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Thermistor in dual speed motor connection

V

Vincent

Jan 1, 1970
0
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 [email protected]

Thanks

Vincent
 
S

SQLit

Jan 1, 1970
0
Vincent said:
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 [email protected]

Thanks

Vincent

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?
 
R

Ross Mac

Jan 1, 1970
0
Paralell connection should work fine, you just won't know which one is
firing the relay....of course that may well be obvious!
 
C

cdr at pandora dot be

Jan 1, 1970
0
you can put thermostors only in serie,

A thermistor is a PTC, if you connect thep parralel, the thermistor-relais
will not work!!!!

Carl
 
R

Ross Mac

Jan 1, 1970
0
I stand corrected!.....thanks, Ross

cdr at pandora dot be said:
you can put thermostors only in serie,

A thermistor is a PTC, if you connect thep parralel, the thermistor-relais
will not work!!!!

Carl
 
R

ripper

Jan 1, 1970
0
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!
 
R

ripper

Jan 1, 1970
0
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.
 
V

Vincent

Jan 1, 1970
0
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
 
V

Vincent

Jan 1, 1970
0
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
 
R

ripper

Jan 1, 1970
0
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.
 
V

Vincent

Jan 1, 1970
0
The motor is 50 HP. Do you mean that motor size is very critical to
determine the thermistor trip condition.

Thanks

Vincent
 
R

ripper

Jan 1, 1970
0
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.
 
V

Vincent

Jan 1, 1970
0
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
 
R

ripper

Jan 1, 1970
0
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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