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
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
When you connect motor to mains, you see full current. Including reactive current.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD and induction motor power factor
RE: VFD and induction motor power factor
And, please, I do know the difference between PF and eta...
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD and induction motor power factor
RE: VFD and induction motor power factor
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
RE: VFD and induction motor power factor
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
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD and induction motor power factor
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
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. 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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD and induction motor power factor
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
Take two aspirin and call us in the morning.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD and induction motor power factor
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
Respectfully
RE: VFD and induction motor power factor
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD and induction motor power factor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD and induction motor power factor
RE: VFD and induction motor power factor
RE: VFD and induction motor power factor
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
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
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
It is an expensive technology for 50Hp but for 100's of Hp though.
RE: VFD and induction motor power factor