Field loss relay

[ptonsparky]

Helped a customer with controls for a 250 hp dc motor today. Once we figured out his BF we had to set the various parameters. One of the items mentioned that we were to disable in this process was a field loss relay. He did not have one. I took a wag that this would be used to apply external braking tocertain loads if needed. Was I even close?

 

[gar]

110819-0802 EDT

How does the speed of an unloaded DC shunt field motor vary relative to the current in the field?

Induced voltage in the armature of a DC motor or generator is proportional to RPM and flux density. In generator mode this means that at fixed speed the output voltage increases as the flux density increases, and flux density is proportional to current up a saturation region.

In motor mode the counter EMF (induced voltage is the armature) closely equals the applied voltage. The difference of these two voltages is the I*R drop in the armature due to load current, a small value.

Thus, in a DC shunt motor if the flux density is reduced the motor runs faster. If current to the field is lost, then under unloaded conditions the motor speed may rise to a level where the centrifugal force tears apart the armature.

The purpose of the field loss relay is to monitor the field current and if current is lost remove power to the motor armature to mechanically protect the motor.

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[ptonsparky]

We were setting the armature current limit of approx 390 amps. Procedure was to lock the armature from turning, remove field wires, apply power to armature at full voltage and set current trip point. Obviously not much torque with the field removed.

I was thinking that the load may overpower the motor and require braking to stop it. I had forgotten about the high rpm possibility.

As always, thank you.