Re: Monitoring motor temperature

In article
<662bb1a5-31ad-40ef-a13d-bfe15292213a@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
John <johnmanders@xxxxxxxxxxxxxxxxx> in uk.rec.models.engineering
wrote:

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.


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 *** (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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