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Background

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.

Problem

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:

  1. A big single point of failure - one unit fault causes many motors to fail.
  2. Many of the motors are limited to being driven at the same speed, when individual speed control is sometimes needed.
  3. Costly.

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).

Solution

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.

Question

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.

Specs

  • 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.

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  • \$\begingroup\$ my gut feeling is - it sounds like you would be taking on a lot of liability doing this project... but I applaud your enthusiasm \$\endgroup\$ – vicatcu Jan 29 '11 at 2:02
  • \$\begingroup\$ @vicatcu: I know, but I've been thinking about how to do this for several years, spent many hours googling and reading up on the theory that I want to at least have a go. If it doesn't work, or it turns out to be too dangerous then maybe I can finally give up this silly idea! \$\endgroup\$ – BG100 Jan 29 '11 at 2:10
  • \$\begingroup\$ +1 for a question well written :) \$\endgroup\$ – ElectronS Apr 26 '18 at 19:27
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I spent 13 years designing electronics of this exact nature: three phase induction motor reduced voltage soft starters and variable frequency AC drives. I spent the last few of those years as a VFD applications engineer helping customers select and configure this equipment for various loads and industries around the country as well.

You will not be able to build something that is cheap and safe. The voltages and currents involved are well beyond the safety margin of a hobbyiest, especially someone who is openly avoiding buying commercial units in order to save money. Don't do it!

While the theory behind AC motor control is very straightforward, the detail level work (heat sink sizing, snubbering, gate drive requirements, de-sat protection, motor overload calculations, bus capacitor protection, etc.) can be quite tricky to get down, especially with heavy duty cycling and regenerative power modes which a carnival ride will CERTAINLY be generating. I strongly caution you against trying to build something of this nature unless you have significant experience not only in microcontrollers and embedded systems design but also significant experience in power electronics and three phase circuitry. People get hurt and killed building this stuff.

My first question for you is whether speed control is really required, or if you only require a soft start up and slow down. Do you vary the speed of the motor once it is started? If not, you may be able to get away with a MUCH cheaper reduced voltage soft starter. These units act like three phase light dimmers; they only adjust the applied voltage to the motor. You will not have a lot of torque at low speeds, but with the right design of motor (NEMA class D) you can achieve exactly what you're after with a fraction of the cost and maintenance.

If you really do need to vary the full-load speed of the motor then you are more or less stuck using a variable frequency drive. As you are aware these are expensive and if you buy cheap you are likely to replace them sooner due to your high surge current (they call this "constant torque") application. What I would definitely recommend doing if this is the case would be to contact various manufacturers (Allen-Bradley, Cutler-Hammer, SAF drives, Benshaw, Yaskawa, etc.) and ask for reconditioned units. Ask for a drive capable of delivering 150% rated current for 30s (this is usually known as heavy duty) or size the drive 30-50% larger than your nominal current rating. You will also likely be running off of generator power which is notorious for being undersized and prone to brownouts and surges as the load requirements change with the state of the equipment being run. Drives don't like that (voltage sags cause current spikes as the motor starts slipping and surges can cause you to overvoltage the bus capacitors) and have a tendency to either fault out or blow up.

I am all about the little guy building something and saving a buck, but this is not the type of project to do this on. If you really want to build a three phase AC drive, start with a little 10HP 480V motor with a hand brake on a test bench. You have all the potential for experiencing the pants-filling sensation of an H-bridge failure or a bus capacitor explosion two feet from your head but without the potential lawsuits and loss of life (except perhaps your own).

