DC Motor For Lawnmower

I remember seeing at some point (possibly on these forums, but I cannot find it) that power (horsepower ratings) for internal combustion engines (as in a lawnmower) do not carry over to DC motors.
This makes sense to me, given the radically different torque curves of internal combustion (peak torque at high RPM) and DC (peak torque at stall).
My question is: if I have a push mower (not self-propelled) currently running on a 3.8 HP engine, what would be an appropriate speed/torque DC motor to replace it? I’m looking to minimize cost, and I am willing to machine my own gearbox/belt reduction. If anyone has any suggestions, as well as a good supplier for an appropriate motor, I’d appreciate it.

3.8 HP = 2834 Watts

CIM peak power = 321 Watts

You need the equivalent of 8.8 CIMs for your lawnmower. Plus battery(ies).

(Can you see why an economical electric car is so elusive?)

But wait - there’s more. To offset the increased weight that such a large motor (or so many smaller motors) plus a power source, you’re going to need to further increase your load of “CIM equivalents”. You also are going to need the appropriate gearing so that your motors run at the “peak power” point of their curves for you (roughly 1/2 the free RPMs).

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Well, if it is a push-mower, then the weight wouldn’t necessarily have anything to do with the number of motors you would need. However, the gearing selection might have enough efficiency loss (trying to combine ~10 motors into one gearbox? Yeesh.) to increase the number of motors you need. At the same time, you might not necessarily need all 3.8 HP, so you could reduce the requirements somewhat there.

In any case–I probably wouldn’t recommend putting together a bunch of CIMs for this application!

I can tell you its going to be alot heavier:rolleyes: …

A 4hp DC Motor will be quite(50+pounds) heavy…not to mention overkill…

A 3/4HP motor(with chain reduction) is more than enough to power a 2-person go-cart to 20+MPH

as for a lawnmover…the size of the blades?..could be a factor for motor selection…

Just because the lawnmower engine is rated at 3.8 Hp, does not mean it developing 3.8 Hp in operation. Without knowing the torque curve of the engine, the speed at which it develops rated power, and the torque at its normal operating speed, I think that there is no simple way to compare them. I would look at commercially available DC electric lawnmowers and see what the manufacturer used. Compare similar sized DC electric and gasoline lawnmowers to develop a rough idea of what would be equivalent to your 3.8 Hp engine.

Why over-think this? Have you looked into the motors used on commercial electric mowers? http://www.blackanddecker.com/ProductGuide/Product-Details.aspx?ProductID=2503

A starter motor from a junkyard is cheap and powerful, and will easily spin the blades of a push mower at, say, 1000-1500 RPM. I am not an expert, some types will overheat under continuous load, and others will not. I think you want a reduction drive type.

In college I worked on an electrical vehicle, it was a lotus elise that we converted to electric. The 5hp motor we used was enough power to drive the car at 55mph on flat roads, could have possibly done more however because of the installation we could not shift gears while moving.

Just an interesting data point.

6.5 amp time 110 volt is about 700 watts for an 18" mower. You could probably get by with a 1 kW continuous DC motor. Like the others have said though, this is going to be a power hog. Unlike an FRC bot or drill, this is near coninuous use for however big your lawn is. You will probably want something on the order of a 1 KwHr pack if your yard requires 30 minutes to mow (many batteries you do not want to drain down too low). When sizing a battery, you need to verify Power and Energy. KW-Hr is energy, and KW would be a power rating. Most batteries do not specify it that way though. Since batteries have a relative constant voltage (12 volt will go between 15 and 10 volts), the instead like to use amperage.

"Battery power rating is measured by two standards. The more popular of these, cold cranking power, determines the amount of current (amps) a battery delivers for 30 seconds at zero degrees Fahrenheit while maintaining a minimum terminal voltage of 7.2 volts. The higher the number, the stronger the battery. Think of those old Diehard commercials, with all those non-Sears batteries giving up the ghost in the cold and snow, and this will perhaps give you a visual image.

The second standard is called reserve capacity rating. This is a warm weather rating (80 degrees Fahrenheit), which estimates the amount of time it takes the terminal voltage of a fully charged battery to dip below 10.2 (or 1.7 volts per cell) at a continuous discharge rate of 25 amps. The rating is expressed in minutes. For example, a rating of 120 means the battery will run for two hours (120 minutes) before ceasing to function"

Roughly speaking this means the battery could deliver 25 amps * 12 volts or 300 Watts for 2 hours, or in our case 700 watts for about 50 minutes.

If you are using lead acid, you will want a deep cycle battery. this has thicker plates and is less likely to give out on you. Most new cheap car batteries are substantially lighter. In lead acid rechargeables, this is bad. If you deep cycle one of these, consider them junk. The plates are really thin and get ruined during a deep cycle.

This sounds like a fun project (do i hear robot mower…). If you are merely looking for a fun DC Power electronics project, look into E-boost bikes. These are fun and you can pick up a cheap bike at a garage sale.

www.surpluscenter.com has many things that may be of interest to your project

A lawn mower would be the perfect application for a brushless motor. They are more efficient that regular brushed DC motors, but they need special speed controllers. Here are two brushless motors which would probably work just fine, most likely with just a single reduction off the motor shaft via timing or V-belts.


Thank you everyone who answered, particularly artdutra for those links. I’ll keep looking around a bit and hopefully find something within my price range.

Sounds like there are quality issues with the 3KW motor. Lots bearing and magnet failures. Perfect size though.

Holy necro!

If we are necroing a decade old thread…

Brushless motors would work best in these situations both in terms of torque and energy efficiency.
I’d drive the wheels separate from the blades.

I’m just amazed a spambot knows the word “Voluntold”