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VFD and induction motor power factor

VFD and induction motor power factor

VFD and induction motor power factor

(OP)
Hello,
  I have a VFD driving a 50 HP induction motor (3 phase, 460V).  At 60 Hz, the motor draws around 70 A.  If I bypass the VFD and connect the motor directly to line voltage, the current draw is about 62 A.  I have already taken into account the difficulty of measuring current from a VFD (I use the internal readout).  I think that the reason for the difference in current draw has to do with the high frequency noise coming from the VFD.  The VFD noise is effectively reducing the motors power factor, and so the motor requires more current to do the same amount of work.  Does anyone know if this is true?  Does a VFD have an effect on the power factor of a motor?

thanks,
Andy

RE: VFD and induction motor power factor

It is a little simpler than that. A VFD has a power factor close to 1. So, the VFD takes less current from the mains than it outputs to the motor. The reactive power circulates between motor and DC link - never needs to go "on line".

When you connect motor to mains, you see full current. Including reactive current.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

Glad you agree with the PF being close to 1.  My EE says you can get the PF to 1.1 if you specify it that way. In another post I was told the PF was always less than 1....

RE: VFD and induction motor power factor

PF > 1? That means perpetual motion - or something...

And, please, I do know the difference between PF and eta...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

Displacement power factor is the cosine of the angle between the voltage and current, so it can never be more than 1.0. It can be lagging or leading, but never more than unity.   

RE: VFD and induction motor power factor

Yes. And PF, be it displacement or distortion, is P/S. And since S = sqrt(P^2 + Q^2) it also follows that no PF, how ever defined, can be more than 1.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

With the PF more than 1. in your plant, you can pull up the overall PF.  This is an alternative to adding capacitors.

RE: VFD and induction motor power factor

The difference can depend on the line voltage vs the VFD output voltage. If you set the VFD to 460V motor it will output 460V even with a different line voltage (within limits of course). The change in voltage could affect the output current. I've seen this mostly be a factor when the motor voltage setting is set too high.

Of course, I've just seen higher currents on VFD's before and it is likely due to the extra losses from the carrier frequency. More current capacitive couples to ground too.

RE: VFD and induction motor power factor

dcasto,

We just said that you cannot get higher than 1.

Having leading PF is another thing. But it cannot be higher than 1.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

I would say Lionelhutz's  response is more accurate and in-line with what the OP asked. Some of otherwise more knowledgeble respondents appear to have misread the question, even if their statements may be true on their own.

If pf were to deteriorate when "bypassing" the VFD,  the current would increase. OP said that the current when motor is directly on line is less than while on VFD. It is not to be confused with input and output current of the VFD.

RE: VFD and induction motor power factor

Quote (eeprom):

Does a VFD have an effect on the power factor of a motor?
(underline emphasis added).
I agree. Ihe question, as asked, relates to the motor's power factor, not what the line sees. To that question, the answer is; maybe, but does it matter?. If anything, the VFD can improve the effective pf of the motor when running at reduced speeds because it will be altering the motor power all together to match the load. At full speed however you may be right, but most likely the display of the load current is going to take that consideration into it's calculation, or at least it should. The only one who could really answer that however would be the engineer at the VFD manufacturer who was charged with designing that algorithm.

Bottom line though, I seriously doubt that this is the reason why you have two different readings. You are off by about 10-1/2%. Figure 3% error factor in the VFD's display reading (if that low) and a potential of 2% error in whatever you used for the across-the-line reading, and now you are only off by 5-1/2%. A VFD couple easily have that much in switching losses at full speed, depending on the technology, age, design, if there are reactors etc.

RE: VFD and induction motor power factor

Yes, I did read the OP as a question about the installation. Not the motor as such.

Yes. Of course, the output of the VFD has a deep impact on the PF of the motor. As said by others. And jraef's error calculation is also something worth considering.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

Don't know where that word "couple" came from, should have been "could". Must have been a screwy spell checker correction, sometimes I whip through those too fast I guess.

RE: VFD and induction motor power factor



When a motor is supplied by the utility the voltage and current are close to a sinus wave. When a VFD feeds the motor an integrated wave, some times far apart from a sinus wave is fed to the motor. Distortion could be the reason for increased current figures. The type of measuring instrument could play a role too. Could you sample the wave form with a scope?

RE: VFD and induction motor power factor

Sinus wave??

