correctly checked to prevent other components from being destroyed. For example, a capacitor will be destroyed if the contacts or coil of a starting relay are bad.
THREE-PHASE MOTORS
Three-phase motors are rugged, reliable, and more dependable than other types of motors. The most common type and the type often used in heating, cooling, and refrigeration is the squirrel cage induction type, shown in Figure 9.36. This motor will be the only three-phase motor discussed in this section.
Fig. 9.36: Three-phase induction motor
Three-phase motors are considerably stronger than single-phase motors because of the three phases that are fed to the motor. Three-phase current actually supplies three hot legs to the device, rather than the two hot legs supplied by single-phase power. Therefore, instead of having a two-phase displacement, a three-phase displacement is available without using starting components. Three-phase motors are common to the industry; thus the technician should understand their operation.
Operation
Three-phase motors operate on the same principles as the single-phase with the exception of the three-phase displacement. A rotating magnetic field is produced in the stator. This interacts and causes a magnetic field in the rotor. However, the three-phase motors require no starting apparatus, because none of the phases are together. In the sine wave of the three-phase motor, none of the phases peaks at the same time. Each phase is approximately 120 electrical degrees out of phase with the others. For this reason, there is no need to use any device to cause a phase displacement, as is needed in the starting of single-phase motors.
Three-phase motors can be purchased in any voltage range desired. For example, a dual-voltage, three-phase motor can be operated on two different voltages with minor modifications in the wiring.
Three-phase motors have two basic types of windings. They are the star winding or wye (Y) winding, as shown in Figure 9.37, and the delta winding, as shown in Figure 9.38. There is no operational difference between the two types, but it does allow designers more latitude in three-phase motor design.
Fig. 9.37 Schematic diagram of the star winding of a three-phase motor
Low-voltage hookup:
- L1 to T1 and T7
- L2 to T2 and T8
- L3 to T3 and T9
- Tie T4, T5, and T6 together
High-voltage hookup:
- L1 to T1
- L2 to T2
- L3 to T3
- Tie T4 to T7, T5 to T8, and T6 toT9
Fig. 9.38 Schematic diagram of the delta winding of a three-phase motor
Low-voltage hookup:
- L1 to T1, T6 and T7
- L2 to T2, T4 and T8
- L3 to T3, T5 and T9
High-voltage hookup:
- L1 to T1 L2 to T2 L3 to T3
- Tie T4 to T7, T5 to T8, and T6 to T9
Troubleshooting
A three-phase motor can be checked by reading the resistance of the winding with an ohmmeter. If a resistance reading of 0 ohm occurs, the motor is shorted. A reading of infinite resistance indicates an open winding. A reading of some measurable resistance is usually from 1 ohm to 50 ohms, depending on the size of the motor. The larger the motor, the smaller the resistance. The smaller the motor, the larger the resistance of the winding. Care should be taken because of the chance of a spot burnout in the winding. Experience should give service technicians the ability to diagnose any type of electric motor.
ELECTRONICALLY COMMUTATED MOTORS
The refrigeration, heating, and air-conditioning industry has a made a concentrated effort to increase the efficiency of equipment that is being placed into operation. This can be accomplished by decreasing the electrical power consumed by the equipment by decreasing the load on equipment. The largest electrical loads in refrigeration or air-conditioning systems are the motors that supply mechanical energy to operate compressors, fans, pumps, and other devices requiring rotation. Electrical resistance heaters also require large amounts of electrical power but are seldom used because of high energy costs. Most other electrical loads in refrigeration and air- conditioning systems require only minimal electrical power. The major emphasis for increased efficiency and reduction in system power consumption has, therefore, been the electric motor.
Equipment typically operates at full-load capacity equipped with some type of capacity control, which is seldom the case in residential installations. Most equipment could easily satisfy the heating or cooling load of the structure while supplying only a fraction of its rated capacity. For example, if the capacity of an air-conditioning system was 48,000 Btu/hr and the building’s heat gain was only 24,000 Btu/hr, conventional equipment would supply 48,000 Btu/hr for the heat load of only 24,000 Btu/hr. This would result in frequent system starting and stopping, not to mention the increased chances of high-humidity-related issues. Stopping and starting an electrical motor requires a great deal more electrical power than normal running conditions. Recall that the locked rotor amperage, which is the amperage draw of the motor on initial startup, is about five to seven times greater than the motor’s normal running amperage. It would, therefore, be advantageous if the equipment operated for a longer period of time at a reduced capacity. This would result in using less electrical energy than the frequent stopping and starting alternative of forcing the equipment to operate at full capacity for shorter periods of time. In addition, less frequent starting and stopping reduces the wear and tear on the system’s mechanical components. One method of accomplishing this goal is to incorporate variable-speed motors into the system. Variable-speed motors have the ability to vary their speed to match the load of the structure. As a result, equipment efficiency is increased and electrical power consumption is reduced as the equipment capacity will closely match the structure’s heating or cooling load.
