Power Factor of Utility

[glenngoh]

hey guys ...

I would like to know what is the typical power factor at the utility (or mains supply) when connected to an AC Induction Motor??? ....

Is there a difference in the power factor when the AC Induction Motor is loaded or not??? ... If yes, are we to expect a higher power factor when the motor is loaded or when it is no load??? ...

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

Thanks.

 

[peebee]

An unloaded motor draws almost 100% reactive power, killing the power factor -- total kVA is low, though, so the effect is not too big on the total facility power factor if there are other connected loads. A fully loaded motor draws mostly real power.

Motors generally don't draw too much harmonic current and loading won't really affect this (more accurately, there's a negligable harmonic current at any loading, and I couldn't tell you how that current changes as a function of loading). If you're on a VFD, then the VFD will draw a larger harmonic current at full load.

 

[glenngoh]

hey peebee ...

Thanks for your reply ... seems to clear some doubts of mine ... But I reckon I might have made or mistake or did not actually make myself clearer ...

From the main utility of 415V at 50Hz, I have connected a PWM controller which I reckon is what u meant by VFD(Variable Frequency Drive)??? ....

From the PWM controller, I then connect it to the AC Induction Motor ... as we all know, THD of line current is in percentage ...

So my question now is ... do I expect to have a higher percentage of THD(say around 100%) when the AC Induction Motor is loaded or when there is no load??? ....

And in regards to the same connection, do i expect the power factor (not DPF or displacement power factor) to be high (say around 0.8) when the AC Induction Motor is loaded or when there is no load??? ...

Thanks.

 

[jraef]

Starting with your first question, typical pf for a motor connected to the line and running at full speed is guestimated at .80 when doing power calculations such as determining kVA of transformers feeding it etc. The motor power factor will go down to as low as .20 when fully unloaded.

Now that you added the inverter into the original equation everything changes. The power factor as presented to the utility will be maintained by the inverter at about .95 throughout the speed range of the motor. With the inverter connected between the line and motor, pf of the motor becomes irrelevent.

THD % from the inverter will increase as the motor load drops, but again, total kVA is dropping at the same time, so the effect on the line is not as great. This is why IEEE is changing specifications for harmonic distortion to relect Total Demand Distortion (TDD) in the near future. TDD increases as the load increases and reaches maximum when the motor is fully loaded.

Subvert the dominant paradigm... Think first, then act!

 

[glenngoh]

hey jraef ...

thanks for your info ... what u said abt the pf of the motor when connected to the line and after when the inverter is connected between the motor and the line is truly useful ... thanks once again ....

 

[glenngoh]

hey jraef ...

I need to clarify a few things with you ...

Firstly, u said that if I am to investigate the supply power from the inverter to the motor ... I should be obtaining power factor of about 0.95 throughout the speed range of the motor due to the inverter ... am I right?

Secondly, if I am to investigate the supply power from the main utility to the inverter throughout the speed range of the motor, what is the likely power factor that I should be obtaining for this part of the power supplied? I have to comment that from the voltage and current waveforms that I obtained, I noticed huge THD in the line current waveforms.

Thanks.