OBJECTIVES
After studying this unit, the learner will be able to:
• describe the basic steps in the operation of the following types of motors:
- repulsion motor
- repulsion start, induction run motor repulsion-induction motor
• state the basic construction differences among the motors listed in objective 1.
• compare the motors listed in objective 1 with regard to starting torque and speed performance.
Repulsion-type motors are divided into three distinct classifications: the repulsion motor; the repulsion start, induction run motor; and the repulsion-induction motor. Although these motors are similar in name, they differ in construction, operating characteristics, and industrial applications.
REPULSION MOTOR
A repulsion motor basically consists of the following parts:
Laminated stator core with one winding. This winding is similar to the main or running winding of a split-phase motor. The stator usually is wound with four, six, or eight poles.
Rotor consisting of a slotted core into which a winding is placed. The rotor is similar in construction to the armature of a dc motor. Thus, the rotor is called an armature. The coils which make up this armature winding are connected to a commutator. The commutator has segments or bars parallel to the armature shaft.
Carbon brushes contacting with the commutator surface. The brushes are held in place by a brush holder assembly mounted on one of the end shields. The brushes are connected together by heavy copper jumpers. The brush holder assembly may be moved so that the brushes can make contact with the commutator surface at different points to obtain the correct rotation and maximum torque output. There are two types of brush arrangements:
1. Brush riding — the brushes are in contact with the commutator surface at all times.
2. Brush lifting — the brushes lift at approximately 75 percent of the rotor speed.
Two cast steel end shields. These shields house the motor bearings and are secured to the motor frame.
Two bearings supporting the armature shaft. The bearings center the armature with respect to the stator core and windings. The bearings may be sleeve bearings or ball bearing units.
Cast steel frame into which the stator core is pressed.
Operation of a Repulsion Motor
The connection of the stator winding of a repulsion motor to a single-phase line causes a field to be developed by the current in the stator windings. This stator field induces a voltage and a resultant current in the rotor windings. If the brushes are placed in the proper position on the commutator segments, the current in the armature windings will set up proper magnetic poles in the armature.
These armature field poles have a set relationship to the stator field poles. That is, the magnetic poles developed in the armature are set off from the field poles of the stator winding by about 15 electrical degrees. Furthermore, since the instantaneous polarity of the rotor poles is the same as that of the adjacent stator poles, the repulsion torque created causes the rotation of the motor armature.
The three diagrams of figure 23-1 show the importance of the brushes being in the proper position to develop maximum torque. In figure 23-1A, no torque is developed when the brushes are placed at right angles to the stator poles. This is due to the fact that the equal induced voltages in the two halves of the armature winding oppose each other at the connection between the two sets of brushes. Since there is no current in the windings, flux is not developed by the armature windings.
In figure 23-1 B, the brushes are in a position directly under the center of the stator poles. A heavy current exists in the armature windings with the brushes in this position, but there is still no torque. The heavy current in the armature windings sets up poles in the armature. However, these poles are centered with the stator poles and a torque is not created either in a clockwise or counterclockwise direction.
Fig. 23—1 Repulsion motor operation: a. No torque created, equal
voltage values oppose each other (soft neutral); b. No torque even though
current value in armature is high (hard neutral); c. Counterclockwise rotation;
brushes in correct position.
Fig. 23—2 Reversing the direction of rotation of a repulsion
motor
In figure 23-1C, the brushes have shifted from the center of the stator poles 15 electrical degrees in a counterclockwise direction. Thus, magnetic poles of like polarity are set up in the armature. These poles are 15 electrical degrees in a counterclockwise direction from the stator pole centers. A repulsion torque is created between the stator and the rotor field poles of like polarity. The torque causes the armature to rotate in a counter clockwise direction. A repulsion machine has a high starting torque, with a small starting current, and a rapidly decreasing speed with an increasing load.
The direction of rotation of a repulsion motor is reversed if the brushes are shifted electrical degrees from the stator field pole centers in a clockwise direction, figure 23-2. As a result, magnetic poles of like polarity are set up in the armature. These poles are 15 electrical degrees in a clockwise direction from the stator pole centers. Repulsion motors are used principally for constant-torque applications, such as printing-press drives, fans, and blowers.
