Problem with DC Drive

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Hey Guys,

Any help here is super appreciated! We have two 700HP DC motors wired in series, controlled by a PowerFlex DC Drive. We start the drive at about 10RPM and the motors start but surge on and off we read as high as 19RPM on our feedback, almost like its hunting.

Here is what I know, I've disconnected each motor at the junction boxes, and disconnected feeders from VFD. Megg'd the motors from GND through brushes, comm and armature and see 1.5+ G ohms at each motor, feeders are full scale. We use a generator connected to rotor as a tach for feedback to drive. Each motor has its own tach but we only use one at a time, they test out ok. Another piece of info, we are just coming out of shutdown, so this drive has been powered off for about 3 months. We had the south motor fail last year and installed our spare, during the this shutdown we had our spare rebuilt and reinstalled on the south. I have also, gone through all control circuits in the LV cabinet and they appear ok. I was on this on the backshift last night, and Rockwell techs have been onsite since about 9am today. I'm heading in and was thinking about pulling apart the SCR banks and testing each of them individually to see if we have an SCR that isn't firing properly.

Any other ideas thoughts, thanks in advance!
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Hey Guys,

Any help here is super appreciated! We have two 700HP DC motors wired in series, controlled by a PowerFlex DC Drive. We start the drive at about 10RPM and the motors start but surge on and off we read as high as 19RPM on our feedback, almost like its hunting.

Here is what I know, I've disconnected each motor at the junction boxes, and disconnected feeders from VFD. Megg'd the motors from GND through brushes, comm and armature and see 1.5+ G ohms at each motor, feeders are full scale. We use a generator connected to rotor as a tach for feedback to drive. Each motor has its own tach but we only use one at a time, they test out ok. Another piece of info, we are just coming out of shutdown, so this drive has been powered off for about 3 months. We had the south motor fail last year and installed our spare, during the this shutdown we had our spare rebuilt and reinstalled on the south. I have also, gone through all control circuits in the LV cabinet and they appear ok. I was on this on the backshift last night, and Rockwell techs have been onsite since about 9am today. I'm heading in and was thinking about pulling apart the SCR banks and testing each of them individually to see if we have an SCR that isn't firing properly.

Any other ideas thoughts, thanks in advance!
Are the motors mechanically coupled together?
Are they series, shunt wound or compound wound machines. If compound, the shunt and series fields should be connected to additive i.e. not cancel each other out.
Was it running OK before you changed the motor?

Can you put a scope on the current feedback? You should see six equal 60 degree pulses for each full cycle of the power supply frequency - presumably 16.67 ms in your case.

Out of interest, what is the application? I think there have been just a couple of applications where I have come across DC motors wired in series. One was a large steel mill drive about 3,000 HP with both motors in the same shaft. The other was a traction application.
 
Are the motors mechanically coupled together?
Are they series, shunt wound or compound wound machines. If compound, the shunt and series fields should be connected to additive i.e. not cancel each other out.
Was it running OK before you changed the motor?

Can you put a scope on the current feedback? You should see six equal 60 degree pulses for each full cycle of the power supply frequency - presumably 16.67 ms in your case.

Out of interest, what is the application? I think there have been just a couple of applications where I have come across DC motors wired in series. One was a large steel mill drive about 3,000 HP with both motors in the same shaft. The other was a traction application.

Thanks for reply, I'm heading in shortly.

1. Series
2. Yes it was running fine, our spare motor is designed such that it can be swapped to either the north or south motor (junction box feeders on either side of motor), so they want our spare on the ground if the north motor were to fail.
3. We don't have a scope in our tools, I'm pretty sure the Rockwell guys will have one in their truck.
4. Each motor has its own gearbox that drives our kiln, this configuration allows us to have 1400HP available power, with enough torque to pick up the kiln if were to stop loaded, its a pretty big kiln.
 
Just a quick note on the feedback signal, we have a secondary set of motors and gearboxes (auxiliary drives, across the line) that we run in the event of a main drive problem. They run to turn the kiln to prevent the shell from warping. When we are on the auxiliary drives I can see the tach feedback signal displayed in RPM on the main drive VFD HIM, and its a steady signal reflecting an accurate reading.
 
I'm still at "two motors in series" :huh:

I hear yah Larry, it took me a bit to wrap my head around it at first. Here is a drawing so you can visualize it. On a side note, the Rockwell Automation tech discovered that our drive had somehow loaded its default values, the default motor has a substantially lower FLA than our two motors, so the drive was shutting off the SCRs when it thought it was in overcurrent. We now have the settings loaded into the HIM as well so we can easily get them back into the drive if this happens again.

PIC 1.jpg
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Thanks for reply, I'm heading in shortly.

1. Series
2. Yes it was running fine, our spare motor is designed such that it can be swapped to either the north or south motor (junction box feeders on either side of motor), so they want our spare on the ground if the north motor were to fail.
3. We don't have a scope in our tools, I'm pretty sure the Rockwell guys will have one in their truck.
4. Each motor has its own gearbox that drives our kiln, this configuration allows us to have 1400HP available power, with enough torque to pick up the kiln if were to stop loaded, its a pretty big kiln.

Sounds like a motor issue. Is the motor running in the same direction as it was previously?
It is likely that the motors have some residual magnetism. If you reverse the current in the series field it be in opposition to the residual. That could possibly result in instability.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190325-0840 EDT

The first post was very incomplete with respect to critical information.

