Oregon Electric Vehicle Association

A Brief Introduction to Motor Sizes

By Jim Husted, Hi-Torque Electric, taken from an EVDL post, March 17, 2007.

In a post to the EVDL, a member asked:

Hi everybody. I've heard mention of 6.7" motors that can be picked up relatively inexpensively. Would something like that be appropriate for a Fiat 500 conversion project? (That's the tiny air-cooled Fiat that looks like a shrinky-dink Beetle, as I bet almost all of you know, but just in case ...)

Jim Husted replied:

First off, when you hear people talking about motor sizes like 6.7, 7, 8, 9, 11, and monster 13's it's in reference to the motors diameter. That really doesn't tell you the whole story being those same diametered motors come in different lengths (not to mention windings).

Motors in the 6.7" sizes are very common and come in hundreds of flavors. They start at about 10" in length using just 4 brushes and weighing about 50 lbs., to motors like the L91-4003 which is something like 15" and uses 8 brushes at around 90 lbs. Being I'm not at the shop I'm just using rough numbers from memory to illustrate that not all motors in a set diameter size will do the same job.

Looking at an electric motor there are a number of things to concider. First is how much workload can it do, then how long can it do it before it heats up to much, and then how long can it do that repeatedly day after day.

For fun, I'm gonna compare a "motor" to "me" for a very simple look at how I see motors and their uses.

I'm a fairly small guy, 150 lbs. (wet, hehe). I can take an empty wheelbarrow and run all over my backyard all day without breaking a sweat, I'm not doing alot of work but my duty cycle is all day, this is like a 13" in a small car.

Now load it with a hundred pounds of dirt and I not only can't push it as fast but I have to stop and take breaks a lot more often as I begin to peter out and get hot, this is more like a 9" in a big car.

Load that wheelbarrow with 2000 lbs. of gold (hey I can dream, hehe) and, well, I'll pop a nut just trying to pick it up, and even if I could lift it I'm not going very far, LMAO! This is like a small 6.7 in a Hummer!

Let's go back to the dirt loaded wheelbarrow. Even though I get tired and hot and have lets say my daily duty cycle of work I do on any given day I can rest and eat and do it again tomorrow. Now after 30 or 40 years moving that sucker around I start to wear out, my back hurts, my muscles aren't as strong (insulation in a motor) and eventualy I burn out and die (where a new dirt hauler throws his load on top of me, LMAO.)

Anyway that's a simple look at motor sizes and both short and long term duty cycles.

So when I hear stuff like "will a 6.7" do "X" job I think, "Well, which one?" The only 6.7 that I'd recommend for a full sized car would be the L91-4003, the others just don't have to mass to do the job. You could wind a slot car motor for 200 volts, don't mean you can throw it into your conversion, hehe.

Even on a 7" motor they really need to be blower cooled to increase their duty cycle for most peoples needs. Marko uses a 7" on his Fiat, "Fiamp" but it gets pretty hot and I wouldn't advise anything smaller.

Anyway if you're off to the scrap yard to hunt up a motor, look for something in that 100lb. + size with 8 brushes and not a smaller palletjack 50lb. 4 brush 6.7" sized motor.


Jim Husted
Hi-Torque Electric

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