380 and 440 volt capacitors

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A true starting capacitor is a dry cap that is taken out of the circuit within a few seconds they will burn up if left in the circuit. It's disconnected by a start relay which can be a potential relay or a current relay.Running capacitors usually seen in HVAC units are only used on single phase equipment and are in the circuit at all times usually on compressors and condenser fan motors You can replace a cap rated at 370v with a 440 but not vise versa. The voltage rating is determined by the motor not the applied voltage at the unit

Many 3 phase roof top units have 3 phase compressors and usually 3 phase indoor blower motors but the condenser fans may be single phase or 3 phase 208 or 480volt. Single phase will have a running capacitor
 
I'm still confused where 370 and 440 comes from. Would it not be 240 and 480? I've been asked before about this and have no idea what to reply.
 
Eddie, a 1ph PSC has one cap inline with the start coil, this cap is that "start" and "run" cap, it's a continuous duty cap. One cap morphs from "start" to "run" because of how the EMF builds up, which reduces the amps from about LRA back down to run amps.

Often we say "start cap" that is a helper cap, it parallels the PSC cap for a brief moment to get a little more phase shift into the start coil, which allows a hard-to-start PSC to start with a bit more start torque, hence using less power at "start" time.
 
Start helper caps may or may not be the EL type.
 
FionaZuppa said:
Eddie, a 1ph PSC has one cap inline with the start coil, this cap is that "start" and "run" cap, it's a continuous duty cap. One cap morphs from "start" to "run" because of how the EMF builds up, which reduces the amps from about LRA back down to run amps.

Often we say "start cap" that is a helper cap, it parallels the PSC cap for a brief moment to get a little more phase shift into the start coil, which allows a hard-to-start PSC to start with a bit more start torque, hence using less power at "start" time.
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So, what are the kind with a start cap and a centrifugal switch (with or without a run cap)?
 
LarryFine said:
So, what are the kind with a start cap and a centrifugal switch (with or without a run cap)?
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That's a CSCR motor. Will have two caps and the Start one will parallel the Run cap and when the "centrifugal relay" gets enough rpm it will cut out the start cap.
 
Yes, @FionaZuppa is correct CSCR uses start and run caps. The stuff I have seen is usually on refrigeration compressors (not as likely on AC compressors) this is because refrigeration compressors sometimes have to start against a high differential pressure between suction and discharge. AC compressors Usually have time for the pressures to equalize between starts.

Ever cycle a window unit thermostat quick and here the compressor hum and not start? That"s because the refrigerant pressure has not equalized yet and the compressor is CSCR can't start with differential pressure. It will start when you wait longer and the pressures equalize.

If a single phase ac compressor won't start the relay and start cap can be added to make it a "hard start kit". Now for smaller compressors you can by a "universal" hard staart kit at a refrigeration supply house capacitor and relay in a box so to speak. Compressors usually use a potential relay or a current relay to take the start cap out of the circuit. CSCR caps are oil filled, CSCR will have a oil filled run cap and a dry start cap. They don't use a centrifugal switch
 
Eddie702 said:
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Ever cycle a window unit thermostat quick and here the compressor hum and not start? That"s because the refrigerant pressure has not equalized yet and the compressor is CSCR can't start with differential pressure. It will start when you wait longer and the pressures equalize.
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And what usually happens is that the internal overload opens when the motor does not start, and the time it takes the overload to reset is usually sufficient to allow the restart to be successful. You can also see this happen when you unplug and replug almost any equipment which contains a hermetically sealed compressor, or where you quick cycle the thermostat on a whole house AC.
Have you seen equipment which incorporates the delay into the control system?
 
I suppose the easy answer is that it allows those two voltage classes to work in the largest percentage of electrical distribution networks.

Assuming a PSC motor, the run cap is in series with the auxiliary winding. The voltage across the cap will be a function of line voltage and the turns ratio of the two windings.

So yes, a 370 VAC cap might be able to used on a 480 VAC motor. Of course a 600 VAC cap would last longer.
 
GoldDigger said:
And what usually happens is that the internal overload opens when the motor does not start, and the time it takes the overload to reset is usually sufficient to allow the restart to be successful. You can also see this happen when you unplug and replug almost any equipment which contains a hermetically sealed compressor, or where you quick cycle the thermostat on a whole house AC.
Have you seen equipment which incorporates the delay into the control system?
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Some residential thermostats have a time delay built in. Some equipment (usually the larger stuff) have TDs built in
 
Russs57 said:
So yes, a 370 VAC cap might be able to used on a 480 VAC motor. Of course a 600 VAC cap would last longer.
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On a PSC, the run cap voltage (across the cap terminals, tested with a V meter) when running is usually higher than line voltage. 1.5x line V is what I see most caps rated for.
 
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