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Q: VARIABLE FREQUENCY DRIVE: POWER IN = POWER OUT
 
21 APR 2009
 
  Art,  
 
I have a question about the operation of VFD's [variable frequency drives] in general. Lately, we are having an issue with high amp draw on a motor (33.8 full load amps). We have been running the VFD at 35 Hz and I have seen that the voltage, on the display, decreases. I had always thought that the Hz (frequency) of the wave would be what varies, not the voltage. If I were putting out 480 VAC at 35 Hz would my amp draw be lower? This is how I always assumed it to be. It would appear to be working differently right now. Can you explain this and let me know why it happens and how a VFD is supposed to work? Thanks for the help.
 
     
  Regards, M  
     
A:    
 
 
21 APR 2009
M,  

I love this question. I'm going to wave a magic wand in this discussion and remove heat loss so everything operates at 100% efficiency and power factor is (1) unity. First let me state that power in = power out. this is only true in Art's world. Now power = volts * Amps (no power factor for now). So volts * Amps on the line side = volts * Amps on the motor side. Now let me tell you that a motor will develop torque as a function of voltage, frequency, and current.
Magic wand again.

Ohm's law:
E=IR
Volts = Amps * Current
With respect to AC Power and motors, the resistance piece is made up of frequency and inductance. inductance in a motor is constant so as frequency goes up and down the resistance piece of Ohm's law goes up and down. OK, so we know Volts, Amps, and frequency can go up & down and inductance remains constant.
Torque output in a motor is mostly related to current. To do this an AC Drive tries to maintain the Volts per Hertz (v/f) ratio constant through the speed range.
volts frequency ohms graph chart
From 0-60 Hz the AC Drive operates in steady Volts/Hertz mode to hold torque response constant.

Look at Ohm's law again.    Volts = Amps * [frequency & inductance]

To develop good torque, Amps must remain constant. From the graph above and the last equation we see that inductance can't change and Amps will remain constant to develop good torque, so if frequency goes up then volts must go up and vise versa.

Now back to one of my opening remarks. Power in = power out.

ac drive vfd line output volts amps

Line side volts are constant at 480V. Power company sees to this. On the output side we need amps to be stable to develop torque. What can vary is amps in and voltage out.

Without making this answer any longer, but modifying my position a bit, i will add that if your application is a centrifugal pump or fan then torque increases and decreases as speed (frequency) increases and decreases, but the AC Drive primarily adjusts voltage as frequency changes to assist the motor in developing torque while managing current.

Thanks for taking the time to write us with this question.

Art
Goldberger Controls, Inc.

 
     
     
     

 

 

 

 

 

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