SEW Servo drawing current at zero speed

This on is a little off topic but here goes.

Got a call at 5 this morning to check a problem with an SEW servo. It was an old Movidyne drive and if it was fixable, it was too late since the first electrician on the scene damaged it while trouble shooting.

Had a new Movidrive as a spare, so decided to swap it over. After a little problem with the resolver caused by pin 6 and pin 9 being reversed (upside down 69 is still 69), got it running around 12:30.
Now this is the problem, at zero speed it was drawing around 30 to 50% current, the motor was getting hot and the unit was tripping out on 'Protection Fault'.

Try as i might, I couldn't get the current down. It didn't appear to be any of the parameters for the PI controller, or the hold controller. It was a conveyor positioning system, so it wasn't holding a load. The set point was only a few increments from the actual position. Changing the gain, intergral, feed forward had no effects.

Eventually, I added an 'inhibit mode' bit to allow the motor to rest when it was not in operation. This resulted in it cooling down to tolerable levels. This was enough for me to go home and get some rest. I will be having a second crack at it on Tuesday, unless it falls over between now and then.

If anyone has any ideas on what could be wrong, I would be most appreciative.

The only thing I havn't tried yet is replacing the motor.

Doug

If the input signal to the drive is analog, take a look at it with a scope to see that it's dead nuts steady. I've see drives that have high freg. signals comming in (noise or controller generated) that'll cause the drive to do as you describe.

Thanks Bleary,

I don't think it could be that.

The drive was a simple positioning type drive with the set point communicated by Profibus.
The PLC would down load the required distance to move, then would set a 'move' command bit to tell the servo to go. The drive would simply increment the conveyor by that distance.
The command used was the SEW 'Go relative' command.
When in position, the drive would set a relay. This could have been done by profibus as well, however originally the drive was not set up for Profibus, and the relay was left over from the previous setup.

Doug

I worked in a stamping plant that got a brand-spanking-new steel feedline with a fixed SLC500 with a 1746-HS controlling a servo motor. After 2 weeks trying to get this running, 2 techs from AB spent 2 days going over the machine, the PLC program & the HS's parameters.

What they ended up doing was 2 things:
1. Billing us for 36 hours & expenses.
2. Telling us to run ground wires from every point to every other point. (The power & encoder cabling were already seperate & isolated from any other wiring and each other.)

Me & my guys spent the next morning running ground wires between:
PLC to Motor, PLC to operator station, PLC to machine base
HS card to motor, HS card to operator station, HS to machine base
Operator station to motor, operator station to machine base
Control panel to machine base
Adding a ground rod behind the machine base & running 5 wires around, etc, etc.

When all these were done (this part of) the machine ran OK.

Also, servo drives do usually send DC power to the motor in a stopped state to hold the motor in place. Maybe the braking effort on the newer unit is stronger than the old one, or there may be a seperate braking force parameter to look for.

Have you the Motor phasing /comutation correct. Servos can still run the correct direction and the correct speed if the commutation and motor phases disagree, but you do get high current consumtion at all steeps inc zero. The only thing you may see ais a hicup at realy low speeds.

Motor power leads are not as well marked as they should be a lot of the time.

Update:

Changed the motor out, checked the cables, re-terminated the resolver.

Results, nothing has changed.

Noticed something though, the current increases as the load on the table increases.
The application is a 'brick indexing conveyor', what it actually does is add spaces between bricks, the whole row is then stacked on a kiln car and sent of to the kiln for firing. the spaces are to allow for correct air flow and even firing.
I found that the zero speed current drawn is only a few percent when the conveyor is empty, and it increases as the number of bricks increase. Talking to the SEW technical support guru, it was thought that this may be a reaction current to an imposed load from the conveyor trying to move slightly. This was un-expected, since there is no braking on the conveyor and at power off it just does not move, especially with a few bricks on it. However, it does fit the evidence.

I am endevoring to re-program it to not power the motor when it is in position.
Over the last few days I have hit the books and now know more about these Movidrives than I did before. I used to know the old Movidynes fairly well, and this allowed me to set up and run with the newer Movidrives when they came out, but I never got the the high detail of knowledge I needed. So this was a good outcome.

Another good outcome is that the company has now realised that we need better knowledge of these systems, and so have authorised the building of a servo training bench where we can train our electricians and experiment with some of the advanced features of these drives.

Overall, it looks like this problem had been with us since the motor was put in back in '97 or '98, it was just that the original drive did not detect it, and the motor was running hot for all this time.

Thanks again to those who replied.

If anyone else gets problems with this brand of servo drives, I am now much better equipped to give advice and help out.

Doug

I'm not familiar with that brand of servodrive, but I do know that on our (slightly outdated) bds5's if our kv/kvi settings get out of wack, the motors will draw current while standing still because they try to oscillate back and forth without actually moving anything.

Just a bit more on servo control motion,
There is always an extra "holding" current that appears when there are external forces applied. That is in its nature. The problem becomes more or adversly apparent when the windings on the motor are no longer balanced. You will also notice the motor has less torque if rotation is achieved and may appear to have a lopsided effect, heat is a byproduct of energy transformation, wasted or not. I have also noticed this same phenominon when the amplifiers have "inherently become unbalanced" more apparent in a push-pull Hfet type output.

Just a......

Bitmore

Depending on which fault you're getting, this sounds like it could be very normal.

If the motor is energized and the Movidyn or Movidrive is not sending it anywhere, because the controller is not receiving a setpoint or destination from the Profibus, the motor could very well draw 1/3-1/2 motor rated current just holding its current position. This is not necessarily a problem unless the motor begins to overheat.

Generally, if there are long stretches where the motor has nowhere to go, you should "Inhibit" the controller or remove the "enable" thereby applying the brake (assuming it has one) and de-energizing the motor (and saving power).

If you're getting a temperature sensor fault from the thermostats in the motor windings, this is simply telling you that the windings are reaching a temperature that could damage the motor insulation. Again, de-energize the motor if it's not being used for long periods of time.

If you need to have the motor energized for long periods of time with little or no movement, then you may want to install a constant speed fan on the motor. It doesn't sound like you're there yet though.

If you've got some other motor or controller utilization fault, then the problem might be slightly different.

Best,
Will