Single-Phase Motors (part 2)

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Capacitor-Start Induction-Run Motors

The capacitor-start induction-run motor is very similar in construction and operation to the resistance-start induction-run motor. The capacitor-start induction-run motor, however, has an AC electrolytic capacitor connected in series with the centrifugal switch and start winding. Although the running characteristics of the capacitor-start induction-run motor and the resistance-start induction-run motor are identical, the starting characteristics are not. The capacitor-start induction-run motor produces a starting torque that is substantially higher than that of the resistance-start induction-run motor.



Recall that one of the factors that determines the starting torque for a split phase motor is the phase angle difference between start-winding current and run-winding current. The starting torque of a resistance-start induction-run motor is low because the phase angle difference between these two currents is only about 40 dgr.

++++ Capacitor-start induction-run motor.

When a capacitor of the proper size is connected in series with the start winding, it causes the start-winding current to lead the applied voltage. This leading current produces a 90 dgr. phase shift between run-winding current and start-winding current. Maximum starting torque is developed at this point.

Although the capacitor-start induction-run motor has a high starting torque, the motor should not be started more than about eight times per hour. Frequent starting can damage the start capacitor due to overheating.

If the capacitor must be replaced, care should be taken to use a capacitor of the correct microfarad rating. If a capacitor with too little capacitance is used, the starting current will be less than 90 dgr. out of phase with the running current, and the starting torque will be reduced. If the capacitance value is too great, the starting current will be more than 90 dgr. out of phase with the running current, and the starting torque will again be reduced. A capacitor-start induction-run.

++++ Dual-voltage windings for a split-phase motor. T5 T6 Start windings T1 T2 T3 T4 T7 T8 Run windings

++++ High-voltage connection for a split-phase motor with two run and two start windings. 240 VAC -- Start windings -- Run windings

++++ Low-voltage connection for a split-phase motor with two run and two start windings.120 VAC

Dual-Voltage Split-Phase Motors

Many split-phase motors are designed for operation on 120 or 240 volts. ++++ schematic diagram of a split-phase motor designed for dual-voltage operation. This particular motor contains two run windings and two start windings. The lead numbers for single-phase motors are numbered in a standard manner. One of the run windings has lead numbers of T1 and T2. The other run winding has its leads numbered T3 and T4. This particular motor uses two different sets of start-winding leads. One set is labeled T5 and T6 , and the other set is labeled T7 and T8.

If the motor is to be connected for high-voltage operation, the run windings and start windings are connected in series. The start windings are then connected in parallel with the run windings. If the opposite direction of rotation is desired, T5 and T8 are changed.

For low-voltage operation, the windings must be connected in parallel. This connection is made by first connecting the run windings in parallel by hooking T1 and T3 together and T2 and T4 together.

The start windings are paralleled by connecting T5 and T7 together and T6 and T8 together. The start windings are then connected in parallel with the run windings. If the opposite direction of rotation is desired, T5 and T6 should be reversed along with T7 and T8.

Not all dual-voltage single-phase motors contain two sets of start windings.

++++ the schematic diagram of a motor that contains two sets of run windings and only one start winding. In this illustration, the start winding is labeled T5 and T6. Some motors, however, identify the start winding by labeling it T5 and T8.

Regardless of which method is used to label the terminal leads of the start winding, the connection is the same. If the motor is to be connected for high voltage operation, the run windings are connected in series and the start winding is connected in parallel with one of the run windings. In this type of motor, each winding is rated at 120 volts. If the run windings are connected in series across 240 volts, each winding has a voltage drop of 120 volts.

By connecting the start winding in parallel across only one run winding, it receives only 120 volts when power is applied to the motor. If the opposite direction of rotation is desired, T5 and T8 should be changed.

If the motor is to be operated on low voltage, the windings are connected in parallel as shown in. Because all windings are connected in parallel, each receives 120 volts when power is applied to the motor.

++++ Dual-voltage motor with one start winding labeled T5 and T6 -- Start winding Run winding.

++++ Dual-voltage motor with one start winding labeled T5 and T8.