There are several methods used to control the speed of electric motors. The speed of an alternating current electric motor is determined by the number of motor poles and the frequency of the power supply. Alternating current is difficult to regulate and control, making motor speed modulation difficult to accomplish by regulating the frequency of the alternating cur rent power supply. The speed of AC motors can easily be varied by changing from one pole arrangement to another, a feature commonly found on many multispeed alternating current motors. When the speed of an AC motor is varied by altering the frequency of the power supply, variable frequency drives (VFDs) are used. These tend to be bulky and very expensive for many applications, but advances in technology are helping to remedy these setbacks.
A very easy way to produce a variable-speed motor is by converting alternating current to direct current. Direct current is easier to control and regulate, enabling a motor to operate at variable speeds. The most common methods used to vary the speed of electric motors, short of using VFDs, are using direct current converters and electronically commutated motors (ECM). The direct current converters convert the alternating current that the power company supplies to direct current, which can then be regulated to vary the speed of the motor. The electronically commutated motor actually reverses one-half of each alternating current cycle to form a single directional current, which is then utilized to vary the speed of the motor.
The increase of electronically commutated motor usage has been phenomenal over the past ten years—from 100,000 motors in service in 1995 to over one million in 2005. ECM-driven appliances are found in residential, commercial, and industrial markets. Probably the greatest increase in the usage of the ECM has been the residential market, where motors controlling fans are usually smaller than 1 horsepower. Electronically com mutated motors are manufactured for 120-, 240-, and 277-volt inputs and typically range from 1/3 horsepower to 1 horsepower for primary air movement in residential air-conditioning systems. Figure 9.39 shows an ECM used as the primary air mover in a residential gas furnace. ECMs have also become popular in smaller sizes with 120- and 240-volt inputs that are used in furnace draft inducers as shown in Figure 9.40. The ECM is utilized in many other applications, such as fan terminal boxes in variable air volume (VAV) systems and air movement in refrigeration display cases, vending machines, and walk-in coolers. Most major air-conditioning manufacturers are using the ECM in their high-efficiency equipment lines.
Fig. 9.39 ECM used as primary air movement in residential gas furnace
Fig. 9.40 ECM used as furnace draft inducer
Construction
The ECM is a brushless DC, three-phase motor with a permanent magnet rotor shown broken down in Figure 9.41. The ECM is actually a two-part motor. One part is the motor and the other part is the control. Both parts are shown in Figure 9.41. Motor phases are sequentially energized by an electronic control, energized by a power supply. A control module is attached to one end of the motor that uses input signals to obtain and maintain the correct motor speed. The motor control is shown in Figure 9.42.
Fig. 9.41 ECM broken down in sections
Fig. 9.42 The control module of ECM; Fig. 9.43 The stator of ECM
Fig. 9.44 The rotor of ECM
The ECM stator is a laminated interlocked stator wound like a three-phase motor, as shown in Figure 9.43. The rotor of the ECM turns with the aid of ball bearings and is equipped with three permanent magnets attached to it at 120-degree segments, as shown in Figure 9.44. The magnets are attached to the rotor with heavy-duty glue and are magnetized by the manufacturer.
Operation
The operation of an electronically commutated motor is determined by the inputs to the motor control. The input signals vary depending on the mode of system operature. Line voltage alternating current is supplied to the motor at all times. The alternating current is converted to direct current and sent to the inverter. The signal from the motor control, also sent to the inverter, ultimately determines the speed of the motor. The ECM experiences a soft start when initially energized. This means that the motor starts at a low speed and gradually ramps up to its final desired speed, which depends on the system conditions. Since the motor ramps its speed up and down, the ECM allows for a wide range of blower or fan speeds. Figure 9.45 shows a block diagram of the ECM and the control. At present it is almost impossible to cover all of the different inputs of the ECM because each manufacturer uses a unique signal or interface strategy.
For more information on the manufacturer-specific signals and interfaces, the technician should consult the equipment manufacturer. Electronically commutated motors have many advantages over the PSC motor, which has been used almost exclusively in low-torque applications prior to the introduction of the ECM. These advantages include quieter operation, higher motor efficiency, better performance, greater comfort for the consumer, and better humidity control in the occupied space.
Fig. 9.45 Block diagram of ECM and control:
Troubleshooting
Electronically commutated motors are more difficult to troubleshoot than most other motors used in the field because of the various interfaces, motor control strategies, and modules that are used by the various equipment manufacturers. The technician should always refer to manufacturer’s information when troubleshooting these motors. Some inherent actions of the ECM, such as the rocking of the stator or the rumbling sound on startup and shut down, may lead the technician to condemn the motor. When the motor is rocking, trying to go both directions, it is determining the location between the stator and the rotor. The technician should isolate the sound rather than change ECMs when the noise is objectionable. ECMs should always be tested under load because motor operation without a load can be erratic. The line voltage supplies to the ECM should never be disconnected or connected with the power on as this action could cause damage to the control module.
