Thermistor in dual speed motor connection

Dear Sir :
I have one dual speed motor and i found that there is 2 thermistors inside the motor.Can I connect these 2 thermistors together to only one thermistor
relay.If yes,how is the connection ? In series or in parellel .
You might reach me at snipped-for-privacy@tm.net.my
Thanks
Vincent
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Last time I dealt with thermistors was on a pump application. We had 3 on the motor, one for each bearing and one for the windings. Each went into a module that would light and then cause a trip of the motor through the controls.
I guess you could hook in series, but the ohmic value might be weird for you controls. As for parallel I am not sure how that would work.
What are you trying to do?
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I intend to control the motor in manual selection for high and low speed. I want to protect the motor with only one thermistor relay because if the motor been control in high speed,the thermistor for low speed might not needed and wise vessus. no matter i connect the thermistors in series or parellel, the resistance will be differect and there is no setting on the thermistor relay. Do you have any suggection ?
Vincent

you
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Paralell connection should work fine, you just won't know which one is firing the relay....of course that may well be obvious!

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you can put thermostors only in serie,
A thermistor is a PTC, if you connect thep parralel, the thermistor-relais will not work!!!!
Carl

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I stand corrected!.....thanks, Ross

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Not exactly, you also get NTC thermistors. Thermistors are basically resistors and their resistance changes with temperature. PTC increasing resistance with increasing temperature, NTC decreasing resistance with increasing temperature. They are usually connected in series. However, in larger motors they may be connected individually to their own thermistor relay, or multi-thermistor sensing unit. To connect in parallel is wrong!

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And thermistors are intended to work with a relay. Usually a relay from the same manufacturer of the sensor.
The usual intent is that the resistance is very non-linear, and goes from "low" to very high around a desired temperature (they aren't very accurate devices).
When any one of the thermistors trips, it causes the relay to trip, and it is the relay that does the switching work (higher voltage and amp capacity).
However, should the sensing circuit become shorted, the relay won't work. Conversely, should it become an open, the relay will trip.
jtiggr
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Are you trying to say that should the thermistor becomes open circuit then the thermistor relay will trip due to the high open circuit resistance? And should the thermistor becomes short circuited the relay would not trip due to the low resistance of the short? Correct in the case of PTC's but not NTC's. However, thermistor relays manufactured today operate in a resistance band. So should the resistance stray outside the band the relay will trip. This solves both high and low resistance faults.

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1. If i connect these thermistors in series ,the resistance value will be changed. Will this affect the thermistor relay trip in the wrong condition. If yes, how to solve this problem.
2. How to know the thermistor in the motor is PTC or NTC type.
3. Normally the thermistor relay will trip at which temperature ?
Thanks
Vincent

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1. You dont say what size of the motor you have. 2. Measure the resistance of each thermistor and post your findings. 3. PTC is most commonly used in motors, but there are odd times when you will get the NTC. 4. The best way to find out what you have is to measure the cold resistance then run the motor for half an hour with the thermistors disconnected. Stop the motor and measure the resistance again. If the resistance has increased then you have a PTC. 5. Thermisotrs in motors usually depend on the class of insulation you have. But typical trip values are 130 to 160 Degress C.

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Ripper: You are correct. I'd become accustomed to either the Siemens or Texas Instruments PTC thermistors for three phase low voltage motors (NEMA frame stuff).
We'd had difficulty with a customer buying a motor with thermistors, and separately ordering a relay (either at a later time on a different order, or from a different supplier).
Mixing the two sources (Siemens thermistor with the TI relay, or visa-versa) always resulted in a problem, and usually a warranty claim.
While most users thought that they were installed 1 per phase (three total). But if you look at the way 3 phase random wound motors are physically constructed, there really isn't a way to attribute one thermister to a specific phase.
jtiggr
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The motor is 50 HP. Do you mean that motor size is very critical to determine the thermistor trip condition.
Thanks
Vincent

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No, on large motors (50 HP is not) it is common to have multi thermistor protection. For example, we have some large submersible pumps (250 kW, 335 HP), they have three thermistors embeded in the windings. These are connected individually to a pump condition monitoring relay.

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I found one of motor control circuit in our factory,the internal thermistor like item is connected in series with the contactor coil. Possible this item is one type of thermistor ? If not,what is that ?
Vincent

very
amp
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This is what we refer to as a klixon. Yes it is a temperature protection device and is basically a thermostat. Again these are common in submersible pumps.

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