AB Powerflex 753 Over Current

Hi guys,

We have an Allen Bradley Powerflex 753 drive powering a simple blower (37kW) motor which we have just powered up. At 50Hz the blower is meant to pull 27kW and 40A at our outlet pressure but it is pulling about 45kW and 80A.

We've tried every different setting we can in the VSD and have mechanically tested the blower. We've tested the RPMs on the motor at 50Hz and they are correct at 2950RPM. We've narrowed the large and abnormal current draw down to either the VSD or the motor.

Motor is meant to be programmed as a constant torque motor and we have the VSD set on Induction SV mode.

Has anyone come across this issue before with the AB 753 and are we missing something?

Was this a VFD that was used and had a previous configuration? If so, you may consider resetting to default parameters just in case.

Other questions:

Motor wired correctly?
Checked current balance across each phase? Not single-phasing?
Blower lubricated correctly and installed properly?
Motor sized correctly for blower load?

If you can decouple the motor from the blower and run it up to 50hz, it should draw appoximately 1/3 of full load amps. If it pulls significantly more, you have a motor/VFD issue. Otherwise the issue is likely in the blower or the load you're asking of it.

Your numbers tell me that the motor is a 2 pole motor 3,00rpm at 50 hz 3,600 at 60 hz
you don't say what the motor is base speed it from the information the it was to be 4 pole
1,500 rpm at 50 hz the load on a fan goes up by the log of he speed.
so any motor speed over 1,500rpm would cause the load it increase my guess would be at about 1,600 to 1,700 the motor would start overloading the 2,950 rpm would be way over loaded
Lower your frequency to 25hz and I bet the motor will be at the full load ratings

Do you have the motor data set up for a 4 pole or 2 pole motor in the Powerflex?

Check motor wiring for Star/Delta connection links.
Look at the name plate for correct connection for supplied voltage.
Not all motors are the same.

Hi all,

Thanks for the responses.

Rupej - this is a brand new VSD on a brand new blower so we started from default settings. The motor is wired correctly (been checked numerous times) and the blower is mechanically fine (also been checked), has the same current over the three phases when operating. Motor pulls about 18A when decoupled which is what we'd expect.

GaryS - this is a 2 pole motor (supposedly) and we have input this information into the VSD. If the manufacturers have accidentally put a 4 pole motor on and we tell the VSD that it's a two pole motor would this cause the motor at 50Hz to run at 2 pole speed even if it's a 4 pole motor?

Jasondelane - we have input the motor info into the VSD as shown on the motor name plate (415V, 65A FLA and 2950RPM speed)

Thanks for your help guys!

Jack

Yes you have a 2 pole motor, the motor rpm's and the output frequency tell me that
but I think the fan should have a 4 pole motor on it.
while 2 pole motor a common on fan loads that's probably why you have a 2 pole motor on it. but in this case the fan was designed for a 4 pole motor. I can understand how that mistake was made. to prove the theory out set the vfd out to 25hz and measure the motor current as I said before I would be the motor load would fall very close to what you are looking for.
2 pole motor base speed at 50 hz is 3000 rpm
4 pole motor base speed at 50 hz is 1500 rpm
as you can see the speed is reduced by 50% the load would be reduced by abut 65%
a good rule is the load on a fan or pump increases by the log of speed
Frankly I am surprised that your load only doubled I would have expected to be much higher with doubled speed.
how did you determine the expected load / motor amps, are getting it from the motor name plate
I would not expect it to run at the motor full load amps I would expect 75%
would be a good continuous running amps
Everything tells me that you should have a 4 pole motor on that fan or if it is coupled with a belt then a 2:1 pully step down would work.
ant I makes no difference if you set the vfd to a 2 or 4 pole the motor seed will be determined by the freq and the number of poles in the motor. the vfd set up is for it to calculate the way to display the motor speed and handle the electronics in the vfd.

