OT: VFD Marketing BS

[dash]

Directly from a drive spec sheet and have seen similar statments from other Mfg's. Note that this states nothing about loads, motor / load mismatch, etc.

"Displacement Power Factor: 0.98 across entire speed range."

 

[Tom Jenkins]

Well, Dash, I understand your problem with the power factor thing, but if your objective is power factor reduction then VFDs are the wrong technology. And, at the time most of this advertising and such was written power factor was measured in the same way most of the utilities did it.

The reason to go with a VFD is because variable speed will provide a reduction in total energy required or because equipment operation demands variable speed for proper function. The claims made for energy reduction need to be analyzed, but in my experience the VFDs do what they claim.

 

[DickDV]

While I am not a pf expert, I suspect something is out of whack when distortion is reported OVER 100%.

I have a feeling that distortion and power factor is somehow getting entangled and the measurement data is being affected.

It would be valuable to get some VFD manufacturers' engineers engaged in this discussion. There are at least some drive manufacturers out there that do not deliberately deceive and there input on this subject would be highly informative, I suspect.

 

[rpcaron]

A modern AC drive power structure consists of three basic stages.​

The converter stage consists of a three phase, full wave diode bridge, though SCRs are sometimes used in place of diodes. If this stage were isolated from the rest of the power structure, we would see a DC voltage with a 360 Hz ripple at the DC bus connection when 3 phase power is applied to the input.​

A filter is required to smooth out the ripple on the DC bus in order to run the IGBT inverter. Therefore, a second or “filter” stage is required. Primarily, this consists of a large capacitor. Often an inductor or “link choke” may be added. The choke, when used, helps buffer the capacitor bank from the AC line and serves to reduce harmonics.


The third stage is the inverter section. This section uses high-speed transistors to apply a “Pulse Width Modulated” or PWM waveform to the motor. Taking advantage of the fact that a motor is basically a large inductor, and that current does not change very fast in an inductor, the DC bus voltage can be applied in pulses of varying width in order to achieve current in the motor that approximates a sine wave.


For the most part, it is the rectifier and the filter that have an affect on the power line. Upon application of AC power the capacitor will charge up to the peak of the applied line voltage through the diode bridge. When a load is applied to the DC bus, the capacitor will begin to discharge. With the passing of the next input line cycle, the capacitor only draws current through the diodes and from the line when the line voltage is greater than the DC bus voltage. This is the only time a given diode is forward biased. This only occurs at or near the peak of the applied sine wave resulting in a pulse of current that occurs eery input cycle around the +/-peak of the sine wave.


As load is applied to the DC bus, the capacitor bank discharges and the DC voltage level drops. A lower DC voltage level means that the peak of the applied sine wave is higher than the capacitor voltage for a longer duration. Thus the width of the pulse of current is determined in part by the load on the DC bus. It is the pulsating input current that gives us the term “nonlinear load” since the current does not flow in proportion to the applied voltage. In fact, with a nonlinear load, current may not flow at all for a major part of the applied voltage cycle.

Power is only transferred through a distribution line when current is in phase with voltage. This is the very reason for concerns about input “power factor”. Displacement power factor in a motor running across the line can be explained as the cosine of the phase angle between the current and voltage. Since a motor is an inductive load, current lags voltage by about 30 to 40 degrees when loaded, making the power factor about 0.75 to 0.8 as opposed to about 0.95 for many PWM AC drives. In the case of a resistive load, the power factor would be 1 or “unity”. In such a case all of the current flowing results in power being transferred. Poor power factor (less than 1 or “unity”) means reactive current that does not contribute power is flowing.


Neither [font=TimesNewRoman,BoldItalic]harmonic nor [font=TimesNewRoman,BoldItalic]reactive current flowing through a system produce power. The power infrastructure has to carry these currents causing heat loss due to increased I^2*R drop in the wire and higher flux in transformer iron. Transformers and distribution lines in some cases may need to be upsized to handle the burden of this additional non power producing current.


If we looked at input current to three identical 100 horsepower drives in the same facility running at equal power levels, we would most likely see three distinct harmonic spectrum patterns from each drive. Each could have a current THD level of say 20%. Looking up stream before the three branch circuits for each drive we would see a total current for each drive about equal to the three drive RMS currents added together. However the THD in current at the same point upstream might only be 7%. Be cautious of anyone who tries to interpret “point of common coupling” as anyplace other than the utility interface. They may be trying to sell equipment that might not be needed.


Furthermore, the displacement power factor that one might see with a drive might be 0.95 as opposed to 0.75 power factor for the same motor across the line. This frees up ampacity in the system. Some of this may be used up by the increase in harmonics but in most cases the over all effect is a net benefit by using a drive. In most cases sizing the transformer and power feed lines as if the motor were running across the line is more than adequate to handle any harmonic currents from an AC drive.

One of the simplest solutions in reducing harmonics is to add a reactor at the line input side or in the DC link. This reactor or inductor will not allow current to change fast. It forces the capacitor bank to charge at a slower rate drawing current over a longer period of time. The addition of this component can reduce typical distortion levels from more than 80% to less than 20% THD depending on source impedance.


Conclusion:

While it is true that in some cases AC drives can cause harmonic related problems, it is important to recognize these instances are not the norm. Often what drives add to the system in harmonics they make up for with improved input power factor actually freeing up KVA in the power distribution system. This is especially true when a link choke is included in the drive. Though many elaborate harmonic mitigation solutions exist (for drive harmonics), it is often an unnecessary expense. IEEE-519 needs only to be satisfied at the Point Of Common Coupling and not within a given facility.​

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