AC Motor Types
Define AC motor and identify the main parts of an AC motor and state their operating functions.
- An alternating current (AC) motor is a motor that uses alternating current to produce rotation.
- The main parts of an AC motor are the rotor and stator.
- A rotor is the rotating part of an AC motor.
- A stator is the stationary part of an AC motor.
Explain how a shaded-pole motor operates and give examples of its usage.
- A shaded-pole motor is a single phase AC motor that uses a shaded stator pole for starting.
- Shading the stator pole is the simplest method used to start a single phase motor.
- The shaded pole delays the magnetic field in the area of the pole that is shaded. Shading causes the magnetic field at the pole area to be positioned approximately 90° from the magnetic field of the main stator pole. The offset magnetic field causes the rotor to move from the main pole toward the shaded pole.
- Common applications of shaded-pole motors include small cooling fans found in computers and home entertainment centers.
Explain how a split-phase motor operates and give examples of its usage.
- A split-phase motor is a single phase AC motor that includes a running winding (main winding) and a starting winding (auxiliary winding).
- A split-phase motor has a rotating part (rotor), a stationary part consisting of the running winding and starting winding (stator), and a centrifugal switch that is located inside the motor to disconnect the starting winding at approximately 60% to 80% of full-load speed.
- When the motor reaches approximately 75% of full speed, the centrifugal switch opens, disconnecting the starting winding from the circuit. This allows the motor to operate on the running winding only. When the motor is turned off (power removed), the centrifugal switch recloses at approximately 40% of full-load speed.
- Split-phase motors are commonly used to operate washing machines, oil burners, and small pumps and blowers.
Explain how a capacitor-start motor, capacitor-run motor, and capacitor start-and-run motor operate and give examples of their usages.
- A capacitor motor is a single phase AC motor that includes a capacitor in addition to the running and starting windings.
- The addition of a capacitor in the starting winding gives a capacitor motor more torque than a split-phase motor.
- A capacitor-start motor operates much the same as a split-phase motor in that it uses a centrifugal switch that opens at approximately 60% to 80% of full-load speed.
- In a capacitor-start motor, the starting winding and the capacitor are removed when the centrifugal switch opens.
- The capacitor used in the starting winding gives a capacitor-start motor high starting torque.
- A capacitor-run motor has the starting winding and capacitor connected in series at all times.
- A lower-value capacitor is used in a capacitor-run motor than in a capacitor-start motor because the capacitor remains in the circuit at full-load speed.
- This gives a capacitor-run motor medium starting torque and somewhat higher running torque than a capacitor-start motor.
- A capacitor start-and-run motor uses two capacitors.
- A capacitor start-and-run motor starts with one value capacitor in series with the starting winding and runs with a different value capacitor in series with the starting winding.
- A capacitor start-and-run motor has the same starting torque as a capacitor-start motor. A capacitor start-and-run motor has more running torque than a capacitor-start motor or capacitor-run motor because the capacitance is better matched for starting and running.
- Capacitor motors are used to operate refrigerators, compressors, washing machines, and air conditioners.
Explain how a three-phase motor operates and why no extra starting methods are needed.
- In a three phase motor, a rotating magnetic field is set up automatically in the stator when the motor is connected to three phase power. The coils in the stator are connected to form three separate windings (phases). Each phase contains one-third of the total number of individual coils in the motor.
- Each phase is placed in the motor so that it is 120° from the other phases.
- Three-phase motors are self-starting and do not require an additional starting method because of the rotating magnetic field in the motor.
Describe a single-voltage, three phase motor.
- A single-voltage motor is a motor that operates at only one voltage level.
- Common single-voltage, three-phase motor ratings are 230 V, 460 V, and 575 V.
- All three-phase motors are wired so that the phases are connected together in either a wye (Y) or delta (A) configuration.
- In a single-voltage, wye-connected, three phase motor, one end of each of the three phases is internally connected to the other phases.
- In a single-voltage, delta-connected, three phase motor, each winding is wired end-to-end to form a completely closed loop circuit.
