# Thread: Ex-7 - Electric RX-7 conversion

1. So...at 100 lbs for 20 hp, then a 100hp motor weighs...500 lbs?! Seriously?!! Geez, no wonder the weight is such a problem on an electric car!

How much does your motor weigh and how much hp does it have?

2. Originally Posted by Bugbyte
So...at 100 lbs for 20 hp, then a 100hp motor weighs...500 lbs?!
That isn't the point - but more or less - YES. From the Baldor motor catalog, for 3 phase motors, a 20HP motor weighs 416 Lbs; and a 100HP motor weighs 1,573 Lbs. Don't compare this to the power / weight ratio of my motor directly. Mine has an aluminum case, is made for automotive (intermittent) duty and is MUCH more expensive. All these trade off on weight. I pulled up a representative example in a catalog to show a simple comparison.

A motor connected directly to a wheel has no gear reduction, so the Torque requirement for the motor is MUCH higher. To get more torque, at low RPMs especially, requires more windings = more copper = more cost and more weight.

When the drive is transmitted through the transmission and the rear end, you get about 14 times greater torque at the wheel because of the gear reduction. 1st gear is 3.475:1 and the rear end is 4.1:1 in the RX-7. You get the complete ratio by multiplying them - 3.475 x 4.1 = 14.2475:1 So the RPM at the wheel is divided by that, and the torque is multiplied by that. The power stays the same since it's the product of torque and speed.

It's torque that gives us the peppy starts we all like. and that's why we have transmissions is a car. Ever try to start out in a car in 4th gear? It doesn't work very well because there's not enough torque at the wheel to accelerate the mass of the car. If you had a 1,000 HP Caterpillar engine, it would pull out in 4th gear (Lots of torque), but for your 150 HP Honda engine you need a gear reduction to accelerate.

Originally Posted by Bugbyte
How much does your motor weigh and how much hp does it have?
My motor weighs 88 Lbs and makes about 40 KW = 53 HP and about 90 Nm = 66 Foot-Lbs of Torque.

I don't know about those 'on the wheel motors'. Maybe they have a planetary gear reduction built into them. But that's my point: 4 motors, 4 gear reducers, 4 times the complexity, 1/4 the reliability. A car with 1 larger motor, 1 transmission, and 1 drive isn't necessarily worse than one with 4 of each. I think it's better.

3. Hmm. So, I'm confused. A non-automotive motor has a hp/weight ratio of
Baldor 20hp/416lbs = 4.8 lbs per hp
Baldor 100hp/1573lbs = 6.3 lbs per hp
Automotive 53hp/88lbs = .60 lbs per hp

So the automotive motor appears to be much more efficient but as you note, much less heavy duty, than the non-auto motors. Is this the reason that you need a gearbox? I'd always heard that electric motors produce maximum torque at the lowest rpms, which is what you want for starts.

I guess the automotive motors don't produce a huge amount of torque -probably because they have fewer windings?

Do you think it is possible the pancake style motors have the same hp to weight ratio as your motor? If so, it might only increase the sprung weight by 50 lbs or so if you replace the brakes with the motors.

4. So the automotive motor appears to be much more efficient but as you note, much less heavy duty, than the non-auto motors. Is this the reason that you need a gearbox? I'd always heard that electric motors produce maximum torque at the lowest rpms, which is what you want for starts.

That's true for DC motors and only for some AC motors. But cars are heavy. It takes a LOT of torque to accelerate them up to 60 in a few seconds or to climb hills. For the industrial motors I referenced, the rated power is at the rated RPM. Most of them are not designed to start under heavy load. Car engines must start under full load.

I guess the automotive motors don't produce a huge amount of torque -probably because they have fewer windings?

Again, there are many trade-offs: Weight, Total Power, Volts, Amps, Torque, RPM Range, Cost, Size, Reliability, Ruggedness. You can't have it all! Oh wait! I just saw a reference on their website to 'coolant temperatures' Yes that's right, coolant piped into each motor! More complexity.

I finally found their datasheet! The motor weighs 70 Lbs! That's not including the wheel & tire.

Do you think it is possible the pancake style motors have the same hp to weight ratio as your motor? If so, it might only increase the sprung weight by 50 lbs or so if you replace the brakes with the motors.

