current measure with frequency drive

Hi,

Someone asked me something, and I can't answer him.

He has an old Danfoss Frequency Converter; no display.

He wants to know the current that the Motor takes,

BUT:

CAN U MEASURE the output current of a frequency drive ?

I've tried with a Micromaster MM440 from Siemens; the current I measured was not the same as the drive gave me...


How can I know the right current ?

If a drive measures its own current, then it does so with a coil arround one of the phases (I once had a peek inside a Siemens MM430 drive), just like a clamp am-meter. So it should be possible.

I think that the trick is to get an am-meter that measures "True RMS".

A VFD actually produces a DC sawtooth waveform, not a true AC waveform. You will need a true RMS voltage or current meter/sensor to measure the outputs.

There are a variety on the market that may be used to input to a PLC or connected to a display.

See if this helps: http://www.nktechnologies.com/content/download/ApplicationNotes/measuring_vfd .pdf

http://us.fluke.com/usen/products/features.htm?cs_id=30405(FlukeProducts)&category=ELW(FlukeProducts)

FREQ measurement

At work we have a Fluke VOM with a frequency measurement. Sorry I am not there (day off thankfully) and I don't remember the type number.

Would it work on a VFD and give a "true" reading.

ALSO the same question goes for the simpler clamp on ammeters. Since voltage is pulsed on VFD the amperage would be also.

ALSO while I am at it.
We have lots of VFD at work. Boss has us measure volt and amps at the input terminals to the VFD or disconnect feeding it. This is a good thing to do I admit.
HOWEVER
It does not give us the whole story I think. How do we know what the VFD power supply is doing - are all three voltages within tolerance ie about 3% imbalance?? I beleive we should be measuring volts and current actually drawn by the motor. Either take them at the output of the VFD or open up the motor terminal box.

What do you guys think??

Dan Bentler

Most VFD's have a display that will give you volts, frequency, current, rpm etc.

To check it you need a true RMS meter which actually averages values over time.

I use the Fluke 87, the new model 87V is even better:
http://us.fluke.com/usen/promotions/better_dmms/87+chart.htm

DISPLAY data

I would have to look
BUT
I do not believe the displays on our units give volts or amps for each phase.

SO
Even if they did an independent reading would double check (calibrate ??) that the display is close -- would it not???

Dan Bentelr

Originally posted by leitmotif:

ALSO the same question goes for the simpler clamp on ammeters. Since voltage is pulsed on VFD the amperage would be also.

Click to expand...

This would be true in a purely resistive circuit. However, a motor is a highly inductive load. The inductive nature of the load will attempt to keep the current in the motor at a constant level. So, within a single PWM period, the current will be relatively stable. You will still want to use a true RMS meter, though. The current will not be COMPLETELY stable as it would be off a stiff AC supply.

[Originally posted by leitmotif:[/I]

It does not give us the whole story I think. How do we know what the VFD power supply is doing - are all three voltages within tolerance ie about 3% imbalance?? I beleive we should be measuring volts and current actually drawn by the motor. Either take them at the output of the VFD or open up the motor terminal box.

Click to expand...

This is correct. A drive is basically a POWER converter. Within the limits of it's efficiency, the voltage-current product at the drive supply terminals will be proportional to the voltage-current product at the drive output terminals. So, for example, assume the motor is accelerating from standstill to base speed at a constant accel rate which requires the drive to use 100% of it's rated current. At the motor terminals you will see a relatively constant current with a steadily ramping voltage. At the drive supply terminals you will see a constant voltage with a steadily ramping current.

Keith

Whats a trip is when you check one that is running and has a blown fuse. In some cases a VFD can provide output to a 3PH motor with a single phase input, as long as the fuse that blows isnt part of the control supply. This can also happen with DC drives.

The drive, if operating properly, can usually "sense" an unbalance and/or many other problems. You have to check the drives manual to determine what you can display, view, or monitor for troubleshooting purposes. In many cases the drive can/will provide an abundance of information for troubleshooting.

One of the most recent drives I dealt with is Toshiba G7
ftp://ftp.tic.toshiba.com/pub/_drives/man_g7_oper07_20_04.pdf

If you look at page 44 you will see what can be viewed in monitor mode, to list a few:
Output Frequency
Frequency Reference...setpoint
Output current
Bus Voltage
Output Voltage
Torque
Load
KW
the list goes on.

