# Motor sizes



## joethemechanic (Feb 26, 2009)

Everything I read on the forum reguarding motor sizes refers to them by a mesurement in inches.

I am used to sizing motors by HP or KW.

What part of the motor are you mesuring when you are stating size in inches?

Oh and I am working with forklift motors just in case that means anything. 

So in a KW rating from the  motor plate, about what size would you use to power an S-10? Or how big on a Motorcycle?


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## major (Apr 4, 2008)

joethemechanic said:


> Everything I read on the forum reguarding motor sizes refers to them by a mesurement in inches.
> 
> I am used to sizing motors by HP or KW.
> 
> What part of the motor are you mesuring when you are stating size in inches?


Hi joe,

Diameter of the frame.



> Oh and I am working with forklift motors just in case that means anything.
> 
> So in a KW rating from the motor plate, about what size would you use to power an S-10? Or how big on a Motorcycle?


Wide open subject. Depends on all sorts of things. Take a look at EV calculators and the EV album. Read other threads here on the subject. Motor sizing is up to the guy doing the project and spending the money. You'll get a lot of opinions. I personally stay away from specific recommendations, but will comment on items I see way out of wack.

Regards,

major


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## Greenflight (Sep 13, 2007)

Yup, very suggestive. One way to look at it, to help you get a ballpark, is to convert kw into hp. 746w/1hp.

Since most motors can take surges of *rougly* 5 times their rated power, this is a good number to use. Say you have a 15 kw motor. Multiply that times 5 to get 75 kw. Divide by 746 to get about 100 hp, which is pretty good for most cars.

Since electric motors have such different torque curves, the hp ratings can vary a lot between ICE and electric and still get similar performance. The best way to get a feel for it (this is what I always tell people) is to check out evalbum.com and look at the setups people are using.


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## joethemechanic (Feb 26, 2009)

Well I wasn't looking for anything specific. Although I would like to get a feel for what sizes people are having success with. That way when I do my calculations I know if I missed something if I come up with a number that is way out of the ballpark.

I might want to mention that I have an associates degree in electrical engineering from about a million years ago. I also sold and serviced AC motors and large drives used in water treatment and chemical processing. So I am familiar with working in standard units.

It is just to me, when someone says " nine inch motor",,,,,well to me that is not a standard unit.


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## Georgia Tech (Dec 5, 2008)

joethemechanic said:


> It is just to me, when someone says " nine inch motor",,,,,well to me that is not a standard unit.


Well Really its not...this term really only works for the standrad Advanced DC motors and the Warp motors. With a forklift motor 9 inch motor can be a small part of the story. When talking size there is more to it than Diameter, such as Stack length. Diameter and stack length work together to form the size of a motor. But even with that I have seen some peuny built 9 inchers and there ans some STOUT built nine inchers. With this the wire sizes and comes into play and usually the more the better, again USUALLY that is...


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## joethemechanic (Feb 26, 2009)

Here is an example.

I have a whole pile of series field DC motors from Crown "walk behinds"

The nameplate data is 2.4 KW and they are 1 hour rated. They have 7/8" keyed shafts. They are roughly 6 1/2" by 9"

I also have some from Raymond lifts that are almost identical in dimensions. But they are 3.4 KW

Why, as far as the EV community goes, are you using inches and not KW?


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## major (Apr 4, 2008)

joethemechanic said:


> Why, as far as the EV community goes, are you using inches and not KW?


Why in the gas car community do they use cubic inches instead of HP? It is just the way things are done and the way they are sold. But get into the production EVs or HEVs and you see no mention of motor diameter, just kW ratings.

Somewhat relates to my nameplate theory. Especially relevant to electric motors as they are used in EVs. Those that live by the nameplate, die by the nameplate. 

Regards,

major


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## MalcolmB (Jun 10, 2008)

joethemechanic said:


> Why, as far as the EV community goes, are you using inches and not KW?


The main reason seems to be that the motors we use are mostly taken from other applications where they are under-rated to guarantee long service. We tend to run them at roughly two to three times their rated voltage (or even more), which immediately increases power output by a factor of two or three. We also run at them at two or three times their rated (continuous) current, which increases power output by another factor of two or three. The overall result is that a motor with a nameplate rating of 5 kw could have a peak output of 20 to 45 kW in an electric conversion depending on how brave (or foolhardy) the builder is. In other words the nameplate power rating really only applies to the original intended application of the motor. I've not come across any standard for rating motors for EV use yet. 

Other things being equal, the diameter of a motor gives a useful indication of the torque it can develop. The mass of a motor is also a useful indicator of suitability for an EV as it reflects the amount of copper in the windings and the ability to soak up heat without something melting. I'd definitely recommend reading through Jim Husted's thread on choosing a motor (look for Hi Torque Electric in the Motors section) to get a feel for what's important, as there are several other factors to consider apart from size


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