AB Powerflex 753 Over Current

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.
 
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.

The first steps that I would have suggested have all been listed, plus a few ;)

Aside from wiring for 200V on a 6-lead motor instead of 400V ... which you've checked ... the motor won't draw that kind of amps without having *WAY* too much load. So I'm not seeing anything obvious.

I was going to ask if the motor is direct drive or pulley-and-belts. Maybe the motor sheave is too large and fan sheave is too small? Maybe I will let that question stand. Spinning the fan too fast will take a lot more current, if the sheaves are mounted backward.

This is not very likely since you have been working on it for a while already, but it's all that comes to mind.

You mentioned rated outlet pressure. Is the outlet pressure correct when the blower is running?
 
Jraef
Go back to basics for little bit
Stop and think about what going on here.
The speed of an AC motor is determined by the supplied frequency and the number of electrical poles.
As we know we can’ change the number of poles in a motor while is running so the only way to chnge the speed is to change the frequency of the power supplied to the motor.
Base Speed = 120 * Frequency / Number of electrical poles (120 * 50 ) = 6000 / 2 = Motor base speed 3,000 RPM. I assume you read the motor speed with a tach at 2,950 that means the motor is trying to run at 3,000 rpm’s but the motor slip is 50 rpm’s. You must keep in mind that a standard squirrel cage motor will never be able to run at the base speed in this case 3,000 rpm even with no load connected if the motor rpm’s = base speed the torque produced by the motor will be 0 and the rpm’s will slip back
The rpm slip from base speed to actual speed reflects the actual load it called slip
Unless the information posted is wrong then (and I have no reason to believe it’s not correct). All of information and my knowledge of motors tells me it’s clear the motor in this case is a 2 pole motor that can’t be disputed.
The motor amps is a reflection of the work performed by the motor in this case as posted it is 80 amps
Line voltage is 410 VAC so (410 * 80) = 3280 * 1.732) = 56,810 watts or 56.81 KW
In this case the actual power consumed by the motor at this time is 56.81 KW
56,81 / 2 = 28.4 KW that looks real close to the expected 27KW
According to the post the fan should pull 27KW at 40 Amps that number really doesn’t quite add up but close enough.
If you do the math it is 28.257 KW
The fact is that the motor is using over twice the power the fan should be using at it’s rated speed and flow.
Ask yourself this question
What would make a fan motor pull twice the rated amps, the answer is simple, it running at way over the rated speed, nothing else will do that.
Actually I am surprised that the current drawn by the motor in not much higher but I think the drive has reached its max current limit 200% and there in no more current available on the buss for the motor
And it’s limiting the current supplied. If you keep running it at that load the motor will burn up no question about it. Even with a 1.5 service factor but they are almost imposable to find anymore most motors now are 1.0 or 1.15 service factor.
If you recalculate the numbers using a 4 pole motor
((120*50)= 6000 / 4) = a motor base speed of 1, 500 rpm you reduce the speed by 50% this would also reduce the work / load by about the same amount.
It’s telling that power actually used is almost 2 x what it should be. So it would make since a 4 pole motor would reduce the load and bring it down into the expected range.

I keep seeing people on here calling for you to auto tune again why, no auto or manual tune for that matter will make a 50% change in current / power draw of the motor a few amps reduction would be the best you could expect.
We have asked several times for the motor name plate data and the fan curve data but they are never posted. If they were I am sure they would bear out what I am saying.
Motors are rated by Volts to Hertz ratio
410V at 50Hz = 8.2 Volts per Hertz
400V at 50Hz = 8 Volts per Hertz
480V at 60 Hz = 8 Volts per Hertz
An electric motor will draw the amps necessary to do the work asked of it, so you have to limit the work you ask of it. How do you do that on a fan. Simple reduce the speed or restrict the air flow through it.
As I said before on a Fluid load ( Air, Water, Oil are all fluids to a pump) the load (Work required ) goes up by the log of speed. A small increase in speed will make a large increase in power required
As I said before most fans are designed to use a 2 pole motor they are cheaper and easier to build 2 pole verses 4 pole but with the motor running twice a fast but they don’t last as long more ware on the bushings.
The drive should have tripped the OL fault while trying to run so it’s also clear that you have not set the motor full load amps in the drive. Don’t feel too bad about that almost all the jobs I been on lately the FLA on the drives were not set at all.
You should contact the manufacture of the fan or the equipment and get the necessary information and select the correct parts for the job.

An interesting thing to think about: in a vacuum pump as you draw the air out and get closer to a pure vacuum the motor amps actually go down ( Moving less air = less load)

So go right ahead and keep wasting your time playing with the Auto Tune it’s your time to waste.
I offer this to help you and others to understand this, not to put anybody down.
 
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.

Since we are all guessing due to the lack of information from the original poster , let me add another guess. Are the design parameters set up for ducting attached, and you are testing with no ducting?

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?

After reading the thread I do not see any one suggesting running the "blower" straight across the line at 50 Hz. this should eliminate any question of a VFD issue. If the motor performs at expected parameters you have a VFD issue. If it operates the same as when running on the VFD, then its a engineering issue.
 
A quick response... I've seen two issues exactly like this, and in both instances, the motor was not sized correctly to the application. In both instances, I had to adjust a current limiting feature as a Band-Aid until they replaced the motor and drive with the correct rated equipment. My answer might be simplistic, but it's the first thing that comes to mind when I see this situation. I've seen the same situation with pumps, as well.
 
One of the fan laws:
kW1/kW2 = (RPM1/RPM2)³
As we have doubled expected power, one reason would be that we have increased the speed by the cube root of 2. In otherwords running the fan at 62.5Hz when it should be running at 50Hz. Or using a 6 pole motor instead of an 8 pole motor.

An 8 pole motor is a rare beast, so it is cheaper / easier to get spare parts in a hurry if you use a 6 pole motor and run the VFD at a maximum speed of 75%. As squirrel cage notes are constant torque capable, if you needed 27kW out of a motor running at 75%nominal speed, you would need 27/0.75 = 36kW nominal motor (or higher. Next standard motor size up is 37kW).

And so, my "guess the solution while we wait for the OP to show the fan curve and motor nameplate and VFD settings" is: motor is 37kw 6 pole 50Hz. Running VFD at 50Hz. Fan curve datasheet says "speed: 750rpm"
 
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We've tested the RPMs on the motor at 50Hz and they are correct at 2950RPM.
On re-reading the post it seems the motor is running at 2pole 50hz speed after they mechanically measured(?) the shaft speed.
So my next guesses are:
Fan curve says 2200rpm, running motor at 2950rpm
OR
Fan curve says 27kw @ nominal pressure and airflow. Testing fan with no Product or ductwork resistance, so airflow is increasing, so power is going up.
 
We are all arguing and speculating over complete guesses. We need to know some very basic info first:
1. What sort of device are we talking about when we say "fan"
2. What is the curve for the device with system curve?

Lets start there and then try and figure out what could be wrong.
 

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