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AC drives and PF


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#1 brad

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Posted 15 September 2003 - 09:48 PM

The information I have seen on the PF of Ac Drives states that the PF is .95 or better, and I have read the information on the Imphotonics web site stating that the real PF is around .7. Is there any more information available showing the .7 PF?

#2 BigMax

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Posted 19 September 2003 - 05:16 AM

Hi Brad,

It may be helpfull to consider that there are actually two 'types' of power factor:o

In any system where the Voltage and current waveforms are not exactly superimposed we say that the power factor is less than unity. For example, with an inductive load such as a motor, there is a phase angle difference between current and Voltage. In other words the two waveforms are displaced when compared over time, not surprisingly this is therefore known as displacement power factor and is familiar to most people.

However, consider also where the Voltage waveform is effectively sinusoidal but the current waveform is not, i.e. current waveform harmonic distortion as would be expected on a supply to a VSD with an uncontrolled input converter. Perhaps surprising to some is that once again the power factor cannot be unity as the Voltage and current waveforms are not perfectly superimposed. This is known as distortion power factor.

So, connecting a VSD to a power source may indeed result in a displacement power factor of 0.95, but the distortion power factor may be around 0.7 unless other devices (ie line reactors etc) are used.

I hope this helps.

Cheers!

BigMax;d;

#3 marke

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Posted 19 September 2003 - 09:09 AM

Hi Bigmax

Well described!! That is a very nice simplistic way of explaining what can be a very difficult mathematical problem.

Brad
Most manufacturers quote cos(phi) at 0.95 which is correct. We then often interpret this as power factor as in the case of a standard motor, this would be the power factor. As Bigmax has already covered, there is poor power factor due to the angular displacement between the voltage and the current, and there is poor power factor due to distorted currents.
In the case of an inverter, the current flowing into the drive, is rectified and converted to DC. Current will flow in each phase only to charge the DC bus capacitors. If there is no DC bus choke and no line reactors, the current will flow for a few degrees on the crest of the waveform only. A very high current for a very short period of time. If DC Bus chokes and line reactors are used, the current will flow for longer periods of time, reducing the distortion, but with a three phase system, at any instance in time, the current will only flow on the phases with the highest voltage so the maximum conduction will be 120 degrees per half cycle. This still yeilds a lot of current distortin and a consequent drop in power factor. Expensive active or passive filtering is required to eliminate the distortion and lift the distortion power factor.
Recheck the data sheets and see if the use the term power factor or cos(phi)

Best regards,

#4 theDOG

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Posted 08 October 2003 - 03:03 AM

Please be aware that the previous posts all relate to Voltage Source Inverters (VSI).
In a Current Source Inverter (CSI), the displacement power factor is dependant on the motor power factor, as the output of the drive is not "buffered" by the DC link capacitors.

#5 marke

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Posted 08 October 2003 - 06:41 PM

Hello TheDog

I agree that there are no DC Bus capacitors, but there is still a DC link with an inductor providing the smoothing rather than the capacitors. I would not have expected that the motor power factor could be reflected back to the supply?

Do you have any papers on this?
Best regards

#6 theDOG

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Posted 14 October 2003 - 03:39 AM

Hi Marke;

Unfortunately, I do not have any papers on this but I'll attempt to explain.

In a VSI, the DC bus voltage is fixed. This is acheived by using a diode bridge and DC Bus capacitors. The inductive reactance required by the motor for magnetising current is provided (circulated?) by the DC Bus cap's. Therefore, the motor PF is not reflected to the line.

In a CSI, the DC Bus voltage is not fixed. Rectification is achieved by SCR's that control the DC Bus voltage to the level that is required by the motor. Therefore, motor current (and PF) is reflected to the line.

#7 toprange

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Posted 08 August 2004 - 02:15 PM

I am wondering if most of AC drives with better than 0.95, why the power company still request the factory to install the cap bank to improve the pf? If my factory do not have any VSD, rectifier etc., then I should not need to install cap banks, is that right?

#8 toprange

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Posted 08 August 2004 - 02:15 PM

I am wondering if most of AC drives with better than 0.95, why the power company still request the factory to install the cap bank to improve the pf? If my factory do not have any VSD, rectifier etc., then I should not need to install cap banks, is that right?

#9 marke

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Posted 08 August 2004 - 07:00 PM

Hello toprange

There are two types of power factor. These are displacement power factor (cos thi) and distortion power factor.
Displacement power factor is caused by reactive loads, (generally inductive) and distortion power factor is caused by rectifier and non linear loads.
Capacitive correction is used to improve a poor displacement powerfactor. Distortion power factor is much more difficult to correct.

All motors have a high inductive current and this contributes to a poor displacement power factor. Unloaded motors can be as low as 0.1 and large loaded motors can be as high as 0.95

For more information, have a look at http://www.lmphotonics.com/pwrfact.htm

Best regards,




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