Protean motor website shows a torque curve showing about 400 N-m. My RX-7 motor makes 90 n-M. Multiply that times the gear ratio, and it comes to about 1300 at the wheels - divide it by two wheels and it's 650 / wheel. So the performance is greater than a 2-wheel version and less than a 4-wheel version. Without liquid cooling or 70 extra pounds on each wheel. Yes, with this I would be rid of transmission, driveshaft, differential. But you need a beefier suspension to manage 70 extra pounds at the wheel. So I don't see any weight or power advantage.

So I'm going to ask you a question: Why would anyone want to do that? When there are already efficient & effective drive trains, suspensions, gearboxes, etc., what would the advantage of an 'on the wheel' direct drive system? I can't imagine a single advantage!

5. Originally Posted by no_hazmats

So I'm going to ask you a question: Why would anyone want to do that? When there are already efficient & effective drive trains, suspensions, gearboxes, etc., what would the advantage of an 'on the wheel' direct drive system? I can't imagine a single advantage!
Not to mention the problems with syncing the traction wheels when turning.
When you have a diferential of sorts, the inner wheel spins slower than the outer wheel when turning.
An enging in each drive wheel, needed to sync this in sorts.
If the inner wheel turns faster than is should, you loose grip and spin out.

6. Originally Posted by no_hazmats
So I'm going to ask you a question: Why would anyone want to do that? When there are already efficient & effective drive trains, suspensions, gearboxes, etc., what would the advantage of an 'on the wheel' direct drive system? I can't imagine a single advantage!
Dunno. There's a lot of complexity in both setups. Differentials and gearboxes are sources of inefficiency. But four motors means (I'm guessing) four controllers and it sounds like some kind of cooling, whether passive or active.

Weight seems to be the enemy whether sprung or unsprung. I've been interested in turning my Beetle into an electric when the engine dies but I have to admit I'm a little disappointed in the range you've seen so far on this project. Seems like much of that is related to the weight of the batteries/motors with additional losses due to heat and inefficiencies. Stripping an existing car down doesn't seem like the way to go.

7. Originally Posted by Bugbyte
Dunno. There's a lot of complexity in both setups. Differentials and gearboxes are sources of inefficiency. But four motors means (I'm guessing) four controllers and it sounds like some kind of cooling, whether passive or active.

Weight seems to be the enemy whether sprung or unsprung. I've been interested in turning my Beetle into an electric when the engine dies but I have to admit I'm a little disappointed in the range you've seen so far on this project. Seems like much of that is related to the weight of the batteries/motors with additional losses due to heat and inefficiencies. Stripping an existing car down doesn't seem like the way to go.

The range is because of the sealed lead acid batteries he is using. Heavy, low cycle count, low energy density, but CHEAP!

Spend real money on LiFePo4 batteries, and you can have great range. Lithium is "the way of the future" right now. I will be putting 8kWh-12kWh in my car once I design the controller/manger/charger/balancer for them. Expect to pay \$400 to \$700 per kWh though.

8. Originally Posted by Bugbyte
Differentials and gearboxes are sources of inefficiency.
1.Differentials and gearboxes or here (pdf)
2.Differentials and gearboxes (zip)

9. Originally Posted by aldogrech
Hi,

Thanks for all the contributions to this Blog. I am at the early stages of building what I believe is the first Citroen DS EV (amanoauto.blogspot.com) and reading your Blog has been extremely helpful.

Thanks, Aldo.
I will be looking this porject, it is awsome!

10. I think a big advantage of the dual or four motor pancake style setup IS the ability to actively control wheel speeds. Though, at the same time, it requires 4 inverters instead of just one, and some sophisticated controls to manage each. It WOULD, however, result in a VERY good handling vehicle, with some of the best traction control capabilities possible. Not really that doable at the hobby level though.

Also, the pancake motors don't necessarily NEED gearing to get the torque they need, as they can simply run with more poles to get the same effect. A 26" diameter wheel needs to spin 1100RPM to go 85mph. A motor with that design speed in mind would be well suited to the application, and would require no extra gearing.

As for the protean motors, they've got plenty of torque. Their zero-speed torque hovers around 400Nm - On that same 26" diameter tire, that's 272lbf from each wheel. That should be plenty to get the ball rolling.