Fan motor stalls when start capacitor disconnected at full speed.

[shahvir]

Hi Guys,

It's common knowledge that Single phase capacitor start induction run motor keeps running on single winding even after start capacitor is disconnected by centrifugal switch. But in practice, table fan motor stalls and hums when I disconnect starting capacitor when fan motor reaches full speed.

Why one phenomenon works in certain motors but fails in fan motors. Are centrifugal switch type motors designed differently to keep running normally even after disconnection of starting capacitor at 80% rated speed?

Thanks,
Shahvir

 

[edison123]

Table fan motors have centrifugal switches?

Muthu

http://www.edison.co.in

 

[Skogsgurra]

Normally not. I guess that the OP tried to disconnect the run capacitor and wonders why the fan stops. The answer, I think, is that it was designed to have he cpacitor permanently connected and that the available torque at full speed drops below demand when the capacitor is removed. All motors are not designed equal [Orwell].

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[shahvir]

So what are the design differences to enable a split phase motor to run on a single winding after reaching rated speed?

 

[che12345]

Dear Mr shahvir

Q. Why one phenomenon works in certain motors but fails in fan motors. Are centrifugal switch type motors designed differently to keep running normally even after disconnection of starting capacitor at 80% rated speed?
A. For single-phase motors they require a capacitor for starting. There are basically [two different methods]. i.e. a) [capacitor start] type, where the (starting capacitor) is [switched off] (by the centrifugal-switch or current-relay) when motor is about 70-80 % speed. This is the type of single-phase motor that is widely used in the industry, e.g. a/c compressor etc.
b) [permanent capacitor run] type, where the capacitor is [permanently connected] in the start winding circuit. This type of motor is widely used, e.g. table-fan, ceiling fan and toilet fan etc. Since the capacitor is [permanently connected] in the start winding, there is [no] centrifugal-switch/current-rely. Removal of this capacitor will stall the motor.

FYI: The difference between a) or b) motor type is dependent on the starting torque produced by the motor to over-come the load torque during starting.

Che Kuan Yau (Singapore)

 

[jraef]

There are multiple types of single phase motors, 3 of the types have at least one capacitor.

Cap-Start/Induction-Run (CSIR), one cap, disconnected at run via centrifugal switch. Medium starting torque, higher efficiency.

Cap-Start/Cap-Run (CSCR), two caps, the start cap is disconnected at run via centrifugal switch. Higher starting torque, medium efficiency

Permenent Split Capacitor (PSC), the cap is in series with the start winding and is never removed. Low starting torque, medium to low efficiency but no servicing of the centrifugal switch required.

Table fans, if they have a cap motor at all (some are Shaded Pole) will use a PSC motor because of no service required but efficiency is not important and starting torque is irrelevant.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Compositepro]

In a PSC fan motor the size of the capacitor controls the torque output of the motor and thus the speed of the fan. They are also designed to be stalled and not overheat because the likelihood of stalling is high.

 

[waross]

PSC motors the starting winding with the permanent capacitor supplies part of the running torque.
With the cap disconnected, the motor may not develop enough torque to continue to run on a single winding.
Or, the motor wired incorrectly.

Bill
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"Why not the best?"
Jimmy Carter