Current control loop in PWM Inverter

Discussion in 'The Projects Forum' started by Chinmay Harkare, Mar 8, 2015.

  1. Chinmay Harkare

    Thread Starter New Member

    Nov 24, 2014
    I am working on a current controlled PWM inverter where I will use PI control to force the current through the inverter to follow a reference provided as is done in Active Power Filters, Grid connected inverters and in some forms of vector controlled ac drives.
    I am currently stuck at getting a faithful measurement on the inverter current. I tried ACS712 yesterday but it's performance is just too low for such an application in terms of noise and speed.
    I have ordered LEM sensors HX 10 and LTS 6-NP and hopefully they should do the trick.
    Now could someone help me in terms of the following:
    1) Let us say that I use an RL load R=4ohm and L=10mH which means Time constant L/R = 2.5ms (if I am not wrong). How do I decide PWM frequency?
    2) I read that the sampling frequency for such current control loops is very high (about 30kHz) but I don't understand as wouldn't the PWM frequency be 12-16kHz max?
    3) Am I right that sampling and calculations are done every PWM cycle? So where does this 30kHz come from?
    4) So what should be my sampling frequency? Should it be the same as my PWM frequency?
    5) What should be the cut-off frequency of the Anti Aliasing Filter and it's order and allowable phase lag in case of higher order filters?
  2. uwed


    Mar 16, 2015
    Higher switching frequency means lower current ripple (resulting in smaller grid- and load-side filters, and reduced motor stress), but higher semiconductor switching losses (reduced system efficiency, higher junction temperatures and reduced reliability, larger heat sink). Switching frequency is an application-dependent key optimization parameter. For inductrial drives in the kW-range it is typically 4kHz-8kHz. If people are around, it is >16kHz because of noise. Switching frequency is dependent on the power. For MW-drives employing IGCTs the switching frequency is well below 1kHz while for small inverters of a few 100W it can be several 10s of kHz.