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Generators & Electric Motors General Discussion

How will the uF of an AC induction motor capacitor affect the torque of the motor?


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Old 05-27-2015, 03:50:28 AM
realflow100 realflow100 is offline
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Default How will the uF of an AC induction motor capacitor affect the torque of the motor?

How will the uF of an AC induction motor capacitor affect the torque of the motor?

Will it have more torque or is there a "balance" that has to be biased to have enough torque?

like if someone replaced a 3.5uF capacitor with a 6.5uF capacitor regardless of whether it heats up faster will it have more torque?
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Old 05-27-2015, 04:26:23 AM
Roland Hayes Roland Hayes is offline
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

Hi and welcom to the Stak.

The fractional hp AC motor on my small cement mixer would struggle to get going again on a full bucket if the power supply was interrupted momentarily (happened occasionally) which I cured by changing its start capacitor from 6uf to 10uf. The increased starting torque was very noticeable and fixed the problem and there was no sign of the motor running any hotter after. The change was made years ago and the mixer still works well so I don't think there is any harm to doing this, the opposite really as there is nothing worse for an electric motor than sitting stalled
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Old 05-27-2015, 07:09:48 AM
armandh armandh is offline
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

there is a balance between the cap and the inductor [winding]
a bit more may increase the torque
too much will be a near short feeding the differently positioned winding

the idea is that a winding powered through a cap will lag in timing [phase]
as the [non polar] cap charges and re charges in the opposite direction.
the difference in timing and the different position of the winding
create the extra torque.

with a given winding
too large of a cap will act more like a wire and not provide much if any lag
thus
having two similarly powered windings positioned differently is detrimental

6 to 10 worth a try; 6 to 60 not worth a try

so yes that HVAC compressor with a big heavy winding might use a 40 MFD run cap
a small fractional Hp motor much much less

Last edited by armandh; 05-27-2015 at 07:20:48 AM.
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Old 05-27-2015, 07:13:51 AM
RobW RobW is offline
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

Do not go to extremes or the winding in series with the capacitor will overheat. The original design of the winding was matched to a certain size of capacitor.
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Old 05-27-2015, 11:35:25 AM
Railroads Railroads is offline
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

Quote:
Originally Posted by RobW View Post
Do not go to extremes or the winding in series with the capacitor will overheat. The original design of the winding was matched to a certain size of capacitor.
Don't forget, AC caps are either +/- 5% or 10% of their rating. Meaning a 6.5 UF cap could be either bigger or smaller than what the label says. The only way to know for sure is to use a capacitor checker.

Robert
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Old 05-27-2015, 12:12:19 PM
len k len k is offline
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

At work we used a small ~ 5 watt 2 phase (90 deg) motor in a device. Had a customer that wanted to power it from a 1 phase supply. I had to trail and error test the cap value for quickest start time (highest torque). If you graphed cap value vs start time it was a bell curve.
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Old 05-27-2015, 01:21:35 PM
Vanman Vanman is online now
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Default Re: How will the uF of an AC induction motor capacitor affect the torque of the motor

It would also be well to differentiate between start caps and run caps. In the case of a start cap on a capacitor start motor, increasing it's value will only increase the start winding current during starting, and may not be detrimental, if within reason. As was mentioned, there is an ideal maximum, beyond which winding current increases, but torque decreases.

But in the case of a permanent split capacitor motor (like HVAC compressor motors usually are) the auxiliary winding is designed for a specific value of cap and a specific value of continuous winding current. To increase the starting torque here, a potential relay and an appropriately sized starting cap would be used.

I've seen instances of folks erroneously increasing the run cap size until the lowest value of LINE current is obtained. This is wrong, and usually burns out the auxiliary winding.

Same applies for the run cap on a cap start cap run motor.

As was stated previously, in the absence of the known value, one would have to determine start cap value experimentally. Just remember that, with each successive test, the windings will be warmer and their resistance greater, thus altering the result. Would be fun though

Keith
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