Tuesday, January 13, 2015

Single-Phase Motors : Introduction , Single-phase induction motors and Resistance-start, induction-run motor.

Single-Phase Motors
INTRODUCTION

There are a number of types of single-phase motors. The types covered in this unit are the single-phase induction motor, the repulsion motor, the repulsion-induction motor, the series motor, and the shaded-pole motor. Three-phase motors generally perform better than single-phase motors. However, in many instances, only single-phase service is avail- able. Most single-phase motors have fractional horsepower ratings. In general, their use is limited to commercial and residential applications.

SINGLE-PHASE INDUCTION MOTORS

There are two main types of single-phase induction motors: the resistance-start, induction-run motor and the capacitor-start, induction-run motor. These motors have fractional horsepower ratings. The resistance-start induction motor is used in appliances and with other small loads where a strong starting torque is not required. The capacitor- start induction motor is used on refrigerators, compressors, and similar loads. Both types of motors are low in cost, are rugged, and have good performance. These induction motors are also called split-phase motors. That is, the capacitor or resistance is used to obtain a phase change for one winding, resulting in a rotating field.

RESISTANCE-START, INDUCTION-RUN MOTOR

Construction

The basic parts of a resistance-start, induction-run motor are

• the stator (stationary part).

• the rotor (revolving part).

• a centrifugal switch, located inside the motor.

• two end shields bolted to the steel frame; these shields house the rotor shaft bearings.

• a cast steel frame; the stator core is pressed into this frame.

Stator. The stator consists of two windings, held in place in the slots of a laminated steel core. The windings are made up of insulated coils. The coils are placed so that they are 90 electrical degrees apart. One winding is the main (running) winding. The other winding is the auxiliary (starting) winding.

The running winding is made of a heavy insulated copper wire. It is located at the bottom of the stator slots. A small wire size is used in the starting winding. This winding is placed near the top of the slots above the running winding.

At start-up, both windings are connected in parallel to the single-phase line. Once the motor accelerates to two-thirds or three-quarters of the rated speed, a centrifugal switch disconnects the starting winding automatically.

Rotor. The rotor of the resistance-start, induction-run motor (Figure 18–1) is the same as that of a three-phase, squirrel-cage induction motor. The cylindrical core of the rotor

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consists of steel laminations. Copper bars are mounted near the surface of the rotor and are brazed or welded to two copper end rings. The rotor may also be a one-piece case aluminum unit. This type of rotor requires very little maintenance. It contains no windings, brushes, slip rings, or commutator where faults may develop. Fans are provided with the rotor to keep the temperature of the windings at a reasonable level.

Centrifugal Switch. The centrifugal switch is mounted inside the motor. It disconnects the starting winding after the rotor reaches a predetermined speed. The switch has a station- ary part that is mounted on one of the end shields. This part has two contacts that act like a single-pole, single-throw switch. The centrifugal switch also has a rotating part that is mounted on the rotor. A typical centrifugal switch is shown in Figure 18–2. This switch is commonly used on split-phase induction motors.

The operation of a centrifugal switch is shown in Figure 18–3. When the rotor is not moving, the pressure of a spring on the fiber ring of the rotating part of the switch keeps the contacts closed. At three-quarters of the rated motor speed, the centrifugal action of the

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rotor causes the spring and fiber ring to release its pressure and opens the switch contacts. As a result, the starting winding circuit is disconnected from the line.

Frame and End Shields. The stator core is pressed directly into the cast steel frame. The two end shields are bolted to the frame. The shields contain bearings that support the rotor and center it in the stator. Thus, the shaft rotates with little friction and does not strike or rub against the stator core.

Principle of Operation

At the instant the motor circuit is closed, both the starting and running windings are energized. Because the running winding uses large-size wire, it has a low resistance. But the running winding is placed at the bottom of the stator core slots. Thus, its inductive reactance is high. Because of its low resistance and high inductive reactance, the current of the running winding lags behind the voltage.

The current in the starting winding is more nearly in phase with the voltage. Small wire is used in this winding, resulting in a high resistance. Because the winding is near the top of the stator slots, the mass of iron surrounding it is small. This means that its inductive reactance is small. Because the starting winding has a high resistance and a low inductive reactance, its current is more nearly in phase with the voltage.

Figure 18–4 shows the relationship between the currents in the windings and the volt- age. The current of the main winding (IM ) lags the current of the starting winding (IS) by nearly 90 electrical degrees. When a current passes through each of these windings, the resulting pulsating field effect gives rise to a rotating field around the inside of the stator core. The speed of this rotating magnetic field is determined in the same way it was found for a three-phase induction motor.

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Synchronous Speed. Consider a single-phase, resistance-start, induction-run motor with four poles wound into both the main windings and the starting winding. If this motor is energized from a 60-Hz source, the synchronous speed of the revolving field is

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While traveling at the synchronous speed, the rotating field cuts the copper bars of the squirrel-cage winding. Voltages are induced in the windings and cause currents in the rotor bars. A rotor field is created. This field reacts with the stator field to develop the torque that causes the rotor to turn.

