I would appreciate comments on a personal project that I am scoping out.

My project is inspired by the rising cost of motor fuel. It seems to me that the economics of a battery-electric car are looking better with every petrol price increase. I plan to build a battery-electric car by fitting out a small passenger car with a 3-phase induction motor, speed drive, and a battery pack.

MOTOR :

A 2-pole 15KW 3-phase induction motor. This will spin out to just short of 3000rpm at 50HZ, I am hoping for a little higher when driven by the VSD. This motor will drive through the original car gearbox, via a primary drive with a drive ratio that gives a speed of 110 Kph at 3000rpm. My mechanical gurus are still debating whether chain or gear drive is likely to be best, but I am assured that either is able to be done.

BATTERY :

The battery pack will consist of about 36 x 12V deep-cycle lead-acid batteries of 35-40 Ah each.
(It would be nice to use lithium batteries but while they are available they are still quite expensive.) The fitting of the batteries will be one of the biggest challenges in this project, I think. A tray across the top part of the engine compartment will hold over half of them, and the rest will go to the rear in the space vacated by the fuel tank.
The batteries will be series-connected for a DC bus of about 400V.

VSD :

The VSD will be supplied from the 400V DC bus, and will drive the motor. I am told that there are already turbine-electric busses on the road that use relatively standard VSDs to drive from a DC supply. As well as providing drive to the motor, it would be good to have regenerative braking too.

Also on board this car will be a switchmode battery charger that I expect I will have to design and build, this will accept a 230V 10A supply as input and will series-charge the batteries while monitoring each battery in the pack and equalising the charging.

If it all comes together, the result should be a car that will do a little more than 100Kph, and will travel 100km on a full charge. Recharging should take about 8 hours from a 230V 10A outlet. It should have a similar level of acceleration to a car with a 1000cc petrol engine.

This is, in brief, what I want to do. Question is, is this a reasonable proposition, or am I overlooking one or more obvious flaws/problems? I would appreciate any wisdom that anyone may care to contribute.

Thanks, and best regards

Mark Monson

very tough project . I wish you good luck.
rolling resistance and wind resistance may cause problems if you've only got 20 hp.
passenger cabin comfort items such as heating and cooling will cause some headaches

just a thought

I think that it is certainly very viable. The yellow buses that cruise around Christchurch City in New Zealand, are powered by a 75KW motor and drive and a large bank of batteries. They charge the batteries over night, plus the bus has a turbine generator which runs on demand to recharge the batteries.

One of the issues to consider, is the safety of the batteries if there is an accident. There is a lot of fault current available and you need to keep that in mind.
You can also use the regen braking to recharge the batteries during deceleration, reducing brake wear and making the charge last longer.

Best regards,

A number of issues to consider -

The battery bank may be of sufficient resistance that regenerative braking is not possible to the degree needed to stop the vehicle with suitable deceleration, this plus safety considerations for a drive fault will require some level of collaborative control between the normal hydraulic brakes and the regenerative braking.

Depending on the breakdown torque rating of the motor - you should be fine over speeding the motor to 200% of rated speed . . . constant torque until you reach maximum bus voltage and then constant power until the decaying breakdown torque catches up with the decaying running torque at higher than synchronous speeds.

I am aware of a small ford pickup (courier) that had a 4-pole 20HP Baldor motor directly coupled to the drive shaft of the truck with deep cycle batteries in the bed wired in series for a 360V DC bus. The Baldor motor was tapped on the 230V leads and was run at speeds to 3600 rpm (65+ mph down hill!) No gears . . . just the driveline. It didn't accelerate very well, but the guy didn't care and he plugged it in only at work and had a 20 mile round trip commute. The drive was a 100Amp rated inverter that was sinewave commutated from a Delta Tau PMAC (closed loop vector control) and was programmed to provide elevated flux levels at low speed for better torque at stall - also allowed plenty of computational HP and interfaces for a display, current draw, bus voltage levels, etc.

I would say that a PM motor would be more efficient I would think - the draw back being that faster than synchronous speeds might be a challenge unless you implement a BEMF suppression routine.

Sounds like you have a fun project.

Thanks for the input, KBrown.

As far as braking goes, I had thought that at least initially I would set up limited regenerative braking when foot comes off accelerator, but to leave the existing braking systems separate and unmodified. As well as being the simplest to set up, this also avoids any difficulties convincing the vehicle certifiers that the braking system is safe and up to the job.

It will be interesting to see just how fast the motor can be spun up to, but without wishing to pooh-pooh the advice of someone who probably knows better than I, surely 200% is pushing it just a bit? I was hoping that a 3000rpm 2-pole motor might get up to 4000rpm.

I am picking that by retaining the clutch and manual transmission, the acceleration should be quite reasonable, especially if I can afford some of the you-beaut lithium batteries. I calculate that lead-acid batteries will contribute a bit over 400kg to the weight of the car, but lithium would weigh less than half that.

Believe me, if suitable DC motors were available, I would cheerfully consider them, but as least in this part of the world they are pretty much unobtainable new. 3-phase induction motors though, are readily available and at very reasonable prices, and speed drives likewise. I did want to build something that could be repeated, using common, off-the-shelf componentry.

Hi Mark,

I am a motor rewinder repairer from Christchurch New Zealand
I have been down your path...even rewound three phase motors for 12 - 24V controllers for other peoples uni projects etc
I was just about to put together an electric motorbike based on my research when Myself and wife had a baby boy. My savings went into the mortgage and I'm not likely to find funds for my would-be project now. What I have for you is this.


http://www.pmlflightlink.com/motors/wheelmotors.html

http://www.whispergen.com/


Forget Batteries
Forget Fuel cells
Forget Tesla designed motors

and for gods sake forget hybrids. They are an abomination.

Clayton Martini

PS K.I.S