ECMs must have high and low voltage at the appropriate connections to operate. This should be the first item that a service technician should check if the motor is not operating. The motor’s failure to operate could be caused by the absence of either line or low voltage. The technician should check each voltage source and verify that the measured voltage is within accept able ranges. If the motor starts but runs erratically, the problem could be anything from loose low-voltage connections to moisture in the control. Common indications of motor/controller problems include constant ramping of the motor speed, either up or down, and constant motor speed during periods of varying load. When problems such as these occur, a troubleshooting chart from the manufacturer could be helpful. The common-sense method of troubleshooting an ECM is to first check the motor. The motor must be disconnected from the control module to check it properly. The resistance readings of the motor should be similar to those of a three-phase motor. Each winding should have the same resistance. If each winding reads the same resistance and the motor is not grounded, the problem is probably elsewhere. Once you have determined that the motor is good and all connections are good, the next item to check is the control module. Figure 9.46 shows an electronic module that is available to check the ECM.
Fig. 9.46 Electronic module used to troubleshoot ECM
HERMETIC COMPRESSOR MOTORS
Hermetic compressors are becoming increasingly popular because of their low cost. Hermetic motors are of the induction type. They are designed for single- and three-phase current. There are four basic types of single-phase motors used in hermetic compressors. The split-phase motor is used on small equipment (fractional horsepower). The capacitor-start motor is also used on small equipment. The permanent split-capacitor motor is used on most window units and small residential units. The capacitor-start—capacitor-run motor is used on any application that requires a good starting and running torque. Many hermetic compressors are built with three-phase motors; usually these are used on the larger equipment.
CAUTION: Make sure all compressors are properly grounded.
Operation
Hermetic compressor motors are totally enclosed in a shell with refrigerant and oil. Hence, they require special considerations. Nothing can be used inside the shell that is capable of causing a spark or that has to move on the crankshaft, such as a centrifugal switch. Therefore, no starting apparatus can be incorporated inside the compressor shell. Starting relays and capacitors must be mounted and wired outside the motor. It must be remembered that hermetic motors operate the same as other motors with the exception of the enclosure.
CAUTION: Oil and refrigerant can spray out of a hermetic compressor when an electrical terminal of the compressor is vented.
CAUTION: The protective covering of the electrical terminals of a hermetic compressor should always be in place in the event of terminal venting.
Terminal Identification
All single-phase motors have a common, a start, and a run terminal. These terminals are sometimes wired directly into an open-type motor and are difficult to find. The common is the junction point of the start and run terminals. The start and run terminals are connected to one end of the windings while the common is connected to the other end. The schematic diagram of a single-phase compressor motor is shown in Figure 9.47 with the terminals identified. Of course, each of the windings of a three-phase hermetic motor is the same because no starting apparatus is required.
In single-phase motors, it is important for the service technician to deter mine the common, start, and run terminals. This task can be performed simply and easily by using an ohmmeter to obtain the resistance of each winding with respect to common. Figure 9.48 shows the resistance values of a single-phase motor after the resistance has been measured at each terminal on the compressor. To find the run, start, and common terminals, the following procedure should be followed:
1. Find the largest reading between any two terminals. The remaining terminal is common (in Figure 9.48 the reading between A and B is largest; C is common).
2. The larger reading between common and the other two terminals identifies start (C to A is 2 ohms and C to B is 10 ohms; therefore, common to B is larger and B is start).
3. The remaining terminal is run (A is run).
This procedure is important, especially in installing the external electric devices, although it is not necessary if a good, readable diagram is available. In a three-phase motor, the resistances among all three terminals are the same.
Fig. 9.47 Schematic diagram of a single-phase compressor with the terminals identified: Compressor terminal box; Fig. 9.48 Terminals of a single-phase compressor with ohmic values given
Legend:
R: Run terminal
C: Common terminal
S: Start terminal
Troubleshooting
Troubleshooting a hermetic compressor motor is often difficult because of its physical makeup and because it is totally enclosed in a shell and cannot be visually inspected. Small hermetic compressors usually have some type of external overload, as shown in Figure 9.49, whereas large hermetic compressors usually have internal overloads, as shown in Figure 9.50. The winding layouts of single-phase hermetic compressors are similar regardless of motor size. The only difference is the size of the windings, which will vary the resistance readings of the motor windings. Three-phase hermetic compressor motors are generally produced in sizes above 3 horsepower. Through experience, the service technician will be able to determine the approximate resistance of the motor windings in a hermetic compressor.
