Hi,

My 200 k W 400 V submersible pump tripped on PTC fault . The motor was meggered and the insulation test was very low, in the region of 10 kilo ohms. I believe that it is quite normal that the insulation resistance is low as the motor is found under water. Is there any test I can do to ascertain the condition of the motor?

Bob

A PTC fault would indicate a high winding temperature, not a winding fault per se. PTC is short for Positive Temperature Coefficient thermistor, a device that rapidly increases resistance as temperature rises. These devices are embedded inside of the motor windings, connected in series. 2 wires are run from the motor up to the control panel. In the control panel there is either a Thermistor Trip Relay, or a PTC input on a drive or soft starter. That PTC sensing device looks for no more than a certain amount of resistance, usually 750ohm. If the resistance ever goes higher, as in when the motor temperature rises beyond a threshold point, the sensing device trips. It can also be caused by a broken PTC thermistor wire, either in the pump or going between the pump and controller, because once the circuit is broken, you have infinite resistance..

Hi Bob

Yes, I agree, the PTC trip indicates that the motor is running too hot. This can be because of the motor being overloaded, or it can be because the motor does not have sufficient cooling. This could result from a low water level, or a low flow rate. If you run the pump continuously against a closed valve, you will heat the water and consequently the motor.

The low resistance is on the low side of normal from my experience but may not indicate a problem. There is probably no relationship between the low resistance measurement and the PTC trip, but it is possible that the insulation has gone, resultig in a shorted turn and this would also cause the motor to heat up and trip the PTC.

I would suggest that you try to run the motor at about half flow and see if the current is normal and if it trips on PTC. If there is a problem with the motor, I would expect it to show up, even at half flow.

Best regards,

Hi Marke,

I refer to the above and would like to add some few comments.
The above pupms water to an irrigation system, comprising of other similar arrangements, whereby many outlets are fed from. A submersible pump obviously is started with the outlet valve opened. I have noted the following

1. When starting with the closed valve, we noted that the current of the pump rose to more than 600 Amps., after the starting time 5 secs, the current went down to ± 402A. per phase which is too high for the pump. The rating current is 328A causing an overload trip.

2. We cannot start the pump when the other pumps in the irrigation system are working because the 200 k W
motor overloaded as soon as we switched on with the valve open. We had a rotor locked trip.

3. We stopped all the other pumps, we opened the valve at about 2½ turns and started the pump. It ran normally with a current of ± 300A. We noticed that if we opened the valve, the current tends to be lower. We had a current of about 280A with 5 turns opened valve.

Another point worth noting is that we cannot start the pump with protection ON when the outlet valve is opened, we always have locked rotor trip in spite of a locked rotor current 8 times nominal...as per pamphlet of the manufacturer.

Your expert comments are most welcomed.

Bob

Hello Bob

Many pumps will start best into a closed valve. This reduces the start torque and the pump just cavitates of spins the water with the impellor.
Some multistage pumps behave more like a positive dispalcement pump and you can not dead head these pumps. If you need to reduce the flow,you need to slow the pump.
It sounds as though you have a constant flow type pump in which case you can not use it in a variable flow installation without changing it's speed.
Have a look at the pump curves and you will ge a good clue.

Best regards,