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# AC Motor Series Capacitor

#### Athosworld

##### Member
Why does adding a capacitor in series to an AC motor increase its speed? The capacitor does boost the voltage somehow because the input voltage is 132v (mexican mains) and coming out from the capacitor are 230v. This has to do with the inductance because it wont work without the motor. I made it, photos below:

You are likely creating a "Series tuned circuit" - a resonant circuit, though there could also be interactions with the phase shift caused by the pole shading within the motor.

I've had that sort of thing happen with a mains-powered relay. With certain values of capacitor in series with the coil, the voltage across the coil was larger than with the capacitor shorted out.

With a relay, the inductance changed a lot when the relay was energised. The electrical characteristics of the motor will change depending on how fast it is rotating, but I don't know how much.

It only works for that motor inductance and may cause the motor coils to get hotter then the magnets get weaker.

Somehow you are affecting the frequency, an induction motor, especially a very inefficient type such as shading coil type, operates on frequency, Not voltage.
To what degree does the RPM increase?

Somehow you are affecting the frequency, an induction motor, especially a very inefficient type such as shading coil type, operates on frequency, Not voltage.
To what degree does the RPM increase?
Little shaded pole induction motors are not very efficient and there will be a lot of slip from synchronous speed. Increasing the voltage may reduce the slip and increase the speed a bit. The OP didn't say how much the speed increased.

Little shaded pole induction motors are not very efficient and there will be a lot of slip from synchronous speed. Increasing the voltage may reduce the slip and increase the speed a bit. The OP didn't say how much the speed increased.
The speed increased around 2 to 3 times. It’s certainly way faster and it hums more

An induction motor ALWAYS runs at a slower speed than the synchronous speed, a phenomenon called slip.

The only way that an induction motor could run 2 or 3 times faster, is that the capacitor with the motor’s inductance tunes it to the powerline’s 2nd or 3rd harmonic.

An induction motor ALWAYS runs at a slower speed than the synchronous speed, a phenomenon called slip.

The only way that an induction motor could run 2 or 3 times faster, is that the capacitor with the motor’s inductance tunes it to the powerline’s 2nd or 3rd harmonic.
Sorry, I just estimated the speed change
I do not have a tachometer to measure the rotation speed. It does appear to run faster

Are you running from the mains electricity or from an inverter? Some inverters have a larger harmonic content. You can't assume that the mains creates a good sinewave.

Can you measure the voltage across the motor when the capacitor is in place?

Are you running from the mains electricity or from an inverter? Some inverters have a larger harmonic content. You can't assume that the mains creates a good sinewave.

Can you measure the voltage across the motor when the capacitor is in place?
Im running it directly from mains. The sine wave looks acceptable on oscilloscope.
I did measure the voltage, its 234v. Mexican mains is 129-134v

It's certainly possible get an increase in voltage that large with a capacitor.

Have you got any way to measure the speed of the motor, with and without capacitor? Can you measure the audible frequency with a smart phone?

It's certainly possible get an increase in voltage that large with a capacitor.

Have you got any way to measure the speed of the motor, with and without capacitor? Can you measure the audible frequency with a smart phone?
Sadly, the motor from the picture decided to fail just now
Luckily i still have 3 more AC motors. Here i used a different motor from a space heater and a 2.2 uF capacitor. This gives 184v.

One flaw though is that the original wire from the motor broke and it would keep breaking a lot, so I put the coil wire to the chassis/core and i used it as one terminal, so now it’s live at 184v and has a non polarized plug.

That does not appear to be a shaded pole motor, but cap start/run.?
Is that the start cap just showing in the pic?
It would be interesting to find out the difference in torque with a cap in total series !!?

That does not appear to be a shaded pole motor, but cap start/run.?
Is that the start cap just showing in the pic?
It would be interesting to find out the difference in torque with a cap in total series !!?
Yeah i know it looks like a cap start motor because its got two windings but its not, it did not have a start capacitor in the space heater. The capacitor was added by me to boost its speed. Here it is with the orange capacitor

Sadly, the motor from the picture decided to fail just now
Perhaps that should be telling you something?.

Operating the motor above it's rated voltage by whatever means, will saturate the magnetics and greatly increase the current, leading to overcurrent failure (as you observed).

Last edited:
Operating the motor above it's rated voltage by whatever means, will saturate the magnetics and greatly increase the current, leading to overcurrent failure (as you observed).
It did not fail because of that
It failed because I accidentally dropped it and damaged the rotor. And i reutilized the wire to make a transformer.

It did not fail because of that
It failed because I accidentally dropped it and damaged the rotor. And i reutilized the wire to make a transformer.
Doesn't change what I said otherwise.

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