CAPACITOR-START MOTOR Motor Starting Methods


The capacitor motor is slightly different from a splitphase motor. A capacitor is placed in the path of the electrical current in the start winding (Fig. 12 4). Except for the capacitor, which is an electrical component that slows any rapid change in current, the two motors are the same electrically. A capacitor motor can usually be recognized by the capacitor can or housing that is mounted on the stator (Fig. 12 5).

A capacitor is placed in the path of the electrical current in the start winding

Adding the capacitor to the start winding increases the effect of the two phase field described in connection with the split phase motor. The capacitor means that the motor can produce a much greater twisting force when it is started. It also reduces the amount of electrical current required during starting to about 1.5 times the current required after the motor is up to speed. Split phase motors require three or four times the current in starting than they do in running.


capacitor can or housing that is mounted on the stator


An induction motor will not always reverse while running. It may continue to run in the same direction but at a reduced efficiency. An inertia type load is difficult to reverse. Most motors that are classified as reversible while running will reverse with a noninertial type load. They may not reverse if they are under no load conditions or have a light load or an inertial load

One of the problems related to the reversing of a motor while it is still running is the damage done to the transmission system connected to the load. In some cases it is possible to damage a load. One of the ways to avoid this is to make sure that the right motor is connected to a load

Reversing (while standing still) the capacitor start motor can be done by reversing its start winding connections. This is usually the only time that it will work on a motor. The available replacement motor may not be rotating in the direction desired, so the electrician will have to locate the start winding terminals and reverse them in order to have the motor start in the desired direction.

Figure 12 6A shows a capacitor start, inductionrun motor used in a compressor. This type uses a relay to place the capacitor in and out of the circuit. More details regarding this type of relay will be given later. Figure 12 6B shows how the capacitor is located outside the compressor



Capacitor motors are available in sizes from 1/6 to 20 hp. They are used for fairly hard starting loads that can be brought up to run speed in under 3 seconds. They may be used in industrial machine tools, pumps, air conditioners, air compressors, conveyors, and hoists.

capacitor start, inductionrun motor used in a compressor

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