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Anonymous Poster #1

Current Measurement in VFD

10/16/2012 6:37 AM

I measured current at both end (incoming and outgoing) of variable frequency drive by tongue tester (clamp meter) while 195KW 3 phase induction motor is running with VFD, But I could't get appropriate value of current. While motor running DOL (without VFD) tongue tester measured proper value of current. Why measured current by tongue tester at incoming and outgoing of vfd is not accurate ? Please explain?????
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Joshi
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#1

Re: Current Measurement in VFD

10/16/2012 7:10 AM

Due to presence of strong harmonics.
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NotUrOrdinaryJoe
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#4
In reply to #1

Re: Current Measurement in VFD

10/16/2012 12:17 PM

It is not harmonics that is causing the problem!

The motor will run at what ever frequency the VFD drives it. The VFD may cause harmonics on the power line but that is not reflected to the motor!!!!!

Your instrumentation is likely not calibrated or compensated to run at the frequency you are using. You must have a "TRUE RMS" meter and current probe to accurately measure the motor current. Anything else will only measure accurately at the frequency that it was designed for (50 or 60 Hz). Cheap meters do not read accurately for frequencies outside their design parameters.

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PWSlack
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#2

Re: Current Measurement in VFD

10/16/2012 7:21 AM

Is this vfd CE-marked?

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Phys
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#3

Re: Current Measurement in VFD

10/16/2012 8:33 AM

Joshi hit it on the head. Put a scope on the VFD output sometime and you will be amazed that any meter can measure current. VFD's are very noisy beasts. (be very careful, but I did it once for a customer to show him why his motor was running at a low current per the tongue tester) A tongue tester is just a best guess instrument in a normal situation with standard AC, add in all kinds of dips and peaks in the ac waveform and the tester is not going to read precise current.

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Aghvel Niazi
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#5

Re: Current Measurement in VFD

10/16/2012 4:43 PM

Whatever I Understand, if a motor runs same Load on two different type of starters i-e D.O.L and VFD

a. Why when motor is connected to VFD, Current at the Input of VFD is differnt from the Current at the output of VFD (motor terminals)?

b.Why for the same Load,Current consumed in case of D.O.L is different from the current consumed when the motor is connected to VFD ?

Case-a: Differance in Current Value at the Input and Out Put of VFD

At Input VFD takes Voltage and Current in the form of sine wave. VFD when supplies power to the motor, only current is in the form of Sine wave. Volatge is not in the form of Sine wave. Also magnitude of Volatge is changed to maintained V/F ratio. VFD applies its own algorithms for voltage and current(Vector Control, Scaler Control etc ). So value of current now will be changed. It will differ from the value of current as it was at the Input of VFD. This is one of the reason

Case-b:Differance in Current in case of Two different Starter

2. Consider a 18.5 KW, 400 V, 50 Hz, 35.5 A, P.F 0.83, 1465 RPM motor. It is connected to a Constant torque Load (Belt Conveyor) and consumes 12 KW when connected to a D.O.L starter.

12000 = 1.732 * 400 * I * 0.83 (Electrical Power Consumed)

I = 20.86 A

Mechanical Power delivered = 12000 * 0.9062 =10.874 kW

Load Torque = 70.88 N.m

When motor is connected to D.O.L, It consumes 20.86 A and Load torque of Belt Conveyor is 70.88 N.m

In case of D.O.L we have Constant Volatge in magnitude, waveform and have constant frequency

When the Same Belt conveyor runs on the Same motor with same load (12 kW) and same Load torque (70.88 N.m), using VFD, many things will change

1. Value of Volatge will be adjusted according frequency to keep V/F ratio to maintained torque.

2. Wave form of Voltage will now not be Sinosidel at the terminal of the motor as previously it was.

3. Change in Voltage causes change in current

4. Even motor runs at 50 Hz, it is not necessary that V/F ratio would be 8. It will be decided by the algorithm of VFD

5. Though V/F is 8 at 50 Hz, then at 50 Hz you don't have Sinosidel waveform of Voltage.

6.Also account for losses in VFD

Thanks & Kind Regards
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jack of all trades
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#6

Re: Current Measurement in VFD

10/16/2012 5:07 PM

This question has been asked on CR4 a few times before.

Try a forum search using "VFD current" for more details regarding pulse width modulated signals and averaging vs true RMS current measurement techniques.

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gckent
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#7

Re: Current Measurement in VFD

10/16/2012 10:38 PM

I want to know where I get one of those tongue testers and does it hurt???????
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Phys
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#9
In reply to #7

Re: Current Measurement in VFD

10/17/2012 8:35 AM

Yeah - I wasn't sure of the terminology when I first read this post, but it does appear when you Google it, so it must be an accepted synonym for a "clamp-on-meter" in some part of the world. I used the term because the writer of the post had used it. Had the term not appeared on Google, I may have suggested a change of the terminology to the more common clamp on current meter.

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gckent
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#11
In reply to #9

Re: Current Measurement in VFD

10/17/2012 5:05 PM

I've heard them called "Tong" testers before but never tongue testers
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Phys
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#12
In reply to #11

Re: Current Measurement in VFD

10/17/2012 5:10 PM

Hey - you have to admit - they do look like a tongue.

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vargaalex
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#8

Re: Current Measurement in VFD

10/17/2012 8:23 AM

I agree that it is harmonics, which could be induced by the VFD itself as it is modulating.
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thccontrols
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#10

Re: Current Measurement in VFD

10/17/2012 10:01 AM

what is wrong with the VFD display?

The incoming current... if you say its not appropriate (? or do you mean not correct??), what are you measuring it against, or is it calculated?

Please explain why you have, in your opinion, the wrong readings!!
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rameshchandsharma
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#14
In reply to #10

Re: Current Measurement in VFD

10/18/2012 5:01 AM

I am measuring the current on incoming side of vfd operating on a 110 kw motor which is running on full load and still showing 20 A only in tongue tester and the protecting relay as well.
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rodeh
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#13

Re: Current Measurement in VFD

10/17/2012 10:32 PM

Hi,

A VFD will draw the same motor power ( shaft power ) from the supply at almost unity displacement PF hance the current drawn at the VFD input is slaightly lower than DOL motor since PF is almost unity. However since teh VFD will draw harmonc currenthen then the total rms current will increase. I must say that it is important that you are using true rms meter to be able to establish an accurate measurement.

Also the output of the VFD is not oure sine wavform (pulses with 600VDC peak to peak) making it difficult for a standard meter to maesure any current without stauration.

Usually VFD suppliers will state the input rms current and also the outout for each size at a nominated supply KA rating.
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Users who posted comments:

Aghvel Niazi (1); gckent (2); jack of all trades (1); Joshi (1); NotUrOrdinaryJoe (1); Phys (3); PWSlack (1); rameshchandsharma (1); rodeh (1); thccontrols (1); vargaalex (1)

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