Need help with a motor running at excessive speed.

Thread Starter

BestAppliance

Joined Mar 21, 2016
4
I own an appliance repair company and had an unusual call. Had a customer that has an over the range hood whose motor runs much faster than normal. It is a "squirrel cage" type blower, 120 volt. It has 3 speeds and is borderline violent at high speed. He bought it open box on sale, and has been like this since its initial install this week. I didn't do the install, he called when the motor had problems. Interestingly, if the capacitor is disconnected after start up, the run speed drops to normal. Could this be the issue? I'm almost positive that it is a start capacitor. Also when i test the voltage at the motor harness terminations, the motor is running at almost 180 volts, yet the line in voltage remains at 120 volts. Im pretty good with appliances, but this is puzzling me. Any and all help is appreciated. Thanks in advance.
 

MaxHeadRoom

Joined Jul 18, 2013
19,421
Did you try the motor on 120v direct?
Also if a squirrel cage type fan, is the inlet or outlet blocked? If so it will remove the load from the fan.
Max.
 

Thread Starter

BestAppliance

Joined Mar 21, 2016
4
Most range hoods aren't strong enough to need a capacitor. Tell more. Is there a centrifugal switch inside the motor to disconnect the capacitor when the motor gets up to speed?
If there is a centrifugal switch, I do not hear it disengage when the unit reaches speed. Based on the wire schematic I do not believe there is. It is very unusual for a range hood to have a capacitor, but this is a very large range hood.
 

Thread Starter

BestAppliance

Joined Mar 21, 2016
4
Did you try the motor on 120v direct?
Also if a squirrel cage type fan, is the inlet or outlet blocked? If so it will remove the load from the fan.
Max.
That is something I didn't fully check. I hear the dampener falling shut when the unit is cut off, and the duct work expand and contract, so some air is moving, but that no guarantee of proper air flow. I'll check it out.
 

MaxHeadRoom

Joined Jul 18, 2013
19,421
Also if a induction motor of what ever flavour it will not run higher than just below synchronism regardless of the voltage you give it!
The violence would not be due to out of balance by any chance?
Max.
 
Last edited:

#12

Joined Nov 30, 2010
18,167
If there is a centrifugal switch, I do not hear it disengage
That might be the whole problem...or it might be that the fan noise is concealing the "click".
ps, a centrifugal switch would be inside the motor. Got more than enough wires for the 3 speeds? Check for complicated circuitry.
 
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