Cap start/cap run vs PSC motors

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Electrical Contractor
Looking at a motor speed controller. Are these statements contradictory?

"Not to be used with Capacitor Start or Capacitor Run Motors."

"Control provides infinite variable speed for shaded pole and PSC motors."

Are or are not PCS motors the same as capacitor-run motors? :unsure:
 
There is a difference between a CSCR and a PSC motor. This speed controller would not work on a CSCR motor because it would have a centrifugal start switch.
 
No.
In a Cap-Start/Induction-Run (CSIR) or Cap-Start/Cap-Run (CSCR) motor, the Starting capacitor and aux. winding is only in the circuit until the motor gets to about 80-90% speed, then a centrifugal switch shorts it out while the motor is running. So if you use them with a VFD and slow the motor down past the point at which the centrifugal switch operates, it places the Starting capacitor and aux winding back in the circuit. Not only are they not made to be in the circuit continuously, but the capacitor charging current looks like a short circuit to the VFD transistors, and the VFD transistor pulses cause extreme heating in the capacitor, so it becomes a race to failure.

In a PSC motor, the capacitor is in SERIES with one of the windings, so it is connected all of the time, there is not centrifugal switch. But because there is the added impedance of the winding in series, the VFD doesn't see the cap charging current as a short and the cap doesn't see the VFD pulses as much.
 
LarryFine said:
Is there such a thing as a cap-run-only (no start cap) motor?
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No. It's Cap-Start/Induction-Run (CSIR) or Cap-Start/Cap-Run (CSCR). CSIR have more starting torque and are cheaper, CSCR run smoother, less pulsating torque and slightly better efficiency.

PSC are cheaper yet and no moving parts, but have less starting torque, so are mostly used for centrifugal pumps and fans where that's not needed.
 
Context: Slowing a bench grinder or bench belt sander motor.

I am familiar with the centrifugal switch and start windings.

I don't know what kind of motor the yet-unbought tool might have.
 
A VFD and a 3ph motor are going way beyond the scope of this little project.

My friend wants to sharpen knives for a living. He's balking at spending $100.

He's the kind of guy who has a new idea to make money every month or two.
 
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I'm sure you guys are right about the cap start. The belt unit has a box on the motor with a switch in it.
 
ptonsparky said:
I bought a Harbor Freight bench grinder with speed control.
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Probably a DC motor on it. If it’s a permanent magnet DC motor, that’s fine. But a brushed DC or a Universal AC /DC motor (also with brushes) is a bad idea on a grinder. The grit will wear out the brushes and commutator really fast.
 
Jraef said:
Probably a DC motor on it. If it’s a permanent magnet DC motor, that’s fine. But a brushed DC or a Universal AC /DC motor (also with brushes) is a bad idea on a grinder. The grit will wear out the brushes and commutator really fast.
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Nothing open on it, and to tell the truth, I thought 'hmmm speed control maybe that's good for something'.
Even at HF, I try not to buy the least expensive of items offered. The H stands for Hobby. IDK what the F is.
 
LarryFine said:
Is there such a thing as a cap-run-only (no start cap) motor?
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That there is basically the description of a permanent split capacitor motor.

They have low starting torque and likely wouldn't be a good choice for the belt sander.

Bench grinders are usually 3450 RPM I don't know that I have ever seen a PSC motor that is high speed, likely would take forever to accelerate to speed, and if you tried to reduce the speed to say 50% may not have that great of torque at that speed either.

DC motors or VFD driven AC motors likely would be better design choices for both applications when it comes to available torque at lower speeds.
 
electrofelon said:
Ok so back to the original topic as I have always been unclear on this:. Can someone summarize which speed control options exist for the aforementioned single phase motors?
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If it has a centrifugal operated switch for controlling the start capacitor, you can not slow it down or the start capacitor will be switched back into the circuit.

In theory if you can disable introduction of the start capacitor after initial starting, you should be able to vary speed to at least some degree.

In fact that is exactly what they do with variable frequency drives that are specifically designed to run single phase submersible pump motors. They eliminate the start capacitor and the potential relay that controls it altogether and the drive is programmed to supply optimal phase differential for the running conditions to the two motor windings.

For 2 hp and below is about as easy or even less costly to just use a three phase motor and a VFD with single phase input if you need a higher torque motor at low speeds than a PSC can deliver. PSC is mostly only good for fans if varying speed.
 
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