VFD causing High current 6

[Alij23]

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?

 

[itsmoked]

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

 

[waross]

Slow the fans down a couple of RPM.

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

 

[rockman7892]

Itsmoked

What do you mean when you say that current cannot be measured with a regular meter on either side of the vfd?

 

[ScottyUK]

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!

 

[itsmoked]

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

 

[dfopiso]

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

 

[edison123]

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.

 

[jraef]

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

 

[waross]

From Canada, thanks for the tip, Jeff. LPS

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

 

[Alij23]

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.

 

[PHovnanian]

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.

 

[Phaneas]

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

 

[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 .

 

[edison123]

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.

 

[Alij23]

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

 

[edison123]

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.

 

[FloatingGND]

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.

 

[waross]

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.

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

 

[Alij23]

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.