VFD causing High current
VFD causing High current
(OP)
We have got couple of VFD controller for our Cooling fans, fans are 600V motors, when we run them locally (bypassing the VFD) the voltage and current is normal (600V and 90Amps) but whenever we use our VFDs we are getting 480V and 120Amps therefore the controller gets hot, some times it causes alarm, any body has any idea why does it happen? What can we do to eliminate this problem?






RE: VFD causing High current
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD causing High current
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD causing High current
Itsmoked
What do you mean when you say that current cannot be measured with a regular meter on either side of the vfd?
RE: VFD causing High current
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If we learn from our mistakes I'm getting a great education!
RE: VFD causing High current
Typical meters all have to show RMS as everything relates to RMS. 120VAC, 480VAC, etc., etc.
However most meters don't read actual RMS they read a strange internally rectified DC. The designer then multiplies this by a fudge factor that gives the actual RMS value from that rectified result, if-and-only-if the measured AC signal is a sine wave. Anything having to do with a VFD is NOT a sine wave, so the meter will not give any form of a meaningful reading.
You must use a "true" RMS reading meter that actually integrates the measured signal correctly and gives the equivalent RMS value.
Unfortunately once you actually have an appropriate meter you are faced, probably, with a clamp-on current transformer that cannot faithfully transduce the same highly harmonic currents.
If the OP is referring to what a meter shows this is likely why the currents are not what are expected. If he is reading the current value off of the VFD display then that will be the properly calculated, actual RMS current, and we will have to look elsewhere. Like as Bill suggests maybe the speed is set too high.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD causing High current
you mentioned, w/o vfd: 600V and 90A -- i assume are operating input values to the motor. assuming balance 3-phase and power factor of 0.9; thus the motor consumes 84.2 Kw
if w/ vfd: 480V and 120A -- again are operating input values to the motor, then the motor consumes 89.9 Kw
did notice if the freq (hz) of the vfd registered less than 60 hz? maybe the reason for overheating is that the input current w/ vfd is higher than the rated current of the motor
RE: VFD causing High current
Why is your VFD set to 480 V ?
It is required to mention frequency whenever you talk VFD numbers.
RE: VFD causing High current
I've seen that before in Canada; people buy VFDs from the US because they are less expensive, not realizing that US VFDs, even if capable of 600V, are going to have default settings for 480V motors. On a couple of occasions I have seen Canadians connect 600V to 480V rated VFDs and get away without destroying them, but were unable to figure out why they couldn't set the voltage higher than 480V.
As Edison123 said, you need to supply more information when discussing VFDs, such as product rating, settings, operating frequency etc.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: VFD causing High current
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD causing High current
RE: VFD causing High current
The problem (as others have pointed out) is that the non sinusoidal currents drawn and supplied by a VFD can have harmonic components which manifest themselves as spikes with high peaks but not much duration. So they don't represent much RMS current. Using a meter which measures the peak by rectifying the AC input, holding the peak value (by charging a cap) and displaying a scaled down value (assuming a peak to RMS ratio of 1.414 for a pure sine wave) will give you an incorrect reading.
But we're all just guessing until we know how your current figures were obtained. There's still a possibility of a different explaination.
RE: VFD causing High current
And if the motor voltage is too low the VFD will try to compensate that missing voltage with a higher current
RE: VFD causing High current
RE: VFD causing High current
If your motor is rated for 600 V, 60 Hz, at 55 Hz, you need to set the VFD output voltage to 550 V. V/Hz must be a constant for the motor. If V/Hz higher than rated, ovefluxing and overcurrent will result. If V/Hz is lower (which is what you are doing now), underfluxing and overcurrent will result.
Experts like jraef will teach you how to program to your VFD if you post the VFD details he asked for.
Special metering (as mentioned ny itsmoked and others) is still required to read accurate currents and voltages, whether you like it or not.
RE: VFD causing High current
RE: VFD causing High current
But 575 V seems to be an odd voltage. Is it a standard voltage in US or Canada ?
Hopefully jeff and other drive aces will have more to say about the drive.
RE: VFD causing High current
RE: VFD causing High current
In some parts of Canada the 575 volt source is nominal voltage. NOPE, That's the standard motor voltage.
The Canadian Electrical Code gives the voltages as 120V, 208V, 240V, 277V, 347V, 416V, 480V, and 600V.
aLIJ23, CHECK YOUR VFD DEFAULT VOLTAGE SETTING AS JRAEF SUGGESTED, or go to a less knowledgeable forum for an answer that you like.
Perhaps, Alij23 you will be happier asking on a different forum.
One last time: VFD's have characteristics that are not like anything else you may have worked on. The basic principals still apply, but some things that in the past were taken for granted have changed.
As a result, the old ways of troubleshooting may not be valid for VFD's.
You don't seem to understand the difficulty of measuring the output current of a VFD, let alone the reasons for that difficulty.
Because of this, you want to disregard meter issues. Yes, by all means disregard the meter issues, not because you don't understand them, but because they are probably inaccurate.
