Few Words About Capacitor-Start (CS) Motors

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Start-run capacitor single-phase induction motor (Source ABB)
Start-run capacitor single-phase induction motor (Source ABB)

Split-phase induction motor //

The capacitor-start (CS), or more precisely, capacitor-start, induction-run motor, is a modified split-phase induction motor used for hard-to-start loads. CS motors are efficient and require starting currents about 5 times their full-load currents.

The schematic Figure 1 shows that the CS motor circuit is the same as the SP motor circuit, except that it includes a centrifugal starting switch and a small-value AC electrolytic capacitor in series with its starting winding.

Single-phase motors - Capacitor start
Figure 1 – Single-phase motors – Capacitor start

The typical torque–speed curves for a CS motor, Figure 2, show that it provides about twice the starting torque of an SP motor. The capacitor lowers the motor’s starting current and increases the phase difference between currents in the running and starting windings to 90°.

(This is about 60° more than the phase difference in SP motors.)

Speed–torque curves for a typical capacitor-start motor
Figure 2 – Speed–torque curves for a typical capacitor-start motor

The capacitor functions only when the CS motor is started, so it can be relatively small and inexpensive. Both the starting winding and capacitor are disconnected by the centrifugal starting switch when the CS motor reaches about 80 percent of its running speed.

The motor then continues to run with only its main winding energized.

CS motors are rated from 1/8 to 3/4 HP.

They run at constant speed under varying loads, offer high running and starting torques, and high overload capacity. Their range of full-load synchronous speeds matches that of SP motors—865 to 3450 rpm when powered from a 120/240-V AC line.

CS motors are used to drive machine tools, refrigerator and air-conditioning compressors, conveyors, pumps, blowers, and fans.

The major components of a fan-cooled CS motor are identified in the exploded view Fig. 10-6. The capacitor in this motor is mounted outside the motor frame in a removable protective housing, to make it easier to replace if necessary.

Replacing a Motor Start Capacitor (VIDEO)

Reference: Handbook of electrical design details – Neil Sclater, J. E. Traister
(Get this book from Amazon)

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About Author


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Mohit gupta
    Jun 05, 2014

    Quality information (awesome)

  2. Yogesh Patil
    May 31, 2014


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