Soft starter driven motor with thermistor protection

[Bijoyq8]

We need to check the proper functioning of PTC thermistor protection for a 3 phase motor driven by soft starter.The motor name plate says the max measuring voltage of 2.5 volts dc for PTC. However for our application the measuring voltage is derived from the Soft starter and the PTC wires from motor is wired to soft starter
Your advice and suggestions awaited.

 

[jraef]

Not sure what you are asking here. PTC thermistors are resistive devices, voltage is somewhat irrelevant in that the detector (in this case your soft starter circuit) is simply looking for a sharp change in the resistance of the circuit to know that the thermistor is hotter than the setpoint threshold (the "Tc" point).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Bijoyq8]

Hello jraef,

When the PTC is connected to a voltage source . a large current corresponding to the low ( cold0 resistance flows.But as the motor runs , the thermistor embedded in winding self heats ( thereby increasing resistance)and reducing the current.In the unfortunate incident of fault , the winding heat will cause the PTC resistance to increase and correspondingly the current is reduced .All through the applied voltage on PTC is relevant.
For PTC there is maximum operating voltage and maximum measurement voltage.

Now back to the original query;

We have a soft starter driven 500 KW motor , the motor has PTC winding protection. The voltage for the PTC is derived from the soft starter. We need to verify the proper functioning of PTC and its circuitry start from Soft starter to the protected motor.

 

[waross]

If all else fails do you have a Voltmeter?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bill West]

The starter's detailed specs may show the sensing voltage used and the intended sensing resistance. This story [link]https://electrical-engineering-portal.com/thermistor-motor-protection[/url] gives a good discussion of PTC protection. I don't see how the overall system trip point can actually be checked however. The most I can see is to disconnect the PTC side of the circuit and measure its resistance. Then connect a potentiometer of a far larger resistance to the soft starter and increase the resistance until the starter demonstrates that its side of the PTC circuit operates.

Bill

 

[Marke]

I agree with Jeff. The resistance of the PTC is low when it is cold and although it is called a PTC, the temperature does not rise significantly until it reaches it's rated temperature, or threshold temperature, at which point the resistance rises very quickly. These PTCs can not be used to measure the actual temperature of the windings, they give an indication that the temperature has exceeded the threshold temperature.
You select the PTC to suit the winding insulation. The trip temperature can not be adjusted except by replacing the PTC with one that has a different threshold temperature.

Typically, the cold resistance is in the order of 200 ohms and there are often three connected in series giving a common operating temperature of around 600 ohms.
The trip threshold of the sensing circuit is typically around 2.8 KOhms to 3 KOhms.
The protection can be tested by replacing the connection with a 4 - 5 K Ohm resistor. It should trip. Commonly, the circuit is tested just by removing the connection to the sensor (open circuit).


Mark Empson
Advanced Motor Control Ltd

 

[Bijoyq8]

As Marke said,PTC cannot measure the actual temperature.

The insulation class of motor is F.There is two PTC in motor , the PTC which is rated for alarm at 135 DEG CELSIUS read 100 ohms ( all three connected together at TB of motor) and the PTC rated for TRIP at 150deg celsius read 220 ohms ( all three connected together internally and two terminals brought outside at TB).
I am chasing the Soft starter vendor if the threshold temperature can be set at a low value ( in soft starter parameter setting) and then observe
a.Alarm
b.Trip
c.sensor break.

The measuring voltage ( at soft starter) came very low around 6 millivolts only across PTC terminals in soft starter !
The resistance of PTC was 106 ohms ( thus the control wire has 106-100 = 6 ohms)