How to calculate the resistance of a wash pump

[sparer]

Hi,

I have a wash pump that I think is faulty but I cant find any data on what the resistance of the windings should be. I have a photo of the pump (attached). It is single phase with a L and N and measures 12 ohms. Does that sound low for a 470W pump?

 

[flameport]

Any readings of DC resistance on an AC motor are irrelevant.

If it's not working, then a failed capacitor is far more likely than any problem with the windings.

 

[jurassic sparkk]

Windings probably okay. As said above test the capacitor and the wires to it. Even the PCB has a higher chance of failing than ruling out a motor problem.

 

[sparer]

It blows a 4A fuse when the motor starts. Would a failed capacitor do that? I've also noticed some water staining on the motor housing that could have crept onto the windings. I guess I could hook it up to the mains directly and see if I blow a plug top fuse (or trip the breaker).

 

[bernardgreen]

Does the motor try to rotate. ?

When a motor rotates it creates a reverse current ( like a generator ) that opposes the current due to the voltage from the mains,

230 Volts applied to a resistance of 12 Ohms will create a current of 20 Amps if the motor is not rotating. The reverse current at full speed will be approx 18 Amps which reduces the current taken from the mains to the rated 2 Amps.

( The actual physics are bit more complicated )

 

[sparer]

Does the motor try to rotate. ?

When a motor rotates it creates a reverse current ( like a generator ) that opposes the current due to the voltage from the mains,

230 Volts applied to a resistance of 12 Ohms will create a current of 20 Amps if the motor is not rotating. The reverse current at full speed will be approx 18 Amps which reduces the current taken from the mains to the rated 2 Amps.

( The actual physics are bit more complicated )

That's interesting.

I don't hear the pump move and the fuse blows right away when it should run.

So would I be right in saying is that if the pump is stalled or will not start (perhaps due to the capacitor) then the windings will take 20A and blow my 4A fuse?

For info, I just tested the unit with the pump disconnected and a multimeter in place. The unit operates normally and doesn't blow the fuse. The multimeter shows 240V when the wash pump should run so I am happy that it is the wash pump at fault and not the PCB etc.

 

[bernardgreen]

Yes If the motor stalled then it will take 20 Amps. . Most likely a failed capacitor or a broken connnection to one or more of the motor terminal is the reason for the motor not starting.

It may be a bit less than 20 Amps as the inductive motor windings will present an impedence to an AC voltage. This will be a few Ohms higher than the resistance the windings will present to a DC voltage

 

[sparer]

Yes If the motor stalled then it will take 20 Amps. . Most likely a failed capacitor or a broken connnection to one or more of the motor terminal is the reason for the motor not starting.

It may be a bit less than 20 Amps as the inductive motor windings will present an impedence to an AC voltage. This will be a few Ohms higher than the resistance the windings will present to a DC voltage

Ok. I see now. I just ran the pump again and it started to spin albeit slowly. The windings started to smell so I turned it off quickly. The motor spins by hand (bearings seem a little dry) so I'll order a new capacitor. I'm presuming that that is the best course of action?
Thanks

 

[sparer]

New capcitor fitted and it runs the same. I took the motor to bits and the bearings and seals are shot and the windings have overheated so a new (expensive) pump it is. Thanks for the help guys.

 

[Justin Passing]

WHat you can get is a "shorted turn" (or turns). It only takes two of the adjacent winding wires to be connected. There's a small voltage induced in the turn of wire, but almost no resistance, so huge current. The windings res reads about right on DC.
YOu can detect them with a growler, which is a thing which generates a strong magnetic field, etc etc. Haven't seen one for years.