Motor Thermistor

The thermistor is a resistive device that, as temperature changes, so does its resistance. If the temperature of the motor rises above a certain value, the resistance gets high enough to tell the circuit to shut off current to the motor.

I would say that you need to replace the thermistor. A good place to start is the motor vendor. Call them and explain your problem. They will have much more specific knowledge of troubleshooting/repair than that which you are likely to find here.

Mike

"thermistor: a semiconductor device having a resistance that decreases rapidly with an increase in temperature. It is used for temperature measurement, to compensate for temperature variations in a circuit, etc. "

Definition comes from a specialised link. Easy to check via google.

Sorry to correct but again and again we are responsible to the one expecting an answer.

Even if short the answer has to be basically correct.

Nick Name, not so fast on the correction, there are multiple types of thermistors. There are one that go up in resistance as the temp goes up, these are called PTC or Postive Temperature Coefficient thermistors. Then there are thermistors that decrease in resistance as the temp goes up, these are called NTC, or Negative Temperature Coefficient thermistors. I'm not in the electric motor field, so I'm not sure which are more commonly used there, but we use both types in our electronic designs. Wikipedia's Thermistor entry describes both types.

Thermistor

Tom

I made the mention because the PTC are mostly used as current limiters as a fuse (polyswith for instance) and, as far as I know NTC are used for in a potentiometric scheme in series with fixed resistors in order to obtain a voltage signal which has same trend as temperature and is used with a reference threshold for control of cooling for instance.

You are right I should have mentioned both from the beginning.

Thermistors are used to supplement the thermal modeling of the drive's microprocessor. The reason is, the thermal model is looking only at energy going into the motor. At very low speeds, there is a lot less energy going into the motor. But at the same time, there is also very little cooling. So at low speeds, the thermal model becomes less effective in protecting the motor from over heating and the thermistors serve as a backup.

Generally a motor has 3 thermistors, it is intended for you to wire them in series. If you have only wired one of the 3 to the thermistor trip input of the drive, it may never trip.

A GA for that Jraef. Would you agree that if his motors have only one thermistor (I'm assuming NTC) he'd be wiser to consider a bridge solution?

Hi JRaef,

The schematic is directly from the Microchip website (AN893), so I think it is viable. After all of the replies, wouldn't just one be sufficient? I would think that, in an overload situation, all of the motor coils would be affected the same.

Mike

Mikerho,

The schematic you posted is for a BLDC motor drive, not an AC VFD; different applications. The apparent thermistors shown in there are likely "Current Limiting" thermistors; a completely different application. In that case, they use the NTC versions because when the drive is first powered up, the NTCs are "cold" and have a high resistance, which limits the inrush. Once they heat up, the resistance drops and they are virtually out of the circuit. It's a common use of NTC themistors.

If your motor has one thermistor, add a second one in series to delay tripping and check all your operating parameters to make sure there is no incipient fault.

JOSEPH