# VFD issue with Motor Current



## CAUSA (Apr 3, 2013)

basball321 said:


> This is a strange one...
> 
> I have an 480VAC Eaton SVX VFD on a screw compressor motor. It has been seemingly showing very high amps while running and so I put a clamp on meter and it's actually only pulling 1/3 of what is showing as motor current on the VFD keypad. The kW reading on the keypad seems to match the amperage of my meter after doing the math.
> 
> ...


What math formula did you use??









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## MikeFL (Apr 16, 2016)

Hello @basball321 & welcome to the forums. 
As @CAUSA suggests above, please fill out your profile by clicking your Avatar and selecting Account Settings. Specifically we need your Electrical Trade filled out (this site is for industry pros only). 

You're in good hands. We have several really good VFD/ motor guys on here (sorry but I'm not one of them).

Cheers & stay safe.


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## basball321 (12 mo ago)

CAUSA said:


> What math formula did you use??
> 
> 
> 
> ...


typical 3 phase kw Calc
√3 × PF × amp × volt / 1000

175HP motor
VFD keypad kW = 27kW
VFD keypad amps = 111A
Meter amps = 37.7 amps


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## gpop (May 14, 2018)

basball321 said:


> typical 3 phase kw Calc
> √3 × PF × amp × volt / 1000
> 
> 175HP motor
> ...


Its a vfd so you can ignore power factor. 

voltage with out hertz equals heat. Which means when we reduce speed we also reduce voltage. For a amp reading to be 1/3rd i would expect the motor to be running at reduced speed (30-40 hertz)


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## basball321 (12 mo ago)

gpop said:


> Its a vfd so you can ignore power factor.
> 
> voltage with out hertz equals heat. Which means when we reduce speed we also reduce voltage. For a amp reading to be 1/3rd i would expect the motor to be running at reduced speed (30-40 hertz)


Yes, VFD is running at 19Hz for the readings given.


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## CAUSA (Apr 3, 2013)

What is the Hz display? At amperage reading?

I would start with confirming the Basics.

voltage of input to VFD for each L1-L2-L3 is 480 VAC.

check amp drawing before the VFD. Is there a difference to the out put. If yes how much + -.

All terminal conditions ok?

cooling fans operating properly?

Does the unit ramp up and down and does the amp draw adjust with the same scale as the speed of the compressor. With a amp clamp. At zero amps is the display 0 or something else?

does the Hz adjust with the same scale as a calibrated meter on the motor circuit? At full Hz what is the actual Hz measurements?


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## gpop (May 14, 2018)

quick vfd basics

Input is ac (rms).
We then use a diode bridge to convert it to dc. ( ac rms x 1.414 so on 480 we would expect 678v dc)
Now we need to smooth out the lumpy dc so we use caps to give us smooth dc.
Now we use high speed gates to send pulse width modulated dc to the motor. The pulse will always be at dc buss voltage the width will be controlled by the vfd to represent a ac sine wave at a given voltage compared to the speed.

Dam its easier just to google it as they have nice pictures.

roughly at 19 hertz the motor will be seeing around 200v (at lower speed the voltage has to be increased slightly due to the motor. At higher speeds its closer to volts/hertz)

√3 × 1 (PF) × 111 × 200 / 1000 = 37 ish amps at 480v rms input


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## gpop (May 14, 2018)

Oh forgot to answer the original question.

nothing wrong just the vfd doing what a vfd does.


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## basball321 (12 mo ago)

gpop said:


> quick vfd basics
> 
> Input is ac (rms).
> We then use a diode bridge to convert it to dc. ( ac rms x 1.414 so on 480 we would expect 678v dc)
> ...





gpop said:


> Oh forgot to answer the original question.
> 
> nothing wrong just the vfd doing what a vfd does.





gpop said:


> Oh forgot to answer the original question.
> 
> nothing wrong just the vfd doing what a vfd does.


All makes sense. Thanks.

If I'm understanding correctly, my inherent issue is on the controller for the Compressor. It uses Percent FLA (0-100%) for a means to calculate load of the compressor. 
The FLA rating for the compressor at 480V is a static value in the controller (197A). The controller uses output amps from the VFD as an input to calculate the percent FLA by dividing output amps by the 197A. Now, this seems to be skewed as a measure of load, if using the output amps of the VFD. It would make more sense to use the amps on the RMS input for this calculation, wouldn't it?