++++Low-voltage connection for a split-phase motor with one start winding.

++++ Determining direction of rotation for a split-phase motor. Clockwise Counterclockwise

Determining the Direction of Rotation for Split-Phase Motors

The direction of rotation of a single-phase motor can generally be determined when the motor is connected. The direction of rotation is determined by facing the back or rear of the motor. ++++a connection diagram for rotation. If clockwise rotation is desired, T5 should be connected to T1 . If counterclockwise rotation is desired, T8 (or T6) should be connected to T1 . This connection diagram assumes that the motor contains two sets of run and two sets of start windings. The type of motor used determines the actual connection. For example, the connection of a motor with two run windings and only one start winding. If this motor were to be connected for clockwise rotation, terminal T5 would have to be connected to T1, and terminal T8 would have to be connected to T2 and T3. If counterclockwise rotation is desired, terminal T8 would have to be connected to T1 , and terminal T5 would have to be connected to T2 and T3.

Capacitor-Start Capacitor-Run Motors

Although the capacitor-start capacitor-run motor is a split-phase motor, it operates on a different principle than the resistance-start induction-run motor or the capacitor-start induction-run motor. The capacitor-start capacitor run motor is designed in such a manner that its start winding remains energized at all times. A capacitor is connected in series with the winding to provide a continuous leading current in the start winding. Because the start winding remains energized at all times, no centrifugal switch is needed to disconnect the start winding as the motor approaches full speed. The capacitor used in this type of motor is generally of the oil-filled type because it’s intended for continuous use. An exception to this general rule is small fractional-horsepower motors used in reversible ceiling fans. These fans have a low current draw and use an AC electrolytic capacitor to help save space.

The capacitor-start capacitor-run motor actually operates on the principle of a rotating magnetic field in the stator. Because both run and start windings remain energized at all times, the stator magnetic field continues to rotate and the motor operates as a two-phase motor. This motor has excellent starting and running torque. It’s quiet in operation and has a high efficiency. Because the capacitor remains connected in the circuit at all times, the motor power factor is close to unity.

++++ A capacitor-start capacitor-run motor.

++++ Capacitor-start capacitor-run motor with additional starting capacitor. Start Capacitor Run Capacitor Overload Relay Squirrel Cage Rotor Centrifugal Switch

++++A and B Potential starting relays.

++++ Potential relay connection. – Thermostat; Run capacitor; Start capacitor

Although the capacitor-start capacitor-run motor does not require a centrifugal switch to disconnect the capacitor from the start winding, some motors use a second capacitor during the starting period to help improve starting torque. A good example of this can be found on the compressor of a central air-conditioning unit designed for operation on single-phase power. If the motor is not hermetically sealed, a centrifugal switch will be used to disconnect the start capacitor from the circuit when the motor reaches approximately 75% of rated speed. Hermetically sealed motors, however, must use some type of external switch to disconnect the start capacitor from the circuit.

The capacitor-start capacitor-run motor, or permanent split-capacitor motor as it’s generally referred to in the air-conditioning and refrigeration industry, generally employs a potential starting relay to disconnect the starting capacitor when a centrifugal switch cannot be used. The potential starting relay operates by sensing an increase in the voltage developed in the start winding when the motor is operating. A schematic diagram of a potential starting relay circuit. In this circuit, the potential relay is used to disconnect the starting capacitor from the circuit when the motor reaches about 75% of its full speed. The starting-relay coil, SR, is connected in parallel with the start winding of the motor. A normally closed SR contact is connected in series with the starting capacitor. When the thermostat contact closes, power is applied to both the run and start windings. At this point in time, both the start and run capacitors are connected in the circuit.

As the rotor begins to turn, its magnetic field induces a voltage into the start winding, producing a higher voltage across the start winding than the applied voltage. When the motor has accelerated to about 75% of its full speed, the voltage across the start winding is high enough to energize the coil of the potential relay. This causes the normally closed SR contact to open and disconnect the start capacitor from the circuit. Because the start winding of this motor is never disconnected from the powerline, the coil of the potential starting relay remains energized as long as the motor is in operation.

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