Total Harmonic Distortion of Line Current

[glenngoh]

hey guys ...

I have connected an AC Induction Motor and am investigating the line current drawn.

Is the Total Harmonic Distortion of the line current higher when an AC Induction Motor is loaded or when there is no load??? ...

Thanks.

 

[electricpete]

I will take a guess:

If you are talking about current THD caused by the motor itself (slight non-linearity of the magnetizing current), then THD will be higher when the motor is unloaded, because the non-sinusoidal magnetizing current forms a smaller portion of the total current when under load.

If you are talking about motor current THD created from the line voltage distortion... it's probably a lot more complicated. I can certainly think of a scenario when it appears that current THD would be highest when motor is unloaded. Let's say the voltage distortion consists of very sharp notches and spikes in voltage. When applied to the equivalent circuit, that high frequency voltage does not cause much change in the inductive magnetizing current, but causes a change in the load current. So increasing the load would increase proportion of input-spike-sensitive resistive load current which would increase THD. This result is the opposite of what was predicted above for motor is source of THD.

 

[glenngoh]

hey electricpete ...

Sorry for my mistake ...

Actually, from the output of the utility ... it is connected to a PWM controller before it is being connected to a AC Induction Motor ...

Hence, I reckon the THD of the line current is due to the PWM Controller??? ....

As u should know, THD is usually in percentage .... So would I expect the THD to have a high percentage of say over 100% when it is loaded or when there is no load??? ...

Thanks.

 

[electricpete]

In that case I would guess current THD would be highest at high load. The reasons given in my last paragraph. The no-load current component is inductive and does not respond to the harmonic voltages. The load component is resistive and does respond to the harmonic voltages.

At least that's what I get using the equivalent circuit. From physical considerations I don't see any reason I can't apply it in this manner.

 

[jbartos]

Suggestion: Visit

http://www.ece.utexas.edu/~grady/C4_Sources.pdf

for:
Even though lower load levels have higher I THD, the harmonic amperes do not vary greatly with load level because fundamental current is proportional to load level.
The voltage THD is dependent on a location of Point of Common Coupling (PCC), upstream system impedance, etc.
When it comes to current harmonic contents for loads over 100%, the harmonic contents, ITHDs, usually increase depending on the nonlinear characteristics of the loads.