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Is dv/dt an issue in DC drives?
3

Is dv/dt an issue in DC drives?

Is dv/dt an issue in DC drives?

(OP)
I know that VFD's have high dv/dt problems requiring special motor winding wires.

Do the DC drives have such dv/dt issues? One OEM is claiming the DC drive is causing the failure of the shunt field winding in their new DC motor. The field winding came completely cooked to my shop as shown below. All the wire enamel was flaking off more due to overheat and not due to dv/dt which typically are insulation puncture faults, in my opinion.

What do you drive gurus think?


Muthu
www.edison.co.in

RE: Is dv/dt an issue in DC drives?

Is the shunt field power by a PWM circuit or just fed a constant DC?
Historicaaly, shunt fields were fed from full wave rectified sources with a trim resister.
Some Ward-Leonard drives used a DC exciter generator but many used a static shunt field excitation.
PWM and the resulting dv/dt issues is used for the armature circuit.
What is the source of the shunt field excitation?
Hint:
Look for damage where the leads enter/exit the shunt winding.
That is where the highest dv/dt effect will be.
A dv/dt issue may cause shorted turns which will lead to overheating but there this will be initiated by an insulation failure at or near the winding ends.
Check for a mismatch between the shunt field rated voltage and the drive rated shunt voltage.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Is dv/dt an issue in DC drives?

(OP)
Hi Bill

Client says it’s a Parker 590 P drive with ripple factor of less than 5%. I have no idea if it's a good DC drive or not.

The field winding did not have any visible damage but the DC resistance was way too low and the motor was reaching full speed at half the armature voltage. Hence, I suspected wrong no. of turns or severely shorted turns in the field and the latter proved to be right.

I see the winding failure only as a thermal issue not as an electrical issue. May be the drive gurus here will prove me wrong.

Muthu
www.edison.co.in

RE: Is dv/dt an issue in DC drives?

I agree with you.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Is dv/dt an issue in DC drives?

Was field economizing on in the DC Drive? Many DC motors will over heat if the motor is stopped for an extended time with the full field current applied to it. Most DC Drives have a field economizing function which reduces the field current when the motor is stopped. It can usually be enabled or disabled.

RE: Is dv/dt an issue in DC drives?

(OP)
VicP

Is there any application that would require field fully excited in standstill condition? This motor has an external blower and that should cool the winding in case of such an application provided of course the winding is sufficiently designed for that.

I have found in this case the OEM has penny pinched on the conductor size which has led to this overheating. We are looking into increasing the copper area as much as possible and also using VFD duty magnet wire.

My doubt about dv/dt in DC drives still remains. :)

Muthu
www.edison.co.in

RE: Is dv/dt an issue in DC drives?

3
Some newer DC drives are using PWM for the output, but mostly for smaller motors than what you show there. The Parker 590 is a good industrial drive, but is thyristor based, using conventional phase angle fired line side voltage control. So no, dV/dt would not be the same as it is for AC PWM output drives. They are clutching at straws.

A couple of things though;

The Parker drive has an input for motor thermistors and although you can’t disable the motor temperature protection circuit in the drive, you can put a jumper across the input terminals of the thyristor input. I’m willing to bet they have done that.

The field on that drive is internally tied to the armature output voltage and has “standstill protection” built in, UNLESS you supply the field externally and program the drive to tell it that. If you do, they warn you to add external overload protection on the field output.


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

RE: Is dv/dt an issue in DC drives?

Muthu
As long as an external blower is operating, the motor should not overheat with rated field current. Does the blower operate 24/7? The overheating will happen if there is full field current and no air flowing through the motor.

RE: Is dv/dt an issue in DC drives?

(OP)
Thanks Jeff. That was the wisdom I was looking for. A converter can't possibly have high dv/dt as a converter-inverter was my thought. The OEM are clutching at straws is right.

Agreed, VicP.

Muthu
www.edison.co.in

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