VFD and THD

Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
I monitored the load side of a VFD and found that it has an average voltage total harmonic distortion of 18%. I would assume this is excessive and maybe that is why it's been tripping?

Is there anyway to mitigate the harmonics?
 
ptonsparky said:
I would be surprised if you didn't have distortion on the load side.

What is the fault code telling you?
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I need to go back to the building to check the fault code, but I am just thinking about all the possibilities for now. Also, the FLA of the motor is 22.9A but the maximum spike was 140A. Could this maybe have caused it?
 
Tainted said:
I need to go back to the building to check the fault code, but I am just thinking about all the possibilities for now. Also, the FLA of the motor is 22.9A but the maximum spike was 140A. Could this maybe have caused it?
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You need to know what the parameters are for the VFD.
The fault code is your first step if it has been running. Check for a mechanical failure while you're there.
 
The output of a VFD in general is terrible from a harmonic standpoint. This is why some specifiers ask for an inverter duty motor. Often they put a line reactor on the input side of the VFD to avoid harmonic propagation upstream, but load reactors are found on many projects when motor winding are damaged.
 
Tainted said:
I need to go back to the building to check the fault code, but I am just thinking about all the possibilities for now. Also, the FLA of the motor is 22.9A but the maximum spike was 140A. Could this maybe have caused it?
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How was the 140A maximum spike measured?
 
The IGBTs inside a VFD switch the output voltage on each phase quickly between the + and - DC bus voltages in order to create a PWM (pulse width modulated) voltage waveform that will approximate the desired sinusoidal output after some form of lowpass filtering. To a large extent, the inductance of motor windings will perform such lowpass filtering because the current through an inductor is the time integral of the voltage across it, and the magnetic field (and therefore the torque) which the windings produce is proportional to this current (and not directly to the voltage itself). But the PWM voltage waveforms at the VFD output will contain a high level of distortion not only at harmonics of the desired output, but also at other frequencies related to combination of these harmonics and the carrier frequency of the PWM. The bottom line is that measurements of THD on the VFD output voltages would not be particularly meaningful, but they would be expected to be very high.

Although a motor can operate directly on a VFD output, the fast rising and falling edges of the output voltages can cause undesireable issues such as reflections on cables to the motor which result in excessive voltage spikes that can damage motor windings as Ron has said. This becomes more problematic as the length of the cables increases. And so load reactors or filters close to the VFD output are sometimes used reduce the higher frequency content of the PWM output. There are others on this forum that have much more practical experience than me in addressing such issues in the field, for example @Jraef.
 
TwoBlocked said:
Check to see if the meter reads true RMS Amps. If not, the meter won't be accurate. Wish I knew which way it will be off... Anyhoo, the amps reading from the VFD is probably the most accurate.
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My rule of thumb in measuring the output of a VFD with a meter is that if the meter cost you less than 4 figures to the left of the decimal point, it’s going to give you relatively meaningless data.

Output current harmonics can be relatively high (compared to across the line) but as mentioned, the motor itself IS the low pass filter. So that distortion results in some added heating in the motor. This is the reason why you will see in motor specs that if a motor has a 1.15 Service Factor, it will tell you that if run from an inverter, the Service Factor becomes 1.0. That’s because the harmonic heating caused by the drive output harmonics is essentially the equivalent of adding 15% more load.

So if you have an issue with the DRIVE tripping, it’s pointless to start speculating on output harmonics being the “cause”. Every VFD in operation right now has this issue and is not tripping. So START with what you KNOW and work toward what you don’t know. Don’t start off with a speculation about an unknown.
 
Jraef said:
My rule of thumb in measuring the output of a VFD with a meter is that if the meter cost you less than 4 figures to the left of the decimal point, it’s going to give you relatively meaningless data.
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The meter I have cost about $5000 each, I monitored the load and line side of the VFD lol
 
Jraef said:
My rule of thumb in measuring the output of a VFD with a meter is that if the meter cost you less than 4 figures to the left of the decimal point, it’s going to give you relatively meaningless data.
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I wonder if one of the 1960s era HP VTVMs would give at least a semi-accurate reading, I think I saw some of them at Urban Ore (Berkeley, CA) recently.
 
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