VFD and induction motor power factor 1

[eeprom]

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

 

[dcasto]

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.

 

[sreid]

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)?

 

[Marke]

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

 

[sslobodan]

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

 

[ozmosis]

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.

 

[Valvecrazy]

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