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linear induction motor/maglev project

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BigBad

Fri Jun 20 2014, 01:58AM

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

Building a multiphase linear induction motor and inverter from scratch.

Ordered the parts for a software driven 3/4 phase, 3kW (peak) inverter. Have more or less simulated it in LTSpice, although the exact components didn't seem to have models. Power on transients were surprisingly large. Going to be driving it from an arduino I have. I went this route because it was cheaper and I wanted to understand it, and I also wanted it to do some weird stuff, run it as a servo, and also add another 3 phases to have two independent motors; I didn't think I'd be able to do all this with a standard inverter.

The electronics and heatsinks arrived today, I showed them to my father, who's a battled hardened switch-mode power supply designer, he started mumbling about 'blowing up' and 'transients' although he didn't find any actual specific problems. LTSpice certainly showed impressive transients at times, and closer examination showed the undervoltage was out of spec. This was fixed in the simulation with an RC snubber across the drain and source of the FETs I'm using, together with some resistors on the gate and Vs to adjust the ringing and turn on speed.

I've modelled the actual motors in FEMM; seems OK. However, FEMM is 2D and the reality is 3D so I'm expecting discrepancies. The power dissipation is fairly frightening, but did the thermals on it today, apparently I should be able to drive it at full power for about 2 seconds before the insulation catches fire; but I was planning on mostly driving it at greatly reduced power, so not particularly bothered.

I've mostly wired up the half bridge for one of the phases of the driver, I'm testing that on its own, initially at low voltage to see how big the glitches are.

At one point I thought I needed a huge inductor to smooth the current from the mains, but I showed that a small panel resistor in conjunction with my smoothing capacitor forms an RC which limits the current very well, so it's under control (the slightly idealised simulation was predicting 15-60 amp pulses even though the average was a long way below 13A.)

It's quite an interesting circuit really.

I could do with a slightly larger panel resistor to smooth out the current taken from the mains, although it still works. I have written some inverter control software for the arduino. With some fiddling, it's running at a chopping period of about 5 microseconds, which is fairly good and gives a pretty reasonable 200hz waveform on four channels simultaneously.

Assembled 8 FETs on the heatsink and made a low inductance +ve and -ve bus using unetched straight copper PCBs. Currently 4 phase, since I have the FETs and the microcontroller and I have plans to do clever stuff. I have only built a single hi-lo driver so far though.

Sourced the U-cores (powder cores for high frequencies and pretty decent saturation ~1.4 T.)

Gonna have to start winding some coils...

Ok, I've run my inverter half bridge up at full mains voltage now, but switching at very low output amplitude. Test coil got to about 45C or so, but none of the inverter components were even faintly warm. I checked various points on the circuit looking for excessive spikes, but it seems OK. The scope doesn't handle it very well in differential mode which made doing that entertaining and somewhat confusing, but so far as I can tell, everything is within spec.

Also, I found a bug that could have been blowing the fuse during power-on. The gates were floating during power-on, and probably switching on due to Miller capacitance. I've added 1k pulldowns from gate to source and that should fix it.

Steve Conner

Fri Jun 20 2014, 10:14AM

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

BigBad wrote ...

a battled hardened switch-mode power supply designer, he started mumbling about 'blowing up' and 'transients'

Sounds about right. He is obviously familiar with Murphy's law of power electronics

The 3-phase motor drives I've seen use IGBTs switching at relatively low frequencies, with no special care taken to avoid transients. Current sensing and limiting is by resistors in series with the low-side device emitters in small drives, and Hall effect CTs on the outputs in larger ones.

The biggest problem you'll probably come across is if using integrated high-side drivers like the IR2181, they are easily destroyed by excessive spikes, negative ones in particular. Drivers with true optical or magnetic isolation will make your life easier.

Thomas W

Fri Jun 20 2014, 11:31AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Wow! i can't wait to see this, i have been hoping to do somthing like this myself oneday, please, do show all the pictures you can! im very interested in this.

BigBad

Fri Jun 20 2014, 08:17PM

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

Steve Conner wrote ...

The biggest problem you'll probably come across is if using integrated high-side drivers like the IR2181, they are easily destroyed by excessive spikes, negative ones in particular. Drivers with true optical or magnetic isolation will make your life easier.

