Fan motor stalling on running winding

[b.shahvir]

TL;DR Summary

Fan motor stalls when start capacitor disconnected at full speed.

Hi Guys,
Single phase capacitor start induction run motor keeps running 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 clutch type motors designed differently to keep running normally even after disconnection of starting capacitor at 80% rated speed?

Thanks,
Shahvir

 

[Asymptotic]

Fan motors are often of the PSC (Permanent Split Capacitor) variety, and in these, the capacitor is always in circuit. Two clues - the capacitance rating will be much lower than a run cap (typically in the 5 uF to 20 uF range), and is of a metallized film construction rather than an electrolytic.

 

[b.shahvir]

But I don't find any design difference between centrifugal clutch motors and normal fan motors. The latter should function exactly as centrifugal clutch motor at rated speed!

 

[AZFIREBALL]

I am not sure but, I think there is a difference in the two. In one the clutch disconnects the start winding after start up. In the other the start winding is made ineffective by an induced back emf from the run winding at top rpm.

 

[b.shahvir]

The induced back emf wouldn't affect an already disconnected start winding at full speed.

 

[AZFIREBALL]

The induced back emf wouldn't affect an already disconnected start winding at full speed.

That is true. I am talking about the one where the start winding is NOT disconnected by a switch.

 

[Asymptotic]

But I don't find any design difference between centrifugal clutch motors and normal fan motors. The latter should function exactly as centrifugal clutch motor at rated speed!

I'd flip this reasoning around and suggest, since as you've observed

• a PSC motor stalls when the start winding capacitor is open/disconnected
• opening the start winding circuit with a centrifugal switch is how other types of split phase motors operate

there are other design differences that may be subtle enough to avoid casual visual inspection that nevertheless exist.

Read this overview from Twin City Fans.
https://www.tcf.com/wp-content/uploads/2018/06/Single-Phase-AC-Induction-Squirrel-Cage-Motors-FE-1100.pdf

and page 1 from this paper regarding PWM control of PSC motors.
http://lipo.ece.wisc.edu/2000pub/00-04.PDF

The capacitor value is chosen so that the total impedance in the auxiliary
winding (auxiliary winding impedance plus the external capacitance)
produces a sufficient phase shift in the current so that a
rotating MMF is developed inside of the machine. The rotating
MMF is developed when the phase currents are in quadrature.
The series connection of the capacitor and the inductive auxiliary
winding effectively creates a resonant tank which raises the
applied voltage across the auxiliary winding. In order to keep
the MMF produced by this winding equal to that produced by
the main winding, the auxiliary winding has additional turns, a
greater resistance and a reduced current causing the PSC induction
motor to be asymetrical.

 

[jim hardy]

Try removing the fan blades and see if it makes a difference.

Unloaded, it should run with just one winding (once started) ..