Direct Drive - any advice? - DIY Electric Car Forums
Go Back  

DIY Electric Car Forums > EV Conversions and Builds > All EV Conversions and Builds

Register Blogs FAQ Members List Social Groups Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Display Modes
  #1  
Old 06-06-2017, 10:15 PM
slloyd slloyd is offline
Junior Member
 
Join Date: Jun 2017
Posts: 14
slloyd is on a distinguished road
Default Direct Drive - any advice?

hi all, i'm in early planning stage (finally!) for an EV build. i'm trying to find good info on how to do the mechanics for a direct drive. any build diaries? more detail and dumbing down the better.

i like direct drive because its simple. i have access to a machine shop (6' long lathe, bridgeport mill, forge, etc.) so will make whatever parts i need. i'd prefer to make a part using shop or send 3D CAD drawing to a fab shop if need be rather than pull from a donor car.

i am planning a KIT car build, ~1500lbs, on relatively flat ground. not too interested in highway driving so it if can meet that speed limit great, if not, no big deal. mostly low mileage city driving, on sundays. rewinding my own motor to get voltage/current desired as per IvansGarage methods. have my own 3-phase AC controller high performance drive.

ps. mizlplix, i beleive in your powerglide discussion.. you convinced me! but i need to keep this simple and cheap. so i was thinking direct drive would be better.. unless for sure i am going in wrong direction, then please let me know!
Reply With Quote
Sponsored Links
Advertisement
 
  #2  
Old 06-06-2017, 11:34 PM
Duncan Duncan is offline
Administrator
 
Join Date: Dec 2008
Location: Southland New Zealand
Posts: 4,512
Duncan will become famous soon enough
Default Re: Direct Drive - any advice?

Hi
Direct drive as in the wheel rpm is the same as the motor rpm is silly
The reason is that (roughly) torque is proportional to weight so you need a very heavy motor roar a lot of torque

If you are thinking about direct drive to the diff - which gives about a 4:1 speed reduction then that is sensible

I'm using a Subaru diff with 4.1:1 - and it works very well

http://www.diyelectriccar.com/forums...ghlight=duncan
Reply With Quote
  #3  
Old 06-07-2017, 01:52 AM
brian_ brian_ is offline
Senior Member
 
Join Date: Feb 2017
Posts: 264
brian_ is on a distinguished road
Default Re: Direct Drive - any advice?

Along the same lines as Duncan's comments...
What do you mean by "direct drive"? Essentially no one connects the motor directly to a wheel. The closest would be driving a wheel with no speed reduction (no gears, no chain), and it would be very strange to do that with a differential so it would likely be one-motor-per-wheel. A motor for each wheel is fine, but running motors at wheel speed makes no sense.

What most people in the DIY EV discussions seem to mean by "direct drive" is either single-ratio (instead of a multi-speed transmission), or without using the original transmission in a conversion (which amount to the same thing in most cases).

Most commercially produced EVs are single-speed... but they also use permanent magnet AC motors, which have the broadest power band of any motor type. Tesla uses induction motors with their single-speed system, but even induction motors have a much better speed range than brushed DC motors (and Tesla's motors are presumably designed with acceptable high speed operation in mind). I suggest having a hard look at how much power you need at what road speed, and what that means for required gearing. Keep in mind that those commercially-produced EV motors are good to much higher rotational speeds than most people are willing or able to run their DIY motors.
Reply With Quote
 
  #4  
Old 06-07-2017, 02:29 AM
brian_ brian_ is offline
Senior Member
 
Join Date: Feb 2017
Posts: 264
brian_ is on a distinguished road
Default Re: Direct Drive - any advice?

Quote:
Originally Posted by Duncan View Post
If you are thinking about direct drive to the diff - which gives about a 4:1 speed reduction then that is sensible

I'm using a Subaru diff with 4.1:1 - and it works very well
It can, and is brilliantly straightforward, although I suspect that for most applications more reduction would be desirable. Fortunately, higher reduction is available for some final drive units.

It would be nice to eliminate the heavy and bulky transmission from most of the conversion projects that I've seen in this forum. In the "conventional" (front longitudinal engine / rear wheel drive) layouts, the motor usually takes much of the engine compartment, leaving not enough room for battery packs, and battery mass carried too far forward. It would be nice to tuck the engine in the transmission space; unfortunately, the motor which works well with only the final drive reduction may not fit there well. Duncan's big motor (11 inches in diameter and weighing 102 kg) is in the transmission location, allowing the battery pack to fit properly in the usual engine space from the front axle line back, but this is in a custom frame. It would be even nicer to mount the motor directly to the input of the final drive, but that has even greater fit problems for most vehicles.

One issue I have with this motor-to-shaft-to-final-drive idea: a typical final drive has a hypoid ring-and-pinion gearset. This design works fine, but wastes a few percent of the power transmitted due to friction, compared to a non-hypoid bevel gear set. That's acceptable in most vehicles, but a few percent mechanical loss means a few percent less power available to the wheels, and a few percent less range... both issues for most EVs. As far as I know, there is no commercially produced EV with a hypoid final drive.
Reply With Quote
  #5  
Old 06-07-2017, 05:27 PM
slloyd slloyd is offline
Junior Member
 
Join Date: Jun 2017
Posts: 14
slloyd is on a distinguished road
Default Re: Direct Drive - any advice?

wow, thanks for the responses. yes i mean direct drive to differential. no transmission. i'm not concerned for a couple of percentage loss in efficiency due to gear type as i'm not trying to optimize the design i'm trying to simplify it while still getting reasonable to good results.

thanks for the direct drive link duncan, and confidence that this could work! Electric lotus 7 was exactly what i had in mind too!! i'm going to read your build log in high detail now

Last edited by slloyd; 06-07-2017 at 08:50 PM.
Reply With Quote
  #6  
Old 06-07-2017, 05:33 PM
dcb dcb is offline
Senior Member
 
Join Date: Dec 2009
Posts: 1,930
dcb is on a distinguished road
Default Re: Direct Drive - any advice?

