Best motor/control technology for and electric vehicle

Thread Starter

strantor

Joined Oct 3, 2010
5,291
First off, I have no intentions of modifying my car; this thread is about theory/opinion.

So, I have been reading over the past few years bits and peices about EVs and hybrids. At some point I want to make an EV; probably just a go-kart or something to start. I was wondering what you folks opinions are regarding the best type of technology for an EV. Most popular kits are based on DC motors with a DC controller and voltages around 120V. The most popular hybrid, the prius, uses a 3phase AC motor. My allegiance is mostly with 3phase AC but the big problem there is getting the high voltages needed. DC is simpler. Anyone want to put in their 2c?
 

#12

Joined Nov 30, 2010
18,210
Considering the smaller size of a go-cart, PWM is what hits my mind first, just because it's simpler. Purely a personal bias.
 

PackratKing

Joined Jul 13, 2008
847
I share interest in the project, tho' would lean toward higher voltage DC motors found in aircraft applications.

I thought also of treadmill motors, which are quite powerful, though drawing 90 ± VDC [ non PWM * ] off series/paralell batteries to achieve voltage and current potential, introduces a unique set of problems with cartage weight and charging of adequate batteries.
Not to mention control components to handle that kind of draw........could be acheived w/ v-reg pass transistors......or .......would scooter-chair companies be a viable source of control parts ?

10,000 questions............:D

* what [ would be the ] the advantage of PWM ?
 

#12

Joined Nov 30, 2010
18,210
PWM is a method of varying the applied power. It can be done very accurately and give very good control when running at partial loads.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,291
Considering the smaller size of a go-cart, PWM is what hits my mind first, just because it's simpler. Purely a personal bias.
If the subject vehicle was larger than a go-cart, say, an offroad vehicle like a competiton enduro truck, what say you? still PWM? (yes I realize that's a bad example; those things have to fill up with gas along the way)

I share interest in the project, tho' would lean toward higher voltage DC motors found in aircraft applications.

I thought also of treadmill motors, which are quite powerful, though drawing 90 ± VDC [ non PWM * ] off series/paralell batteries to achieve voltage and current potential, introduces a unique set of problems with cartage weight and charging of adequate batteries.
Not to mention control components to handle that kind of draw........could be acheived w/ v-reg pass transistors......or .......would scooter-chair companies be a viable source of control parts ?

10,000 questions............:D

* what [ would be the ] the advantage of PWM ?
I will have to look more into these aircraft DC motors. most aircraft motors I have read about are 400hz AC.

I'm thinking a little more balls than what a treadmill or scooter controller could handle. If I were going to build a go-kart I would want it to be on par with the competiton gokarts that go 80mph+ and accelerate fast enough to snap your neck without support.
 

tom66

Joined May 9, 2009
2,595
Another good reason to use three phase is the motor can be smaller, cheaper and lighter. Having less copper also slightly improves the efficiency. However saving money on the motor can be offset by the fact that it costs more to drive the motor.

The Prius has a battery-to-wheel efficiency exceeding 99%, which is impressive. Still, as it uses 70kW+ motors, the driving circuit needs cooling.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,291
Another good reason to use three phase is the motor can be smaller, cheaper and lighter. Having less copper also slightly improves the efficiency. However saving money on the motor can be offset by the fact that it costs more to drive the motor.

The Prius has a battery-to-wheel efficiency exceeding 99%, which is impressive. Still, as it uses 70kW+ motors, the driving circuit needs cooling.
Really? Wherever you read that number, I bet they were factoring in regen braking. Even then the number seems a bit high. If it were to run @ >99% efficiency I would think it would generate nearly zero heat, in which case the driving circuit wouldn't need cooling.

So, what we have so far:
3phase AC
pros: efficiency
cons: batteries, cost

DC:
Pros: simple(r)
cons: efficiency
 

tom66

Joined May 9, 2009
2,595
Really? Wherever you read that number, I bet they were factoring in regen braking. Even then the number seems a bit high. If it were to run @ >99% efficiency I would think it would generate nearly zero heat, in which case the driving circuit wouldn't need cooling.

So, what we have so far:
3phase AC
pros: efficiency
cons: batteries, cost

DC:
Pros: simple(r)
cons: efficiency
1% of 70kW is 700W. That's like having a room heater in the engine compartment. So it is liquid cooled.

You're right - I was only looking at converter efficiency. The motor is 85-95%, depending on torque/RPM; the converter is ~98%. Here's a report on it: http://www.osti.gov/bridge/servlets/purl/890029-WIfqPO/890029.PDF
 

THE_RB

Joined Feb 11, 2008
5,438
A DC brushed motor (series wound) is close to unbeatable for traction as it's magnetic field increases with load. Especially in a "go cart" type design where cost is also an issue I would go with PWM and DC series motor.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,291
A DC brushed motor (series wound) is close to unbeatable for traction as it's magnetic field increases with load. Especially in a "go cart" type design where cost is also an issue I would go with PWM and DC series motor.
RB I've seen your posts on the mobility scooter so I know you know your stuff when it comes to this. What's you opinion about scaling it up? Say if the application were a full size car, would your opinion remain the same?
 

shortbus

Joined Sep 30, 2009
7,761
To me a three phase motor and a VFD without the AC input(front section) would be ideal for a full size vehicle. Using a battery bank and a gen set to recharge batteries when needed would round out the combo.

