8mm Forum: Motor Burn Out

This is topic Motor Burn Out in forum General Yak at 8mm Forum.

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Posted by Simon McConway (Member # 219) on February 22, 2014, 10:34 AM:

Nothing to do with projectors...I have recently bought a brand new extractor fan from Ebay...totally unused, but had been built around 1976, so had been standing in it's box for 38 years! On switching on, all was well until a smell of "hot windings". The wattage of the motor is 65, but I measured it pulling around 90 at this stage. It ran for a few hours, then slowed down, drew 800w and blew the fuse on the supply. This is the second time I've had this happen...that is, a brand new, old stock fan burning out. The wiring is certainly correct (as is supply voltage), so that cannot be a problem. The fan was also very free turning, so it wasn't as if it had seized. My question is why has this happened twice now and how should I commission a motor like this in the future...i.e. one that's new but been standing round for years?

Posted by Pasquale DAlessio (Member # 2052) on February 22, 2014, 10:51 AM:

Can you see if there is any corrosion on the plates inside the windings? You may have to dismantle the motor to see inside. The motor shaft will turn by hand but when voltage is applied if there is a lot of corrosion or rust the magnetic filed is blocked and it will over heat.

Posted by Simon McConway (Member # 219) on February 22, 2014, 03:16 PM:

Everything looked pretty shiny. I wondered if I should have cleaned all of the internal parts of the motor by spraying with switch lubricant spray, before switching on?

Posted by Simon McConway (Member # 219) on February 23, 2014, 04:48 PM:

On closer inspection, there was some surface rust inside on the stators (? I think that's the term) which I could have removed with switch cleaner spray and steel wool. Too late now though, as it's already burned! Do you think this caused the burn out? It is pretty minor surface rust.

Posted by Paul Mason (Member # 4015) on February 24, 2014, 07:36 AM:

Just a guess but the motor was obviously stored in damp conditions hence the corrosion. The motor winding inter-turn insulation may have had low resistance leading to an inter-turn short-circuit. This is very difficult to spot by testing. Another time I would test the motor insulation and then store it for a couple of weeks in a warm dry place like an airing cupboard. Take it out and retest. if everything is OK try it out.

Posted by Andrew Woodcock (Member # 3260) on March 08, 2014, 09:30 AM:

Test the motor windings insulation to earth at 500v dc using a megger. You would expect a reading in excess of 2 megohms on a fit for purpose winding.
Also if there is rust across any of laminations this can cause eddy currents which in turn will cause hot spots on that section of the core pack. Cleaning the rust can cause other hot spots as the laminations may short out with one another.

[ March 08, 2014, 02:48 PM: Message edited by: Andrew Woodcock ]

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