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#72651 - 12/06/06 03:53 PM PSC Motor Windings-Follow-up for Scott  
wendel  Offline
Member
Joined: Aug 2004
Posts: 55
Scott, you did a great job illustrating the way voltages are varied to achieve different speeds for PSC motors.

One question. Would a weak capacitor cause a PSC motor to work on high speed setting (full voltage) but not on low speed setting?

Does a weak capacitor just not shift the phase as much as it should and thereby affect the running torque?

At lower speed there are lower rpms but does the torque remain the same? Would the current draw remain the same for all speeds while the voltage changes would be responsible for the horsepower changes throught the three speeds?


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#72652 - 12/08/06 08:40 PM Re: PSC Motor Windings-Follow-up for Scott  
Scott35  Offline

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Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Wendel,

Here are a few "Attempts" per the latest questions regarding PSC Motors:

Quote

Would a weak capacitor cause a PSC motor to work on high speed setting (full voltage) but not on low speed setting?


I am not 100% sure, but it seems likely that a Capacitor having a "Partially Open" scenario may attribute to an operational problem at the lowest speed setting - most likely if trying to start at lowest speed setting first, as this type of start will be prolonged even with a known good Capacitor.

If the Capacitor is "Shorted" or "Partially Shorted", this would create odd behavior on any speed selection, as the Auxiliary Winding is "Highly Saturated", instead of being "Lightly Covered" with Current / Flux, as would be normally seen.

The Capacitor's value is small (2.2 µF upto 8.5 µF), and so limits the amount of Current flowing through the Auxiliary Winding - in addition to achieving a higher degree of Phase Offset (as opposed to the offset seen from the X/R ratio of the Auxiliary Winding as only a "Resistance-Start" option).

Quote

Does a weak capacitor just not shift the phase as much as it should and thereby affect the running torque?


Yes and No.

If "Weak" = Partially Open, then the Auxiliary Winding does not achieve the necessary level of Current (and consequentially, produce the required Flux), so the overall Motor performance is affected.
This will affect the overall Speed (make it slower) as the overall Torque produced by the Motor is also reduced.

The Phase Offset in the Auxiliary Winding will become less with a partially open Capacitor.

If "Weak" = Partially (or fully) Shorted / Closed, then the Auxiliary Winding will be fed as if it was part of a "Resistance-Start" Motor, and the only Current Limiting Factor + Phase-Offset will be from the Winding's Resistance-to-Reactance ratio.

This would create a higher level of Current flowing in the Auxiliary Winding, yet would reduce the overall Phase Offset.

Quote

At lower speed there are lower RPMs but does the torque remain the same? Would the current draw remain the same for all speeds while the voltage changes would be responsible for the horsepower changes throughout the three speeds?


At lower speeds, the Motor Torque is also reduced - which is the reason why the Motor spins at a lower RPM.

The input VA (Volt-Amps) is different across the various speeds - with the highest (or Motor's Rated) VA at High Speed.
The input VA is reduced as the Voltage is also reduced to the Motor Windings (by selecting one of the "Speed Selection Taps").

Since the reduced "Volt-Amp Packages" carry within them the True Power (Wattage, or in this case, the delivered Horsepower), the overall True Power, in each case of Speed Selection, will be reduced according to the input selected on the Autotransformer.

The lowest level of True Power / VA drawn from the supply will be made at the lowest speed selection tap; with values increasing as higher speeds are selected - upto the Motor's rated levels being drawn from the supply, when the "High Speed" selection is used.

So, in a nutshell, the primary reason for the Motor's slow speeds is due to the low level of True Power drawn from the supply, when using the low speed taps.
It spins as fast as it may, moving the largest level of Air possible for that amount of True Power (Horsepower).

Selecting the lowest speed tap setting allows the lowest amount of True Power to be drawn from the supply, and therefore the Motor may only move a small amount of Air.
(The Motor will spin at the highest speed it can - moving the highest volume of Air it may, according to the level of True Power it may access from the supply).

As higher speed tap selections are "made", the Motor may draw increased levels of True Power, and therefore move a higher volume of Air - and consequentially, rotate faster.

When the "High Speed" tap is used, the speed and volume of Air moved will fall into the Motor's Maximum ratings.

FYI:

The "Blower" is what determines the speed of the Motor. ("Blower" = Fan Blades)

If the Blower has small blades, with very minimal tilt, the amount of Air moved will be low, so the overall speeds will be high.

If the blades are long, and/or have large tilt, the amount of Air moved will be larger, so the overall speeds will be lower.

Remove the fan / blower from the Motor, and run it (shaft only). All speed selections will spin as fast as the high speed selection does!!! (or very close to it!)

Let us know if you have further questions.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#72653 - 12/11/06 03:55 PM Re: PSC Motor Windings-Follow-up for Scott  
wendel  Offline
Member
Joined: Aug 2004
Posts: 55
Many, many thanks, Scott. Great explanation!!



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