Might be a stupid question, but have you done the autotune procedure? I did an autotune on a motor that ended up having to be replaced with a model that was exactly the same spec-wise but when I tried to run the new motor I was getting really strange numbers and behaviour. Ended up having to redo the autotune for the new motor and everything worked great afterwards.

What control method are you using in the drive? V/Hz, SVC, or Flux? You may want to try using V/Hz to eliminate tuning issues.

Along with what GaryS is saying, do you have the fan curve data? The fan specifications should say what fan speed the fan is intended to run. If the fan speed is suppose to be 1500rpm (50hz on 4pole motor) and you have a 2pole motor (3000rpm at 50hz) the fan flow and thus power requirement will be much much higher.

What sort of fan is this? Are you sure that you have the fan going the correct way? On some fans/blowers, if they are going in reverse you will get low flow and very high current draw. This is due to the running very in-efficiently and basically self-heating inside the fan. I have seen this on a high-pressure blower that was running reverse, no-one could figure out why it was running so high load and flow was down.

Are you sure on the pressure ( I assume this is discharge pressure you are measuring?) If the pressure you are measuring (and I assume controlling??) is lower than you think, then the fan could be moving a higher volume of air than you think, thus causing higher power consumption.

You might try using a analog input Solid State Relay {SSR}.

This type of relay does not have contacts but uses a transistor type device called a SCR or a variation. Instead of controlling frequency, it controls voltage by
limiting the peaks of the sine wave. I would use either a 0-10vdc or 0-20ma output from the PLC for control and start the Fan with full voltage then decrease it by as much as 50% after the fan is spinning for several seconds.

You can find one example at https://www.alliedelec.com/product/carlo-gavazzi-inc-/rm1e40aa50/70314237/ along with a datasheet that says

"This relay is suitable for control of heaters, lighting and
slightly inductive loads such as small fans. The relay can also
be used for soft turn-on of high-power incandescent lamps."

It should generate very little heat but you may want to mount the SSR on a metal panel or use a heat-sink.

Let us know if it works if you try it.

^ nothing personal but I don't think SSRs like that are suitable for controlling a 3-phase motor, especially one of this size.

mylespetro said:

Might be a stupid question, but have you done the autotune procedure? I did an autotune on a motor that ended up having to be replaced with a model that was exactly the same spec-wise but when I tried to run the new motor I was getting really strange numbers and behaviour. Ended up having to redo the autotune for the new motor and everything worked great afterwards.

Click to expand...

This is the first thing I thought of as well, in my experience if you fail to do an Autotune procedure, the drive has to use a basic motor equivalent circuit model and in doing so, can exhibit unexpected behaviors, including over saturating the motor, which leads to high current values.

Helliana said:

What control method are you using in the drive? V/Hz, SVC, or Flux? You may want to try using V/Hz to eliminate tuning issues.

Click to expand...

Generally this is true, but on the newest drives like the PF750 series (or any make using the Gen 6 IIGBTs, i.e. since 2017), the way the transistors work now results in very steep leading edges (high rate-of-rise) on the PWM pulses, so Autotuning is now necessary even if using V/Hz mode.

why do you want to keep trying to auto tune with the speed load curve this far out it's not possible to tune it. it should run without the auto tune but it's clear the speed on the fan is to high and that will cause the motor to run in overload
As was said before check the fan curve and motor I am sure that you will find that the fan should be mounted on a 4 pole 1500 rpm at50 hz motor
or you could connect the across the line and see what it draws as 60 hz my guess is the at 60 hz you can't even run the motor the current will be way high over heat the motor and blow the fuse

GaryS, with all due respect, the OP has not confirmed that the fan was supposed to have a 4 pole motor. Yes, that is USUALLY the case, but we don’t know that for sure. In fact, we don’t really know if this is a “fan” or a blower, people tend to use the terms interchangeably even though they are not always the same thing. For instance I am working with someone right now on some high pressure turbine blowers that are on 2 pole motors and running at 200Hz, my customer is constantly referring to them as “fans”, I have given up on correcting them. You may still be (probably are) right, but we just don’t know until the OP checks in again.