100 Hp Motor Running 3 Hz drawing 115 amps ?

Good Evening ,

I have 2 colleagues working on a problem with a 3 phase 480 vac 100 hp motor for a conveyor. The motor has a brake. Brake is releasing.
This motor is drawing about 115 amps and running at only 3 hz , and it does not fault out. What do you think this problem could be. I told them to look at the brake , but they say the brake is releasing. They told me that it has a jerking motion when it runs. It is running on a Cutler Hammer SVX 9000 AF Drive.

Thanks in advance

That's pretty slow. Are you using an encoder feedback?

Is it being commanded to run at 3 Hz, or is it running at a speed that's consistent with 3 Hz ?

Has it run smoothly previously ?

Unless the drive has been reprogrammed, I suspect that you're looking at a mechanical problem and not something electrical.

So a 100HP motor rated 460V 60Hz would be roughly 124A FLC, you are running at 115A. Where is the problem?

Are you thinking that just because it is running at 3Hz, the current should be lower? Because that is incorrect thinking. Current equates to TORQUE, not speed. So if, at 3Hz, your conveyor still needs full torque, full torque it is! And with full torque comes full current. There is no problem here, unless of course you have a fan cooled motor, because the FAY is not going to be moving any air at 3Hz, so you will need a separately powered blower running full speed. But if the motor is TENV and designed for inverter use, should be fine.

if you are using simple V/hz, control you could be losing large amounts of torque, especially at that low speed. Try using sensorless vector or flux vector using an encoder if possible.

Try putting the VSD into manual mode and confirm that operation is correct (i.e. you should be able to ramp speed up and down, if so it is a input/program issue).

A 100 hp Motor

Yes Jraef is right a 100 hp motor should draw around 115 Amps, there is no issue here, you should have stated your problem that the motor can't run above the 3Hz, I don't see a mechanical problem either, because in this case the current should exceed the motor rated Amps & trip the protection.

Also running at 3 Hz will heat up the motor, in this case a force cooling (separate cooling fan should be used)

Then the problem should be re stated to CAN"T RUN A 100 HP MOTOR BEYOND 3 HZ

Hagos

Rob,

i have to ask, why have a 100 hp motor running at 3 hz when you can use a smaller motor and a gear reducer to accomplish the same thing?

this motor can run at a higher rpm and would be more stable.

we had 50 hp motors running gear boxes for conveyors 300+ feet with no problems.

regards,
james

The jerking motion is called "cogging" and it is common when running a conventional induction motor below 6 or 10 Hz.

James called it - unless you really need a 20:1 speed range you should get a gear box. That will give better control, allow use of a smaller motor, and avoid over-heating issues.

I agree with all that has been said so far. I also suspect that the command speed is much higher than 3Hz, and that is why the post is worded as it is. I think his problem is that the motor:
a) cannot overcome the initial torque requirement to get up to speed. This can be due to running in V/Hz mode and/or a mechanical issue.
b) has a wiring fault or ground fault somewhere between the VFD or in the motor.

I would start with a basic ohm test on all three phases including the wiring all the way from the drive through the motor and with any disconnects closed. You should get a very low ohm reading and they should be balanced within a few percent. Next, I would do a megohm test to ensure that there is not some undetected ground fault.

If those tests turn out okay, then you have proven (97% of the time) that the wiring and motor is okay.

Hopefully the OP will come back with more details and findings.

Key question: Is the speed command at 3hz or is it above that and the motor is being restrained from increasing speed further? At 115amps the motor is not even fully loaded.

Observation: True, 100hp is not needed to run this slow but there are plenty of applications that require high hp at full speed but occasionally have to operate down in very slow speeds. Hopefully, that is the case here or else we have a massive misapplication of motor and drive.

Well I saw nothing indicating that the motor was COMMANDED to run at anything but 3Hz, only RobS can answer that, so in the absence of information to the contrary, I took it at face value.

Current = Torque, so if you have a load on a conveyor that needs 304 ft. lbs. of torque to move it, you will need 304 ft. lbs. at 3Hz or 28Hz or 60Hz.

Now should we decide to go off into conjecture land, if the motor IS being told to run at a higher speed and it is NOT, then there is a perfectly logical explanation that can result in this exact scenario. The user has programmed a Current Limit of 115A into the drive, or a torque limit of 300 ft. lbs. As the drive accelerates, the load on the conveyor is requiring MORE than 300 ft. lbs. of torque to accelerate it, so it reaches that limit almost immediately, at 3Hz, and goes into torque / current limit, which on a drive, also results in a speed suppression. Bottom line, not enough motor or too much load, whichever way you want to look at it.

A quick look at the SVX9000 manual shows that Eaton calls it "Current Limit" and the default setting is 1.5X the drive amp rating. So if someone went in and changed that value to 115A, then it is doing exactly what the programming said to do. But I did notice that the SV9000 manual did NOT describe HOW the current is limited. I just know that overriding the commanded speed is the only real control that VFDs have to limit current.

I am sorry. The drive is suppose to run at 60 hz , but will only run at 3 hz , at now about 145 amps. Could it be a mechanical limit ?

Is this a new install or has it worked OK before

Did someone try reversing the rotatation of the motor

How many terminals are in the terminal box and how are they connected

My reading of post 1 along with info from post 13 would have me thinking you are hitting the setting for the drives current limit

To me it would appear to be a motor wiring issue with the limited data you have supplied

I agree with Gil47. I suspect that this drive is failing to ramp up to 60 Hz because it is being current limited. When I first saw this thread, I thought that you were intentionally operating the motor at a very low frequency.

That could be caused by an issue with the motor wiring, but it could also be a mechanical issue such as a worn bearing. It could even be due to a shorted turn in a motor winding.

How difficult is it to disengage the motor and attempt to run it unloaded?