Fig. 9.49 Small hermetic compressor with external overload; Fig. 9.50 Large hermetic compressor with internal overload (cutaway)
Electrical troubleshooting of hermetic compressor motors is done by taking a resistance reading of the windings with a good ohmmeter. Determining the condition of the windings is easy if the problem with the motor is open windings, shorted windings, or grounded windings. Figure 9.51 shows a schematic representation of these three conditions.
CAUTION: If arcing sounds (sizzling, sputtering, or popping) are heard inside a compressor, immediately move away; this sound indicates a possible compressor terminal venting situation.
Fig. 9.51 Schematic representation of good, shorted, open, and grounded compressor windings with internal overload: Good, Shorted, Open, Grounded
Most single- or three-phase hermetic compressor motors have three terminals on the outside of the casing that connect the motor to the external power wiring, as shown in Figure 9.52. Some large hermetic compressors have more than three terminals, such as dual-voltage, part winding motors or two-speed motors, as shown in Figure 9.53. The resistance readings of single-phase motor windings are not the same because the compressor has a start winding and a run winding connected by a common wire, as shown in Figure 9.54. The physical makeup of a single-phase motor will allow the service technician to match the resistance readings to determine the condition of the windings. The sum of the resistance readings of the start to common terminals and the run to common terminals should equal the resistance reading obtained between the run and start terminals, as discussed in the terminal identification section. If the readings do not match, a spot burnout of the winding is likely. Three-phase motors will have the same resistance in each winding; if not, the motor is bad because of the spot burnout. The service technician must be careful, however, before condemning a hermetic motor whose winding resistance readings vary, because the problem may actually be bad connections, a faulty meter, or a misreading of the meter. A good service technician should use every possible diagnostic tool to ensure that no good hermetic compressor is condemned.
CAUTION: When removing a compressor, make sure that electrical power supplies have been disconnected and the refrigerant recovered.
Fig. 9.52: Spade-type hermetic push-on terminals ; Several terminal arrangements on hermetic compressors. Screw terminals: 24-Volt compressor sensors; Line voltage c’ case heaters; Compressor power; Two-speed motor connections
Fig. 9.53 Terminals on a large hermetic compressor
Fig. 9.54 Schematic of windings of a single-phase compressor with internal overload (ohm readings for winding shown)
Fig. 9.55 Open compressor winding being checked with an ohmmeter.
Fig. 9.56 Shorted compressor winding being checked with an ohmmeter.
Fig. 9.57 Grounded compressor windings being checked with an ohmmeter.
Diagnosing an open, shorted, or grounded hermetic compressor motor is easy because the resistance readings obtained are definite and exact. An open winding in the compressor motor means there is no continuity or no complete circuit; it gives an infinite resistance reading, as shown in Figure 9.55. A shorted winding in a compressor motor means the winding has burned together; it gives a zero ohm reading, as shown in Figure 9.56. A grounded winding in a compressor motor means that part of the winding is contacting the compressor body; it gives a resistance reading between the shell and the terminals of a compressor, as shown in Figure 9.57. Good contact on the compressor shell must always be maintained if the motor is grounded; therefore, any paint must be removed from a small section of the compressor. The open and shorted windings should be read on a low ohm scale (R x 1), but the grounded winding should be read on the R x 10,000 scale or higher. A grounded compressor can be dangerous because the technician or customer can receive an electrical shock if he or she touches the casing of a slightly grounded compressor. A resistance reading as high as 500,000 ohms indicates a grounded compressor that should be changed. Grounded compressors, if allowed to operate, will often operate at a higher-than-normal temperature; the warmer the windings, the lower the resistance of the ground in most cases.
CAUTION: To ensure safety and prevent damage to the motor, restart only after determining the cause of stoppage.
CAUTION: Before resetting a circuit breaker or fuse, check for a short circuit to ground.
Before condemning the compressor, service technicians should make certain that the internal overload of the compressor is not open. This condition can easily be determined by touching the compressor; if the compressor is hot, it is a good indication that the overload is open. The internal overload of a single-phase hermetic compressor is located in the common conductor that connects the run and start windings, as shown in Figure 9.58. Internal overloads used in three-phase hermetic compressor motors are connected at the common junction of the windings, as shown in Figure 9.59. There are many reasons for an internal overload to open in a hermetic compressor: for example, low refrigerant charge, locked-down compressor, faulty starting components, and high discharge pressure.
CAUTION: When troubleshooting electric motors or hermetic compressor motors that are extremely hot, make sure they have ample time to cool before condemning them.
Mechanical failures in hermetic compressors often seem like electrical problems, especially when the compressor is locked down or when the internal overload opens because of some mechanical failure. The technician must make certain the problem is truly electrical before an accurate diagnosis can be made.
Fig. 9.58 Schematic of a single-phase compressor with internal overload; Fig. 9.59 Schematic of a three-phase compressor with internal overload.
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