Strange VFD Readings 5

[edison123]

Telemacanique Altivar 71 VFD driving a 440 V, 100 HP, 60 Hz, 350 RPM vertical motor (vertical machining centre)

Parameters set in VFD:

Motor voltage - 440V

Motor current – 169A

Motor KW – 75

Motor frequency – 60Hz

Motor rpm - 350

Running Frequency - 23.3 Hz (136 RPM)

Output as measured by VFD metering: Voltage-169 V, Current -98.5A, Speed-136 rpm & Frequency-23.3Hz. (Power & power factor could not be measured)

Input as measured by external Schneider Power meter ION6200/0.5 class accuracy : Voltage-424V, Current-12.6A, 11.8A, 7.4A , Power Factor-0.93 & Power - 5.2 KW

Why such a wide unbalanced VFD input currents ?

Why the power of 5.2 KW does not seem to tally with the average current of 10.6 A and PF of 0.93 and voltage of 424 V ?

Why the VFD does not show power and pf on the motor (VFD output) side ?

BTW, the no-load current of the motor is high at around 70% of FLC but that is normal for such a low speed motor.

 

[edison123]

And if I need to check the VFD output parameters (voltage, current, power and pf), what sort of power analyzer should I use ?

I have a Fluke Power Analyzer 435. Would it work properly on VFD output ?

 

[DickDV]

Your numbers seem about right to me. Input kw should be just slightly higher than output kw. Not amps, since the voltages are different.

Do not try to measure anything on the drive output. It is nearly impossible to get accurate readings even with very good test equipment. Instead, use the drive keypad to report output conditions.

 

[Skogsgurra]

Slightly unbalaced voltages often result in highly unbalanced currents. This is mostly because the three-phase voltage is rectified and feeds the DC link, which represents the peak voltage plus ripple.
If one phase voltage is lower than the others, the current drawn from that phase will also be lower. And since it is the difference between peak voltage and DC voltage that drives current, even a slight voltage difference produces a large current difference.
That is why most VFDs have problems with voltage unbalance as small as 2 percent.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

In this country, the standard configuration for that drive is no DC bus choke and no AC line reactor. In this configuration, the bridge rectifier is directly across the capacitor bank and acts like a peak rectifier. The current is very peaky with a narrow conduction angle. If you add a 3% DC reactor or 3% AC reactor to the circuit, you will get much more even currents with a wider conduction angle.
Without the reactors, most current flows on the higher two phases. A small imbalance in supply voltage can lead to a large imbalance in input current.

In trying to relate input and output currents, you always have trouble because the input and output power factors and voltages are not equal. The true KWS on the input will be almost equal to the true KW on the output, the difference being the losses in the VFD.

When you calculate the input power, you need to take into account the true power factor, not the quoted or displacement power factor.
With no reactors, the true power factor could be more like 0.7 than 0.93 due to a low distortion power factor. (High current distortion of around 80% THDi)

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[jcarrr]

The great value of the fluke will be to make all of the current and voltage waveforms visible. After you see those, the difficulties of accurately metering just before and just after an ASD will be clear.

Best to figure that if the drive is not smoking or on fire, almost all of the power going to the drive is going to the load. Believe the upstream power meter. Kind of.

The power indicator outputs of the dives I have worked with always seem to give pretty accurate estimates of power.

 

[edison123]

Thank you gentlemen. I didn't know that voltage balance is so critical for drives. I will check whether this drive had a dc bus choke or ac line reactor.

In this particular drive, the client says there is no provision to read the drive output power and pf. In such cases, how does one find out the motor power ? And, if the drive's own meter is the only one that should be relied on, how to cross-check or calibrate that meter ?

 

[Skogsgurra]

I think that your customer should look once again. The Altivar 71 is a high-end drive and has either a full graphic operator's panel or (for the smaller drives) a seven segment display plus push-buttons.
It is possible to read all pertinent parameters and operational data from either display. And, I would believe what the display is saying. Never seen a standard drive where the display is showing wrong numbers.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Thanks Gunnar. I will ask the client to recheck the drive meter.

How do these meters measure anyway ? Fourier analysis ?

 

[Skogsgurra]

They usually do not measure directly. Instead, there are torque and speed actual values internally in the drive and the power is calculated as torque times speed. Can't get any better.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Thx again. I can understand speed, but how does the drive measure torque internally ?

Sorry for so many questions.

 

[Skogsgurra]

There is a direct relation between the so-called torque current (Iq) and motor shaft torque. The vector drive keeps track of Iq and Id (magnetizing current) and uses them in the two control loops for torque and excitation.
It does that very precisely - it is often the speed actual value that has the least precision (due to slip, that sometimes is not known very well).
There is an FAQ faq237-1062 that describes some of the inner workings of a vector drive.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Thank you Gunnar. Jeff, Dick & you have done a very good job of explaining drives concept for motor-heads like me.

Wish I could give you another LPS.

In this particluar motor, it is sensorless. And the drive would't auto-tune. May be it is due to excessive no-load current - 70% of FLC ?

 

[ozmosis]

I'm wondering if your drive maybe undersized.
If the drive has been sized on hp then it will be too small.
The motor you have is quite a special and the current rating is higher than the 'standard' 100hp ATV71. If you haven't increased the drive rating to the next size up (or more) then any amount of tuning will cause problems with the internal algorithm that is trying to define the attributes for the motor control (sensorless vector performance).
As an example, the ATV71HD75N4 indicates 100hp but the maximum current rating at 380Vac is 167A, therefore the maximum current rating at 440Vac will be about 150A.
My guess is that you would need a drive rating even up to the ATV71HC11N4 that indicates it is actually a 150hp drive rated at 173A @ 460Vac.

 

[edison123]

Thx ozmosis. I will check with the client. But he was saying the drive was going into current limit often, before he fiddled around with the parameters. Not sure what he exactly to avoid the current limit.

What does the drive do during auto-tuning ?

 

[edison123]

Hopefully this thread isn't considered dead yet.

Where can I see on the web the typical (actually measured - not theoretical) waveforms of a VFD output voltage and output current ?

 

[DickDV]

The difficulty in describing drive output voltage waveforms is that they change as the get further and further from the drive output terminals.

They start out as nice clean rectangular pulses with a height of the DC bus voltage and a frequency of whatever the carrier frequency is. But the width varies from very narrow to wide as the current sine wave is formed.

As these pulses get further from the drive, they start overshooting and undershooting, ringing, combining with reflected waves, etc etc, and the result is an incredible mess of random everything. After years of doing drive work, I am still amazed that the motor finds anything useful to run on at distances over 100 feet from the drive.

 

[Skogsgurra]

That's a very good description: "an incredible mess of random everything".

Imagine trying to sort out EDM events from such a mess. We do that. And that has made us measure lots and lots of actual voltages and currents. I shall put some of those measurements together and put on our home-page. It will take a day or two. Stand by.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Thanks guys for that messy tip. I look forward to seeing those waveforms.