A car basically only uses around 20 -30 horsepower while cruising so any more is just bragging rights. One thing I would also include in the drive train is a planetary type transmission, some thing like a single stage Lenco transmission. This would allow torque multiplication for starting the car moving and allow keeping the motor in the best RPM range, instead of over driving the VFD frequency.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,291
Shortbus,
That's exactly how I felt but after looking at the price of batteries to achieve the voltage needed, it gets expensive quick. If you were going to go with 480VAC then you would need to feed 650-700VDC into the inverter DC bus. that's a crazy battery bank. I guess you could go with a 230V motor but it's hard to find a 230V inverter for that high power. Also hard to ruggedize an inverter for automobile use.
anyways, That's the conclusions That I've made, which is why I posted.
 

GetDeviceInfo

Joined Jun 7, 2009
1,729
First off, I have no intentions of modifying my car; this thread is about theory/opinion.

So, I have been reading over the past few years bits and peices about EVs and hybrids. At some point I want to make an EV; probably just a go-kart or something to start. I was wondering what you folks opinions are regarding the best type of technology for an EV. Most popular kits are based on DC motors with a DC controller and voltages around 120V. The most popular hybrid, the prius, uses a 3phase AC motor. My allegiance is mostly with 3phase AC but the big problem there is getting the high voltages needed. DC is simpler. Anyone want to put in their 2c?
The 'best' for someone tinkering with a gokart is going to be a lot different than a team building the future automobile.
I personally believe the brushless PM motors with high power densities will win the day, and are in thier infancy as applied to automotives. High pole count with skip frequencies combined with multi phase (not necessarily 3).

For your go kart, a starter motor and a stack of batteries with PWM btween, and your hair will be flying. Costs about right as well.
 

CraigHB

Joined Aug 12, 2011
127
I'd also like to play around with an electric vehicle at some point. Not thinking in terms of a regular 1-1/2 ton car, more like a street legal mini-car around a half ton.

I was thinking in terms of a smaller high rpm BLDC motor geared down on each wheel, possibly using a rare earth rotor.

How feasable is a BLDC motor with outputs of 20KW and 20K RPM?
 

THE_RB

Joined Feb 11, 2008
5,438
RB I've seen your posts on the mobility scooter so I know you know your stuff when it comes to this. What's you opinion about scaling it up? Say if the application were a full size car, would your opinion remain the same?
Well I've only seen under the hood of one full sized electric car, and it used a DC series motor and was a standard electric vehicle kit package from the USA retrofitted to a typical small Australian car.

The thing with traction is that you get can get very high intermittant peak loads and the series motor does that well from a small cheap motor while still allowing reasonable efficiency at cruising. For stop start city use it might be the best all round motor.

I've read a bit on the electric car competitions, they do one here in Australia thousands of miles across straight desert roads, and those cars seem to use very expensive high tech (probably custom built) brushless PM motors. Their goal is cruising efficiency at all costs, and they don't need peak loads like getting up a driveway or over a speedbump etc.

There lots of commercial kits to fit to real cars, it's worth checking them out as I might be 10 years behind the times... ;)
 

tom66

Joined May 9, 2009
2,595
For my model aircraft I use brushless three phase motors and a three phase ESC about the size of a two pence coin capable of driving a 12A motor (at 11.1V, that's 133W, or about 1/6th horsepower.) I use it to run a 6-7A motor which works good.
 

shortbus

Joined Sep 30, 2009
7,761
Shortbus,
That's exactly how I felt but after looking at the price of batteries to achieve the voltage needed, it gets expensive quick. If you were going to go with 480VAC then you would need to feed 650-700VDC into the inverter DC bus. that's a crazy battery bank. I guess you could go with a 230V motor but it's hard to find a 230V inverter for that high power. Also hard to ruggedize an inverter for automobile use.
anyways, That's the conclusions That I've made, which is why I posted.
Heres a new one rated at 30HP ; http://www.ebay.com/itm/30-HP-230V-VECTOR-VFD-VARIABLE-FREQUENCY-DRIVE-NEW-/360127613146

Heres a new motor; http://www.surpluscenter.com/item.asp?item=10-2590&catname=electric

Both of those were just the first ones I found.If there are industrial surplus places around you they probably have used ones for less.
 

Smoke_Maker

Joined Sep 24, 2007
126
Strantor

This is a subject that I'm interested in also, having owned 5 EV myself over the years, the last one was a Mazda RX7, I built it for drag racing and held the NEDRA record for several years for my voltage class.

It would be good to start out with a GoCart or better yet build a mini bike, but be careful once you get the EV bug it will cost you time and money. The easiest way to go is with a DC motor (readily available, easy to control, low voltage), BLDC is a little more challenging (finding or building the motor, building the controller, low to medium voltage), AC motors are as challenge as BLDC in my opinion (lots of motors to chose from and maybe rewind, building a controller or modifying a used one, higher voltage).

Now if I was to build another EV it would be a AC motor, but those days are past me now. I have a deposit down on a Nissan Leaf. I truly think that for city folks EV's are the ticket for cheap transportation (American made fuel, solves city pollution and the teenagers can only get so far away from home before you get the call)
 
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