# Vdf low voltage high current problem



## ouchen108 (May 17, 2017)

Hi guys 
I have a vfd for exhaust fan 40 hp and 380v/62a 
the vfd when reach the 45 hz the current reach the current limit 68a and the voltage is very low 140v
Any ideas what is the problem

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## MDShunk (Jan 7, 2007)

I have a hard time believing that the VFD doesn't alarm out for something well before that. What's the fault on the VFD?


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## ouchen108 (May 17, 2017)

MDShunk said:


> I have a hard time believing that the VFD doesn't alarm out for something well before that. What's the fault on the VFD?


The vfd give warning that reach the current limit thats why the wont increase anymore it stop in 45 hz

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## frenchelectrican (Mar 15, 2007)

What power supply are you on ? 50 HZ or 60 HZ?

What brandname that VSD is that ??


I know many VSD will throw a fault code way before that. 

Did someone mess up the setting before ?

How did you get 140 volts on that? by the display or using the voltmeter ?

If latter you need RMS voltmeter to read it otherwise the VSD will throw your meter out of wack.


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## ouchen108 (May 17, 2017)

frenchelectrican said:


> What power supply are you on ? 50 HZ or 60 HZ?
> 
> What brandname that VSD is that ??
> 
> ...


Power supply 60 hz
Brand GE AF600
The voltage and amperage i check it in readout data in vsd and i measured it by a regular clamp meter but the readings are the same

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## paulengr (Oct 8, 2017)

Sorry you’re dealing with a generation 1/2 design. GE drives suck. They’re like an early 90s drive. Noisy and bad output issues even brand new.

Sounds like torque limit or current is going somewhere it shouldn’t (fault). You can easily set up a drive to do this and not fault. Check or measure (with a true rms meter!) the output current per phase. Don’t trust the drive...sometimes the feedback electronics fail and you can spend a lot of time chasing your tail. If there is a current imbalance more than 5-10% you have serious motor or cable problems and need to address it. Second check ground current either with the drive after establishing the readings are correct or running an amp meter (flexible current probe is easiest but small drives can use iron core).

If you have a scope particularly if you have a filter (load reactor, snubber, dv/dt filter), check for failure (output not clipped). GE drive output is horrendous and wears output filters out quickly, and shreds cables if you don’t add one even with short cables. I don’t care what the GE sales people say. Ask who actually made the drive and then look at their old discontinued model.

Also if you have external filters check these too. More than once I’ve narrowed it down to a failed load reactor/filter.

Moving on then magnetic fields are produced by current, not voltage. The drive manipulates voltage to control current. In an open loop V/Hz drive it outputs a fixed voltage and just trips if the current is too high. In a vector drive the magnetic flux is almost purely inductive (reactive power) and the torque is almost purely resistive (real power). The drive measures the currents and manipulates voltage to achieve the desired speed. In a Volts/Hz drive it just faults if the current is too high but doesn’t manipulate voltage unless it just current limits. In full field oriented (vector) control up to current limit the drive varies voltage to get the required current. After that it limits or faults. So in most cases at this point the problem is excessive load torque, not really a drive problem.

So load wise there are two issues. You might have a bearing problem. Uncouple it and turn the shaft or if you have one check with vibration analysis.

That leaves process issues. First thing I usually find is holes in the ductwork or dampers adjusted incorrectly. Centrifugal fans vary a lot with pressure so seemingly small changes cause big problems. On big fans usually the starting procedure is with the dampers fully closed and operators always forget. I can’t tell you how many times I’ve heard “we never had to do that before” and then find out not lately.

Next has the fan been changed or the gear box/belts been changed? Power draw is exponential with speed and diameter so again small “innocent” changes drastically affect it.

A centrifugal fan power is proportional to lbs per hour of air moved. So as temperature goes up/down air density changes. Cold startups or running at night or humidity are major factors.

Once you verify the drive/motor is fine move onto process because almost every time it’s a load problem not a motor/drive problem but because nobody understands the motors/drives or mostly because electricians are trained to troubleshoot, it becomes an “electrical” problem and since electricians don’t understand drives and process problems it becomes a “drive” issue.



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## gpop (May 14, 2018)

Does the fan rotate when the drive is not turned on. If so check that you haven't dropped a phase to the motor. Believe it or not some vfds will not report a loss of a output phase.


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