Describe a dual-voltage, three phase motor.
- Most three phase motors are made so that they may be connected for either of two voltages. Making motors for two voltages enables the same motor to be used with two different power line voltages.
- The normal dual-voltage rating of industrial motors is 230/460 V.
- Dual-voltage three phase motors are wired so that the phases are connected in either a wye or delta configuration.
AC Motor Maintenance
Describe motor nameplates and enclosures.
- For the safest service possible, the information given on the motor nameplate should be checked before putting a motor into operation. The nameplate should be checked to ensure that the proper voltage and current are being used.
- Typical enclosures available are open motor enclosures and totally enclosed motor enclosures. When replacing a motor, it is important to ensure that the motor enclosure meets the proper specifications.
Explain the effects of long motor starting times and over-cycling.
- A motor must accelerate to its rated speed within a limited time period. The longer a motor takes to accelerate, the higher the temperature rise in the motor.
- Over-cycling is the process of turning a motor on and off repeatedly. Over-cycling occurs when a motor is at its operating temperature and still cycles on and off. This further increases the temperature of the motor, destroying the motor insulation.
Explain how excessive heat affects a motor.
- The life of motor insulation is shortened as the heat in a motor increases beyond the temperature rating of the insulation. The higher the temperature, the sooner the insulation fails. The temperature rating of motor insulation is listed as the insulation class.
- All motors produce heat as they convert electrical energy to mechanical energy. This heat must be removed to prevent destruction of motor insulation. Motors are designed with air passages that permit a free flow of air over and through the motor. Airflow removes heat from a motor.
- Overheating can also occur if a motor is placed in an enclosed area. A motor overheats due to the recirculation of heated air when a motor is installed in a location that does not permit the heated air to escape.
Describe an overload.
- An overload is the application of excessive load to a motor.
- Motors attempt to drive the connected load when the power is ON. The larger the load, the more power required. All motors have a limit to the load they can drive.
- Overloads should not harm a properly protected motor. Any overload present longer than the delay time built into the protection device is detected and removed.
- An electrician can observe the even blackening of all motor windings that occurs when a motor has failed due to overloading. The even blackening is caused by the slow destruction of the motor over a long period of time.
- Current readings are taken at a motor to determine an overload problem. A motor is overloaded if it is drawing more than rated current.
Explain how altitude affects a motor.
- Temperature rise of motors is based on motor operation at altitudes of 3300′ or less. A motor with a service factor of 1.0 is derated when it operates at altitudes above 3300′. A motor with a service factor above 1.0 is derated based on the altitude and service factor.
Troubleshooting AC Motors
Explain how a short circuit can occur in a motor.
- In a motor, a short circuit can occur due to the following:
- The insulation on the motor winding breaks down due to overheating, which occurs when the motor winding must carry higher currents or the point of the short is the weakest point of the winding insulation.
- The insulation is nicked (or removed) because of a foreign object entering the motor housing (file shaving, etc.).
- There is a manufacturer fault that occurred when the insulation was placed on the winding and the fault only showed up after the motor was operated (subjected to vibration, heat, etc.).
- If motor insulation breaks down between two windings, there is a phase-to-phase short circuit. If motor insulation breaks down between a winding and the ground wire, there is a phase-to-ground short circuit.
Explain how to determine wye or delta connections.
- A DMM is used to measure resistance or a continuity tester is used to determine whether a dual-voltage motor is internally connected in a wye or delta configuration.
- A dual-voltage, wye-connected motor has four separate circuits. A dual-voltage, delta- connected motor has three separate circuits.
- A DMM is used to determine the winding circuits (T1-T4, T2-T5, etc.) on an unmarked motor by connecting one meter lead to any motor lead and temporarily connecting the other meter lead to each remaining motor lead.
- A continuity tester may also be used to determine the winding circuits on an unmarked motor by connecting one test lead to any motor lead and temporarily connecting the other test lead to each remaining motor lead.
