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VFD causing High current
5

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

You cannot use a regular meter to measure the current on either side of VFD.  Are you?  How are you measuring the current?

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD causing High current

Slow the fans down a couple of RPM.

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

Output is unlikely to be at a standard frequency, possibly not sinusoidal, and laden with high frequency switching hash. Input will be at standard frequency but will be harmonic-laden. Both situations can cause peculiar effects on meters. A true RMS meter would be the normal recommendation, and even then check its behaviour at very low frequencies.
  

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

RE: VFD causing High current

rockman; VFDs send distorted (highly harmonic) waveforms to the motor and draw a distorted current(from rectifiers) on the supply side.

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

hello Alij23

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

If the fans were pulling 90 A at 600 V, then at 480 V, they would draw a higher current in the proportion 600/480 (assuming the speed is the same).

Why is your VFD set to 480 V ?

It is required to mention frequency whenever you talk VFD numbers.

RE: VFD causing High current

4
Methinks maybe you have the VFDs defaulted to 480V motor settings, in other words the wrong V/Hz ratio. If the VFD is only supplying 480V and 60Hz and the motor is expecting 600V at 60 Hz, you are running the motor under fluxed; the voltage is 20% low. because at a fixed frequency, torque is related to the square of the voltage, you will lose about 36% of your torque! The slip will then increase under load, the motor will pull more current and overload.

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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: VFD causing High current

From Canada, thanks for the tip, Jeff. LPS

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

RE: VFD causing High current

(OP)
The VFD frequency is 55Hz ,i measured Voltage and Current directly from output wires of the VFD Cabinet ,i agree that it might be Meter problem , but i do not understand why VFD increase the AMP and decrease the Voltage,can it be because of VFD settings?the Fan motor is 125HP and the buss feeding those Motors is 600V.

RE: VFD causing High current

Quote (Alij23):

i measured Voltage and Current directly from output wires of the VFD Cabinet
With what kind of meter? What kind of current clamp/transducer? Model numbers?

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

Is the DC Link Bus 600V_DC? Cause in this case the max motor voltage you can run is 600V_DC/sqrt(2) = 425V_AC

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

(OP)
The bus is AC 600 Volt, guys we have to put this meter problem aside ,the problem is what can cause high AMPS ?as i mentioned before if i run the fan locally(bypassing the controller) every thing is alright but as soon as VFD comes to picture every thing starts changing (high amps and low voltage) the other thing is my frequency is 55hz right now ,i was wondering if we run the fan under 60 Hz and this interesting phenomena keeps increasing the current,then i have to modify the breaker settings too .

RE: VFD causing High current

Your question has been answered before.

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

(OP)
thanx Edison ,my fan is a 125hp ,575V SF=1.15 and FLA 119Amp and my VFD is a GE-drive AV 300i,
 

RE: VFD causing High current

Assuming it is 575 V at 60 Hz, for 55 Hz the VFD output voltage should be set to 575*55/60 = 527 V.

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

In some parts of Canada the 575 volt source is nominal voltage. Only specific drives can take the higher voltage. Look to your GE manual and see if you can set the native voltage to 575 volt (or 600) That voltage is the RMS. The peak which hits the DC bus inside the drive is well over that. In a 480 volt drive the max bus voltage is 800, beyond that you get an overvoltage fault. The Over voltage fault may occur if the drive is not fitted for the 575 volt nominal. Check your manual.

RE: VFD causing High current

575V in Canada is no odder than 460V in the US. The standard system voltages are multiples of 120V, (120V, 240V, 480V, and in Canada 600V), the standard motor voltages are multiples of 115V. (115V, 230V, 460V, and in Canada 600V).
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.

Quote (Alij23):

The bus is AC 600 Volt, guys we have to put this meter problem aside ,the problem is what can cause high AMPS ?as i mentioned before if i run the fan locally(bypassing the controller) every thing is alright but as soon as VFD comes to picture every thing starts changing (high amps and low voltage) the other thing is my frequency is 55hz right now ,i was wondering if we run the fan under 60 Hz and this interesting phenomena keeps increasing the current,then i have to modify the breaker settings too .
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

(OP)
Dear waross you are not supposed to get mad at me becasue i am asking something i do not understand.the reason i am telling to put the meter probelm aside is becasue i use the same meter for both cases ,the thing i do not understand is why my motor driving more amps when i use VFD? what possibly can happen while my current passing through VFD ?is it VFD firmware setting issue? or some hardware issue?i dont know VFDs well that is why i dont understand and that is why i am asking poeple who may know!
by the way i messure my voltage and amps just from motor side not VFD side.

RE: VFD causing High current

Not mad, but a little frustrated.
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

Alij

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

The VFD can and will display everything you ask about, and correctly!  You may need to find the manual and look up how to select the various displays.  But it WILL show you the DC bus voltage, the supply voltage selected, the correct current value, that is being sent to the motor, etc., etc. ,etc. ,etc. ,etc. ,etc..

And do forget your meter!

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD causing High current

OK, unless the meter is a high quality true RMS unit then throw it away and quit using it. The cheap meters most electrical people like to purchase are useless when measuring voltage and current on a VFD. I would recommend you purchase a higher end Fluke model, such as a Fluke 87 or better, that states it's true RMS and expect to pay $300+ for it.

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

(OP)
thanx guys , i did not know enough about VFDs ,i did read some about them and i got it .

RE: VFD causing High current

Gentlemen - Ali is working with me trying to puzzle this drive issue out.  

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

I was a bit confused, because all of the information I could find on that drive at first stated that the maximum voltage is 460V. But I found a manual with reference to their having a 575V version for Canada, so that cleared things up from a logistic standpoint, but I still have my suspicions (see below). Something is amiss here and the evidence presented in this thread still points to the motor being under-fluxed. So I have to theorize based on what I can see in the available literature. Here are my working theories;

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:

Quote (GE manual):

5.3.2.2. Setting Motor Parameters
The AV-300i drive is factory set for use with 4-pole standard motors, either for 400V or 460V, to be operated up
to the nominal speed. Enter nameplate data recorded for the connected motor to ensure satisfactory performance.
Then later:

Quote:

The BASIC MENU provides the parameters required for the initial commissioning of the drive. The data that is factory set for the default motor concerned refers to a four-pole machine for either 400V or 460V). The motor data in the DRIVE PARAMETER menu should be changed accordingly if different motors are used.
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:

Quote:

Example 2
Use of a standard motor 575 V / 60Hz
Application requiring total immunity from torque disturbances due to main voltage fluctuation.
Base voltage 575 V -15% = 490 V
Base frequency 60 Hz -15% = 51 Hz
The motor works at nominal flux until 51Hz (85% of the nominal speed), supplying full torque.
Over this frequency the voltage is regulated constant regulated, the flux is weakened and the motor supplies
constant power equal to 85% of the nominal (rated) power
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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: VFD causing High current

Jeff - You are hereby renamed Lieutenant Colombo.

RE: VFD causing High current

jraef-You are the detective and the engineer!

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

You know what they say, if it looks like a duck, it quacks like a duck and it floats on the water, it's probably a duck...

 


"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

Awesome jraef.


 

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

RE: VFD causing High current

Try using a low pass filter inbetween the meter amd terminals. Is the DC link in good condition and if so, is the drive output clamped at 480 via a parameter.

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