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Induction generators using VFD

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BigBad

Sat Apr 26 2014, 12:24AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I'm trying to build a 3 phase induction generator. It's going to be a good few kilowatts.

The problem is, I did the maths on the phase correcting capacitors to get leading phase and they're annoyingly big, and heavy, I need it to be light.

I figure I could effectively cross connect the phases and ditch the capacitors with some sort of boost and/or buck type thing running at much higher frequency than the generator and do much the same thing; i.e. using inductors running at high frequencies to ditch the capacitors.

It'll probably be slightly less efficient, but it's for only momentary use; I've got a very low impedance load that I need to put lots of current through for a projectile accelerator.

So I guess I'm trying to do a quadrant IV VFD, with active PFC even though it's linear.

Before I reinvent the wheel, does anyone know off hand if there's any relatively easy strategy to do this?

Patrick

Sat Apr 26 2014, 12:30AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

can we see the schematic and specs on the accelerator? maybe the power solution will appear after we see the intended load.

BigBad

Sat Apr 26 2014, 01:16AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

It's just a 3-phase synchronous PM motor matched to the induction generator.

Sulaiman

Sat Apr 26 2014, 06:19AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

IF you have a 3-phase supply then a ready made inverter may be cheaper than capacitors
e.g.

If you only have single phase supply then something like

either type has provision for setting the maximum speed and the speed ramp up time with control from front panel and/or switches and potentiometer or remote via serial link
as well as other features.

I only did a quick search, some inverter drives can go to a few hundred Hertz
(we once repaired one that went to a scary 60,000 rpm, there are probably even faster ones.)

Ash Small

Sat Apr 26 2014, 06:33AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

OK, the best I can come up with so far is to increase the number of phases, convert from AC to DC, then you only need a 'smoothing capacitor' which will be smaller capacitance compared to the one you previously calculated, then convert back to AC (full bridges, one for each phase), and feed to motor.

At the moment I don't see how some kind of 'boost and/or buck' converter could be used without using capacitors as well, although, as I'm sure you're aware, I'm no expert in this field.

I'll be following this thread with interest.

Steve Conner

Sat Apr 26 2014, 08:57AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Every VFD is 4 quadrant, in that they can all brake the motor regeneratively and feed the energy back to the DC bus.

However most of them don't have the required bidirectional PFC front end to convert the recovered energy back to AC and feed it back to the line. It adds a lot to the cost and complexity as it basically means replacing a plain diode bridge with a second inverter.

So unless the drive is expected to recover a very large amount of energy over its life, enough that the energy savings would pay for the extra electronics, they typically just switch a large resistor across the DC bus to dissipate it. I've never tried it personally, but I'm fairly sure you could connect a DC load to the DC bus and the VFD would let you run the motor as a generator indefinitely.

A projectile accelerator would normally be driven by a compulsator, a special alternator designed for extreme pulsed power output. At the short timescales under consideration, the distinction between synchronous and induction generators gets somewhat blurred, but as far as I know, pulsed alternators mostly use a field winding for excitation, either in series with the output current, or separately excited by a capacitor discharge. This is as opposed to a pure induction generator, which needs a leading power factor in the stator for excitation.

Sulaiman

Sat Apr 26 2014, 10:34AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Steve, there are 1 quadrant VFDs (drive forward only)
2 quadrant drives (drive forward or reverse)
4 quadrant drives (drive or brake forwards or backwards at variable speed (e.g. crane hoist)

All modern VFD drives have diodes inverse parallel with each of the six transistors (igbt)
so stoping FWD or REV a 2Q drive allows the back emf of the motor
to dump energy into the main Bus capacitors.

Small drives just live with this

Medium size drives have a transistor
which switches a 'brake' resistor across the Bus if the voltage rises too high.

Large drives usually have regenerative braking.

Ash, what you described is the typical VFD
I doubt that you could diy one at lower cost than purchasing one.

BigBad

Sat Apr 26 2014, 10:51PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Yes, I don't necessary want to home brew it, but I'm trying to get my head around it.

Presumably if the main bus voltage goes up from the switched currents through the reverse diodes, then because they're driven from it the 3 phase drive voltages tend to go up too with it, until the iron losses or whatever equal the generated power by making the generator less efficient.

So basically the VFD will automatically stand in for the capacitive network by cross coupling between the phases via the main bus.

Ash Small

Sat Apr 26 2014, 11:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've been reading up on VFD's since my earlier post, and I learned quite a bit from the wikipedia page:

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