Filtering the output of a VFD

So after many months of assing about and pestering from the old man (dad) I finally have a working prototype of my sinlge to three phase variable speed drive up and running. The design uses an International Rectifier IRAMX16UP60A IGBT module:

I have the output connected directly to the motor. PWM is around

The problem, i know nothing about how hot a 1/3hp 3phase motor should be running at, this project is for my father, a retired engineer. I can live with the module itself running at its current temp, considering the relative lack of heatsinking. Nothing a bigger heatsink and a fan cant handle.

My question is, should the output be filtered with a capacitor, or is it ok to feed the raw PWM output straight into the motor? The output looks very bad on the cro, but the noise induced into the ground lead makes this hard to prove.

Please excuse my ignorance on this subject, but I really know very little. Google has been very good to me so far, but I am struggling to find an answer on this one.

Thanks.

Andy

BTW. I will do some proper temperature tests very soon!!

I prefered my 389 CID GTO! ;-)

-- Link to my "Computers for disabled Veterans" project website deleted after threats were telephoned to my church.

Michael A. Terrell Central Florida

what, no 9kV 6kA GTOs ?

Thats fine. Small motors are amazingly lossy - they have a great Surface Area to Volume ratio, so can easily dissipate lots of heat, thus its cheaper to make them lossy. So the manufacturers do. Above a few kW this stops being the case, and 100kW+ machines are very efficient, for the converse reason.

As long as your motor case temperature sits below 80-100C you should be OK. IIRC 150C insulation is the norm.

I presume you have fixed V/Hz control, perhaps with LF boost? (to overcome stator IR drop at low Vout). If you just DOL the output, watch for rotor heating after multiple starts - so-called "soft" starters are notorious for cooking rotor bars this way.

if Tj = 150C the module will *DIE* in a matter of weeks due to thermal expansion & contraction. 60-70C is great though.

CBarn24050 makes a great point re. fan cooling at reduced speeds (cube law IIRC).

Make sure your waveform doesnt have a high harmonic content, and the motor should be OK. first-turn failure is a problem with inverter-fed machines, usually only when fed from a long cable (peak voltage doubles, and can even triple if your modulator is bad enough). inverter-rated AC drives have heavy insulation on the first few turns for exactly this reason.

stick filter caps on the output and watch your IGBTs s*it themselves. dV/dt is pretty high, many kV/us, so I=CdV/dt is nasty. Little dutch time bomb, tick tock boom.

Chokes first....and watch them catch fire :)

output filters for VFDs are very tricky. All the commercial ones I have seen get *HOT* - 180C insulation on the chokes, that sort of thing.

I have set a number of large chokes on fire doing this :)

Cheers Terry

Maybe in the future :)

I tried looking on the manufacurers website to no avail. However they did mention some ratings of the bearing at 100degC.

Yup.

If you just DOL the output, watch

This is all under no load btw. My plan is to mount the heatsing in the case with a fan blowing over it. This should keep things cool.

There is a qustion. How can I check the output? I have a high voltage probe for the cro, but it is terrible for noise. The probe is 1000:1.

I will skip output filtering then! Many thanks.

Andy

:)

small motors run stinking hot, which is probably why they dont mention it.

jolly good jelly.

Depending on the IGBTs you use, conduction losses may or may not dominate. I suspect at your Fsw then Psw dominates, so an unloaded motor is a not-bad test. But it will get hotter under load.

make sure you have a heatsink over-temp. 80C is good, and in conjunction with a decent current limit will keep the IGBTs safe in the event of an overload.

I have a bunch of 4th order (IIRC) bessel filters, 50R in & out that I use in conjunction with a P5200 diff probe (Zout = 50R) and a coax cable

• 50R terminator to the scope. A little bit of gain peaking (thanks HP3577), but monotonic rolloff from 6kHz to 50MHz.

Lin = 2.5mH Cshunt = 1.44uF L = 6mH Cout = 1uf

I forget what cores I used (blue, 25mm OD, 14mm ID, 10mm thick) but my tech sectional-wound them with wire-wrap wire :). IIRC they came from Jaycar or DSE. Its built on a piece of Cu-clad PCB, mounted in a 92mm x

You shouldnt need it - unless the drive makes all adjacent electronics go nuts. In which case, use a screened cable from drive to motor, screen co-axially bonded to drive & motor (ignore instrumentation guys who say this is wrong)

You're welcome. Post some pics of the toy.

Cheers Terry

the specs, the specs, my kingdom for the specs.

Cheers Terry

No no! not yet! I am just getting a second cut of the board done. Once i build up version 2, i will post some pics and schematics on my website.

Specs?

The best part was that I did the entire restoration myself, back when I could handle the heavy labor. It ran like a dream! ;-) It could lay rubber on hard packed sand, and idled at 300 RPM. It had a pair of seven and a half foot 3 inch straight exhaust piles that could rattle buildings, at the right engine speed. Ah, I miss the good old days.

That's not a car, its a moneypit. ;(

nice.

that would have sounded great.

The father of a friend of mine recently bought a nice new merc. He's a retired mechanic, so was poking around trying to figure out where to put oil etc. but was unsuccessful. He asked the dealer, who replied "the car will tell you when it needs oil, bring it to us and we'll do it". Sheesh.

Cheers Terry

Andy, At the risk of asking an obvious question, the motor you are using is "inverter rated", right? Otherwise a VFD would cook an ordinary AC motor...

nah, only if there is a long lead, enough that transmission-line behaviour can double the peak voltage seen by the first turn (and, if your modulator is as bad as those used by Allen Bradley, triple it).

Cheers Terry