DC bus correction for inverter VFD via PWM modulation

Question

This is a question concerning an inverter circuit without PFC.

Single phase AC is rectified and pooled on DC bus capacitors. From there the DC voltage is modulated at 5kHz on 6 IGBTs, emulating sinusoidal waves of the required frequency, using an open loop V/F scheme.

Now assume the requested frequency is steady. Due to fluctuations on the AC input, DC bus voltage may vary from 280Vdc to 330Vdc. Which is the best way to modify the PWM signal in order to drive the load at the same power?

At the moment I'm adjusting the PWM duty cycle proportionately to the nominal DC BUS voltage (limiting the maximum adjustment up or downwards).

DutyAdjusted = DutyNominal * (DCBusNominal / DCBusMeasured)

Is that enough? Is the approach dependant on the type of load?

Answer 1

Speed variation due to changes in the mechanical load and limited torque capability at low speed are the principle disadvantages of open-loop V/f control. Speed and torque capability variations due to mains voltage fluctuation will be less noticeable. If you want to improve the performance without abandoning the open loop control scheme, you might consider providing a manual adjustment for V/f to increase V/f slightly at low speed. Also consider a scheme for estimating operating torque and increasing the frequency slightly to compensate for motor slip increase with load torque increase. A scheme to maintain constant V/f with fluctuating mains voltage will improve performance, but will probably provide less improvement that the two schemes that I suggest.