REPULSION START, INDUCTION RUN MOTOR
A second type of repulsion motor is the repulsion start, induction run motor. In this type of motor, the brushes contact the commutator at all times. The commutator of this motor is the more conventional axial form.
A repulsion start, induction run motor consists basically of the following parts:
Laminated stator core. This core has one winding which is similar to the main or running winding of a split-phase motor.
Rotor consisting of a slotted core into which a winding is placed. The coils which make up the winding are connected to a commutator. The rotor core and winding are similar to the armature of a dc motor. Thus, the rotor is called an armature.
Centrifugal device.
a. In the brush-lifting type of motor, there is a centrifugal device which lifts the brushes from the commutator surface at 75 percent of the rated speed. This device consists of governor weights, a short-circuiting necklace, a spring barrel, spring, push rods, brush holders, and brushes. Although high in first cost, this device does save wear and tear on brushes, and runs quietly. Figure 23—3 is an exploded view of the armature, radial commutator, and centrifugal device of the brush-lifting type of repulsion start, induction run motor.
b. The brush-riding type of motor also contains a centrifugal device which operates at 75 percent of the rated speed. This device consists of governor weights, a short-circuiting, necklace, and a spring barrel. The commutator segments are short circuited by this device, but the brushes and brush holders are not lifted from the commutator surface.
Fig. 23—3 An exploded view of a radial commutator and centrifugal
brush-lifting device for a repulsion start, Induction run motor
Commutator. The brush-lifting type of motor has a radial-type commutator (figure 23—3). The brush-riding type of motor has an axial commutator (figure 23—4).
Brush holder assembly.
a. The brush holder assembly for the brush-lifting type of motor is arranged so that the centrifugal device can lift the brush holders and brushes clear of the commutator surface.
b. The brush holder assembly for the brush-riding type of motor is the same as that of a repulsion motor.
End shields, bearings, and motor frame. The parts are the same as those of a repulsion motor.
Operation of the Centrifugal Mechanism
Refer to figure 23—4 to identify the components of the centrifugal mechanism. The operation of this device consists of the following steps. As the push rods of the centrifugal device move forward, they push the spring barrel forward. This allows the short-circuiting necklace to make contact with the radial commutator bars thus are all short circuited. At the same time, the brush holders and brushes are moved from the commutator surface. As a result, there is no unnecessary wear on the brushes and the commutator surface and there are no objectionable noises caused by the brushes riding on the radial commutator surface.
Fig. 23—4 An exploded view of a short-circuiting device for a
brush-riding, repulsion start, induction run motor
The short-circuiting action of the governor mechanism and the commutator segments converts the armature to a form of squirrel-cage rotor and the motor operates as a single-phase induction motor. In other words, the motor starts as a repulsion motor and runs as an induction motor.
In the brush-riding type of motor, all axial commutator is used. The centrifugal mechanism (figure 23—4) consists of a number of copper segments which are held in place by a spring. This device is placed next to the commutator. When the rotor reaches 75 percent of the rated speed, the centrifugal device short circuits the commutator segments. The motor then will continue to operate as an induction motor.
Operation of a Repulsion Start, Induction Run Motor
The starting torque is good for either the brush-lifting type or the brush-riding type of repulsion start, induction run motor. Furthermore, the speed performance of both types of motors is very good since they operate as single-phase induction motors.
Because of the excellent starting and running characteristics for both types of repulsion start, induction run motors, they were used for a variety of industrial applications, including commercial refrigerators, compressors, and pumps.
The direction of rotation for a repulsion start, induction run motor is changed in the same manner as that for a repulsion motor, that is, by shifting the brushes past the stator pole center 15 electrical degrees.
The symbol in figure 23—5 represents both a repulsion start, induction run motor and a repulsion motor.