Quote:
Originally Posted by brian_ View Post
but they also use permanent magnet AC motors, which have the broadest power band of any motor type. Tesla uses induction motors
what?!? As soon as the pm makes peak power you have to start throwing even more power at it to maintain that power output (because you have to cancel the PM field), induction wins "broadest powerband" by a wide margin..


Last edited by dcb; 06-07-2017 at 05:40 PM.
Reply With Quote
  #7  
Old 06-07-2017, 06:20 PM
brian_ brian_ is offline
Senior Member
 
Join Date: Feb 2017
Posts: 264
brian_ is on a distinguished road
Default Re: Direct Drive - any advice?

Quote:
Originally Posted by dcb View Post
what?!? As soon as the pm makes peak power you have to start throwing even more power at it to maintain that power (because you have to cancel the PM field), induction wins "broadest powerband" by a wide margin..

In addition to obviously being conceptual sketches rather than real data, neither of those traces look anything like the AC PM characteristics that I have seen, published by manufacturers of both motors and vehicles. More typically, what I am seeing is:
  • Synchronous three=phase AC PM: roughly constant torque from stall to a point, then roughly constant power from that point up to the maximum operating speed - these are the characteristics shown in the MotoredBikes.com post for an induction motor
  • Three-phase AC induction: roughly constant torque from stall to a point, then torque dropping rapidly enough that power drops steadily from that point up to the maximum operating speed

That graphic apparently came from a post in the AC Induction Motor Ebike Project thread in MotoredBikes.com. I had not heard of motor-assisted bicycles using three-phase AC motors with permanent magnet rotors, so although I'll admit to not having read dozens of posts of graphs posted without sources and not based on measured data, my guess is that the comparison is to inexpensive brushless PM DC motors... not to motors as used in common car-sized EVs. One hint is that the "PM" motor output drops to zero at a speed (2988 rpm) which is far too low for any production EV - that speed would be 28 mph in a Leaf, while a real Leaf runs its AC PM motor up to about 10,000 rpm (and so over 90 mph), producing near its peak (80 kW) power from below 3000 rpm to over 9000 rpm, and hits peak efficiency around 6000 rpm at full load.

The Leaf's AC PM motor:

(This shows only torque, but mechanical power is just the product of torque and speed: 280 N-m @ 2700 rpm, 75 N-m @ 9900 rpm, and all the points between are all 80 kW)

Of course, a new motor for a Leaf (or Volt, or Bolt, or whatever) probably costs as much as most people are willing to put into an entire DIY EV conversion.

Last edited by brian_; 06-07-2017 at 06:24 PM.
Reply With Quote
  #8  
Old 06-07-2017, 06:39 PM
slloyd slloyd is offline
Junior Member
 
Join Date: Jun 2017
Posts: 14
slloyd is on a distinguished road
Default Re: Direct Drive - any advice?

i'll be using 3-phase induction motor. the reason is becausee every major city has motor rewind shop, at least one, and they have oodles of motors in their back room on the shelf that are ready for scrap which they willing to give you for next to nothing. then you rewind it for the voltage/current level you want, and poof, basically new motor for good price!
Reply With Quote
  #9  
Old 06-07-2017, 07:09 PM
dcb dcb is offline
Senior Member
 
Join Date: Dec 2009
Posts: 1,930
dcb is on a distinguished road
Default Re: Direct Drive - any advice?

Quote:
Originally Posted by brian_ View Post
The Leaf's AC PM motor:
The leaf isn't entirely a PM motor, it is part switched reluctance. Maybe if you read more and typed less?
Reply With Quote
  #10  
Old 06-07-2017, 10:42 PM
slloyd slloyd is offline
Junior Member
 
Join Date: Jun 2017
Posts: 14
slloyd is on a distinguished road
Default Re: Direct Drive - any advice?

well, i ran a search for thread title "direct drive" and read every one. i did not see a detailed build showing how this is all done mechanically. i'm picturing motor and yolk as one unit (like this bad boy http://www.evsource.com/tls_transwarp9.php). then to drive shaft. other end of drive shaft connects direct to rear differential with a ratio hopefully close enough around 4:1 or higher. so basically you scour the scrap yard for a few different cars that you know have rear wheel drive with a ratio you want and you grab that rear axle with its differential and grab also the drive shaft. might be you have to change length of drive shaft but that can be done. i'm assuming the rear axle will magically fit perfect in the lotus 7...

Duncan, any chance you have more detailed build diary somewhere with a bazzilon pictures?

Last edited by slloyd; 06-07-2017 at 11:06 PM.
Reply With Quote
Reply

Share or Bookmark this

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

 
Support DIY Electric Car
Sponsors

All times are GMT -6. The time now is 08:22 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Ad Management by RedTyger