Fridge fan motor windings

[Statick]

Hope this is the right location to post.
I asked my friend to give me the motor that he replaced for his fridge.



I checked the 2 terminals with an ohm meter and it showed 470k Ohm.
I then used a megger between the same two terminals at 500V and it showed 0.02M Ohm and 20V (short?)
Could someone help me interpret/explain the numbers?
Thanks.

 

[Marvo]

Testing between the terminals of the coil with an ohmmeter you saw 470Kohms, I would expect a healthy winding to measure a few ohms.

The test you did with the megger tester at 500v, were you testing across the two terminals of the motor winding or between one of the terminals and the frame of the motor? If it was across the two terminals of the winding then it was a meaningless test.

What are you hoping to achieve with this motor? Are you an apprentice or trainee maybe just showing interest in it? Does the shaft of the motor rotate and spin freely by hand?

 

[Lucien Nunes]

When a small motor like this goes open-circuit, it's usually due to a very tiny break in the fine wire of the winding that may arc and carbonise the insulation before failing completely, or corrosion / verdigris. The ends of the wire remain close together surrounded by poorly insulating material. When you apply a low voltage test, the resistance appears high. But the higher voltage applied by the IR tester causes the thin layer of suspect insulation around the gap to break down / flash over, hence the lower reading.

Also, 20,000 ohms is a pretty poor insulation test result and near the bottom of the range for the tester, but hardly what I would call a 'short.' 0.2 ohms would be more like it. On 230V, 20k isn't even low enough to trip an RCD.

 

[Marvo]

When a small motor like this goes open-circuit, it's usually due to a very tiny break in the fine wire of the winding

It might be worth taking the wrappng off the winding and checking to see if there's a thermal fuse that might have blown.

You can often see the bump in the windings if there's one integrated into them but some little fractional horse motors might not have one if their rated as 'impedance protected'.

 

[Statick]

Testing between the terminals of the coil with an ohmmeter you saw 470Kohms, I would expect a healthy winding to measure a few ohms.

Yes I expected either a few ohms or open circuit, instead I got high resistance so it piqued my curiosity and I did the IR.

The test you did with the megger tester at 500v, were you testing across the two terminals of the motor winding or between one of the terminals and the frame of the motor? If it was across the two terminals of the winding then it was a meaningless test.

I did both. Between the frame and terminal, it was as expected, very high resistance. The reason I did terminal to terminal was to see if the high voltage was able to 'break' the high resistance so that I get a open circuit and hence prove that it was open circuit. Instead I got lower resistance, probably due to arcing as explained in the next post.

What are you hoping to achieve with this motor? Are you an apprentice or trainee maybe just showing interest in it? Does the shaft of the motor rotate and spin freely by hand?

I wanted to find out what's wrong with the motor. It's just for interest and learning. The motor spun freely. Thanks for taking time to response.

 

[Statick]

When a small motor like this goes open-circuit, it's usually due to a very tiny break in the fine wire of the winding that may arc and carbonise the insulation before failing completely, or corrosion / verdigris. The ends of the wire remain close together surrounded by poorly insulating material. When you apply a low voltage test, the resistance appears high. But the higher voltage applied by the IR tester causes the thin layer of suspect insulation around the gap to break down / flash over, hence the lower reading.

Also, 20,000 ohms is a pretty poor insulation test result and near the bottom of the range for the tester, but hardly what I would call a 'short.' 0.2 ohms would be more like it. On 230V, 20k isn't even low enough to trip an RCD.

Thanks for helping me understand this better. Much appreciate.