Take two aspirin and call us in the morning.infinity

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: VFD and induction motor power factor

At ful line frequency output of the VFD, there is commonly some distortion of the output waveform resulting in some flat topping, plus the output voltage is typically a little lower than the line voltage, plus there is a lot of capacitive current flowing at the output of the drive due to the capacitance of the cable and motor. Essentially, all these factors will tend to increase the actual output current of the VFD at line frequency. Drop the frequency a little and you get a better sine wave output current and no voltage limiting.
Additionally, you have inaccuracies of measurement. Some VFDs measure the DC current and use that to determine the output current. This can lead to high inacuracy.

The Cos(thi) of the input to the VFD is better than 0.95, but the true power factor is typically less than 0.8 due to the distortion power factor.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: VFD and induction motor power factor

It's good to see you drop in, Marke. Your knowledge of drives is always valuable to the forum.
Respectfully

RE: VFD and induction motor power factor

For Smoked: Yes. Sinus is the original Latin form for sine. That is what this function is called in most of the world outside US/UK/OZ.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD and induction motor power factor

Whoa! Really.  Most interesting.  Thanks Skogs.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: VFD and induction motor power factor

I agree with jraef.  5% loss in the drive itself is common.  I have experienced this many times.  The motor pulls less amps when running across the line, than when controlled by a drive.  The output voltage of the drive cannot be higher than the input voltage.  When using reactors or line filters, even less voltage makes it to the input of the drive.  If the motor is running on 480 volts when supplied across the line, and 460 volt through the drive, the lower voltage must draw higher amperage to supply the same power.  If actual energy used is volts X amps X power factor, then a motor running on a drive always uses more power than the same motor running across the line.

RE: VFD and induction motor power factor

OK, cos theta has to be between 0 and 1, you win.  BUT, you can still install a leading VFD and when you do your calculations with KW and KVA you'll need to know the value of COS theata in the reverse direction so would it be -.9? not 1.1.  

RE: VFD and induction motor power factor

1) an interesting fact and 2) a question.

1) One would think that with three phase rectifing and filtering for DC B+ voltage (ignore diode drops and capacitor ripple),that a VFD could output the same maximum voltage to the motor as the maximum line voltage.  Alas, the maximum output voltage from the VFD is 86% of the line voltage.

2)  Rectifiers followed by filter capacitors creates a lagging power factor.  This is because the charging current happens at the voltage peaks.  In larger drives, is something done to improve the power factor (big line inductors, active power factor correction)?

RE: VFD and induction motor power factor

Hello sreid

1) Some VFDs rectify the three phase supply and apply the rectified voltage directly to the bus capacitors. The DC bus voltage is almost the peak voltage of the sinewave rectified. Others, use AC or DC reactors in series with the rectifiers. This tends to average the voltage applied to the capacitors and the DC bus voltage is reduced.
The maximum output voltage is determined by the DC bus voltage and therefore, by the "filtering" applied before and after the rectifier.

2) The current flows through the crest of the voltage waveform only. This puts the peak of the current waveform in phase with the voltage waveform. Therefore the drive manufacturers quote COS(thi) better than 0.95, but if you look at the angle between the voltage zero crossing and the current zero crossing, you could legitimately argue that the angle thi is close to 90 degrees!!
The true power factor is reduced due to the distortion of the current waveform. The current waveform is discontinuous with current flowing for less than 120 degrees per half cycle. If there are no reactors, the current conduction angle can be as low as 10 - 20 degrees.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: VFD and induction motor power factor

I run trough fields above so I will not mark anything but 2 things
1. power factor can not be larger than 1 it comes from the nature of power it is S*sin(F) + jS*cos(f) power factor is derived from math function and it is that sin(f) or cos(f) as you know transforming equitation from basic math is
sin(f)^2 + cos(f)^2=1 that is why power factor can not be larger than 1. Anyone proves otherwise and I am certain he will get Nobel prize and solve all energy problems in the world :)
2. Difference between currents is that motor is not having his full power over VFD because someone has not set it properly (10V at max or 20mA at max power one of these 2 is a control value for % from full load) Most probably it is used 0-20mA signal for control and PLC settings were made for 4-20mA signal (0% at 4mA) so it loses 1/6 of the max power (never reaches it) If you divide 62 with 6 you will and substract get approximately 50A

RE: VFD and induction motor power factor

I'm not sure you'd get a Nobel prize but certain Active Front End drives can be programmed to provide either a leading or lagging power factor or set to 1.
It is an expensive technology for 50Hp but for 100's of Hp though.

RE: VFD and induction motor power factor

The way most drive manufacturers figure power factor and efficiency can be misleading.  They may say the power factor is .99 and the efficiency is 90% with a drive.  In reality the actual power consumed may be the same as when throttling with a valve at lower PF and efficiency

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