Stator RTD Showing High Value During Running

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Thread Starter

Anurag Bhardwaj

We have HT motor. When motor is at stop, Stator RTD shows 108 ohm and in running 660 ohm.w hen stop its value is 112 ohms. Values are measured by removing all PT100 cable from junction box. What is the reason of this high value n how it can be rectified?
 
>We have HT motor. When motor is at stop, Stator RTD shows
>108 ohm and in running 660 ohm.w hen stop its value is 112
>ohms. Values are measured by removing all PT100 cable from
>junction box. What is the reason of this high value n how it
>can be rectified?

Presumably you have more than one RTD in the stator, do they all read the same? Have you tried reversing the meter leads, you should get the same reading, if not you may have a break. Try a large capacitor (say 100 microfarads) across the leads in case it's induced AC. Make sure the RTD is not grounded at the motor.
 
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anurag_31177

Thanks Roy.

> I should have asked if it's always done that or just started?

Yes we have simplex RTD's (1 RTD per phase). We have asked our customer the same whether he is getting this problem in all or only one. Still reply to come.

Customer has measured the Resistance of PT-100 with multi-meter by removing the RTD connections from the Junction box. There is no meter in between.
 
To get a resistance reading, the battery in a meter sources the voltage that drives a known current through the 'unknown' resistance, creating a voltage drop that the meter reads and interprets as a displayed resistance value.

So the RTD measures 108 ohms at ambient (about 20 DegC) when the motor is off, but when the motor runs the same measurement is 600 ohms (a value way off the Pt100 charts).

Somehow, a dc current with the same polarity as the measurement is created when the motor runs to produce a phantom resistance reading in the meter. Noise is bipolar, so where's the rectification come from? Interesting situation.
 
This is why I suggested switching the leads. if it's a phantom Voltage, the resistance will read High one way but low the other. Another time I have seen a phantom Voltage is where there is a loose connection, and it's acting like a cell, usually some form of corrosion taking place.
 
> switching the leads.

Good idea.

>where there is a loose connection, and it's acting like a
>cell, usually some form of corrosion taking place.

something to keep in mind. Thanks.
 
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