DC Motor issue 2

[edison123]

Our client replaced a running DC motor with a spare one. The motors are of different make but have the same rating (800 V / 2000 HP / 1000 RPM). The load is a wildly fluctuating rubber mill with continuous changes in speed as well as the load.

With the spare motor, coupled no load trial was taken and motor ran to its rated speed ( 1000 rpm ) at rated voltage ( 800 volts ). no abnormality found

Load trial taken. Running of motor up to 2200 amps is found satisfactory .

When the motor tries to draw >2200 amps ( as demanded by load ), the current shoots up to 6000 amps and the drive (ABB) trips on over current fault.

There is no sparking at the brushes , temp. rise is normal and no noticeable vibration.

The motor armature voltage was 335 v , 74 amps field current (rated field current) and the speed was 415 rpm.

Drive tuning done with the help of ABB engineer. Current pulses were checked and found ok.

The original motor ran with the same load and same drive without any problems below 700 RPM. But it had serious sparking issues whenever the speed was over 700 RPM.

With such low speed and low armature voltage, is stalling a natural phenomenon ? If so, why the first motor didn't have this issue ?

I would appreciate any advice / suggestions / ideas.

Thx in advance.


* Life is licking honey off a thorn *

 

[DickDV]

Couple of questions first: 74 amps field current sounds awfully high---are you sure?

Is the spare motor the same field configuration. For example, are they both shunt, stab shunt, or comp shunt. If stab shunt or comp shunt, are you sure you have the series field winding (S1 and S2 leads) wired to aid rather than buck the shunt field.

Finally, what is the nameplate rating for armature amps and are you doing any field weakening?

 

[aolalde]

Edison123:

Apparently the nominal armature current should be 1865 Amps. (2000*746/800). 2200 amps are 18% over current.
I guess if the controller keeps the field constant and the flux is not enough to produce the overload load torque without a drastic speed reduction. Then the induced contra electromotive force (fem) is reduced, and then the armature current increases at a higher rate.

fem = K* PHI*n

Va = fem + Ia*Ra

Ia = (Va- k*PHI*n) / Ra

where Va= armature voltage, PHI= flux per p[ole, Ia = armature current and Ra = armature resistance.

It seems that the two motors are not identical.

 

[TomG33]

The motors are not identical, sure enough, But i am surprised that the controller is is going clear out to >6000 Amps and then tripping on Overcurrent. it almost seems that the controller is loosing control of the current.

From my Experience, this sort of current runaway is purely a function of the controller. Why It happens with one motor and not the other is puzzling. I can imagine the drive is set up so that the motor current limit incereases as the speed or voltage goes down and the figures you mention seem to support this, SO I would assume that there is some finite stall point.

Try taking the field off the motor, (defeating some protection may be necesary) Locking up the armature somehow, and then running the drive into it's Controlled stall. It should stabilize at some specific value. if it cannot maintain a stabilized stall then chances are that the different armature resistance or inductance of the replacement motor has caused the Armature current regulator to change it's charactersitics and not be as stable as it is with the other motor.

maybe a little armature current regulator tuning is required.

Apart from this i imagine you are looking for a hardware failure of some sort, Causing the drive to play up at the higher current levels. this would not be fun to look for


Tom Grayson

 

[DickDV]

The drive programmer may have set the current limit out to 250% to give you plenty of overload torque but, usually, that parameter can be easily changed if a lower limit is preferred.

Either way, if sounds like it will be crucial to identify just how the two motors are different. And why are you calculating armature amps? What is the armature amp level stamped into the nameplate?

 

[CWHart]

The spare is stalling. DC motors can saturate at the poles due to armature reaction and different designs, shunt, stabilized shunt etc can react differently as discussed by DickDV. He is correct in that it is acting like a compound wound differentially connected series field opposing the main field. Make sure that the field is healthy, and that there are no missing windings, and no grounds. The nameplate may indicate the type of design. The motors are not of the same type or have been rebuilt differently. Placement of the poles, type of shims used in a rebuild, movement of the brush rigging, type of brushes and spring tension can all effect high current operation as well. There is always the possibiliy that some one of the internal windings, the interpole(com pole), or one series field pole, could be reversed.

 

[edison123]

I studied the motor first hand this week.

In the drive, the torque limt had been set at 139% by the user. The current limit has been set at 2100 A per bridge which amounted to 4200 A for the two bridges of the drive.
The motor rated current is about 2 KA.

In a duty cycle of about 2 minutes, the peak current was over 6400 A for about a second. The winding and bearing temps < 50 deg C.

In some cycles, the drive would trip on O/C at the peaks of 6.4 KA and over. But, the motor was not stalling at this point. No sparking either.

The current limit was re-set at 1300 A per bridge (2600 A total). The torque limit was also reduced to 110%.

After these re-sets, the motor peak current was reduced to less than 6 KA and was mostly around 4 to 4.5 KA. No more trips. The duty cycle time also did not change appreciably.

So I conclude the drive was the problem not the motor. I leave to the drive experts in this forum how this happened.

 

[edison123]

Tom,

Could you pls expand on the armature current regulator tuning ?

The client is proposing to do just that (no field / lock the armature and steadily increase the armature voltage up to some current of which he is not sure) and I do not know what that means or will achieve.


Dick,

After reducing the current and torque limits, the peak current was greatly reduced. Could you tell how the peaks were shaved off by reducing these limits ?

 

[DickDV]

My first comment is that, when changing out a DC motor, even an identical motor, a retune of the inner current loop is mandatory, absolutely. Without that done, unstable armature current behavior would be expected. Most modern drives will auto-tune this for you.

Second, to answer Edison123, the only tool the drive has to limit current and voltage is firing angle on the scrs in the power bridge. So, assuming the armature current is bumping against the current limit, the drive will alter the scr firing angles to prevent any further increase in current. There is a temptation here to say that the drive limits the armature voltage to limit current and that would be true assuming the armature speed is held constant but, under varying speed conditions, its the current that is controlled without regard to voltage.

 

[edison123]

Thx Dick.

In this case, your view about tuning the drive to suit the motor seems to be correct. No issues with the motor after proper setting of current & torque limits. Client is happy and so am I.

Stars to you and Tom for making me look like a hero.