Apparently there are two DC drives with their output shafts mechanically locked together. This is critical information, and means both motors must run at the same speed.

It is unlikely the fields are permanent magnets. Thus, they are wound fields. It is unlikely the fields are series type. This would mean the fields are what would be called a shunt motor. Further to get maximum torque in a variable speed application the fields would be separately excited at full rated current. It would not matter whether the two fields were connected in series or parallel, most likely they are in series.

Both fields are excited in such a manner that both motors have the same field flux density.

Why connect the two armatures in series? So the same current flows in both motors, and in turn load torque is automatically approximately equally balanced between the two motors.

It turned out the whole problem was in the setup of the VFD. With an understanding of how this system worked and some simple measurements one could have focused on the VFD early on.

This gets back to the need by electricians to understand how things work to be able to efficiently troubleshoot problems.

.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
I'm still at "two motors in series"
I'm not sure why this application was configured this way. Probably so the process would keep going without interruption if one pooped out.

Back in the day, (prior to ~1980 or so) railroad locomotives commonly had one DC motor per axle, and they were all wired in series so that each one could receive the maximum current available and produce the maximum starting torque. As the speed increased and the tractive effort demand fell, a very large relay switched them from series to series-parallel. As the speed increased still more, they were switched into parallel.
http://members.localnet.com/~docsteve/railroad/en_info.htm#transit
They are more-or-less mechanically coupled through the rails and all turn at the same speed, except when one ore more wheels slip under a heavy load.

Today, they mostly use AC motors and semiconductor VFDs.
 
190325-0840 EDT

The first post was very incomplete with respect to critical information.

Apparently there are two DC drives with their output shafts mechanically locked together. This is critical information, and means both motors must run at the same speed.

It is unlikely the fields are permanent magnets. Thus, they are wound fields. It is unlikely the fields are series type. This would mean the fields are what would be called a shunt motor. Further to get maximum torque in a variable speed application the fields would be separately excited at full rated current. It would not matter whether the two fields were connected in series or parallel, most likely they are in series.

Both fields are excited in such a manner that both motors have the same field flux density.

Why connect the two armatures in series? So the same current flows in both motors, and in turn load torque is automatically approximately equally balanced between the two motors.

It turned out the whole problem was in the setup of the VFD. With an understanding of how this system worked and some simple measurements one could have focused on the VFD early on.

This gets back to the need by electricians to understand how things work to be able to efficiently troubleshoot problems.

.

Fair enough bud, a little arrogant, being that you posted how smart you are after gathering all the info even after the solution was presented to you. Its pretty easy to come up with that opionon to solve after you have the solution. I'm not doubting your intelligence, but out of experience on my end ... i've seen more SCRs fail in bigger drives than i've seen them losing their memory. We have a couple cranes that are wound rotor, with step out resistors to control speed. These old girls are way more robust and tougher and reliable than any of the new VFD drives will every be. Obviously the greens want them because they are supposed effiencient ... by the time you cool them with A/C ... is it really worth the difference. That may be for another post.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190327-1630 EDT

BMANN06:

I am sorry you did not understand my post.

Your first post was inadequate for someone not associated with your machine to give you much useful help. You had more knowledge and information about the machine at the time of your first post, and just did not supply that information.

Without you telling us that the motor shafts were mechanically locked together we had to wonder why would you electrically connect two motors in series. Generally it would not make sense to do this.

In your last post I believe you are talking about wound rotor induction motors. These motors are not efficient when a large external resistor is used to adjust for low speed. Zero external resistance would provide the greatest efficiency.

If you have electronic drives that are failing components at a high rate, then the drives have been poorly designed, and not well built. There are lots of factors here, but good control, efficiency, and reliability can be achieved.

I would suggest that you study the basic theory of how AC and DC machinery works. There are many good books in this area dating back to 1900.

.

.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
In your last post I believe you are talking about wound rotor induction motors. These motors are not efficient when a large external resistor is used to adjust for low speed. Zero external resistance would provide the greatest efficiency.
The Improved Static Kramer gives very good efficiency for this type of motor. We had systems operating at 95% efficiency and that includes motor losses.
 

Tony S

Senior Member
This must be one hell of a kiln to need that amount of power to turn it. The largest kiln drive I’ve come across was 600HP and that was well oversized.

Pity the baring gear and generator didn’t work.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
This must be one hell of a kiln to need that amount of power to turn it. The largest kiln drive I’ve come across was 600HP and that was well oversized.

Pity the baring gear and generator didn’t work.
We did a few in cement works that were 2250kW (about 3,000 HP).
These were 11kV wound rotor motors using our ISK drives.
Sorry mods, a bit off topic I know.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190328-2040 EDT

Besoeker3:

I believe
BMANN06's comments were relative to plain resistors being the wound rotor load.

But your discussion on the
Static Kramer is important because it shows what regenerative feedback can do.

Do you have curves of efficiency (output mechanical power to input electrical power) vs load torque for various input or output power levels? Under what conditions did you get 95% efficiency?

.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
190328-2040 EDT

Do you have curves of efficiency (output mechanical power to input electrical power) vs load torque for various input or output power levels? Under what conditions did you get 95% efficiency?
I do - but maybe we should stick to the OP's topic.
 
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