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Hatchy
23-02-2012, 09:45 AM
Hi,

I have an old plant. Large motor (75HP I think) running a Trane compressor on a chiller for AC. Motor not going and I have isolated problem back to a Crompton Parkinson Cat No 187-120027 Temp Switch. I have power on Common to NC. Bridged Common to NO as test and motor fired up. Does anybody have info on this control. I am getting 120V on thermistor terminals. Motor re-wound 6 mths ago and hope not damaged. Any idea's what I should be looking at. Not familiar with this old control that must be 35yr old.

BTW. Motor sounded a bit rough last time on site. Was getting a Harmonic up-down noise.

taz24
23-02-2012, 10:28 AM
.

The thermistor is wound into the windings and we have no way of accessing it.
They do have different readings for different temps so if you can find the values
you could compare these to the actual readings to prove if faulty or not.

If the motor was rewound only six months ago though is it not under a warenty of
some sort?

If it tests out ok and if it is only a thermister fault I would short it out and let the
motor run on until it fails.

If it is under a warenty of some sort then get it back and let them deal with it.

If it was rewound a few months ago they will have details on the thermister so go back
to the rewind company and see what they say.

All the best

taz

.

chilliwilly
25-02-2012, 10:07 PM
120 v seems rather high for a thermistor, they usually are 0-10v or 4-20 ma. If you keep the relay terminals shorted to n/o so the motor runs then monitor the resistance of the thermistor and see if it gives a linear output.

I have lost count of the amount of thermistors I have shorted out over the years because they cause more problems than they are supposed to prevent. If the motor tests out ok then like Taz 24 says keep the amplifier board shorted out and run the motor regardless. As you can't change the thermistor.

Where it is critical you can attach a thermistor to the motor casing and enclose it in metal putty about the size of half of grapefruit, but you will have to work out the offset temperature difference of the stator windings core and the surface temperature of the casing. Then calibrate the amplifier accordingly.