Problem with big old RELIANCE DC motor.

My customer has a SLITTER which is very old, it has a 75 hp RELIANCE dc motor.

Motor starts and runs fine, speed varies as you adust the potentiometer; The problem is when you flip the switch off, motor speeds up for a couple seconds and the amperimeter goes all the way to max 400 A.

DC drive consists of a 3 phase DC bridge with 3 SCRs and 3 diodes, and I tested it with other motors and it works fine. 60 and 150hp.

I will post photo of the motor plate and connections as well as video of the problem when I get home.

Speed going up probably means that the field current is collapsing before the main supply is shut off.
Separately excited machines run faster as the field weakens.

How is the switching of the main current handled, usually there is a contactor or something interlocked with the field power.

Regards, Dan.

The only suggestion, I can think of is that the breaker or contactor switch contacts are contaminated and arccing to bridge the contacts with modulated open / surge close until extinguished.

Either replace worn switch/contactors andor add large RC snubbers.

It could be caused by the windings getting permanently magnetised. Are there any devices buried in the motor, like start relays or centrifugal switches. One other possibilty would be a short circuit turn in a field winding. Once the DC current has settled there would be no problem other then you would be short of one turns worth of magnetic field. If the field was changing, then the winding would act like a transformer with a shorted turn and consume a large amount of current. This would also occur on start up , though it might be hidden by the "normal" inrush.
Because of the large size of this motor, 75 HP = 50 KW, the winding resistance will be really low, but their inductances should be comparable. So I would locate the windings and hopefully you will have at least two to compare. I would feed some low voltage AC (6 V?) into each and see if one consumes a lot more power then the other. This would be the one with the shorted turn.
Frank

To add to Frank's suggestions;

• Use the megohmmeter to measure the insulation resistance of all windings.
• Correct any loose or broken connections.
• Check for signs of heating or “resistive connections”
• Examine the brushes
• Examine disconnect Contactor for contamination, contacts must be cleaned to,reduce arc
  

I'm assuming that your motor is a shunt wound or light-compound one.

On such motors, high speeds occur only because of two reasons:
-excessive armature voltage
-field current weakening

With a DC voltmeter, monitor both the armature voltage and the field current during the startup conditions.

The armature voltage should ramp slowly (or at least in steps), whereas the field current should remain constant and in agreement with the motor's plate ratings.

EDIT: If you find the armature voltage goes full swing and then settles down, it could be that the controller's IR compensation adjustment is set incorrectly for that particular motor.
Please post the plate's photo.

Motor's Photos???

Scope on pot voltage and Vin will tell whats the reason.

Thank you all for your suggestions.

Apparently there was no snubbing in the circuit and the flyback voltage was turning on the SCR's, I added a 0.2 ohm resistor in series with a contactor in paralel with the motor, so , when the electronics remove the trigger to the SCR's the contactor connects the 0.2 resistor to drain the motor's back EMF.

I recorded video and photos but had another project, this is a system built in 1960 and thank God I finally got it working properly.