I don't know how old the Danfoss drive is, but if it earlier than PWM, in other words probably a six step drive, then you can measure the output (motor) leads with a true RMS meter and expect reasonably accurate readings.

On modern PWM drives, my experience tells me that no test meter is going to give you an accurate voltage reading and the current reading will only be around 90% accurate. For my purposes, I NEVER, that's NEVER take readings on the motor leads, instead taking the readings directly from the drive display.

It is true that tests like imbalance voltage and current cannot be read from the display but that doesn't particularly concern me since the voltage output has to be balanced by design and, if the current is out of balance, the drive will protect to the highest phase current. Given the balanced voltage, if the current comes back out of balance, you have a motor that is in trouble.

In cases of extremely long lead lengths, I have seen some of the output current not reach the motor but, instead, pass to ground thru lead to ground capacitance. This can be a nasty problem but, I emphasize, only occurs in extreme conditions--typically leads over 500 ft in length and then it is worse with small motors and drives that with higher horsepowers. The ground current can still be checked with direct measurement by operating the drive with the motor disconnected at the motor. The ground current will show up as output current. Most drives will fault on Ground Fault under these conditions but, if that particular fault is turned off, that's what you will see.

VFD motor check

I NEVER, that's NEVER take readings on the motor leads, instead taking the readings directly from the drive display.

LOTS easier from VFD output I agree.

OK Dick - why not
granted taking the P******head apart
untaping the leads
and having the problem of hot leads hanging out (Its a temporary test after all FAMOUS LAST WORDS) ARE ALL A ROYAL PAIN
AND getting test leads wrapped in motor shaft.
AND maybe the capacitve leak to ground.

Dan Bentler

I don't know how old the Danfoss drive is, but if it earlier than PWM, in other words probably a six step drive

Click to expand...

I believe Danfoss invented the type of transistor driven PWM modulation that is used in all VFDs of today. They never made other kinds of drives before they came out with the first "VLT".

If you cant use a clamp am-meter, then how does the drive measure the current (just curios) ?

It is a clamp type ammeter that is used, the difference is how the result is obtained. The standard method for measuring current is Average Responding this works fine as long as its a "pure" sine wave. VFD's output distorted waveforms..ie waveforms that are not "pure" sinusoidal. The way to measure these waveforms is RMS A True RMS measurement is obtained by first squaring the signal, averaging the squared signal and finally taking the square root of that average The result is the true power (heating value) of the wave. This allows very different waveforms to be compared to each other and to the equivalent DC (heating) value.

I imagine the drives, even older ones, would have to use RMS values for their calculations.

I didnt think about there being a need to see the 3 phases individually on the output side. I have seen a VFD power a 3PH motor with single phase input BUT I have never seen a drive that would work if there was an output phase gone...ie blown SCR or IGBT etc.

I have seen issues where the SCRs were "weak" (technically bad) and there was output but the motors performance degraded and it got hot.

You get a broken wire ie single phase the motor and the current unbalance will fault the drive.

Take a close look inside one when you get a chance....these things are one big MULTIMETER , sometimes O'SCOPE too with a microprocessor that measures, monitors and calculates everything.

VFD Current

Many VFDs use a Hall Effect current sensor instead of a current transformer. The Hall Effect sensor works on both AC and DC current but a current transformer will only work on AC current. The drive current is measured in the DC Link between the converter and inverter sections of the drive and the AC current is calculated from the DC level.

I have found that the most accurate way to externally measure the motor voltage or current is with an analog meter. Digital meters, even True RMS ones, will not be accurate. Fluke claims that its new Model 87V is able to measure VFD outputs. To quote their advertising

"Until now, there hasn't been a multimeter able to accurately measure adjustable speed drives. But the new Fluke 87V is designed specifically to handle these complex signals."

In other words, they are admitting their older True RMS meters were not accurate. I have used a Fluke 87III for several years now and never trusted the readings when measuring drive outputs. At low voltage, I have seen the readings off by at least 50%. Don't throw out the Simpson yet.

Fluke 87V Features

RMS

Little question with a big answer I guess...


What's the difference between the RMS current and the actual current you measure ?

RMS is Root Mean Square, what does it mean, how do you calculate it, is it the amplitude ?


regards

THIS LINK might help explain.

beerchug

-Eric