As the rotor accelerates to approximately 80% of the rated speed, the centrifugal switch disconnects the starting winding from the line. The connections for the centrifugal switch are shown in Figure 18–5. At start-up, the switch is closed. As the motor accelerates to its normal running speed, the centrifugal switch opens. The motor then continues to operate, using only the running winding.

Once the motor is running, current is induced in the rotor for two reasons: (1) the alternating stator flux induces “transformer voltage” in the rotor, and (2) “speed voltage” is induced in the rotor bars as they cut across the magnetic field of the stator. The combined effect of these alternating voltages produces a torque that keeps the rotor turning.

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Energized Windings. A resistance-start, induction-run motor must have the starting winding and the main winding energized at start-up. The motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. However, a single-phase source supplies the motor. Therefore, the motor is called a split-phase motor because it starts like a two-phase motor from a single-phase line. Once the motor accelerates to a value near the rated speed, it operates on the running winding as a single-phase induction motor.

If the centrifugal switch mechanism fails to close the switch contacts when the motor stops, the starting winding circuit will be open. This means that when the motor circuit is reenergized, the motor will not start. Both the starting and running windings must be energized at the instant the motor circuit closes if the necessary starting torque is to be formed. If the motor does not start, but a low humming sound is present, then one winding is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or the switch contacts are pitted.

If only one stator winding is energized, an alternating field, rather than a rotating field, is formed. If the rotor is at rest, this field induces an alternating current in the rotor winding. This current acts as the secondary of a transformer. Rotor poles are developed by this induced current. These poles are exactly aligned with the stator poles. Thus, no starting torque is developed in either direction of rotation.

If a split-phase motor is started with too great a mechanical load, it may not accelerate enough to open the centrifugal switch. Also, if a low terminal voltage is applied to the motor, the motor may fail to reach the speed required to operate the centrifugal switch.

Starting Winding. The starting winding uses a small size of wire, resulting in a large resistance. The starting winding is designed to operate across the line voltage for just three or four seconds as the motor accelerates to the rated speed. Therefore, the starting winding must be disconnected from the line by the centrifugal switch as soon as the motor accelerates to three-quarters of the rated speed. If the motor operates on its starting winding for more than sixty seconds, the winding may char or burn out.

To reverse the rotation of the motor, simply interchange the leads of either the starting winding or the running winding, preferably the starting winding. Interchanging these leads reverses the direction of rotation of the stator field (and the rotor).

Dual-Voltage Motor

In many cases, single-phase motors have dual voltage ratings such as 120 V and 240 V. The running winding consists of two sections, each of which is rated at 120 V. One section of the running winding is marked T1 and T2. The other section is marked T3 and T4. The starting winding consists of only one 120-V winding. The leads of the starting winding are marked T5 and T6. To operate the motor on 240 V, the two 120-V sections of the running winding are connected in series across the 240-V line. The starting winding is paralleled with one section of the running winding.

To operate the motor on 120 V, the two 120-V sections of the running winding are connected in parallel across the 120-V line. The starting winding is connected in parallel with both sections of the running winding.

Figure 18–6 shows the circuit connections for a dual-voltage motor connected for 120-V operation. The connections for 240-V operation are shown in Figure 18–7. In this case, the jumpers are changed in the terminal box so that the two 120-V running windings are connected in series. These sections are then connected across the 240-V line. Note that the 120-V starting winding is connected in parallel with one section of running winding. If the voltage drop across this section of the running winding is 120 V, then the voltage across the starting winding is 120 V. The starting winding may also consist of two sections for some dual-voltage, resistance-start, induction-run motors. As before, the two running winding sections are marked T1 and T2 and T3 and T4. The two starting winding sections are marked “T5 and T6” and “T7 and T8” (Figure 18–8).

The table in Figure 18–8 shows the correct connections for 120-V operation and for 240-V operation.

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Speed Regulation. A resistance-start, induction-run motor has very good speed regulation. From no load to full load, the speed performance of this type of motor is about the same as that of a three-phase, squirrel-cage induction motor. The percent slip on most fractional horsepower split-phase motors ranges from 4% to 6%.

Starting Torque. The starting torque of the motor is poor. Although the main winding consists of large wire, it has some resistance. Also, the starting winding has an inductive reactive component, even though it is placed near the top of the stator slots. As a result, the current in the main winding does not lag the line voltage by a full 90° because of the resistance. The current in the starting winding is not in phase with the line voltage because of the inductive reactance.

Figure 18–9 shows the phase angle between the current in the main winding and the current in the starting winding for a typical resistance-start, induction-run motor. A phase angle in the order of 30° to 50° is large enough to provide a weak rotating magnetic field. As a result, the starting torque is small.