If you look at my profile description, you'll see that I'm involved in building and wiring up Carnival floats that are powered by an 800KVA, 415v, 3 phase Generator. Electrically, they generally have about 10 or 15 motors, thousands of light bulbs and a 20KW sound system.
3-phase induction motors are used to drive big spinning steel structures which carry a lot of weight and have a lot of momentum, and therefore need to ramp up (and down) the speed over a period of around 10s so as not to put too much stress on gearboxes, chains and bearings, etc. The running speed also needs to be adjusted to fit the float design, which needs to be done electrically as changing gear ratios is not practical once built.
To control the motors, an Inverter Drive is needed, which is a very expensive piece of kit that has a huge array of parameters that can be adjusted to control motors in all different situations. We have 2 of these units that control a few motors each. So the problem I have is:
- A big single point of failure - one unit fault causes many motors to fail.
- Many of the motors are limited to being driven at the same speed, when individual speed control is sometimes needed.
As we struggle to raise the funds to build our carnival float every year, we cannot afford to buy any more Inverter Drives (unless someone is kind enough to donate one as they have with the two we've got).
I would love to be able to build my own for a fraction of the cost. I don't know if I'm setting myself an impossible task and am setting my sights way too high, or if this is indeed achievable. Looking inside an Inverter Drive, I can see several SMD circuit boards, some huge capacitors/inductors, and some heat sunk transistors.
I have experience of switching/dimming resistive loads using pic based control circuits, but although I know the theory, controlling a 3 phase inductive load at the moment is beyond my skill level.
Can any one guide me to where I should start with this. I suppose initially I'm looking for a block diagram of the modules (with a little detail) that I would need to build a home-made inverter drive (e.g. PSU, microcontroller, output stages, etc), and how they fit together. I can then work out if indeed its even worth attempting this, and if so work out which parts I can happily design and make myself, and which parts I'll need more help with.
- Input: 240v single phase or 415v 3 phase.
- Output: 415v 3 phase variable frequency motor drive.
- Controls: Stop/Start, Ramp up speed (seconds), Ramp down speed (seconds), Running speed (Hz), Emergency stop.
- Display: Current speed (Hz), Load (A)
- Motor wiring: Star
UPDATE (31st Jan): From the information provided by pingswept and bt2 in their answers, I think I've come up with a simulation of the bare bones of what I need... Rectify the 3-phase to get 586V DC and use 6 IGBTs controlled by an IRS2330 which is controlled by PWM from a microcontroller.