Current loop in VFD for AC Induction Motor

[ecenus]

I would add in a current loop in the code of my VFD for a 3-phase AC induction motor (125hp) to control the current.

As I am using V/f scheme to control the motor, if the current is too large or small, I need to adjust the voltage applied to the motor based on the current values (I can measure the current value through my VFD). Does it mean that I have to define a "dynamic" v/f scheme because voltage variation is fulfilled by changing frequency ....

Thanks for your input in advance!

 

[cswilson]

What are you hoping to accomplish with this current loop? How will you decide what the desired current levels are?

A traditional (open-loop) VFD relies on inherent feedback loops in the operation of the induction motor. The output frequency sets the desired no-load motor speed. The greater the difference between the this speed and the actual motor speed, the greater the slip frequency. The greater the slip frequency, the more current the motor will draw. This "current loop" is inherent in induction motor operation -- there is no "conscious" control loop needed.

My sense is that unless you are willing to go all the way to sensorless vector control, you will not be gaining anything, and you stand a good chance of screwing up the action of the inherent feedback loops.

Curt Wilson
Delta Tau Data Systems

 

[ecenus]

Thank you for your tips, cswilson,

My current loop is just for the current limit as my VFD has the capacity of 450A. If the motor draws too much current which is over than that, it will hurt my hardware. So, I want to add in the current loop to limit the current: if the current is over, say, 400A, I will lower the voltage to reduce the current.At this moment, the motor might not be able to produce much torque but at least, the current value will not go so high to be over than 450A. In order to do that, I have to "dynamically" changing v/f ratio to change the voltage...

My intention of using this current loop is for protecting my hardware. It is still a scalar control.

 

[DickDV]

You have just described how the Current Limit function works in most drives by default.

 

[LionelHutz]

Yes, most current limits in drives seem to either lower the voltage, ie lower the V/Hz ratio, or they lower the frequency which will also lowers the voltage. I think lowering the frequency is better because that maintains the V/Hz ratio.

 

[DickDV]

For further protection of load machinery, you can usually set up the drive current limit function as an electronic shearpin. That means that when the current limit is reached, the drive faults. A reset is needed to restart the drive and a record is made in the fault log.

If that option isn't available in the software, you can usually set up an output relay as a current level detector and then wire it into an external fault loop on the drive digital inputs. Works the same as an electronic shearpin.