Monitoring motor temperature

Hoping someone can help on a little problem of mine !! I have a 2.2KW 3phase extractor fan motor which is bifurcated (inside
seperate ducting within the flue). The fan motor is controlled by a transformer type device rather than an inverter. The motor seems to be getting very hot at slower speeds. I am considering extra cooling by inserting a 4" axial fan (bathroom extractor fan) into the ducting access plate to add extra cooling. Question is, is there an easy way on monitoring the motor's temperature? to avoid burning the thing out? I thought about one of those digital thermometers with a long probe that you can use when cooking turkeys etc. Would this work? What sort of temperature would be normal for a fan motor running at low speeds? Any help would be greatly appreciated. Cheers. Brad.
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wrote:

Monitoring with a digital thermometer is perfectly reasonable. Make sure you get good themal contact between the probe and motor, ideally glue it on with a blob of epoxy. Check the motor spec for the acceptable running temp. - some motors can run at what seems to be frightening temperatures. I once queried Bosch on the subject regarding a system I was testing and they said "...if the paint's not peeling it's fine" ! That's NOT to say the same necessarily applies to your motor, but check the OEM's spec as you may be worrying needlessly.
Richard
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"Richard" <sharkface-pilot at toucansurf dot com> wrote in message

Many thanks for the advice about my motor, I feel quite a lot better now. Cheers. Brad.
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There are various ways of measuring motor temperature. The most suitable depends upon where the motor is and how often you want to check the thing. At the simplest end are self adhesive strips with a temperature scale that changes colour. These can be reversible or not so you can either see the actual temperature or the highest achieved. The digital probe you mentioned are good. Get the probe as close to the windings as possible and with good thermal contact. Watch the battery life if it's on for a long time. You can also use an infra red non contact thermometer. These can be used at a distance but be aware that the read area increase with distance. Remember that the heat is developed in the windings and they're normally the critical point of failure. The closer you can get to them when measuring the temperature the better. The motor name plate should give you the insulation class, probably a letter but I'm a bit out of touch now. This relates to the max temperature the insulation will take. As Richard says, modern motors are designed to run very hot. You mention the motor gets hot at slow speed. This is a common failing on motors driven via a variable speed drive. Their own fan runs at motor speed and it's output reduces dramatically as the speed drops. ISTR it's a cube law. If a motor is to be run at high power output at a lower speed, an auxiliary fan is normally needed to keep it cool. Other points worth checking are that the motor is clean and not covered in a nice thick layer of insulating dust. Also make sure it's not been painted umpteen times. Paint can build up to a surprising thickness.
John
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In article
wrote:

To some extent the slow speed effect is self compensating on a fan.
The OP said it was a bifurcated fan, the sort of thing you find used as a boiler's ID fan. Maybe the heat is coming from the flue.
Another thought, is it a pukka job or something that some sheet (there may be a bad pun here) metal-basher bodged up using a normal fan? If the latter then the motor isn't getting the expected cooling air.
Regards,
David P.
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Brad. wrote:

There is a section on measuring motor temperature in one of Jim Cox's books, I think "Electric Motors in the Home Workshop". He explains that there is not a lot of point in measuring the temperature of the casing; you need to measure the temperature of the windings themselves. He recommends measuring the increase in temperature by measuring the change in resistance of the windings. Obviously you need to be careful not to give yourself an electric shock doing this, but it can be done safely. I've done it in the past and it gives sensible results. I would give you the parameters required for the calculation, but I seem to have misplaced my copy of Jim's book. If you think you'll find the information useful, I might be able to find my copy. But if you use electric motors often, I'd recommend buying the book. It's very informative. Jim Cox also sometimes visits rec.crafts.metalworking using the name Jim Pentagrid.
Best wishes,
Chris
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