I am putting a current limit loop in the VFD for my 125hp 3-phase AC Induction Motor.
My scheme is to get the current feedback from my DSP board. If the current is over a certain limit level, say 300A, I will lower my commanded frequency f so that the voltage drops. (I use v/f scalar control)...
The inverter is from Semikron and the rest of board, i.e. DSP board is made by me and a friend of mine. The VFD's current limit is 450A which is the hardware limit that could not be changed by the code...BTW, I use TI TMS320L2407A chip and download the TI's code (with v/f profile) from their...
Itsmoked-
Thank you for your prompt reply.
The motor does NOT start turning immediately when I use a higher v/f ratio. It takes about 2-3 seconds before the motor spins. The acceleration rate is 0.23Hz/sec. Werid...
BTW, my motor is 125hp 3-phase AC Induction Motor...
Thank you itsmoked.
Maybe I did not put it clearly.
The scenario is when I use the steady-state v/f ratio to start the motor, my inverter trips out directly(within 400ms, by then the motor did not turn at all); but, when set a higher v/f ratio, with the same acceleration rate, the motor can...
Skogsgurra-
Thank you for your detailed answers.
I am still trying to iron out a weird thing happening to my motor: When I try to start the motor, I have to give a higher v/f ratio than the ratio at the steady state so as to turn the motor; Otherwise, if I keep the steady state v/f ratio to...
I am using a VFD to drive a 3-phase AC Induction motor. A v/f scalar control method is used.
Many textbooks define such motor working range is usually from 8 or 10hz to, say, 60hz. I think that is because the torque is insufficient to drive the motor at the low-speed range. So, I would like to...
Thank you for your tips, cswilson,
My current loop is just for the current limit as my VFD has the capacity of 450A. If the motor draws too much current which is over than that, it will hurt my hardware. So, I want to add in the current loop to limit the current: if the current is over, say...
I would add in a current loop in the code of my VFD for a 3-phase AC induction motor (125hp) to control the current.
As I am using V/f scheme to control the motor, if the current is too large or small, I need to adjust the voltage applied to the motor based on the current values (I can...
Yes, I am using VFD, 3 phase AC Induction motor. Spin the motor with no load first.
I actually want to understand more about the motor starting process which has "auto boost" and "voltage boost".
thanks!
What is difference between "auto boost" and "torque boost" during AC Induction motor start? If the motor has no load, is that possible to make the motor work at low speed range, say under 10hz?
Thanks!
I am thinking whether my starting commanded frequency is incorrect. I start from zero, so v = f * ratio, V is small. I should start the commanded frequency from 8 hz, get a higher starting voltage to start the motor...
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...
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...
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...
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...
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...
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...
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!
