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Joined: Nov 2011
Posts: 3
Jeffdc Offline OP
New Member
I am trying to help a customer out with this problem. There is an AC drive running a motor on a paper mill. Not 100% of the time, but frequently, during either the accel or decel, at a specific speed, the current will spike up and trip the drive. There is a 20s accel and decel time. When speed hits around 55% rated motor speed, the current will start climbing until get overcurrent fault on drive. Or, may not do this on accel, but then stop drive, and when speed drops to around 55%, get current spike and bus voltage spike, and get fault also. Can a bad motor do this?

Joined: Nov 2007
Posts: 763
Is this a new install or something that just started with an existing install?
Iím no expert on the subject, but I read something somewhere about a similar issue when using deceleration in place of actual dynamic braking on a VFD motor drive with a no load condition will cause the DC bus current climb and cause a fault due to the motor acting similar to a generator.
I believe the solution was to install a dynamic brake package or simply let the motor stop on its own under no load without using deceleration function.

Maybe someone else with more first hand experience will have some info to add.

Joined: Jul 2002
Posts: 8,443
Likes: 3
Is this motor part of a direct drive system or a belt drive system?
What exactly does this motor drive?
Also what brand/model/size is the motor and it's attached drive?
55% of full load speed sounds far too low on startup, and makes me wonder if the VSD has been commissioned properly.
My bread and butter is working with VSD's on motors, there isn't a problem I haven't been able to cure. wink

Joined: Nov 2011
Posts: 3
Jeffdc Offline OP
New Member
Not sure brand, but motor is 150HP, goes into a gearbox (not sure ratio), then turns a shaft. There are 4 levels of rolls in this paper mill that can be engaged/disengaged with the main shaft with clutches and gears. No belts. The drive is SSD AC890.
We have ran the system with 1 level of rolls, 2 levels of rolls, etc, and seen the problem off and on with all our tests. Drive does not trip unless running with all 4 levels since overall current is lower with less than 4 levels engaged with main drive shaft.
What do you mean by 55% sounds low? The 55% is a speed when this problem seems to occur at. Other than that I am not sure what meaning it would have.
Thanks for your interest in my problem and I look forward to further discussions on this.

Joined: Jan 2005
Posts: 354
pdh Offline
Could it just be a soft start running on L-N until at sufficient speed it switches over to L-L for the remaining accel, and perhaps doing it at the wrong time?

Joined: Mar 2004
Posts: 947
twh Offline
It might be internal to the VFD. Because the problem happens on accel and decel, it probably isn't the load driving the motor. Is the current increase actually through the motor or just line side? Do you have a line side choke installed?

Joined: Nov 2011
Posts: 3
Jeffdc Offline OP
New Member
The drive output current (motor current) is what increases. Not sure if there is a choke on line side.

To give an update on this, I had the customer change the dynamic braking params because they were never setup. They said this seemed to prevent the drive from tripping but still get the current increase. They said they think there may be a mechanical problem that only shows itself at certain speeds. If I find out that it is a mechanical problem I will post that here so topic can be closed.

Joined: Nov 2005
Posts: 827
Likes: 1

I am by no means an expert on VFDs but I have heard of problems with resonance. Sure sounds like that could be your problem.

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