Can DC drives be run without a tacho input? 3

[edison123]

Don't the DC drives need a tacho input for speed & current control?

The client is running the motor without tacho input and with 'armature control' and is facing the problem of drive tripping on overcurrent.



Muthu

http://www.edison.co.in

 

[Skogsgurra]

The motor parameters do not look very different, I do not think that any new tuning needs to be done. At least not to avoid overcurrent tripping. The difference in resistance and inductance isn't big and the fact that GD2 looks quite different reflects the fact that a small increase in rotor diameter changes GD2 as D^4. It is also possible that one of the inertas is given as moment of inertia - that introduces a factor four.

Speed is not given. Do these voltages and current give a speed that is similar in both motors? If not, one may have to look at that.

Otherwise, drives always come with a current limit that normally cannot be overridden much and it is not common to have an instantaneous overcurrent trip if the drive is OK. Which it seems to be. How is the current limit set?

I am not to tell you, Muthu, about interpoles and compensation windings, but if they have the wrong polarity (happened in a large steel mill motor only a few weeks ago) you will have bad arcing and also overcurrent when reversing. I know, I shouldn't even mention it. But I did, sorry.

Speed controller tuning should not be considered yet. This is either the gating unit or the current controller/current measurement that don't behave as expected.





Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[iop95]

I think Bill is right. Old motor have sligthy nominal Ra*Ia voltage (12.1176V) that mean drive force new motor to high armature current (2095A) for same nominal regim.
Maybe overcurrent apear when start after a long pause and motor cool down to 25-30°C. At 25°C armature resistance is much lower than at 75°C and current go to 2600A.

 

[Skogsgurra]

Please, iop. Don't confuse the discussion.

Drives have current controllers and very effective current limiting. So, the armature resistance has nothing with overcurrent tripping to do.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Hi Gunnar. I defer to your knowledge and experience on this. I was not so much concerned with the armature resistance as with the commutating pole resistances. If this is used as a shunt to measure the armature current, then the difference in resistance may cause unintended consequences. I seem to remember something like this being used in some old MG drives in the resistor - amplidyne era.
From your reaction I must assume that this is no longer a common practice.
Thank you for your patience Gunnar.
Yours Bill
PS Have a virtual drink in the Pub, on my tab. I am sure that that is still allowed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Gunnar - The compole and the compensating winding polarity all checked in many ways - from the old fashioned compass method to the actual running on partial load in the shop floor. Also, the client's load (Banbury rubber mill) fluctuates from a few hundred amps to 2000 A and they say no sparking was seen. So, no neutral axis or wrong polarity issues there.

I asked the client for the voltage/speed/current data for both the motors and still didn't get them before they swapped out the motor.

At what point the motor parameters (and which ones) demand a drive tune-up? 10%? 20%?

I also find it odd that the heavier rotor has a lower GD[sup]2[/sup].



Muthu

http://www.edison.co.in

 

[Skogsgurra]

Yes, Bill. One drink doesn't kill. It is the steady flow that is bad for me. Will enjoy that fluid gold.

Muthu, there is no magic limit where new tuning is needed. So, as long as the armature current is steady and not reacting too slow, it shouldn't be necessary to do any adjustments. For a rubber (mixing) mill like that, speed varies as load changes over the cycle, but speed precision or constancy is not very important. Not like a paper mill where you want to regulate down to around 100 PPM.

I think that a proper commissioning where Gating Unit, Current Measurement, Current Controller and, not to forget, auxiliary power shall be checked. In cases like this, it is very easy to forget that +/-15 V (or 12) and +24 V needs to be constant and smooth and if they are not, you will have problems of all sorts.

Also, if it is a Four Quadrant drive, there may be a problem with the torque (current) sensing comparators. One cannot get to grips with these drives without the necessary knowledge and experience. There must be an old-timer laying around in your part of the World. Dust him off and bring him on site. That will help.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[edison123]

Thanks again, Gunnar. Yeah, it's way above my pay grade. Unfortunately, I don't know personally any drive expert around here. My last advice to the client, get in ABB, after all it's their drive.

Muthu

http://www.edison.co.in

 

[Skogsgurra]

Good. Best you can do.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[iop95]

Gunnar, confusion may arise from too few informations.
I suposed is a sensorless driver that use emf controller only, w/o current sensor in current loop.

 

[Skogsgurra]

Such drives do not exist.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.