Good job I'm not using that specific device.... oh wait; doh.

I already downloaded the application note about undervoltages. The issue seems to be mostly stray inductance around the switches; I plan to keep the leads very short.

I have a spare set of switches and drivers, I'm probably going to assemble one leg on its own and see how it goes with an inductive load; just run it PWM as an oscillating voltage source, see how bad the glitches are and to allow me to optimise.

BigBad

Sun Jun 22 2014, 10:42PM

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

I haven't got a LTSpice model of the particular switch I'm using but my SWAG based on modelling other devices is that the undervoltage switch transients with plausible stray inductances is right around the ragged edge of destroying the IR2181 (about -18 volts). The device is hardened to negative voltages down to -20v and it may theoretically work, but...

I might have to build a step-down switch stage to drive the 3 phase switches, and wind the cores to take more current at lower voltage; the switching transients seem to go with voltage.

Or I could bin the IR2181 and use a different drive approach.

hmm

I think I'll build a single phase leg first and run it at low supply voltage and low duty cycle via a small variac we have and see what undervoltage I actually get; it may not be as bad as I think; although probably it will be worse.

Sulaiman

Sun Jun 22 2014, 11:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I'm not endorsing any product but for the price you should consider somethong like this

or similar / higher power.
It will allow you to concentrate on the hard part ( mechanics )
inbuilt protection methods, speed ramping, remote control etc.
later you may wabt to build your own inverter
but when you consider the design effort and cost,
a ready made inverter makes a good choice.

there are ready made inverters for up to 60,000 rpm or 100's kW. (scary!)

BigBad

Mon Jun 23 2014, 10:46AM

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

I get where you're coming from, but I'm doing something slightly unusual, the peak power and current looks too low for what I'm trying to do at the moment, it's also rather heavy, one of my aims is to carry the power supply on the maglev and use passive cooling if at all possible. The standard power supplies are sized for continuous loads.

Anyway, I think I've solved the undervoltage issue, a lot of it is that the reverse diode takes ~1 microsecond to switch on, but LTSpice seems to agree that an RC snubber right across the source and drain of the FET will take out the transient while not burning power at other times; it just shorts out the diode for a moment and gives it breathing space to start to conduct as it reverses and also damps out the stray inductances.

I tried lots of other things from the FET app notes, but they made hardly any difference; I think it's because my circuit is running at relatively high rail voltages, a lot of motor drives run below 100 volts; mine is over 300 volts, so I'm getting proportionately bigger transients.

I'll soon find out whether the real world agrees with the simulation; judging by the simulation behaviour running the real circuit on a variac for testing seems to be a very, very good idea indeed.

BigBad

Thu Jul 03 2014, 12:51AM

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

I've built a single leg, and am powering it from ~36 volts AC on a variac, and additionally pulsing the FETs at a quarter power (giving a 200 hz 'sinusoidal' waveform). It's giving a rather funky waveform right now, I think partly because the rectifier/smoother feeding it is a bit underpowered. But I nevertheless drove a coil from it (about 150 turns, I already had it kicking around) around one leg of an E-core.

It easily lifted a kilogram of iron running it just as a conventional attractive electromagnet. I would have run it at higher power, but the variac is fairly small. and I need to check stuff before turning it up.

It's going to be scary powerful. I'm running it at about 1/20 current at the moment...

BigBad

Mon Jul 07 2014, 07:40PM

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

Have been running it at 1/3 voltage, 100+ volts on a variac.

I tried going above this, but the Vbs transients started getting out of hand- over 20volts doesn't make me or Mr Controller happy, but Vs looks fine. Subtle changes in board layout should (in principle) completely negate all the parasitic inductances on the drain side which are probably a lot of the problem, I modelled it in LTSpice and it looks promising, and I'm ordering some bigger and better capacitors to help damp. I'm expecting it will be able to run at full design voltage/power; my fall-back idea is simply to run it at lower voltage via a step-down, but that would be a pain.

Steve Ward

Thu Jul 10 2014, 12:17AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Having designed somewhat big BLDC drives "from scratch", the main feature id say you should not go without is fast over-current protection. Its real easy to blow up a bunch of silicon with a stalled motor.

Also watch out for braking currents and over-volting the bus from regenerative charging.

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