Motor start and run capacitor

Application of start and run capacitor for single phase motor.

Why we applied a capacitor for single phase induction motor?The reason is to improve performance of motor by generate the rotating field.Design for this motor have a two ( 2 ) stator winding,and one of it is the auxiliary and is connected by a suitable capacitor size.

The capacitor increases the starting torque and the full load power as the current flowing through the auxiliary winding, de-phased by the capacitor, creates a rotating field.The voltage across capacitor usually higher than the mains supply.

The choice of the capacitance is usually empirically made on the motor. This is because of shuttle differences between motor manufacturers.As the torque and the power of the motor also depends on the reactive power of the capacitor.

Starting capacitor (electrolytic)

For starting a single phase motor,it required a high capacitance of capacitor for a high starting torque .Start capacitors are designed for momentary use.Generally this type of motor used a centrifugal switch or Potential relay to disconnected this when approximately 3/4 the speed is reached to avoid overloads of the auxiliary winding.

The capacitance has to be such as to provide a reactive power about double of the nominal power of the motor with a Vc ( motor working voltage ).Start capacitors normally ratings of 70 micro farad or higher.It also have three voltage classifications: 125V, 250V, and 330V.

Running capacitor (non-electrolytic)

After the initial start of the motor this capacitor is also needed to obtain the full load power of the motor where it then remains in continuous operation with the motor and forming an integral part of this circuit.

The motor working voltage ( Vc )  is about 1,5-2 times the mains supply and decrease with the increase of the load.With capacitive reactive power of about 75% of the nominal power of the motor, the comparison of power is slightly lower than that of a three-phase motor of equal size.

For running capacitor are rated in a range of 3-70 microfarad. It also rated by voltage classification from 370Vac and 440Vac.The sizing of rated is a critical part for running capacitor.If the wrong run capacitor is installed, the motor will not have an even magnetic field.

This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheats.


  1. Thanks Joe, for the explanation of the distinction between Start & Run Caps.

  2. Hi. I’m trying to repair my washing machine and found that the motor run capacitor was defective. The motor run capacitor is 9 microfarad 450 VAC 50/60 Hz. I was looking around for a replacement and could only find a motor run capacitor of 10 microfarad 450 VAC 50/60 Hz. I was told that this capacitor of 10 microfarad can be used in place of the 9 microfarad. Is this true? Please help. Thanks

    • Hi Jun..if you cannot find same size for capacitor ( 9 microfarad ),you can used capacitor 10 microfarad.It ok because the different only 1 can effected to life span of motor if different value for microfarad over than 20%…

  3. Andrew Pearce says

    Thanks for so many interesting articles,
    whilst siply trying to fix an old generator I now have a much better underpinning of knowledge…
    Is it possible to calculate the size of capacitor required in a generator? mine has blown right though the rating printed on it!
    Its a 50hz generator but to confuse the maths it is both capable of 230v and 110v outputs (both at 2200w), what will the effect of putting an incorrect rating cap in be? a different output voltage?
    Many Thanks- Andrew Pearce

  4. i have a 125v 50-60hz motor would 3 phase power work with it

  5. sandipan says

    180 w motor single phase what the capacitor needed .(in micro farad)

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