DC motor braking 1

[stantomas]

Hi,

We have a DC motor (150HP) on a dc drive that provided braking when the motor is stopped. The braking is via braking resister and the electronic drive.

We at times, unfortunately will open the drive power supply disconnect while the motor is still coming to a stop. This results in a "fire ball" within the motor.

Question: We realize opening the disconnect is not safe or proper to do.....but what do you think the scenario is when this happens as far as the motor "speed change". For example, might the motor have a sudden speed fluctuation, then it may free wheel from there. Or does the motor just keep slowing down with little speed change immediately after the "fire ball"?

The question comes from damage that we experienced after one of these "fire ball" episodes to the mechanical portions of this system that the motor is "driving". basically we have seen bent shafts. is it possible the motor has a sudden change in speed when the disconect is opened? and the fire ball occurrs?

If you could include some basic "theory" on the motor acting as a generator while slowing, and the transient affects which occur at the instant the disconnect switch is opened....resulting in the motor braking circuit being suddenly "open circuited", that would be helpful to understand how this may lead to the mechanical damage on components that the motor is driving.

Also if you can offer some theory on why the "fire ball" occurs that would be helpful. Our belief is the motor is acting as a generator as it is braking..then when the disconnect is opened, the motor energy must dissipate "instantaneously" and it finds paths within the motor to do so.

Thanks much!

 

[BobM3]

The "fireball" is maybe severe sparking at the commutator? Or from somewhere else?

 

[DickDV]

It might be the shunt field being effectively open-circuited and flashing back as its stored energy discharges.

In any case, I would think that a couple episodes of this would consign the poor motor to the rewind shop.

 

[VicP]

This is commonly known as a regeneration (regen) fault. When the motor is regenerating (braking), DC current is flowing from the motor into the power lines. The AC line voltage reverse biases the SCRs at the appropriate time to turn them off. When the AC voltage is suddenly removed, there is nothing to turn off the conducting SCRs and since the drive power supply does not decay instantly, the next set of SCRs gets turned on while the previous SCRs are still conducting. This results in a dead short across the motor armature. The motor is unable to communicate the resulting large current and causes severe arcing at the brushes. The high current produces a large braking torque, which causes the motor to stop very fast. This can cause mechanical damage.

The same thing can happen if the motor voltage is too high in relation to the AC line voltage when braking starts. A good rule of thumb is to limit the DC voltage to 105% of the AC voltage.

It is common practice to install a high speed fuse in the motor armature circuit to protect against this. Be sure to use a fuse with a sufficient DC voltage rating.