Single-Phase Motors (part 5)

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Universal Motors

The universal motor is often referred to as an AC series motor. This motor is very similar to a DC series motor in its construction in that it contains a wound armature and brushes. The universal motor, however, has the addition of a compensating winding. If a DC series motor is connected to AC, the motor operates poorly for several reasons. The armature windings have a large amount of inductive reactance when connected to AC. Another reason for poor operation is that the field poles of most DC machines contain solid metal pole pieces. If the field is connected to AC, a large amount of power is lost to eddy current induction in the pole pieces. Universal motors contain a laminated core to help prevent this problem. The compensating winding is wound around the stator and functions to counteract the inductive reactance in the armature winding.

The universal motor is so named because it can be operated on AC or DC voltage. When the motor is operated on DC, the compensating winding is connected in series with the series field winding.

Connecting the Compensating Winding for AC:

When the universal motor is operated with AC power, the compensating winding can be connected in two ways. If it’s connected in series with the armature it’s known as conductive compensation.

The compensating winding can also be connected by shorting its leads together. When connected in this manner, the winding acts like a shorted secondary winding of a transformer. Induced current permits the winding to operate when connected in this manner. This connection is known as inductive compensation. Inductive compensation cannot be used when the motor is connected to DC.

++++ Using the series field to set the brushes at the neutral plane position. Armature; Series field; Voltmeter

++++ Using the compensating winding to set the brushes to the neutral plane position. Armature; Compensating winding; Voltmeter

The Neutral Plane:

Because the universal motor contains a wound armature, commutator, and brushes, the brushes should be set at the neutral plane position. This can be done in the universal motor in a manner similar to that of setting the neutral plane of a DC machine. When setting the brushes to the neutral plane position in a universal motor, either the series or compensating winding can be used. To set the brushes to the neutral plane position using the series winding, AC is connected to the armature leads. A voltmeter is connected to the series winding. Voltage is then applied to the armature. The brush position is then moved until the voltmeter connected to the series field reaches a null position. (The null position is reached when the voltmeter reaches its lowest point.) If the compensating winding is used to set the neutral plane, AC is again connected to the armature and a voltmeter is connected to the compensating winding. AC is then applied to the armature. The brushes are then moved until the voltmeter indicates its highest or peak voltage.

Speed Regulation:

The speed regulation of the universal motor is very poor. Because this motor is a series motor, it has the same poor speed regulation as a DC series motor.

If the universal motor is connected to a light load or no load, its speed is almost unlimited. It’s not unusual for this motor to be operated at several thousand revolutions per minute. Universal motors are used in a number of portable appliances where high horsepower and light weight are needed, such as drill motors, skill saws, and vacuum cleaners. The universal motor is able to produce a high horsepower for its size and weight because of its high operating speed.

Changing the Direction of Rotation:

The direction of rotation of the universal motor can be changed in the same manner as changing the direction of rotation of a DC series motor. To change the direction of rotation, change the armature leads with respect to the field leads.

Summary

++ Not all single-phase motors operate on the principle of a rotating magnetic field.

++ Split-phase motors start as two-phase motors by producing an out-of-phase condition for the current in the run winding and the current in the start winding.

++ The resistance of the wire in the start winding of a resistance-start induction run motor is used to produce a phase angle difference between the current in the start winding and the current in the run winding.

++ The capacitor-start induction-run motor uses an AC electrolytic capacitor to increase the phase angle difference between starting and running current.

This causes an increase in starting torque.

++ Maximum starting torque for a split-phase motor is developed when the start-winding current and run-winding current are 90 dgr. out of phase with each other.

++ Most resistance-start induction-run motors and capacitor-start induction-run motors use a centrifugal switch to disconnect the start windings when the motor reaches approximately 75% of full-load speed.

++ The capacitor-start capacitor-run motor operates like a two-phase motor because both the start and run windings remain energized during motor operation.

++ Most capacitor-start capacitor-run motors use an AC oil-filled capacitor connected in series with the start winding.

++ The capacitor of the capacitor-start capacitor-run motor does help to correct the power factor.

++ Shaded-pole induction motors operate on the principle of a rotating magnetic field.

++ The rotating magnetic field of a shaded-pole induction motor is produced by placing shading loops or coils on one side of the pole piece.

++ The synchronous-field speed of a single-phase motor is determined by the number of stator poles and the frequency of the applied voltage.

++ Consequent-pole motors are used when a change of motor speed is desired and high torque must be maintained.

++ Multispeed fan motors are constructed by connecting windings in series with the main run winding.

++ Multispeed fan motors have high-impedance stator windings to prevent them from overheating when their speed is reduced.

++ There are three basic repulsion-type motors: the repulsion motor, the repulsion-start induction-run motor, and the repulsion-induction motor.

++ Repulsion motors have the highest starting torque of any single-phase motor.

++ The direction of rotation of repulsion motors is changed by setting the brushes 15 degrees on either side of the pole pieces.

