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Self-programmed AC IM beach buggy

2495 Views 4 Replies 2 Participants Last post by  justn
Hi guys

I am building a beach buggy conversion, using an induction motor - with a catch though - I've programmed my own motor controller.

The car is a simple Beetle chassis with fibreglass body.

I am going to use a 15kW 220V 3ph induction motor (alluminium - 96kg - would be ideal but I've got a cast iron one at hand, weighing 130kg, double the weight of a 1600cc beetle motor).

I will be linking up 26 lead-acid batteries in series for a bus voltage of 310V or so. (For 220V motor - 220x\sqrt{3} with SVPWM)

I am going to mount the motor directly to the gearbox, where I can easily use 2nd or 3rd gear to obtain my reduction ratio of about 8:1, and can lock it and chuck out the extra gears/selectors to free up weight.

My first issue:

Is this weight too much, especially if it is going to be hanging behind the rear axle. I am not sure that batteries in the front where the fuel tank was ie. by the steering column, will balance the weight out sufficiently. How will this handle turning, having such a large moment of inertia for yaw (<---correct word?) rotation.

Otherwise I can try turn the gearbox around to have the motor midmounted - but this will require lots of modification, but may be worth it.

Onto software/inverter issues...

I have played around with a sample Inf HybridPack 1 but my setup I am using is more 'rustic'.

I am using IGBTs (SKM100GB 123D)from Semikron with a SKHI61 driver, setup with laminated busbars and DC bus capacitors for cancelling out the inductance of the long battery cables.

I am using a DSP (F2812 from TI, 32bit fixed point) for my program. I have written the software using simple field orientated control, where I specify setpoints for the torque and flux. It gets a speed measurement from a rotary encoder and uses the current measurement to predict the rotor flux position.

It is working nicely in testing, but I have to manually set the flux - has anyone come up with clever ways of setting the flux depending on the torque required. It seems inefficient to just set it to nominal flux when you start the car and then just adjust the torque setpoint.

That done I just need to worry about adding a transducer so that the torque setpoint is controlled by the pedal (should I use a potentiometer? A resolver seems more ideal given the repeated use but quite pricey)

Battery weighting issue:

Will it be better for performance to have batteries mounted on the sides of the car/buggy, increasing the cars "lateral/rolling" moment of inertia, or mounted near the centre axis in the front bonnet.

Battery charging issue:

I will be linking up 26 lead-acid batteries in series for a bus voltage of 310V or so. As I am going to program regen braking into my algorithm, are there any concerns for charge balancing of these?

(I am thinking there will only be problems when the batteries are full and i need to float charge them, as this is when some can be overcharged. Maybe I should only allow limited regen when batteries are quite low for now)
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If possible keep the batteries just in front of the rear tires and just at and behind the front tires. Keep the batteries as low as possible. My Ghia could only fit 16 batteries without building ugly battery boxes. That will be one hell of a heavy buggy with that many batteries on board. You will for sure need to beef up the suspension. I'd go for lithium if you want to have that voltage system on such a small vehicle.

As for that motor! I'd for sure build up some extra support for the motor so not all the weight is being held by the four bolts on the transmission. Why such a huge heavy motor? How are you going to charge the thing? What are you going to use for a DC DC converter? Where are you going to sit? Where is your passenger going to sit? Where is your controller going to fit? Buggies are small. Most are shortened by 14 inches and that is precious space for batteries. Not much room to successfully hang that many batteries safely.

Pete :)

· Registered
4 Posts
Discussion Starter · #3 ·
As far as batteries go I'm not really going for range as it is purely a prototype / fun car. Even 10km is more than enough for driving around a track!

I'm fine with the motor weight for now. I don't need a DC/DC converter, as I need to only power my DSP board which I can tap off some batteries.

I plan on sitting in the driver seat... Passenger in the passenger seat :)

My controller is just a DSP board and an inverter, and can easily fit in the back. And yes the chassis is shortened but still plenty of space.

Remember its a beach buggy, not a road legal luxury sedan! Weight is inevitable and I'm fine with lead-acids for now.

Thanks for the reply - any ideas on the other issues?

· Registered
5,007 Posts
Welcome to the forum:)

I would be a little concerned with the weight even if you don't plan on taking it on public roads. How much will that entire battery weigh? For only 10km of range you could probably get away with less than half the battery pack and the weight savings will improve over all efficiency.

So you ended up buying the Infineon HybridPack1? Any reason you chose not to use it on your car?

I can't really help with the technical aspects of the controller (If you read my thread, you likely know this already:p) but there are others that should be able to offer some advice.

· Registered
4 Posts
Discussion Starter · #5 ·
Thanks! For lead-acid I'd say the weight will be about 140kg maximum. I'm in academics at the moment (this is a side project for fun, to demonstrate some motor control algorithms I've been looking into) The HybridPack was a sample from work, and it has an Infineon TriCore DSP chip on it. I've been doing all my programming on a TI DSP so I guess I just stuck with it.
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