Many repulsion start, induction run motors are designed to operate on 115 volts or 230 volts. These dual-voltage motors contain two stator windings. For 115-volt operation, the stator windings are connected in parallel; for 230-volt operation, the stator windings are connected in series. The diagram in figure 23—6 represent a dual-voltage, repulsion start, induction run motor. The connection table shows how the leads of the motor are connected for either 115-volt operation or 230-volt operation. These connections also can be used for dual-voltage repulsion motors.
Fig. 23—5 Schematic diagram symbol of a repulsion start, induction
run motor and a repulsion motor. Fig. 23—6 Schematic diagram of
a dual-voltage, repulsion start, induction run motor.
REPULSION-INDUCTION MOTOR
The operating characteristics of a repulsion-induction motor are similar to those of the repulsion start, induction run motor. However, the repulsion-induction motor has no centrifugal mechanism. It has the same type of armature and commutator as the repulsion motor, but it has a squirrel-cage winding beneath the slots of the armature.
Fig. 23—7 An armature of a repulsion-induction motor: slots for
regular winding; squirrel-cage winding.
Figure 23—7 shows a repulsion-induction motor armature with a squirrel-cage winding. One advantage of this type of motor is that it has no centrifugal device requiring maintenance. The repulsion-induction motor has a very good starting torque since it starts as a repulsion motor. At start up, the repulsion winding predominates; but, as the motor speed increases, the squirrel-cage winding is used most. The transition from repulsion to induction operation is smooth since no switching device is used. In addition, the repulsion-induction motor has a fairly constant speed regulation from no load to full load because of the squirrel-cage winding. The torque-speed performance of a repulsion-induction motor is similar to that of a dc compound motor.
A repulsion-induction motor can be operated on either 115 volts or 230 volts. The stator winding has two sections which are connected in parallel for 115-volt operation, and in series for 230-volt operation. The markings of the motor terminals and the connection arrangement of the leads is the same as in a repulsion start, induction run motor.
The symbol in figure 23—5 also represents a repulsion-induction motor (as well as a repulsion start, induction run motor and a repulsion motor.)
NATIONAL ELECTRICAL CODE REGULATIONS
Regulations for the motor branch circuit overcurrent protection, motor running over- current protection, and wire sizes for motor circuits are given in Article 430 of the National Electrical Code. Refer also to Example 8, section 9 of the Code.
SUMMARY
Repulsion motors are available in three basic designs: (1) repulsion motors, (2) repulsion start, induction run motors, and (3) repulsion-induction motors. Theses motors are easy to recognize because they are ac induction motors but use a commutator and brushes. The important points to remember is that the motors have neutral positions of the brush mountings that yield no motor movement. These neutral positions are referred to as hard or soft neutral. The brushes are shifted off neutral to give the motor the desired direction of rotation.
REVIEW QUIZ
1. What is a repulsion motor, and how is rotation produced?
2. Name one application of a repulsion motor.
3. Describe the operation of a repulsion start, induction run motor.
4. Explain the difference between the brush-lifting type of repulsion start, induction run motor and the brush-riding type of repulsion start, induction run motor.
5. A 2-hp, 230-volt, 12-ampere, single-phase repulsion start, induction run motor is connected directly across the rated line voltage.
a. Determine the overcurrent protection for the branch circuit feeding this motor.
b. Determine the running overcurrent protection for this motor.
6. What size wire is used for the branch circuit feeding the motor given in question 5?
7. Describe the construction of a repulsion-induction motor.
8. What is one advantage to the use of the repulsion-induction motor as compared to the repulsion start, induction run motor?___________
9. Explain how the direction of rotation is changed for any one of the three types of single-phase repulsion motors covered in this unit. __________
10. Insert the correct word or phrase to complete each of the following statements.
a. A repulsion-induction motor has a good ______ and a fairly good
b. A repulsion motor has a high starting torque and its speed rapidly decreases with
c. The centrifugal short-circuiting device on a repulsion start, induction run motor operates at approximately ____ of the rated speed.
d. Both the repulsion start, induction run motor, and the repulsion-induction motor operate as after they have accelerated to rated speed.