240v AC motor blowing fuses

Thread Starter

paul_capri

Joined Jun 16, 2019
1
Hi, I hope someone can help ...

I've got a 240v AC motor which keeps blowing the 13A fuse after running for about 2 seconds. I'm no expert on AC motors, but I'll give you what I can.
The application is a V-twin 10cfm air compressor. I can't see any rating info on the motor - it's integral with a V twin pump, but I'd say it's the size and power of a washing machine motor. It is brushless, so the only windings are those of the stator assembly. It also has a 40 micro-farad run(?) capacitor.

I've removed the stator assembly and visually inspected it. I've also connected it directly to the mains in its 'removed from the motor' state in an attempt to rule out any issues arising from overload or its in-series pressure-switch cut out. The behaviour is exactly the same - it blows the fuse after ~2 seconds. I retried the test with the capacitor disconnected, but the behaviour was exactly the same.

I've measured the DC resistance of the complete assembly (i.e. from the live to neutral connections), and it's giving me a reading of about 25 ohms. I don't believe that the field windings can just 'go bad', so could it be the capacitor? My experience of these is that when they fail the motor just hums, but this one clearly runs albeit very briefly.

Any help would be gratefully received.
 

MaxHeadRoom

Joined Jul 18, 2013
19,067
That is quite a high resistance for a motor of that apparent size.
Have you tried a new capacitor?
You need a AC motor run rated type, no other version should be used.
If it won't run off load with a good capacitor then it points to a shorted winding.
BTW, you should have two windings, a start and a run, you should be able to measure both, the run winding will be the lower one..
Max.
 
Last edited:

Hymie

Joined Mar 30, 2018
708
An earth fault is a possible cause if the installation does not have an earth leakage device tripping.

Ideally you should Megger the Live & Neutral to Earth at 500Vdc; but you could check for continuity with a multimeter – don’t forget to spin the motor to check for an intermittent short.

When further testing by applying a mains voltage, I’d recommend overcurrent protection by a 10A fuse (or less) – a 13A fuse operating immediately will be passing more than a 20A fault current.

Applying 240Vac to the stator alone will cause a high current to flow since there is no inductive effect from the rotor limiting the current.
 

MaxHeadRoom

Joined Jul 18, 2013
19,067
Applying 240Vac to the stator alone will cause a high current to flow since there is no inductive effect from the rotor limiting the current.
Although assuming a standard AC induction motor, there will always be a high induced current in the squirrel cage rotor at switch on.

Edit Add-On: If an induction motor is fed with the rotor removed, the current should be minimal, as there is no shorted-turn squirrel cage present.
Often in older motors that had copper bars with soldered end connectors, they used to throw solder at high temp working, this caused the motor to stop or slow and very little current flowing.
Max.
 
Last edited:
Top