Turbo pumps

[Doug Coulter]

It's a permanent magnet. (nice chip btw, but you maybe don't need that fancy). The main thing is you have to ramp speed slow, so as not to get ahead of it.
The fancier controllers look at the phase of the current to ensure they don't do stupid stuff (say, the original Pfeiffer controller). You can tell if you've lost lock that way.

If you do pwm, you don't need the power supply step voltage thing John used, but things get trickier. Obviously you have to know how to set/change the pulse width in that case.

If I were doing this from scratch, I'd just use one of the SOC type microchips and do it all in software driving pwm outputs. I could probably find one that would do 3 phase
sort of naturally, or run one pwm at 3x speed and gate the output around to the motor phases.

The thing you want to sweat is what happens in an inrush accident -- some sort of current monitoring should save you there. Continuing full power drive at the wrong
frequency will make a lot of things hot -- and may even wreck the magnets. This can be tricky to measure, as what you are interested in is an indication that the current on this phase is different from cycle to cycle -- beating in frequency. When you see that, it's time to shut down or do something smarter than whatever you were doing.

Pfeiffer uses the free wheeling diodes to power the controller long enough to vent and do smart shutdown when the mains power fails. It takes a long time to run down when
the vacuum is good.

With a microprocessor, one might look at the signal coming back from the pump once in awhile with the drives shut down to see if you're in sync. One or two roundy rounds with no drive won't hurt a thing.

So at the start, the tradeoff is basically do you want the simplest thing that can work in ideal conditions, or something that's a little more robust.

I find that it's pretty rare to need all the pump motor has in it. Even during initial pumpdown, it gets out of that situation quite fast. In my system, the turbo power draw controls the forepump, and it all kind of works out via a current limit on the turbo. Basically, it doesn't spin to the point where it can draw excess power until the vacuum is already fairly good, and then it can't draw excess power because there's no drag on it. Both start together, but until the roughing pump gets it down some, the turbo just doesn't spin real fast due to the current limit.

I don't know if Pfeiffer does the "check the phase while dropping drive" thing, but that's how I would do it -- easier that way than measuring a current phase angle to see if we are leading or
lagging the rotor.

[Jerry]

There are quite a few designs, especially on Atmel's site, that use the back EMF for commutation. Atmel even provides all the C code and libraries to do it with one of their uPs.

[johnf]

Jerry
TMP50's are ordinary three phase synchronous motors the rotor is not magnetised

I use current info to decide whether I keep feeding volts to the
output bridge ie sudden inrush of air --current goes through the roof -- controller drops relay from transformer-- DC bus goes to zero

[Jerry]

Ahh, thats why that relay is there.

[Doug Coulter]

I'll defer to John's knowledge on these older pumps -- I've never touched one. The new ones are obviously magnetized (residual, or deliberate I don't know) -- you can watch them "cog" if you start them spinning, and they do run the controller for nearly 20 min if you pull the mains plug, even powering the vacuum gage.

[Jerry]

I am pretty sure my little Pfeiffer is standard induction. I can hold the pump in my hands in front of me and turn around and the rotor tries it darndest not to change orientation. I think if it was a PM motor it would want to turn with the pump housing.

John, on the board you so kindly posted, am I getting these connectors right?

J2
1 Gnd
2 buss Estop relay
3 Buss voltage select? Why?
4 Buss voltage monitor, .0625v per volt
5 +5v


J4
1 Gnd2 Current Feedback
3 freq control voltage
4 VCO Feedback
5 Fault Out
6 Drive IC power enable

The 4018 is set up as a divide by 6 counter. So it should be getting 7200hz in for 1200hz out?
according to the online calculator I get 3430khz/v 5373khz min, 14979khz max. the cap on the IC looks like 3.9pf? Something wrong here...

[johnf]

Jerry
Cap on 4046 = 3900pF
From memory I set the 4046 for 300Hz min and around 8Khz max
Relay tap selects the transformer windings so that at low speed max current into turbo is around 3.5amps so half dc buss volts till 3000hz then full bus volts to final freq of 7200hz (1200Hz three phase out).
My original circuit used a SMPSU flyback that raised its volts along with control volts. but it was a little too complex and couldn't quite get the required power without burning up snubber components.

[Jerry]

Thanks for the info. I may just nix the VCO and use an output from a teensy (http://www.pjrc.com/teensy/) to send a pulse train out. I can also use that to control the rest as well.

[johnf]

Jerry
That's fine
I'll build an analog computer before using the digital kind

been burnt a few times with startup ie all ports going high during reset and NMI's causing untold grief on real time port operations

[chrismb]

Please excuse me if I'm rewinding a bit on the DIY pump controller bit, and also excuse me if my atrophying brain is getting a bit slow on the uptake; if I have three 120deg phased 50% duty cycle outputs, let's say 1, 2 and 3, which are switched (only high or low on each), and I have a delta motor arrangement to drive, let's say with winding connections A, B and C, then do you simply need to connect 1 to A, 2 to B, 3 to C? So in the first 60deg current would flow from A to both B and C, then in the next 60 current flows from A to C and also B to C, then in the third 60 all are high so no current flows, then in the 4th, A is low so current flows from both B and C, in the 5th only C is high so current flows from C to both A and B, and in the last 60 all are low so no current flows? Is that what is meant to happen?