Twin 9 inch vs twin 11 inch

Post by **gholm** » Sat, 10 May 2014, 04:04

Apologies for the noob question but I can't help but wonder why the ev drag racing fraternity seem to prefer the twin 9 inch motors in their DC racing setups to the clearly more powerful 11 inch versions.
Is there some unseen advantage that I've missed?
I would have thought the dual 11" kostov or netgains would be hard to beat under a 2k Zilla or Shiva, no?

Post by **BigMouse** » Sat, 10 May 2014, 06:32

The drag cars have enough trouble putting down the torque from the twin 9" motors! Twin 11" ones would be nuts if you had the controllers and batteries to feed them. Racers always find a way though.

Maybe it's a power/weight ratio thing?

Post by **antiscab** » Sun, 11 May 2014, 06:08

Both the 9s and 11s are power limited by the commutator - they develop the same power, it's just the 9's peak at a higher rpm

both use the same windings, both 200A continuous, both have the same I2R rating (how long you can over current before stuff gets too hot)

the extra mass of the 11s is dead weight, it only gets you more torque and lower rpm. useful if your gearing matches it, but in a racing application you swap the gearing for what you want anyway

Post by **gholm** » Sun, 11 May 2014, 17:47

Interesting. I imagined the 11" motors would have a higher I2R rating owing to their larger thermal mass, but if what you're saying is true, then using the 9inch versions makes total sense.
Thanks all.

Post by **Richo** » Wed, 14 May 2014, 20:52

Does anyone have an idea what peak Nm and kW twin 9" produces?

Post by **Johny** » Wed, 14 May 2014, 21:04

No but I have a photo of a couple of siamese motors that John Wayland sent me when I was tossing up DC vs AC.

Sorry - doesn't help but at least we get an image on this page

Post by **zeva** » Wed, 14 May 2014, 22:28

Some figures for you: John Wayland's White Zombie has a claimed 1050Nm of torque from the twin motors at 2000A. Pretty sure he has ADC motors, not sure if it's the 8" or the 9".

Mike Willmon's Ford Pinto conversion uses dual 9s with a Z2K controller and gets "over 1000Nm".

My RX7 uses dual Impulse 9s so would probably only manage about 750Nm. I should dyno it one day.

From memory, the BMW conversion EV West built for the Pikes Peak Hill Climb uses dual 11s and a Shiva controller.. likely to be the reigning torque king!

EDIT: As far as power goes, with twin motors you'll almost always be limited more by what the batteries can deliver than what the motors can handle, but twin Warp9s should be OK up to about 400kW (briefly) at 5000rpm.