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3 phase motor thermistor

I have some 2.2KW 3 phase AC induction motors from ABB and Siemens, they are all in the same class and very similar, if not identical design. I measured the thermistor on some of them and got about steady

180ohm at 30C room temperature. But on some motor, I get a reading of 180ohm initially for a short moment and it goes to infinity. I don't know if it is normal. Does PTC thermistor exhibit such strange behavior?

Is it easy to change the thermistor? I mean is it easy to open the motor and replace the 3 sensors? And where can I purchase those sensors?

thank you, Andrew

Reply to
ahgu
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Thermisters might come in 3 flavors, I am sure there are 2 different kinds. They do operate some what differently across the range of measured temps. Find out what the thermister is made of and who by then get the infromation on the unit. Also find out the range of the thermister. If you really low on the temp scale you could get some weird readings.

My applications are for motor temp, winding temp, then front and rear bearingings. All of mine go to a collection point/box that converts the signals to analog and channel numbers for our DCS system. I have one 5kv motor that has 7 spots monitored, we only use 4.

Changing them????? Ours are imbedded in the motor and except for the bearing locations we have them changed at the motor shop when the motors are rewound.

Reply to
SQLit

The fact that you are using the thermistor on a motor is relatively unimportant. Ordinary thermistors (negative temperature coefficient) should not have that happen. By the way, there is no such thing as infinite resistance. You should be able to measure it.

My guess is that you have a defective thermistor in which you are not making contact to the leads. It may be due to motor vibration or thermal expansion. There is a good chance it may happen to other as they get cycled.

Bill

Reply to
<salmonegg

It sounds like bad thermistors. Sorry, they are not easily changed (except any on the bearings). If you completely disassemble the motor, you *may* find them just sort of 'stuffed' into a space in the windings. But most of them are laid down right in the slot after the bottom coil is placed, and then the top coil is laid on top of them. So the only way to replace one is to rewind the motor.

FWIW, you can usually 'get away' with just one working thermistor. Although it is common to lay one in each phase, there is no real reason for one phase to be a lot warmer than another, *if* the applied voltages are well balanced. The only time I've seen one phase read a lot higher than the others, it was due to blocked ventilation, a lot of dirt, or imbalanced supply voltage / single-phasing.

daestrom

Reply to
daestrom

I think all 3 thermistors are connected in series. There is only a 2 wire output from the motor. Since you cannot connect them in parallel, it has to be in series. Maybe I will find a motor shop and hear what they say. They should know how to rewind the motor.

thanks Andrew

daestrom wrote:

Reply to
ahgu

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