The effect of THDv on the THDi of an induction motor.
The effect of THDv on the THDi of an induction motor.
(OP)
A recent onsite test carried out to establish the THDi drawn by 6 pumps controlled by VFDs and soft starters yielded unexpected results.
We have a requirement that the THDi at the point of supply with all pumps running is less than 8%.
There are four pumps controlled by soft starters and two pumps controlled by VFDs. One VFD is fed via a Mirus Lineator filter to reduce the THDi.
On paper, it was expected that the THDi with all pumps running would be less than 8%, but this was not the case.
The background voltage distortion, THDv was around 4.5% and the THDi with one pump running on a soft starter was around 9%.
It would appear that the motor, fed by a distorted voltage waveform, was drawing current with twice the distortion.
Does anyone have any information on what the mechanism for this is?
Best regards,
Mark.
We have a requirement that the THDi at the point of supply with all pumps running is less than 8%.
There are four pumps controlled by soft starters and two pumps controlled by VFDs. One VFD is fed via a Mirus Lineator filter to reduce the THDi.
On paper, it was expected that the THDi with all pumps running would be less than 8%, but this was not the case.
The background voltage distortion, THDv was around 4.5% and the THDi with one pump running on a soft starter was around 9%.
It would appear that the motor, fed by a distorted voltage waveform, was drawing current with twice the distortion.
Does anyone have any information on what the mechanism for this is?
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd





RE: The effect of THDv on the THDi of an induction motor.
Look at the harmonic equivalent circuit. Neglecting leakage reactance, due to the very high slip at harmonic frequencies, the impedance for the harmonics can approach locked rotor impedance of approx 0.2pu, in which (at full load), your harmonics would increase by a factor of approx 5 above the voltage harmonics. However the harmonics have higher leakage reactance, so the harmonic impedance is typically higher and increase somewhat lower.
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(2B)+(2B)' ?
RE: The effect of THDv on the THDi of an induction motor.
...so the harmonic impedance is typically higher and increase somewhat lower. ..
should have been
...so the harmonic impedance is typically higher and current increase somewhat lower. ..
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(2B)+(2B)' ?
RE: The effect of THDv on the THDi of an induction motor.
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(2B)+(2B)' ?
RE: The effect of THDv on the THDi of an induction motor.
RE: The effect of THDv on the THDi of an induction motor.
There was a similar discussion, including some posted examples here:
http://mai
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(2B)+(2B)' ?
RE: The effect of THDv on the THDi of an induction motor.
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(2B)+(2B)' ?
RE: The effect of THDv on the THDi of an induction motor.
Neil
RE: The effect of THDv on the THDi of an induction motor.
Motor size 15KW, all the same.
Only one pump running at rated load when the THDi measurement was made, so other drive/filter info irrelevant at this point.
I was expecting the THDi to be close to the THDv, not double.
The primary harmonics present were 5th and 7th.
Pete, your comment about the slip impedance of the harmonics is very valid. I had not thought about the obvious!! I guess that would explain it. This throws quite a slant on things as that suggests considerable slip losses and extra heating in the motor!!
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: The effect of THDv on the THDi of an induction motor.
Neil
RE: The effect of THDv on the THDi of an induction motor.
The pump that was running was running on a soft starter. - no filter. The filter was on a VFD.
The requirement is to get the THDi contribution of the whole site to less than 8%.
The expectation was that by using four pumps on soft starters, - no contribution, one pump on a VFD with a filter, - approx 4.5% contribution, and one pump with no filter, 35% contribution, that the total harmonic current contribution would be less than 8%.
The problem was that the contribution from a single pump running effectively across the line was 9%, so there is no way that the installation willl comply due to the effect of the externally generated THDv.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: The effect of THDv on the THDi of an induction motor.
Neil
RE: The effect of THDv on the THDi of an induction motor.
Yes, that is the cause, refer back to the priginal post. The question was why the THDi was 9% when the background distortion THDv was 4.5%. This has been explained by Pete with the low slip impedance at the harmonic frequencies.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: The effect of THDv on the THDi of an induction motor.
Neil