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  • \$\begingroup\$ Thanks for your very detailed answer, I appreciate information from an expert in the subject! To answer your first question, after ramp-up, the speed doesn't vary however the speed won't be 50Hz, it could be anything between 25 and 60Hz and as the generator supplies AC at 50Hz I need something to change the frequency. So I might be wrong but I don't think I can get away with a reduced voltage soft starter. \$\endgroup\$ – BG100 Jan 31 '11 at 23:53
  • \$\begingroup\$ If your generator is not able to meet its rated speed and you are relying on the VFD's ability to adjust the motor speed to compensate I think that your generator is either woefully undersized or the governor is in need of some repair! Some surge and sag is expected with any engine-driven generator but they should be able to stay within 5% of their rated output frequency under load. \$\endgroup\$ – akohlsmith Jan 31 '11 at 23:56
  • \$\begingroup\$ I think I'll certainly try contacting some manufacturers as you suggest. The generator we use is an 800KVA, and we normally load it up to around 70%, so I don't think brownouts (although we ran out of fuel once). Also, I don't know if it makes a difference to anything you can suggest, but the load that I'm powering has a lot of momentum, but the gearboxes we use have worm gears that prevent free-wheeling. If you want to see it in action, take a look at this youtube video: youtube.com/watch?v=sL2i0SxpKIY \$\endgroup\$ – BG100 Feb 1 '11 at 0:07
  • \$\begingroup\$ To reply to your comment about the generator speed, I think you misunderstood... the generator runs at 50Hz, but I need the motor to run at a fixed frequency say 30Hz which is the speed that I want to configure the motor to run at for the duration it's switched on. \$\endgroup\$ – BG100 Feb 1 '11 at 0:09
  • \$\begingroup\$ yeah I'm aware of the general mechanical concept (and have ridden a few of the rides <g>) -- the generator should be able to keep up with the load... Might be worthwhile getting the generator technician out and explaining to him that the thing can't keep a reasonably steady frequency. The electronic governors are pretty painless but I've adjusted a few oldschool mechanical ones... it's like a Rube Goldberg machine in them, I swear. :-) \$\endgroup\$ – akohlsmith Feb 1 '11 at 0:10
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In the big picture, I think you're talking about a 500-1000 hour job and at least a few thousand dollars in materials and circuit board expenses. If you really want the best solution for the money, get a high-paying job and spend the wages from the same 500-1000 hours to buy used motor controllers off Ebay or Craigslist. You'll end up with more motor controllers.

But that's not much fun. If you wanted to build one, you'd need 6 MOSFETs or IGBTs (one on the high side and one on the low side of each leg of the star). Then you use a microcontroller to turn the switches on and off to simulate a sine wave.

Since the voltage you're talking about is quite high, you'd need what's called a high-side driver-- a chip that can control a MOSFET at very high voltages. The IRS2330 from International Rectifier that will do all three phases. Page 19 of the IRS2330 datasheet has a nice diagram of the basic circuit.

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  • \$\begingroup\$ Thank you, this is really helpful... but are you serious about this being a 500-1000 hour job with costly materials? Looking at the datasheet, and the microchip appnotes 00843a that bt2 mentioned in his answer, this actually doesn't seem too complex. \$\endgroup\$ – BG100 Jan 30 '11 at 0:27
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    \$\begingroup\$ I am serious about that estimate. While it's true that the control components are cheap the switching elements and and large filters you'll need will be expensive. Also, you'll need to pay for circuit boards, which are at least $100 per run for something this size. I guess an experienced electrical engineer could maybe do it in 250 hours and maybe $1500, but that seems very optimistic. \$\endgroup\$ – pingswept Jan 31 '11 at 15:14
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    \$\begingroup\$ I should also add that part of what makes all this hard is the high voltage. Everything would be cheaper and easier if you were running at 48 V. As power goes up, you have to start worrying about more details. \$\endgroup\$ – pingswept Jan 31 '11 at 15:16
  • \$\begingroup\$ Ok, well I don't care how long it takes as I can use my spare time over a year or two if needed. The cost is a problem though as I may as well just buy one! For the filtering, I take it you're talking about the smoothing capacitors for the rectified DC? Given that this doesn't have to be perfect as it will only power the motors on the carnival float for 20 hours in any one year, unlike an industrial grade unit, how little filtering can I get away with? e.g. from DigiKey, I can get 2 x 10000uF 450v caps for $178, giving my 5000uF in series. Can I get away with just this? \$\endgroup\$ – BG100 Jan 31 '11 at 20:55
  • \$\begingroup\$ You won't build a 3-phase AC drive capable of driving IGBTs large enough to safely control a 50-75HP motor in 250 hours, I don't care how good your engineer is. There's a LOT of control software to write if you're after something reliable and safe. See my answer below for more detail. \$\endgroup\$ – akohlsmith Jan 31 '11 at 23:04
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I would check out these app notes from Microchip

http://ww1.microchip.com/downloads/en/devicedoc/70096A.pdf

http://ww1.microchip.com/downloads/en/AppNotes/00843a.pdf

They have a 3 phase development kit, but I believe you have to ask to borrow one of them, and as you're not looking to actually manufacture this, I doubt you'd get high priority. Still, they have schematics and code available for free use.

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  • \$\begingroup\$ Thanks for these datasheets, looking at these and the datasheet that pingswept provided in his answer, I think I can actually work out how to do this. \$\endgroup\$ – BG100 Jan 30 '11 at 0:29

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