Strange VFD Readings
Strange VFD Readings
(OP)
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.
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.





RE: Strange VFD Readings
I have a Fluke Power Analyzer 435. Would it work properly on VFD output ?
RE: Strange VFD Readings
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.
RE: Strange VFD Readings
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange VFD Readings
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
RE: Strange VFD Readings
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.
RE: Strange VFD Readings
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 ?
RE: Strange VFD Readings
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange VFD Readings
How do these meters measure anyway ? Fourier analysis ?
RE: Strange VFD Readings
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange VFD Readings
Sorry for so many questions.
RE: Strange VFD Readings
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: What is Vector Control in a VFD? that describes some of the inner workings of a vector drive.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange VFD Readings
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 ?
RE: Strange VFD Readings
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.
RE: Strange VFD Readings
What does the drive do during auto-tuning ?
RE: Strange VFD Readings
Where can I see on the web the typical (actually measured - not theoretical) waveforms of a VFD output voltage and output current ?
RE: Strange VFD Readings
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.
RE: Strange VFD Readings
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange VFD Readings