Full load current for Active Front End drive

[ElecEng]

Hi all,

I have a question regarding the Full load current in Active Front end drive(AFE) that is used to drive a propulsion Motor.

I learn from my previous projects that the full load current for the AFE is less than the full load current that is sent to the motor from the AFE.
Eg: IF the motor full load current is 2900 Amp then the full load current for th AFE that is used to drive this motor is only around 1600 amp. Why is there a difference ?

Also, do AFE's account for Short Circuit Current ?
Thanks

 

[mike_kilroy]

FM?

FM aside, remember ANY drive, active front end or not, is a power converter with an efficiency. So ignoring the few % inefficiency power in = power out..... so if the drive is running 1/2 speed and rated torque for instance, that means it is basically outputting 1/2 input voltage and full rated current.... hence input amps will be 1/2 the output amps.

Make sense?

 

[petersonra]

There is also the issue of power factor.

The input to the drive is close to unity PF. The output is not. The power is almost the same though.

 

[Jraef]

Yes, it appears you may be equating input and output POWER to input and output AMPS. They are not the same thing. Power factor is the issue as Bob says. The VFD essentially corrects the power factor of the motor. Loaded, unloaded or otherwise, the current drawn from the line by the VFD is always very close to unity PF.

But if you compared POWER, as in kW, the input to the drive is ALWAYS higher than the output of the motor, by a factor equal to the losses inside of the drive.

By the way, AFE or not makes no difference except that an AFE, if providing Active Harmonic Elimination, is even a little bit BETTER at maintaining that power factor closer to unity by cleaning up the harmonics and thus the distortion power factor as well as the displacement power factor.