PSC motor behavior when changing run capacitor value.


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Old 07-31-17, 11:52 AM
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PSC motor behavior when changing run capacitor value.

Hi.

I was browsing and read alot of articles and forums around the web
and i didn't find a full explanation about the behavior of the current
and the voltage in the PSC motor.
Here are two situations that i would like to understand(and please,
correct me if i wrong about something).
1)if i will connect an higher value capacitor than the original rated,
the impedance in the series capacitor-start winding branch will be lower,so
i assume that the current and the torque,when the motor will be turn-on,will be higher
by using this higher value cap and the current will be also higher(of course,
compare to the original cap value)during the normal operation of the motor.
In that case,what will happen in the main winding?
Does the current will be the same/higher/lower,when we start the motor?
Does the current will be the same/higher/lower,during the normal operation of the motor?
2)In the other hand,if i will connect a lower value capacitor than the original rated,the impedance
in the series capacitor-start winding branch will be higher,so i assume,that the
current and the torque,when the motor will be turn-on,will be lower and
the current will be lower,also,during the normal operation of the motor.(along the start winding).
As in the first question:what will happen in the main winding?
Does the current will be the same/higher/lower,when we start the motor?
Does the current will be the same/higher/lower,during the normal operation of the motor?

Thanks in advance.
 
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Old 07-31-17, 12:00 PM
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The run cap changes the phase angle on the start winding. It's hard to say which direction a larger/smaller value will change the operating current.

A higher value or a smaller value can both cause the motor to draw excessive current.
 
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Old 07-31-17, 01:47 PM
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In general, if you increase the capacitance value you will increase the startup torque. However, you also increase the motor current as well, so there's a limit as to how much you can increase the capacitor value. Also, there is a point of diminishing returns, where the torque will actually decrease as you continue to increase the capacitor value. Below is a link to an article that explains this better than I can.

https://gearmotorblog.wordpress.com/...acitor-motors/
 
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Old 07-31-17, 01:51 PM
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A weak run cap will result in a higher amp draw.

Any time that I measure a compressor running RLA I know to check the run cap because it will likely read below specs.
 
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Old 08-01-17, 06:53 AM
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Thanks for your response,but you answered very partly to my question..
My main interset is in the current aspect.(at the run capacitor vs the starting capacitor according to my descriptions)

If i connect larger capacitor(+10%):what will be the current in the run winding
and what will be the current in the start winding at the moment that i start the motor
and what will be the current in the run winding and what will be the current in the start winding at the normal operation of the motor.(lower,higher,the same,compare to the oem capacitor).

And in the other hand,if i connect smaller capacitor(-10%):what will be the current in the run winding and what will be the current in the start winding at the moment that i start the motor
and,also,what the current will be(on the start and run winding)at the normal operation of the motor.(compare to the oem capacitor).

For example:
1a)You said that if i install +10% larger capacitor,the current on the start winding will be higher when i will start the motor,but will the run winding,also,be higher at that starting moments?(compare to oem capacitor).
1b)At the normal operation of the motor,with this 10% higher capacitor,the start winding current will be higher,but will the run winding,also,be higher?(compare to the oem capacitor).

2a)If i install -10% smaller capacitor,the current on the start winding will be lower when i will start the motor,but will the run winding,also,be lower at that starting moments?(compare to the oem capacitor).
2b)And what happens with the currents at the run winding and start winding at the normal operation of the motor with that -10% smaller capacitor,compare to the oem capacitor?

BTW,I am not talking about locking rotor,as Houston204,mentioned,which of course,will cause the current be higher(RLA).


Thanks.
 
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Old 08-01-17, 06:57 AM
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You're asking questions that are beyond the scope of this forum. This forum is here to help people that are having problems with their A/C systems. None of us here (to my knowledge) are motor design engineers. You may have to find a forum that deals specifically with motor design.
 
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Old 08-02-17, 04:46 AM
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Okay,i understand.
I was pretty sure that there are,also,AC practical engineers in this forum
,while the Air Conditioner compressor has PSC motor.
So,i will ask it in different way.
If the run capacitor in the AC becomes weaker(has less capacitance),but it still
manages to start and run the motor,will the current be higher during running compare to a new capacitor?
Will the motor be hotter during running compare to a new capacitor?
 
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Old 08-02-17, 09:06 AM
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Your question may be answered by an electrical engineer, or better, a college EE professor. Most of us are AC techs here, so we may or may not be able to help you much. However, here is a dumb way that I am thinking. why don't you put different capacitors on and measure the currents real time. This should be a very easy test, and you get your answers immediately.
 
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Old 08-02-17, 12:07 PM
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What I believe happens with a weak Capacitor is the Motor draws higher amps and runs hotter.
 
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Old 08-03-17, 04:35 AM
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This is exactly what i was thinking to do,clocert,but i didn't have
a smaller and a bigger capacitor to make the test.
Buying capacitors only for that purpose is not the best idea.(a waste of money).
Anyway,thanks for all the responses and the link you attached.
 
 

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