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Figure 33-5 Relative positions of running and starting windings of a four-pole split-phase motor.
The running winding is made up of four sets of coils: A, B, C, and D. A is wound in a reverse direction to B, etc.
The starting winding is 90 electrical degrees displaced from the running winding and consists of coils A', B', C', and D', with every other coil reversed as was the running winding.
The difference in the conductor sizes of the running and starting windings, as well as the difference in the number of turns in the two respective windings, gives a phase displacement and thus a rotating field.
At start, both the running and starting windings are connected across the line. When the motor gets up to % to % the running speed, a centrifugal switch opens, cutting the starting winding out of circuit and the motor pulls into its running speed. Figure 33-6
Figure 33-6 Schematic of a
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illustrates a schematic of a split-phase motor, showing the centrifugal switch that cuts the starting winding in or out of the circuit.
To reverse this motor, the starting or running winding is reversed, but not both. Also, the motor must come to a standstill before reversing. See Figure 33-7.
These motors are also wound for dual voltages, as illustrated in Figure 33-8. Note the starting winding is a 115-volt winding only.
The capacitor-start motor has become an outstanding single-phase motor. It is built very similarly to the split-phase motor, except that the starting winding is heavier. These motors have excellent starting torque. They came into general use with the development of the electrolytic capacitors of large capacitance and of small size. They are practically always dual-voltage motors and can be reversed the same as the split-phase motors. The capacitor is in series with the centrifugal switch, so it is only in circuit during start and for a short period. The number of starts and stops per minute are limited because of the electrolytic capacitor. (See Figure 33-9.) They are reversed in the same manner as the split-phase motor.
Capacitor-Start, Capacitor-Run Motor
This motor has the high starting torque of the capacitor-start motor and is quieter in running than the split-phase motor or the capaci-tor-start motor, as the running capacitor continues to split a phase and the system is more like a polyphase motor. See Figure 33-10.
There is another version of this motor that uses the capacitor to start and to run, but has no centrifugal switch. The starting torque is low, there is no centrifugal switch, and the capacitor stays in circuit all of the time, so the capacitor can’t be of the electrolytic type. See Figure 33-11. These motors are used on fans and oil burners, etc., where low starting torque is required.
CENTRIFUGAL SWITCH Figure 33-7 Reversing a split-
\ phase motor.
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Figure 33-8 Connections for a dual-voltage split-phase motor.
Split-phase and capacitor-type motors are all two-winding motors and are reversed in the same manner as was described previously.
On the repulsion motor the stator is wound with running windings only, and the rotor is an armature (wound) with a commutator. The
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Figure 33-9 Dual-voltage capacitor-start motor.
Figure 33-10 Capacitor-start, capacitor-run motor.
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commutator brushes are short-circuited. The direction of rotation is reversed by brush shifting. See Figure 33-12.
These motors are very similar to the repulsion motor with shorted brushes. The difference is that as the motor comes up to speed, a centrifugal mechanism moves a short-circuiting necklace out, shorting all of the commutator bars, and it then runs as an induction
Figure 33-12 Repulsion motor—shifting short-circuited brushes from A-A' to B-B' changes direction of rotation.
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motor. The starting torque of both the repulsion motor and the repulsion-induction motor is high. Both of these types of motors have been replaced to a large degree by the capacitor-start motors.
1. Can a three-phase motor be started on one phase and, if so, how? Explain fully.
2. Describe the shading-coil motor, and explain how it operates.
3. Explain what a universal motor is.
4. Describe a hysteresis motor and how it operates.
5. What is a split-phase motor?
6. Sketch a dual-voltage, split-phase motor.
7. How do you reverse a split-phase motor?
8. Describe dual-voltage, capacitor-start motors.
9. Describe a capacitor-start, capacitor-run motor.
10. Describe a repulsion motor.
11. Describe a repulsion-induction motor.
Grounding and Ground Testing
Grounding and ground testing seem to mystify many in the electrical industry. This need not be so. Grounding is as good as the precautions that are taken to see that there is a low resistance of the electrical ground circuits to earth.