Repulsion-Type Motors

Repulsion-Type Motors

Repulsion-type motors are divided into several groups including repulsion­ start, induction-run motors; repulsion motors; and repulsion-induction motors. Regardless of the varying types, there are certain construction characteristics which are common to all of them:

• Each has a stator similar to the running winding on a split-phase motor.

• The rotor consists of a slotted core with embedded windings con­ nected to a commutator at one end of the rotor.

• Bearings are mounted in the end plates to support the rotor shaft.

• Carbon brushes are fitted in holders and ride on the commutator to provide a path for current flow between commutator segments and each brush.

• Brush holders are supported in the motor, either on the front end plate or on the rotor shaft.

REPULSION-START, INDUCTION-RUN MOTORS

The repulsion-start, induction-run type of motor is of the single-phase type, ranging in size from about Yto hp to as high as 20 hp. It has high starting torque and a constant-speed characteristic, which makes it suit­ able for such applications as commercial refrigerators, compressors,

pumps, and similar applications requiring high starting torque.

There are two different designs of repulsion-start, induction-run motors: the brush-lifting type and the brush-riding type. In the former, the brushes are automatically moved away from the commutator when the motor reaches approximately 75o/o of its full speed. In the brush­ riding type, the brushes ride on the commutator at all times. The brush­ riding arrangement (with an axial form of commutator) is used almost ex­ clusively on smaller motors, whereas the brush-lifting type (with a radial form of commutator) is used in both small and large motors. See Fig. 4-1.

REPULSION MOTORS

The repulsion motor is distinguished from the repulsion-start, induction­ run motor by the fact that it is made exclusively as a brush-riding type and does not have any centrifugal mechanism. Therefore, this motor both starts and runs on the repulsion principle. This type of motor has high starting torque and a variable-speed characteristic. It is reversed by shifting the brush holder to either side of the neutral position. Its speed can be decreased by moving the brush holder farther away from the neutral position.

The stator of the repulsion motor is like that of the repulsion-start, induction-run motor, and the stator is generally wound for four, six, or eight poles. Four leads are normally brought out of the motor for dual voltage operation.

The rotor consists of an armature constructed much in the same manner as those used in de motors with laminations and is also generally skewed. The windings may be either hand or coil wound and are connected either with lap or wave windings. The commutator is of the axial type, and the brushes always ride on the commutator.

REPULSION-START-INDUCTION MOTORS

In the repulsion-start-induction type of motor are combined the high starting torque of the repulsion type and the good speed regulation of the induction motor. The stator of this motor is provided with a regular single-phase winding, while the rotor winding is similar to that used on a de motor. When starting, the changing single-phase stator flux cuts across the rotor windings, inducing currents in them; when flowing through the commutator, a continuous repulsive action upon the stator poles is present.

This motor starts as a straight repulsion type and accelerates to about 75o/o of normal full speed when a centrifugally operated device con­ nects all the commutator bars together and converts the winding to an equivalent squirrel cage type. The same mechanism usually raises the brushes to reduce noise and wear. Note that when the machine is operat­ ing as a repulsion type the rotor and stator poles reverse at the same in­ stant and that the current in the commutator and brushes is ac.

This type of motor will develop four to five times normal full-load torque and will draw about three times normal full-load current when starting with full-line voltage applied. The speed variation from no load to full load will not exceed 5o/o of normal full-load speed.

The repulsion-start-induction motor is used to power air compres­ sors, refrigeration, pumps, meat grinders, small lathes, small conveyors, stokers, and the like. In general, this type of motor is suitable for any load that requires a high starting torque and constant-speed operation.

Most motors of this type are less than 5 hp.

Troubles occurring in this type of motor can be found in the com­ mutator, brushes, centrifugal switch, short-circuited rig, bearings, oil­ soaked insulation, solder thrown out of commutator, too much or too lit­ tle tension on the throw-out centrifugal spring, and opens, shorts, or grounds in the rotor of stator windings. Rotation is reversed by shifting the brushes.

MOTOR DRIVES

The type of motor selected for a given application should be of sufficient capacity to avoid overloading but not so large to cause it to be inefficient and to make the initial cost higher than it should be. It is usually better to install a small motor on each machine being driven than to install one large motor to operate a drive shaft to operate all the machines.

The proper design of motor drives for machines involves the selec­ tion of a method of connecting the motor to the driven machine in an ade­ quate and efficient manner. The selection depends on power supply, space limitations, safety and working conditions for operators, initial cost and operating costs, production and quality of product, and surrounding con­ ditions. The objective is to obtain a working installation which will be the best when all these items are considered.

Motors can be connected to machines in several ways. When the speed of the motor is identical to the speed desired for the driven ma­ chine, a direct coupling is used to connect the motor shaft with the ma­ chine shaft. When the speed of the motor is different from the speed of the machine shaft, a connection must be used which will give the proper speed ratio between the motor and the machine. A speed ratio may be ob­ tained by using a V belt, flat belt, chain or set of gears. With belts, the speed is inversely proportional to the effective diameter of the two pulleys. With a chain or gears, the speed ratio is inversely proportional to the number of teeth on the two sprockets or gears.