Variable frequency drive techniques

[mook johnson]

Folks,

I'm very familiar with PWM and 6-step trapezoidal BLDC motor drives using
Mosfets or IGBTs. I'm looking into some high power (1 - 5K watt) induction
motor drives and the guys are dropping the term "12-step" and "24-step"
drives for 3 phase motors using triacs and SCRs.

Can't find anything on google about 12 step VFD.

While Im vaguely familiar with how SCRs work, could someone here describe
how they accomplish 12 and 24 step commutation? Besides reduced harmonic
distortion, what are the advantages of 12 - 24 step drives over more common
6-step or PWM drives?

thanks

 

[terry]

Folks,

I'm very familiar with PWM and 6-step trapezoidal BLDC motor drives using
Mosfets or IGBTs. I'm looking into some high power (1 - 5K watt) induction
motor drives and the guys are dropping the term "12-step" and "24-step"
drives for 3 phase motors using triacs and SCRs.

Can't find anything on google about 12 step VFD.

While Im vaguely familiar with how SCRs work, could someone here describe
how they accomplish 12 and 24 step commutation? Besides reduced harmonic
distortion, what are the advantages of 12 - 24 step drives over more common
6-step or PWM drives?

thanks

12-24 step drives are exactly what they say they are, as is 6-step. More
"chops" in the waveform = lower harmonic content. SCRs have been around for
a long time, and range from small to ridiculously large. Triacs are only
small things. The advantages of SCRs are primarily cost, but they are pretty
robust. Only thing is they dont like turning off. 12- or 24-step scr drives
will likely have all sorts of wound components - interphase reactors etc.
Commutation behaviour of course depends on topology. There are plenty of
books that deal with this topic in considerable detail eg Schaeffer, but
unless you deal with power systems you wont come across the stuff.

PWM absolutely slaughters 6-12-24-step modulators when it comes to
harmonics, but the penalty is increased switching losses c.f. the machine
derating associated with waveform harmonics (best not to think about fast
PWM edges and their effect on motor insulation, bearing EDM etc). Modern
power electronics mean that PWM IGBTs win from about 2kW to 1MW or so. I am
not up to play with state-of-the-art MOSFETs, but last I looked they won
below about 2kW. And if you have a FET/IGBT switch, then you can implement
any modulation scheme you like, especially if the waveform is generated by a
microprocessor.

That being said, small machines (below 10kW) have quite high impedance and a
very favourable surface-area-to-volume ratio, so can happily tolerate a
really shitty waveform. A 6-step waveform can have the harmonic content
greatly reduced by adding a little bit of PWM around each edge. For
cost-sensitive fixed-speed applications, SCR "soft-starters" are used to
provide a voltage which ramps up over a pre-set start time (basically a
three-phase light dimmer) to full voltage (ie SCRs on whenever forward
biased).

HTH

Cheers
Terry