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elhigh · 10-20-2015, 09:29 AM

Hi Alex,

Frank is the resident skeptic. Don't worry about him.

First, let me advise you not to tell us about car basics. We understand how transmissions work around here. You wasted some time telling us about the gear ratios in your VW transmission. We're already pretty familiar with stuff like that.

Second, completely throw away the idea of adding an alternator to your EV's electric motor. Just dump it. I will explain:

Assume you have 100 watts worth of energy coming out of your battery to go into your drive motor.

The drive motor is - I'll be optimistic - 90% efficient. (Some are better, but not under all loads at all speeds, and we're talking about a variable load/speed application).

That means your motor can generate 90 watts of output.

Now imagine that 90 watts of power is devoted entirely to turning the alternator.

A very efficient alternator is about 75% efficient. Good luck finding one that good, by the way, they mostly exist as prototypes. 90w x .75 = 67.5 watts.

All of this assumes you are only spinning the motor to run the alternator. That's the only thing we're looking at here.

Now we have 67.5 watts to send back to the drive battery to extend the range of the vehicle.

You pulled 100 watts out of the battery so you could send 67.5 watts back. It's better to just leave the power in the battery for moving the car.

If you wanted to be able to use the alternator for regeneration braking, don't worry about it. That's pretty easy to design in, especially if you're using an AC motor. And if you can't afford an off-the-shelf motor controller, don't worry about that either: Welcome to PaulandSabrinasEVstuff.com Electric Motor Controllers These guys are Ecomodder contributors, and evidently just really avid EV homebuilders. If you can follow instructions and solder you can probably build a pretty good motor controller on a tight budget.

If you need the power to run the usual 12v accessories in the car, don't worry about it. That just takes a DC-DC converter.

As for the transmission, you can choose to leave it in or design a vehicle that doesn't need it. Skip the gear reduction at the wheels (also called "portal" axles), that adds a LOT of weight at the wheel, which makes for a really rough ride. You could strip out all the unessential parts to lighten the transmission, leaving in only the ratio you need, or even choose a really low (numerically higher) final drive ratio and drive the axle directly. That would be the most efficient option; every gear reduction you go through wastes some of the motor's power in friction.

This is part of why the Tesla has a single-speed transmission.

VW Beetles are popular for conversion to EV because they're so common and it's easy to find parts if you need to make changes. With the rapidly improving battery technology you could build one with a decent amount of range, certainly enough for modest daily commuting.

10-20-2015, 09:29 AM	#6 (permalink)
elhigh Master Novice Join Date: Jan 2008 Location: SE USA - East Tennessee Posts: 2,314 Josie - '87 Toyota Pickup 90 day: 29.5 mpg (US) Felicia - '09 Toyota Prius Base 90 day: 50.48 mpg (US) Thanks: 427 Thanked 616 Times in 450 Posts	Hi Alex, Frank is the resident skeptic. Don't worry about him. First, let me advise you not to tell us about car basics. We understand how transmissions work around here. You wasted some time telling us about the gear ratios in your VW transmission. We're already pretty familiar with stuff like that. Second, completely throw away the idea of adding an alternator to your EV's electric motor. Just dump it. I will explain: Assume you have 100 watts worth of energy coming out of your battery to go into your drive motor. The drive motor is - I'll be optimistic - 90% efficient. (Some are better, but not under all loads at all speeds, and we're talking about a variable load/speed application). That means your motor can generate 90 watts of output. Now imagine that 90 watts of power is devoted entirely to turning the alternator. A very efficient alternator is about 75% efficient. Good luck finding one that good, by the way, they mostly exist as prototypes. 90w x .75 = 67.5 watts. All of this assumes you are only spinning the motor to run the alternator. That's the only thing we're looking at here. Now we have 67.5 watts to send back to the drive battery to extend the range of the vehicle. You pulled 100 watts out of the battery so you could send 67.5 watts back. It's better to just leave the power in the battery for moving the car. If you wanted to be able to use the alternator for regeneration braking, don't worry about it. That's pretty easy to design in, especially if you're using an AC motor. And if you can't afford an off-the-shelf motor controller, don't worry about that either: Welcome to PaulandSabrinasEVstuff.com Electric Motor Controllers These guys are Ecomodder contributors, and evidently just really avid EV homebuilders. If you can follow instructions and solder you can probably build a pretty good motor controller on a tight budget. If you need the power to run the usual 12v accessories in the car, don't worry about it. That just takes a DC-DC converter. As for the transmission, you can choose to leave it in or design a vehicle that doesn't need it. Skip the gear reduction at the wheels (also called "portal" axles), that adds a LOT of weight at the wheel, which makes for a really rough ride. You could strip out all the unessential parts to lighten the transmission, leaving in only the ratio you need, or even choose a really low (numerically higher) final drive ratio and drive the axle directly. That would be the most efficient option; every gear reduction you go through wastes some of the motor's power in friction. This is part of why the Tesla has a single-speed transmission. VW Beetles are popular for conversion to EV because they're so common and it's easy to find parts if you need to make changes. With the rapidly improving battery technology you could build one with a decent amount of range, certainly enough for modest daily commuting. __________________ Lead or follow. Either is fine.