Motor Capacitor 1

[cenglish]

I must have been asleep in my EE class, but could someone enlighten me on the purpose of a capacitor used with an electric motor? Is it to store energy in order to "jump start" the motor?

I'm looking at a cheap 115vac electric motor for a simple application and it says a 4 uF 370 VAC capacitor is required. How important is it to get a capacitor with exactly these specs? Will the motor run without this capacitor?

 

[JStephen]

I'm sort of fuzzy on that myself. But seems like the motor acts as an inductance, and gets the amps out of phase with the voltage, which requires more amps for same amount of power- and the capacitor helps reduce that effect. I suppose you might get more heat buildup in the motor without it.

 

[electricpete]

The simplest form of single-phase motor produces a forward and reverse rotating field. Once the motor is spinning, it will in a sense "lock on" to the field of interest (call it the forward one).

But before it gets started spinning, it developes no net torque (the forward torque is equal/opposite the backward torque). So it cannot start itself.

To overcome this, single phase motors need some kind of auxiliary winding with a time phase shift introduced. One of the means for doing this is putting a cap in series with that aux winding.

Without the cap, the motor will run but not start (you have to help it get started, which may not be a safe thing to do). I would think 10% or 20% variation in the starting capacitor value will not be a big deal in most cases.

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[electricpete]

By the way, why ask this question on the "mechanical engineer" forum? There is a motor forum on eng-tips

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[cenglish]

Good question electricpete. I'm new to this forum and just posted my question without looking to see if there was a more appropriate place. Thanks for pointing this out.

 

[TheTick]

I was once told the capacitor reduces noise. Not sure whether that meant audible noise or EMI "noise".

 

[Pud]

In the UK, they are called "starting capacitors" so I guess electricpete is correct.

No lawsuits please!


Cheers


Harry

 

[bnrg]

The addition of a capacitor also can reduce current draw due to low power factor.