High current failure in starting AC Induction Motor
High current failure in starting AC Induction Motor
(OP)
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!
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!





RE: High current failure in starting AC Induction Motor
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
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"Why not the best?"
Jimmy Carter
RE: High current failure in starting AC Induction Motor
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!
RE: High current failure in starting AC Induction Motor
Thanks!
RE: High current failure in starting AC Induction Motor
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.
RE: High current failure in starting AC Induction Motor
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....
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
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....
"
RE: High current failure in starting AC Induction Motor
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
RE: High current failure in starting AC Induction Motor
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...
RE: High current failure in starting AC Induction Motor
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 *
RE: High current failure in starting AC Induction Motor
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...
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
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?
RE: High current failure in starting AC Induction Motor
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If we learn from our mistakes I'm getting a great education!
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
Sorry for the TLA (three letter acronym
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!
RE: High current failure in starting AC Induction Motor
RE: High current failure in starting AC Induction Motor
In the mean time, the value of the time you spent trying to debug this design would have bought you a fully debugged design from ABB, Siemens, SSD, Danfoss, Control Techniques, etc. And any of those drives, provided that it was sized correctly for the motor current, would in all likelihood have you up and running by now if we can assume that the motor itself is healthy. Why do it the hard way?
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If we learn from our mistakes I'm getting a great education!