++ The direction of rotation for split-phase motors is changed by reversing the start winding in relation to the run winding.

++ Shaded-pole motors are generally considered to be nonreversible.

++ There are two types of repulsion-start induction-run motors: the brush riding type and the brush-lifting type.

++ The brush-riding type of motor uses an axial commutator and a short circuiting device, which short-circuits the commutator segments when the motor reaches approximately 75% of full-load speed.

++ The brush-lifting type of repulsion-start induction-run motor uses a radial commutator. A centrifugal device causes the brushes to move away from the commutator and a short-circuiting necklace to short-circuit the commutator when the motor reaches about 75% of full-load speed.

++ The repulsion-induction motor contains both a wound armature and squirrel-cage windings.

++ There are two types of single-phase synchronous motor: the Warren and the Holtz.

++ Single-phase synchronous motors are sometimes called hysteresis motors.

++ The Warren motor operates at a speed of 3600 rpm.

++ The Holtz motor operates at a speed of 1200 rpm.

++ Stepping motors generally operate on DC and are used to produce angular movements in steps.

++ Stepping motors are generally used for position control.

++ Stepping motors can be used as synchronous motors when connected to two-phase AC.

++ Stepping motors operate at a speed of 72 rpm when connected to 60-hertz power.

++ Stepping motors can produce a holding torque when DC is connected to their windings.

++ Universal motors operate on DC or AC.

++ Universal motors contain a wound armature and brushes.

++ Universal motors are also called AC series motors.

++ Universal motors have a compensating winding that helps overcome inductive reactance.

++ The direction of rotation for a universal motor can be changed by reversing the armature leads with respect to the field leads.

QUIZ

1. What are the three basic types of split-phase motors?

2. The voltages of a two-phase system are how many degrees out of phase with each other?

3. How are the start and run windings of a split-phase motor connected in relation to each other?

4. In order to produce maximum starting torque in a split-phase motor, how many degrees out of phase should the start- and run-winding cur rents be with each other?

5. What is the advantage of the capacitor-start induction-run motor over the resistance-start induction-run motor?

6. On the average, how many degrees out of phase with each other are the start- and run-winding currents in a resistance-start induction-run motor?

7. What device is used to disconnect the start windings for the circuit in most non-hermetically sealed capacitor-start induction-run motors?

8. Why does a split-phase motor continue to operate after the start windings have been disconnected from the circuit?

9. How can the direction of rotation of a split-phase motor be reversed?

10. If a dual-voltage split-phase motor is to be operated on high voltage, how are the run windings connected in relation to each other?

11. When determining the direction of rotation for a split-phase motor, should you face the motor from the front or from the rear?

12. What type of split-phase motor does not generally contain a centrifugal switch?

13. What type of single-phase motor develops the highest starting torque?

14. What is the principle of operation of a repulsion motor?

15. What type of commutator is used with a brush-lifting-type repulsion-start induction-run motor?

16. When a repulsion-start induction-run motor reaches about 75% of rated full-load speed, it stops operating as a repulsion motor and starts operating as a squirrel-cage motor. What must be done to cause the motor to begin operating as a squirrel-cage motor?

17. What is the principle of operation of a capacitor-start capacitor-run motor?

18. What causes the magnetic field to rotate in a shaded-pole induction motor?

19. How can the direction of rotation of a shaded-pole induction motor be changed?

20. How is the speed of a consequent-pole motor changed?

21. Why can a multispeed fan motor be operated at lower speed than most induction motors without harm to the motor windings?

22. What is the speed of operation of the Warren motor?

23. What is the speed of operation of the Holtz motor?

24. Explain the difference in operation between a stepping motor and a common DC motor.

25. What is the principle of operation of a stepping motor?

26. What does the term bifilar mean?

27. Why do stepping motors have teeth machined in the stator poles and rotor?

28. When a stepping motor is connected to AC power, how many phases must be applied to the motor?

29. What is the synchronous speed of an eight-pole stepping motor when connected to a two-phase, 60-Hz AC line?

30. How can the holding torque of a stepping motor be increased?

31. Why is the AC series motor often referred to as a universal motor?

32. What is the function of the compensating winding?

Industrial/Practical Applications and other Uses:

You are an electrical contractor, and you have been called to a home to install a well pump. The homeowner has purchased the pump but does not know how to connect it. You open the connection terminal cover and discover that the motor contains eight terminal leads marked T1 through T8. The motor is to be connected to 240 V. At present, the T leads are connected as follows: T1 , T3 , T5 , and T7 are connected together; and T2 , T4 , T6 , and T8 are connected together. L1 is connected to the group of terminals with T1 , and L2 is connected to the group of terminals with T2 . Is it necessary to change the leads for operation on 240 V? If so, how should they be connected?

33. How is the direction of rotation of the universal motor reversed?

34. When the motor is connected to DC voltage, how must the compensating winding be connected?

35. Explain how to set the neutral plane position of the brushes using the series field.

36. Explain how to set the neutral plane position using the compensating winding.

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