High current failure in starting AC Induction Motor

[ecenus]

I have met a few problems in starting a 125hp (460V) AC Induction Motor:

1. I use v/f control method. When I start the motor, the motor trips out as my inverter can only handle 400A and the startup current (Locked rotor current) is over than that. For the pull-up torque: T = k * B * I * cos(theta) and B is proportional to v/f. So, if I need to keep current lower than 400A and wanna start the motor, then I need to increase v/f. Is my understanding correct?

2. The situation is: if I keep the accel about 7Hz/sec, the motor will trip out at 0.4 sec due to the high current; If I set the accel about 1.88 hz/sec, the motor will trip out at 0.9 sec...I am confused about it...

Thanks you in advance for your answers and reply!

 

[waross]

Increase your acceleration time. You should be able to start that motor at under 200 amps on a variable frequency invertor.
Volts/Hertz at 460 is about 7.7 . If you have a lot of inertia in the load you must use a longer acceleration time.
The volts per frequency is a function of the motor voltage and the system voltage. If you have been trying to adjust V/H higher you will be increasing the starting current rather than reducing it. The volts per Hertz ratio is not usually something that needs to be adjusted. It should be set properly and then left alone. If it is too high the magnetic iron in the motor saturates and the current rises disproportionately. If it is set too low you lose torque. Even when an expert is setting up a special application such as combined over-voltage, over-speed, over horsepower the V/H ratio is generally constant.
Good luck

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ecenus]

Thank you, Waross.

I just tried to start the motor with NO load, but still failed. So, your suggestion is: I keep the v/f prof still as 460/60 = 7.7 but increase acceleration time, is that correct?

Many thanks, again!

 

[ecenus]

BTW, if it is like what you said, then why do we have a "low-speed voltage boost" when we start the AC Induction Motor? As we know, if we have a low-speed voltage boost, we should have a different v/f ratio for the "low-speed voltage boost"....

Thanks!

 

[jraef]

Torque boost, as it is called in a V/Hz (non-vector) drive, is a feature that does indeed temporarily alter the V/Hz ratio to make the V higher at the low end of the speed range, typically up to about 10Hz. It essentially helps to overcome the fact that when the rotor is moving slowly, the winding resistance becomes a larger part of the total circuit so it increases the voltage to provide more of the available torque; more useful work for the current consumed. But it does so at the cost of increased low end current. Once the motor is past 10Hz or so, if the ratio is still high the windings will saturate, the magnetizing current will rise and less useful work is done per unit of current. So your current skyrockets and the motor does not accelerate.

As waross said, keep your V/Hz pattern set for the motor voltage. Don't play with that on a V/Hz drive or you end up with issues like you are seeing. Vector drives do that, but they are a quantum leap more sophisticated and can alter the ratio on the fly as the motor circuit changes with speed and load.

If you need the motor to accelerate faster then you will need to give it more current. If the drive can't deliver it, you need a bigger drive. If more current is not possible because of power limitations then you need to give it more time. The VFD cannot create magic. Vector mode, open or closed loop, would help a little because it can optimize your torque output dynamically, but that would mean changing your drive if it is not vector capable.

 

[ecenus]

Thank you, Jraef and Waross for your advice.

I tested the AC induction motor with fixed v/f ratio, 7.7 (460/60 = 7.7) with the acceleration time (0-60Hz in 262.2 sec). However, the motor current surges over 400A and the system trips out in less than 200ms.

With the same acceleration time, I changed the v/f ratio to a higher value (v/f = 81) the motor can run for a few seconds and when it reaches the commanded speed of 8 hz, the system trips out...Scope shows the current is suddenly over 400A when system trips out...

Do you have such experience before? I am totally confused....

 

[sreid]

Is your commanded speed 8 Hz? That's at the low end of what you can do with a V/F drive.

 

[ecenus]

I commanded the motor to speed up from 0 to 20Hz. but it trips out when my commanded speed reaching 8Hz. But, at least it spinned for a few rounds if I keep my v/f as high as 80. If I set v/f as low as 7.7, the system trips out due to the high starting current immediately without motor spninning at all.

 

[dpc]

Have you checked the motor full load amps against the rated output of your drive? Are you sure this drive is actually big enough to run this motor?

