# Utility Vehicle motor sizing



## pbiebach (Jun 29, 2010)

I'm trying to figure out what size motor would be suitable for a EV conversion of a light utility vehicle (Kawasaki mule diesel) 
ICE specs:
~20 hp
~40 ft-lbs torque

I'm not trying to make a racing machine here, just want comparable performance to ICE.

I want to keep costs reasonably low. 

Im thinking I need a motor with approx the following stats
~8 hp cont, ~20 hp Peak
~ 40 ft-lbs torque at maximum 

Does this make sense also specific motor suggestions would be much appreciated.


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## major (Apr 4, 2008)

pbiebach said:


> I'm trying to figure out what size motor would be suitable for a EV conversion of a light utility vehicle (Kawasaki mule diesel)
> ICE specs:
> ~20 hp
> ~40 ft-lbs torque
> ...


Hi pbi,

There is a lot of application unknowns here  How does your vehicle and expectations compare to a golf cart or those burden carriers like Cushman or Taylor Dunn electrics? Those run 36 or 48 volt systems with 6.6 inch diameter motors. Take a look at that.

Also, on continuous power ratings for electric motors, it makes a big difference on the ventilation factor. If you have a real dirty application and need a totally enclosed motor, expect the continuous power rating to be half or a third of the same size motor allowed to be ventilated fan cooled. This does not affect the peak rating, but would reduce the frequency of overloads.

Off hand, unless your application is unusual, I don't see the need to match the ICE peak power. In other words, a 3.5 hp continuous rated totally enclosed electric motor with a peak of 12 to 15 hp might be able to fill your bill.

_Reasonably low cost _is pretty wide open. But I'd look at 6.6 and 7.2 inch diameter motors in the 36 and 48 volt range. You can get reasonably priced controllers, contactors, fuses, batteries and chargers. Also, staying below 50V nominal avoids certain OSHA standards, if that applies.

Cool project  Keep us posted,

major


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## TX_Dj (Jul 25, 2008)

pbiebach said:


> Im thinking I need a motor with approx the following stats
> ~8 hp cont, ~20 hp Peak
> ~ 40 ft-lbs torque at maximum





major said:


> In other words, a 3.5 hp continuous rated totally enclosed electric motor with a peak of 12 to 15 hp might be able to fill your bill.



And let me expand on major's comment by way of pointing out your specs about torque: the reason you can get away with a smaller HP motor with electric is because of torque. Electrics make a lot of torque. Torque is what moves you, HP is what makes you feel like you're moving faster, even faster (if that makes sense- it's work done per period time). 

If anything, I would choose a motor/controller combination that gave more torque than the ICE. ICE isn't (usually) good at making torque, so it compensates by making horsepower. Just the same, EV isn't (usually) good at making horsepower, but it compensates by making torque.

ICE will typically make peak torque around 3500 RPM (such as in a gas car/truck) and peak horsepower near 5000 RPM (unless it has a much higher RPM). EV will typically make peak torque almost off of stall, and up to 1000-2000 RPM, and horsepower at low RPM as well.

The reason is relatively simple, because you can't have horsepower without either a large torque number and/or large RPM number. 10,000 ft-lbs at 10 RPM works out to about the same HP as 10 ft-lbs at 10,000 RPM. The difference being, you're not going to move a house with a 10 ft-lbs engine.

This is also why diesel ICE is used in trucks, because it packs a huge amount of torque into a low amount of RPM (a semi rig that makes 400 hp at 1500 rpm is putting 1400 ft-lbs of torque, a lot more than a gas "400 hp" engine rated at 5500 RPM peak output). 

Just like a diesel, the EV hits its torque early. Earlier than diesel. It's also more consistently delivered through the RPM range, very much unlike ICE, which makes a "peaky" curved powerband.

More torque, not enough to tear up your drivetrain, less HP = much more powerful feeling ute.


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## pbiebach (Jun 29, 2010)

Thanks for the replys major and TX_Dj

I have a couple more questions
My utility vehicle needs a bit more power then a golf cart, its designed to be able to do work (pull trailers, etc) here's a picture of what the vehicle.
http://www.kawasakimule.org/images/products/2008/kawasaki-mule-3010-diesel.jpg

Electric seems Ideal since I really only need torque at low speeds, never needs to drive very fast, just powerful.

Are series wound motors the way to go? 
any specific motor suggestions so I can get an idea of specs/price?

I look forward to posting details of the build as I get started


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## major (Apr 4, 2008)

pbiebach said:


> Electric seems Ideal since I really only need torque at low speeds, never needs to drive very fast, just powerful.


You may want to look at the reduction ratio. When you say _powerful_, you actually mean _having a lot of force_. Power being the product of force times speed. So to have high torque at low speed doesn't mean powerful. A low power motor geared appropriately can give you high torque at the axle. With these types of vehicles this ability for low speed force is often measured as _draw-bar pull_. This relates directly to the towing capacity and gradibility.



> Are series wound motors the way to go?


Those can work nicely. But the SepEx can also. And give you some extra features. But is generally more complex to set up. I think you need to look at the motor type selection in conjunction with the controller which you will be using. Possibly procuring the motor and controller from the same source.



> any specific motor suggestions so I can get an idea of specs/price?


Check out web sites for EV parts suppliers, more than one. Maybe even call and get their recommendation. 

major


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