kiln DC drive motors. 1

[Thedroid]

I just replaced one of the DC drive motors on a cement kiln, and now there seems to be some sort of unbalance going on. The kiln is driven by two 75hp DC motors wired in series to an ABB DCS 500 controller. Power is transmitted through toothed Gates power belts, gear box, and a dual pinion, girth gear arrangement. The motor was replaced during an annual shutdown and other areas of the kiln were worked on at the same time, so the motor was not the only thing replaced.

We think that there is an uneven buildup of coating causing the torque requirements to be inconsistent. This was verified by a 200 degree difference in surface temp around the outside of the kiln.

As I watch the kiln turn it seems like the motors are passing the load back and forth and causing alot of noise and vibration for half a revolution. Measuring the armature current shows that there is not a linear increase in current as the heavier side is lifted, but more of a series of peaks and dips as the drive belts go slack and then tighten up again. Watching the voltage panel meters inside of the drive rooms shows that as 1 motors voltage increases the others decreases, then they switch roles and the other motors voltage increase and back and forth. Everyone is convinced that the problem is purely mechanical, but I'm not sure. I've never worked with DC motors in this configuration before, so I'm trying to understand what could be happening. Armatures and fields are wired in series.

 

[waross]

The DC drives that I have seen with motor armatures in series had the fields in parallel on a fixed voltage. Has someone set the proportional band lower on the controller? Sometimes adding a few percent more proportional band will make a lot of problems disappear.

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

 

[itsmoked]

Wow motors in series.. Never seen that myself. After getting my butt kicked with transformers in series in college I would never try that.. I'm here to see what happens.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Thedroid]

I'm pretty sure that both the armature and the fields are both in series, and the drive is rated to put out a little more than double the voltage of arm and fields. This arrangement has worked real well for a long time. We just started up the kiln after the shut down last weekend, and we've been puzzled ever since. I will double check on the fields tommorow. There are some trim resistors in the drive cabinet that would probably make more sense if the fields were in parallel, but I will check on this.

No one has changed any of the parameters on the drive, the only thing that changed electrically was exchanging one of the motors. No adjustments were made to the pinions or the girth gear either, just new refractory, and new shims on the tyre which could be off.

 

[ScottyUK]

Are the motors matched? Has one of them ever been rewound? Does it still match the others?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[edison123]

Aren't the two series motors supposed to have the armatures and the fields cross-connected when working in parallel ? i.e. Armature of first motor connected to the field of second motor and armature of second motor connected to the field of first motor.

 

[Thedroid]

I believe all of them have been rewound. These are the original motors. On each kiln one of the motors is labeled stabilized shunt and the other is labeled special compound. When the first electronic speed contollers replaced whatever was orignally in place all of the motors where rewound, so I'm told. The ABB drives have been in service for at least 6 yrs and prior to that some type of GE drive was used. I'm not sure what kind of arrangement was used prior to this. Armatures are definately is series with one common power sourcre and I will check on the fields tonight.

 

[DickDV]

I would suggest retuning the field loop and the inner and outer armature loop. It is quite possible that rebuilding one motor has changed the parameters enough to require a retune.

When load-sharing two motors like you describe, the tuning is usually quite loose. I would not expect the gains to be really high or anywhere close to instability.

 

[Thedroid]

What type of procedure could I use to retune these loops? Would it involve using the trim resistors? This is my first experience with this type of set up.

 

[DickDV]

If your drive has an autotune feature, use that. Otherwise, it's time to get an experienced DC motor/drive tech in to do it. Field, armature, and external loop tuning can be very tricky especially with motors in series.

 

[Thedroid]

Fields are in parallel and are run through trim resistors. The process has smoothed out on it's own, but I need to do more studying of the motor arrangement.