 

[ecenus]

Yes, I have checked it. Full load LRC is 145A while my drive current limit is 450A. The drive is big enough to run the motor as I did in the following way:

Start the drive with v/f ratio as 80 -> when the commanded speed is over 5Hz, switch v/f ratio to 7.7 (460/60 = 7.7) -> the motor can start now and reach the commanded speed, say 30Hz...

But, if I keep the v/f ratio as 7.7 constantly but increase the acceleration time, I can not start the motor...The concern is posted in the 6th post, which is

"
I tested the AC induction motor with fixed v/f ratio, 7.7 (460/60 = 7.7) with the acceleration time (0-60Hz in 262.2 sec). However, the motor current surges over 400A and the system trips out in less than 200ms.

With the same acceleration time, I changed the v/f ratio to a higher value (v/f = 81) the motor can run for a few seconds and when it reaches the commanded speed of 8 hz, the system trips out...Scope shows the current is suddenly over 400A when system trips out...

Do you have such experience before? I am totally confused....
"

 

[waross]

Check the motor for shorts or opens.
Set the VFD back to the factory defaults, and enter the basic motor parameters. Use a very long acceleration time.
Try the VFD with another smaller motor.
Look for faulty components in the VFD

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ecenus]

I have checked the motor that there is no shorts or opens;
The v/f parameters work for 20hp motor but failed in starting 125hp motor. And, this is why I was thinking of changing v/f profile to start the 125hp motor....

I am just curious why, with the same acceleration time, a higher v/f value (80) at least makes the motor run for a few rounds before it trips out while the low v/f value (7.7) makes the motor trip out immediately with no motor running at all...

 

[edison123]

My two cents

1. Check if the motor turns freely by hand to rule out any mechanical binding.

2. Check the motor connection with the name-plate connection. May be the winding is wrongly connected in delta when it is designed for wye.

3. Bypass the vfd and start the motor DOL and measure the inrush current.

* I would go green if only I were not yellow *

 

[ecenus]

Thank you!

1. I check the motor which can turn freely by hand;
2. I check motor connection. It is correct;
3. I have a purchased VFD for reference. It can start the motor with 10Hz/sec acceleration rate without any problems...

 

[ecenus]

BTW, the inrush current when I used a referenced VFD is 150A, acceleration rate is 10Hz/sec.

 

[jraef]

Looks to me as if you have a bad VFD. Symptoms are similar to ones where I had a cracked transistor. Limped along OK in test until you put some current through it, then it leaks to ground. Jumping to 400A in 200ms is not "load related" IMHO. Most new VFDs will have protection features to prevent you from even attempting this once it detects it the first time, yours may not. What brand and model is it? Have you consulted the manufacturer?

 

[ecenus]

Thank you, jraef.

Our VFD is "home-made". It consists of one DSP board plus an inverter. Maybe I did not say it correctly. The "purchased VFD" I mentioned actually is also made by an engineer who left us quite long time ago. Unfortunately, his work was not documented and I could not find him or his design documents...His "VFD" is acutally a box with a DSP board only. When I connect it to my inverter to drive the 125hp motor, it runs good.

When I use his DSP board connected to my interter, there is no problem in driving the motor with accel rate 10Hz/sec; but I failed in doing that using my DSP board connected to the same inverter to drive the motor with 10Hz/sec accel rate....

But, my DSP board with my inverter is good at driving 20hp motor....

I do not think it is my inverter's problem. Maybe it is because of my DSP board and code itself? If it is my DSP board problem, how can it drive 20hp motor without any problem?

 

[ScottyUK]

Why not just buy a COTS VFD? They are so cheap these days that an in-house design just can't begin to compete on cost, plus the protection and monitoring, not to mention performance, will be far superior with a COTS design.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

COTS? That's a new one on me. Common Off The Shelf?

 

[ScottyUK]

Commercial Off The Shelf.

Sorry for the TLA (three letter acronym ). If you haven't heard it then probably others haven't so apologies to all.

Designing your own widget in a market saturated by mass-produced widgets at commodity prices always seems like a waste of time to me, unless you're designing one to compete in the market or have a lot of money to throw at a custom design for a specific application.


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If we learn from our mistakes I'm getting a great education!