Do some Googling and find out what the DC Bus voltage is in a VFD. It is not the voltage on the MCC bus.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD causing High current
by the way i messure my voltage and amps just from motor side not VFD side.
RE: VFD causing High current
Forget the meter, it probably is NOT giving you the right value anyway, but.
CHECK YOUR DEFAULT SETTINGS TO SEE IF IT IS SET FOR A 480V INCOMING LINE INSTEAD OF A 600V INCOMING LINE AS JRAEF SUGGESTED.
If you need help with that, ask and one of the VFD experts may help be willing to walk you through it.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD causing High current
Did you read and try to understand any of the responses posted above ?
If you did, then all your questions were already answered. If you didn't, then Bill is right.
RE: VFD causing High current
And do forget your meter!
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD causing High current
A couple of people posted that you need to post details of the equipment you used and you ignored them.
Also read what itsmoked just posted. The VFD will display the outut voltage and current internally and it will do it correctly.
If the frequency is 55Hz then the output voltage should be 527V. Your measurement of 480V makes either the meter or the VFD settings very suspect.
Two other settings you must get right are the motor voltage and the motor frequency (sometimes called the knee frequency).
RE: VFD causing High current
RE: VFD causing High current
The drive is a GE AV300i Model 3150, 157 A IEC 146 class 1 rated output, 575V +/-10% input. It is programmed for the motor nameplate data: 575V, 125 HP, 119 FLA, 900 rpm and for 575V Mains Input Voltage. The motor characteristic data was gathered using the drive's auto program features. There are 35 identical units acting the same way.
Ali's meters are Fluke 87 or better. All readings match the GE drives' readout for voltage, motor amps and input amps and the readings from high end analyzers. It is unlikely that it is a metering problem.
The drives have no speed feedback signal and use "sensorless" vector control software to estimate/control the output speed. Speed follows a 4-20 MA input signal from the plant DCS. The drive says the input signal is calling for 820 rpm (55 Hz)output and the output is about 55 Hz, output speed is within a few rpm of setpoint. The 450-480 V drive output is less than expected for 55 Hz (55 x 575/60 = 527V).
The drives all have bypass contactors so the motors can be operated at rated voltage and frequency. When on bypass, the motor currents are under the FLA (depending on ambient air temperature). On VFD, the motor amps are at or above FLA.
Will a vector drive back off the output voltage to minimize torque and save energy? Is that even possible?
We will recheck the drive programming and see if we missed something that did get reset to 480V. The high temperature alarms got cleared for now by activating the room ventilation and cleaning the drive cooling fan filters.
The supplier for these drives almost went bankrupt on the job and will not supply support. Their technical expertise was lacking after their engineer left. GE no longer distributes or supports these Italian made AV drives.
At my suggestion, Ali went on this web site and asked for some help trying to understand how these drives could be working correctly. (I was busy elsewhere.)
We need to take a unit down and do a full check on the programming parameters.
RE: VFD causing High current
1) The drive is set for V/Hz mode of operation and someone turned on the "Variable Torque" mode. Most vector drives I have encountered will not allow you to put a drive in Auto-tune unless you have the control method set to Vector, but this drive doesn't seem to state any such restriction. That means you may THINK you are setting the motor tuning by doing an Auto-tune, but if someone left Fn 321 at setting 3, which is V/Hz mode (default) and 712 to a setting of 3 which is Variable Torque, the motor will still be providing less voltage at a given speed in an attempt to save energy. If the load needs constant torque then it will slow down, increase slip and draw more current.
2) Someone, at some point, got themselves messed up with programming of this drive and reset to the Factory Defaults. UNFORTUNATELY, the factory default values may in fact be for 460V motors, even on the 575V version of this drive! Here's my evidence, pasted from the 575V manual:
Then later: Now if I look at Fn `161, the Motor Voltage setting, it says in this manual that the default setting is in fact 575V. But being that the fact that the 575V version of this drive appears to be an afterthought in their product line (because there is no other literature saying it even exists), it's entirely possible that the mfr messed up and allows the Set to Factory Defaults function to reset the drive to 460V, the original design voltage. Just a hunch, but one based on my experience with other mfrs that attempted to sell 575V drives into Canada. Some of them just "tweak" a 480V design and call it good, and when you do that, nasty little details get missed.If neither of those conditions exist, it also appears that this drive has something called a "dynamic regulation margin" (Fn 889) that artificially limits voltage above a certain speed. Here's how they describe it: If that is enabled, that would cause what you are describing if your load is demanding full torque at that speed.
God I love forensics, I shoulda been a detective... it's just not as satisfying when I'm not in front of the drive though.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: VFD causing High current
RE: VFD causing High current
We checked everything we could in the software, all appeared to be OK, except the motor amps was programmed as 136 instead of tne nameplate 119 amps. We checked voltage and speed on several units and caculated the V/F ratio (actually the V/rpm ratio).
In all cases it was about 0.52 = 460 Volts/886rpm.
It should have been 0.65 = 575 V/886 rpm.
You are correct that it looks like something in the firmware defaulted the drive to the 480V version.
RE: VFD causing High current
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD causing High current
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD causing High current
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD causing High current
RE: VFD causing High current
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