Losing field current in a DC machine 4

[blunt80fi]

Hello!

I'm simulating a DC Motor with matlab. I have this problem with simulating the situation where the field current is lost when the machine is running in steady state. My results show that the armature current becomes very high and the rotation speed drops to zero. But what should happen with the voltages and torque? I have very little experience with these kinds of machines, and I get some results for these also, but they don't seem to be reasonable. I would be very happy for all tips and information anyone can give.

Thank you.

 

[charlierod]

BobM2:

As far as i'm concerned the increase in speed occurs due to an accelerating torque produced by an increase in electromagnetic torque (unless you intentionally reduce load torque at the same moment the field gets lost)
If not, How can you explain shaft accelaration? I really would like to know.

 

[BobM2]

Charlierod - Yes, the shaft will accelerate due to the magnetics until it reaches a speed where the load torque from the friction and windage equals the torque from the magnetics. At that point the shaft will quit accelerating and the speed will remain constant.

 

[BobM2]

Charlierod - Further on the motor torque, over a range of reduced field strength the armature current will rise and you can deliver name plate torque. If the maximum armature current (limited by applied voltage and IR drops) is, say, 4 times the rated current then the field strength could be 1/4 of its rated strength and the torque would match the name plate torque. Field strengths below the 1/4 value can not be compensated by increased current because the current can't increase any more. Taken to the limit, a near zero field would require an infinite current to deliver the name plate torque.

 

[jktwn]

Some years ago we had an 800hp dc motor on an extruder run away with disastordous results. Following maintenance work, it was started with the motor uncoupled from the gearbox and the field wiring was inadvertantly left disconnected.

The armature actually blew through the end casing shooting straight up to the ceiling (40 ft) breaking the roof struts and falling to the floor on the opposite side of the extruder. There was copper shrapnel all over the place and luckily no one was hurt.

We immediately put a procedure in place for maintenance to follow when starting up these motors.

 

[DickDV]

wxashcra, being an extruder motor, it also was most likely a stab. shunt or comp. shunt motor which means it had a small series field as well as a shunt field.

Even with the shunt field disconnected, it had a weak field from its series field. This would be just enough to cause the runaway you describe.

 

[TomG33]

Blunt,
No doubt you have had plenty to digest here, and everybody has raised some very good points. Technically speaking you are right about the torque going to zero with Zero field current, and, if your motor is a pure Shunt connection (which I have never seen in the sizes I deal with) it should stop.

You need to keep in mind the dynamics of the model. In a real motor, the shunt field will have a time Constant of about 4 seconds. If you remove the field excitation and let the field decay naturally, the Field current will decay exponentially to 39% of it's rated value in the first 4 seconds, The remaing drop to 2% of it's rated value will take a further 3 time constants ( 12 seconds) during this time, you need to do intermediate calculations of speed, current and torque.

I think you will find, that if you have a machine running at operating speed, with no restraining load torque, and you remove the motor field excitation, that the motor does in fact run away in speed up until the time the field is so weak that it no longer provides any motor torque.

Unfortunately it takes far less time to accelerate to destruction then it does for the field to decay to a level where the motor stalls. As others have said, IT DOES HAPPEN.

If the motor does have a load torque then the behaviour during the time the field takes to decay, will be dictated by speed torque characteristic of the load.

Most motors above a couple of hundred Kw will have a series component of armature current in the field. This is used to help with commutation and to limit Motor speed droop under load.

I remember, as a young engineer, doing a current regulator CFB ploatiry test on a 300 HP Motor with the field disconnected. We had no shaft brake but I had the wooden handle of a sledge hammer wedged in the shaft mounted fan. The CFB was the incorrect polarity and we had uncontrolled armature current run away. The torque developed was enough to snap the wooden handle, but Luckily it did not let the motor get away. So Much for the "No field, No Torque" camp.

I think it would be remiss of any DC Drive manufacturer, to suggest doing a stall check with no field, without suggesting the armature be restrained in some way.

I know the company I work for ( A well known DC Motor Manufacturer) does not suggest you try that.

Tom