DC Shunt Field Slowly Dropping Off 2

[ngm]

Greetings, I am also pretty new in the motor field.

at our foil rolling mills, we have 3 motors in tandem at our recoiler and 3 at the decoiler.

motor specs: 70 kw, V=141V, Ia=568A, Vf=63/15V, If=15.2/4A, w=400/1200rpm.

the front motor is tied to one drive, while the back 2 motors are tied to another drive. it works well.

the back 2 motors armatures are in series and their fields are in series.

we can select motors as follows depending on torque requirements: 1 motor (front) or 2 motor (2 back motors) or all 3 motors.

when running with all 3 or just the back 2 motors we recently noticed that the field current on the back 2 motors starts at full field and slowly starts to decay from full field of 15.2 A to about 14.3 amps, and is erractic with a band of 0.25Amps. While the front motor holds full field. we are still well below field weakening speed of 400 rpm, so field weakening is not the case.

the decoiler motor fields and control seem to work well.

sorry i can't send you a pic.

any ideas?

 

[ScottyUK]

If you don't have active control of the field current there is a fair chance that Ohm's Law is taking over and as the winding heats up the resistance incraeses, so the current drops.

What scheme are you using - constant shunt field with armature current control, or do you have regulation / control of the shunt field?

 

[Motorwinder]

Are you running blowers to keep the fields cool?

 

[itsmoked]

Does sound like a classical thermal resistance increase.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Check the voltage as well as the current. If the voltage is holding up this may be a case of thermal increase. If the voltage is dropping the controller may be faulty.

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

 

[ngm]

Good Day Gents,

1. Field Regulation: We have doing active field current control, so there is regulation, or should be.
The field pac on the Siemens 6RA70 drive, provides the field regulation.

2. Blowers to keep teh field cool: Yes we have blowers cooling all our motors, and they are working well.
we use flow devices on the vents of the motors to detect a minimum amount of air flow through the motors.

3. Voltage Check: will check the voltage today and come back with some measurements.

Thanks for the assistance thus far

 

[waross]

This could be a poor connection with possible corrosion and possible micro-arcing.

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

 

[ngm]

Dear All,

Thanks for the advice.

I had a dd drives expert look at the field variation and he suggests that there is something not so happy with the field, and needs to come in and look at it.

I will feed back within a month of his investigation.

 

[Skogsgurra]

The 6RA70 constant current excitation normally works very well. It is only when you are working close to the voltage limit that it turns into a voltage source. What is your field supply voltage? And what is your grid voltage? If field supply voltage is close to 80 percent of grid RMS voltage, then you have a problem.

Gunnar Englund

http://www.gke.org

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