# Motor is the weak link?



## Bags (Jun 20, 2010)

What is my best choice for a motor? I am gradually realizing that the motor is the weak link … batteries and controllers are capable of giving “enough” power, but motors can’t take that much, for very long. OK, I know I’m being vague here, so here are the specs:

. Approx 6KWh battery pack. Assume A123 20Ah cells, 32s3p or 96s1p. Maybe 25% bigger if necessary.
. 1000+ amp controller, or Sol. Jr.
. 850 lbs total weight, including driver and everything except the motor (no typo, 850 lbs)
. Single gear (no shifting), but gear ratio can be anything
. (19.5” diameter tires, if you’re calculating)
. Great acceleration (25 ft/sec^2) out to 70-75 mph (only 80 mph top speed).

… and do it for 45-50 seconds, 6 times within 90 minutes. (That is, a good chunk of the 45-50 seconds, not continuous for that long.)

… and be 99.0% likely not to burn up.

What motor will make that possible?


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## Ziggythewiz (May 16, 2010)

So you want 25 ft/sec^2 up to 70 mph or for 45-50 seconds? I think there's a bit of a difference between those two.

Most people get great acceleration from a Warp 9 or two. I only have a 6.7" motor and it's far from the weak link being the only big item in my car not fully utilized.


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## Bags (Jun 20, 2010)

25 ft/sec^2 "on tap" at any speed up to 70-75 mph. But utilizing that level of acceleration for, say, 40% of the 45-50 seconds, a lower level of power for another 40% (maintaining speed), and no power for the remaining 20% (braking).

Tell me about your vehicle?


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## Ziggythewiz (May 16, 2010)

So...you just need to get to 300 mph and maintain it? 25 ft/sec^2 means 0-60 in under 4 and over 300 in 18 seconds. You don't need that kind of performance from your motor, battery, or controller.

I have a slug bug with 1000 lbs of lead. It was pretty zippy with only 600 lbs but with the added weight it's pretty average. Still beats anything around up to 30 mph (I only use 2nd).

In a super-light car a 9" motor should be great for anything off the track. The hard part would be finding the right gear ratio to get you the performance you want all the way up to 70 without changing gears.


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## Ace_bridger (Nov 22, 2011)

I would agree with Zig.

The weak link has always been the batteries but from what I saw on EVTV last night that may be able to change with CALB's new lithium. Jack demonstrated 20C current draw with ease.

So, with 100Ah pack that 2000A. Get these batteries, a 2000A controller and an 11 inch motor or two...put them all in a light weight car and you'll have as much performance as you can handle...the weak point is now the human and how big their genitalia are!!


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## Bags (Jun 20, 2010)

This will be on “the track”, for autocross (so in the parking lot, not literally on the track). 

And yes, 0-60mph in under 4 seconds. Then brake down to 40, then up to 65, etc. Max speed of 80 mph, but more likely 70 … this human can’t move the steering wheel fast enough.

20C for the new CALB’s? Really? I hadn’t seen that; can you point me at it?

9” motor “or two” … 11” motor “or two” … that was the essence of my question. Others have posted that a 9” motor will burn up in a few seconds at 1000A.


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## somanywelps (Jan 25, 2012)

Ace_bridger said:


> The weak link has always been the batteries but from what I saw on EVTV last night that may be able to change with CALB's new lithium. Jack demonstrated 20C current draw with ease.


I only saw him do 5.5C and 12C two weeks ago, but yeah, 20C should be fine.


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## Ziggythewiz (May 16, 2010)

Are you ever really on the throttle for 4 seconds? I thought autocross was usually tighter than that.

Check out the performance forum. I think John Metric has posted data from his Texas mile run which should be more relevant to you than a typical 1/4 mile run.


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## Bags (Jun 20, 2010)

No, probably not for a full 4 seconds, but certainly enough to go 0-50ish.

So that makes the "Texas mile" comparison somewhat apples-and-oranges.

What about specific motor recommendations? If I only have a 96V pack, then maybe the Kostov 9" 96V? Plamen says maybe ... goes to 6000rpm, so that's an advantage for a direct-drive configuration. What if I had a 300-ish volt pack? Kostov 9" 220V? Or do I really need an 11"?


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## PStechPaul (May 1, 2012)

I ran the acceleration on my EV Calculator and it will take about 40kW to accomplish that. Since it is only required for a short time you can probably run the motor at 3x or 4x torque so a 10kW or 15HP motor may suffice. That would be probably about 150-200 lb for a standard 3 phase AC motor. However, the torque on the wheels must be considered. You need 2918 Newtons or 656 pounds, so on 20" wheels that's 547 lb-ft. If you can get 45 HP out of the motor at 1800 RPM, that's 131 lb-ft, so a 5:1 ratio will give you 655 lbft and a maximum wheel RPM of 360 which is 5.25 ft/min or about 214 MPH.

The difficult part will be keeping the total weight down to 800 lb. The driver and the motor will be about 300 lb. 100Ah (360 Wh) batteries are about 8.7lb and you need about 20 of them for 7.2 kWh, so 174 lb. Maybe 26 lb for the controller and other components. So the rest of the vehicle needs to be 300 lb. But if the motor weight is removed you have 450 lb to work with.

Just my rough calculations. Also need to figure in efficiency and stuff...


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## Ziggythewiz (May 16, 2010)

From what I've heard it seems the primary difference between the 11 and 9 is the heat it can soak. For short runs I don't think that'd be much of an issue. You definitely want the voltage though, so K9 220 is probably a good choice. You can always start with one and add another if you still think it's the weak link.

As for the 20C test, I think Ace is referring to the mad science someone did throwing a wrench across a cell. I wouldn't push em past 10C unless you're okay replacing the pack early.


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## ruckus (Apr 15, 2009)

20C Ha!  For a millisecond! What good is that?

While I was impressed enough with the new CALB cells to test them in my next build, I think we have to be careful about throwing out wild numbers.

As 'C' goes up, so does voltage sag. In the end, it may be better to talk about watts. If a 40ah cell sags to 2.6v at 10C (400A) then it is putting out 1kw. If you try to draw more 'C' the cell voltage sags even further and you still end up with 1kw. So nothing gained.


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