All though in both types of Induction Motors, capacitor does the job of phase shifting to produce the rotating magnetic field, their working principle is the same. Then whats the point in disconnecting the capacitor by centrifugal switch for capacitor start, and not disconnecting for the capacitor run motors. I mean is either of the Induction Motor designated for particular power rating, plus i read somewhere that capacitor start Induction Motor has high starting torque. Also when i browsed the internet found this piece of info that capacitor run motor has improved p.f. due to start capacitor, so isn`t it always advantageous to have capacitor permanently to improve motor efficiency.

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    \$\begingroup\$ Please avoid use of the weird abbreviations that your question is littered with. For example, exactly what does "m/r" mean? For that matter, the abbreviation "IM" would mean "Inter Modulation" to most people who work with RF or audio. \$\endgroup\$ Commented Jun 17, 2015 at 4:30
  • \$\begingroup\$ sorry for the abbreviation, corrected them. Didnt know it had several other meanings. dnx to note me @DwayneReid \$\endgroup\$ Commented Jun 17, 2015 at 9:25

2 Answers 2


A Capacitor Start Motor not only switches out the capacitor when up to speed, but also switches out the starting winding which is in series with the start capacitor. Capacitor Start motors have very high starting torque for a single phase AC motor. The same is true for a capacitor start / capacitor run motor WITH a centrifugal switch. In this case, the run capacitor is a different value and just provides some additional phase lag especially to help the motor maintain speed under load, and improve the power factor. A Capacitor start/run motor without a centrifugal switch has a lower starting torque, but good performance under load, and again, an improved power factor.

  • \$\begingroup\$ so if a capacitor start and capacitor start/run motor were designed for the same power rating, and since after start auxiliary winding in capacitor start motor is switched out wouldn`t this motor takes less current than the capacitor start/run as it has the main and auxiliary windings working throughout the motor operation. Does this difference make any sense while comparing the two motors ? \$\endgroup\$ Commented Jun 18, 2015 at 7:58
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    \$\begingroup\$ Induction Motors only take the current they need. They don't draw full load current all the time. A capacitor start only motor has a very high starting torque, but can sag in speed under load. A capacitor start/run motor has a lower starting torque, but doesn't sag nearly as much when heavily loaded. A typical good example of Capacitor start is an air compressor motor. A typical example of Capacitor run is a table saw motor. \$\endgroup\$
    – R Drast
    Commented Jun 18, 2015 at 11:15

Why not experiment with a capacitor-start motor yourself to see what happens if the start capacitor remains connected after the motor has reached operating speed.

You would notice two or three things:

1) the motor starts to get very hot.

2) the motor does NOT reach full speed until the starting capacitor / winding is disconnected.

3) the motor makes unpleasant sounds. Sort of a growling sound.

Like you, I was curious when I was a youngster. So I experimented to find out what happens.

That experience has done me well over the years - it makes diagnosing a malfunctioning motor easy.

  • \$\begingroup\$ so aren`t these problems encountered in a normal capacitor run induction motor as they have the capacitor throughout the motor operation \$\endgroup\$ Commented Jun 17, 2015 at 9:29
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    \$\begingroup\$ @FiazMR: Tweaking the design of an induction motor with two windings and a capacitor to maximize starting torque will require compromises in efficiency and smoothness of operation. Motors which are not designed to use a cap-delayed winding on a continuous basis may, in the interest of maximizing torque, accept compromises in efficiency and smoothness beyond those that would be acceptable for continuous operation. \$\endgroup\$
    – supercat
    Commented Jun 17, 2015 at 17:09

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