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## gpop (May 14, 2018)

basball321 said:


> All makes sense. Thanks.
> 
> If I'm understanding correctly, my inherent issue is on the controller for the Compressor. It uses Percent FLA (0-100%) for a means to calculate load of the compressor.
> The FLA rating for the compressor at 480V is a static value in the controller (197A). The controller uses output amps from the VFD as an input to calculate the percent FLA by dividing output amps by the 197A. Now, this seems to be skewed as a measure of load, if using the output amps of the VFD. It would make more sense to use the amps on the RMS input for this calculation, wouldn't it?


Sounds like you are using a analog output on the vfd rather than a CT. If so look in the manual at AO options for something that represents load rather than amps.


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## MotoGP1199 (Aug 11, 2014)

Not super familiar with that drive. Is there possibly a setting somewhere that is totalling amps across all three legs instead of the average?

Makes no sense to me to have that option as it means nothing but who knows.


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## JRaef (Mar 23, 2009)

basball321 said:


> typical 3 phase kw Calc
> √3 × PF × amp × volt / 1000
> 
> 175HP motor
> ...


kW on the keypad is MECHANICAL kW, not electrical kW. Mechanical kW, like HP, it related to speed and torque.

CURRENT in a VFD is related to TORQUE. A 175HP motor should be in the neighborhood of 210A FLC, which means at FULL LOAD TORQUE, it will be drawing 210A. Your motor is drawing 111A, so from that we can deduce that your motor is only putting out around 53% torque (111/210). Compressors are a constant torque load, constant torque loads on VFDs means that the motor current will be whatever the load demands, irrespective of the speed command. So this is not at all unexpected.

As gpop indicated, if you measured the INPUT current to the VFD, that is the current *at 480V*. The output current would be based on it being roughly *200V to the motor*.


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## SWDweller (Dec 9, 2020)

What is the bottom end for HZ for the compressor? I have seen more than a few that do not like to be less than 20-25% for the bottom end. A reasonable time for ramp up is fine. 
I have seen compressors that over heat when the HZ/ speed is so low. The internal fans were never designed for such a slow speed.


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## just the cowboy (Sep 4, 2013)

A standard amprobe will not give you a accurate reading also, that is why we use the VFD display.


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## paulengr (Oct 8, 2017)

SWDweller said:


> What is the bottom end for HZ for the compressor? I have seen more than a few that do not like to be less than 20-25% for the bottom end. A reasonable time for ramp up is fine.
> I have seen compressors that over heat when the HZ/ speed is so low. The internal fans were never designed for such a slow speed.


This is not an easy question. A rough guess would be 10-15% of FLA.

Every motor has two different kinds of currents, ignoring losses. Inside an induction motor we have a rotating magnetic field that pulls the rotor along. But most electricians are surprised to see about 25% FLA no load on a 4 pole motor. The voltage is circulating a magnetic field around the motor. This requires current but it is doing NO work. It’s just magnetic flux. So it pulls an almost 100% reactive current which lags voltage by 90 degrees and is pure kvars…reactive power. It is almost uniform across all speeds and you need full flux to achieve full torque.

The second current is all torque. This is doing work so it is all kw, 0 degrees lagging, etc.

In a DC motor we can separate them somewhat but not in AC.

So if we just use the basic formula that HP = torque x RPM and knowing that HP x 0.746 = kw and that V x A x PF = kw, we can see that if speed gets close to zero so does HP. And if we cut speed in half, we cut HP and thus kw in half at the same speed. Now going over to the electrical side we need torque so the current can’t be reduced. That means we reduce voltage. At half speed we expect to reduce voltage to about half.

BUT this assumes flux current doesn’t exist. It does exist though so if we are working V/Hz mode then we basically have a minimum voltage of about 10-15% of name plate, often called “boost”. As we approach 0 RPM we gradually roll out voltage until it’s constant.

In sensorless flux vector mode we effectively just separate the reactive and torque currents to mathematically track what is happening. However down below about 1 Hz this doesn’t work (DC does not have a power factor) so we switch to V/Hz. In full sensored vector mode (encoder) we can just measure speed so we don’t switch.

At slow speeds two mechanical issues occur. One is the bearings aren’t turning fast enough to float the rolling elements so they make metal on metal contact. The other is that fans and other similar things are proportional to the square of the speed so the motor in particular overheats with an integral fan below around 25-50% of full speed. Compressors have similar issues.


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