AB 753 VFD Issue for Lifting Application in SVC

[ThegrouchyEngineer]

I have an application using a VFD in sensorless vector control for lifting. This system was designed specifically for us by an outside vendor and worked perfectly on their factory floor and for a week at our facility.

The Issue that we see from time to time is the lift is commanded to lift up and there is a period of several seconds where it doesn't move at all. The nameplate current of the motor is 1.4A and it seems the current hits 1.4A then flatlines. From the plot at 8:00:23, you can see the lift sits there for 7s with the frequency output of the drive close to zero. Then the movement command is cancelled and attempted again. This 2nd time, it takes off.

The motor is a 3/4hp Bauer worm gear motor with an output speed of 29rpm. It should have plenty of torque to move the load according to the vendor, but looking at the graph, once it starts moving the load, the current drops to 1.3amp which seems to be too close to the 1.4A FLA if it is truly oversized.

The drive is a 480V AB 753 with a rated current of 2.1A

I don't recall seeing this issue when the lift is commanded to lower the load and I don't see it with no load on the lift either.

I think that small hz blip at the beginning of each test is due to the backlash of the gearbox, lifting tension of the belt etc. of the system.

VFD Plot

Orange line = start command with 1 being start and 0 being stop
Yellow line = current in amps
Green line = frequency feedback from the drive in hz
The blue line = the frequency set point (In this case it is always 45hz)

Anyone have any thoughts on what is going on? and how to fix it. Would putting in a skip frequency from 0-1hz help?

 

[petersonra]

Does this thing by any chance have a mechanical brake on it that is not releasing?

 

[synchro]

Have your plotted the same waveforms without a load for comparison purposes?

 

[paulengr]

Sensorless vector does not work at all below around 0.6 to 1.2 Hz because there is no vector to detect. Was the motor originally auto tuned with rotation with no load, brake, etc.? Probably either the V/Hz / Sensorless switch setting is too low or the no load current (torque boost) is too low so it gets stuck in “idle” with too low of a current to accelerate (stalls). You may need to redo the auto tune or tweak one or the other setting.

Really this is more of a sensored application. You can’t hold a load in sensorless for instance.

 

[ThegrouchyEngineer]

Does this thing by any chance have a mechanical brake on it that is not releasing?

There is a mechanical brake on the shaft of the motor and I do believe it is releasing each time because the current in the down position is .9A every time.

There is another brake on a guide shaft of the lift too and that one I know has to be releasing because it is strong enough to stop a freefalling load rather quickly

 

[ThegrouchyEngineer]

Have your plotted the same waveforms without a load for comparison purposes?

I have a waveform of the lift lowering with a load, but can't find one without a load. I will have to see if we can get this info. The current while lowering .9A with a load.

 

[ThegrouchyEngineer]

Sensorless vector does not work at all below around 0.6 to 1.2 Hz because there is no vector to detect. Was the motor originally auto tuned with rotation with no load, brake, etc.? Probably either the V/Hz / Sensorless switch setting is too low or the no load current (torque boost) is too low so it gets stuck in “idle” with too low of a current to accelerate (stalls). You may need to redo the auto tune or tweak one or the other setting.

Really this is more of a sensored application. You can’t hold a load in sensorless for instance.

The motor was static tuned by the vendor. We did a rotate tune with the spare motor we ordered (it can't be decoupled from the worm drive gear box) so we did the tune with the brake off and decoupled from any load other than the gearbox. I can't find the info, but the parameters weren't much different with the static vs. rotate.

I will look into the setting that you mentioned to see if that helps!

 

[synchro]

It may be stating the obvious, but did you verify that there's adequate lubricant in the gear box?
Also, did any codes show up when the motor stalled under load?

 

[Jraef]

It may be stating the obvious, but did you verify that there's adequate lubricant in the gear box?
...

Lol, been there, done that, got the t-shirt and the hat! Works great for a couple of days, then not so much.