Shift gears, or direct drive?

http://www.cafeelectricpress.com/blog/?p=22

02 Aug 2007 10:30 pm
Lately I’ve realized that some people don’t realize the excessive strain that running with a single ratio puts on a DC motor and controller. To help clarify things I will copy a few old posts I sent to the EV list to give you all something to think about. Please look these over before choosing to run a vehicle without a shifting transmission.

First, the general info from 3/3/2004:
Unless the car is very light, a Z1K will not be enough controller to do direct drive without overheating. Also it will be slower.

I often drive the 914 on a single Z1K in one gear. Performance is weak and would get much better if I shifted. Also, it gets too hot climbing hills.

Build a strong controller and everyone is always trying to push the limits! I guess that’s the nature of the beast. :-)

For single ratio vehicles over 2000 lbs, a Z2K is a better choice (yes, and much more expensive). Under 2000 lbs. it really depends on many issues.

In general, running single ratio on a car requires twice the controller and twice the motor for normal driving. If you are already planning on two motors and 2K amps of controller, so it is already way overpowered in cruise mode and you can drive your batteries to the max at any time, then there may be benefits to running direct drive. In general, I suggest keeping the transmission in a DC car. You get more performance for much less money.

And some more detail was found on 12/17/2001

Subject: Transmission or direct drive?

There’s one situation that hasn’t been discussed much here which I think is very important to consider before committing to direct drive for street use. That is climbing hills at slow speeds.

Some of you are familiar with Page Mill road. It’s one of my favorite scenic drives for a EV. This road climbs about 1/2 mile vertical in 8 miles of road. So the average grade is 6.25%. Traffic often flows at 20 to 25 mph on this windy road.
This poses a real challenge for a direct drive setup.

When I use the EV Calculator at: http://www.geocities.com/hempev/EVCalculator.html
I find that fourth gear is the gear that I would use as a direct drive setup. If I push the rpm a bit higher than I like to I can hit 100 mph in that gear. I certainly wouldn’t want a lower gear (higher numerical ratio) because I wouldn’t hit the top end that I need.
In this gear, about a 4.11 overall, it takes just a bit under 60 ft/lb at the motors to climb this hill.
If this were a single 8″ motor car, 60 ft/lb would be about 380 motor amps and the motor would only last a couple minutes before overheating.
Fortunately I have two motors. Still 30 ft/lb draws 230 amps in each motor, and the motors might do that for about 20 minutes before overheating, which is just about how long it takes to get to the top of the hill. Add to that the fact that a slow turning motor doesn’t cool very well because the fan is turning so slowly and it’s getting pretty close to dangerous.

Of course, a big external fan could make this safer, but I just wanted to point out that direct drive systems require awfully big motors to be reliable in street use since it can involve going uphill at slow speeds.

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That’s it, I hope this will help some of you from going down a road fraught with overheated motors and controllers. When in doubt, keep that transmission in the loop!

-Otmar

4 Responses to “Shift gears, or direct drive?”

1.
on 21 Aug 2007 at 11:19 am Tom Reed

Look out Zilla, we are making better controller at lower price.
we have technology that will bring your price down.
most controller 120v/600 amps cost about $150 to build. STOP charging so much for your controller. your parts cost about $300.00 to build.

Tom Reed.
2.
on 01 Oct 2007 at 7:49 pm Matt

Otmar,

Can you say a little about AC versus DC controllers, and why AC controllers are so much more expensive? I realize there running an AC motor efficiently is a much more complex process, but is there an intrinsically higher cost, or is a company such as AC Propulsion charging based on their development and engineering time?
3.
on 20 Jan 2008 at 4:48 am Otmar

Hi Tom,
I wish you luck with it.
I think you will find that your beliefs don’t match reality. On the other hand if you can make a cheaper controller that’s reliable and performs as people want, then that is the name of the game, isn’t it?
If you do pull that off, then maybe I can shut down and take some much desired time off. :)
Looking forward to hearing of your success!
-Otmar
4.
on 21 Jan 2008 at 9:08 pm Otmar

Matt,
That is a subject that has been discussed at length on the EV discussion list. Try searching for AC vs DC, or AC DC Flame wars. I will try to summarize my view for and put it in the new FAQ for you.
-Otmar