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Electric Lawn Mower Motors

oldlawnmowerman81

14 years ago

Hi,

For a typical electric lawn mower motor that has a max speed of 15,000 RPM; for the motor to run within the limits of the CPSC-mandated 19,000 FPM, is it true that there must be gears which produce a ratio of about 3.75:1, 4:1 or 5:1 for the motor to achieve a final rotational speed in the 3,750-4,000 rpm, or in some cases, the 3,000-3,250 rpm range?

I noticed modern Black & Decker corded mower motors (the Lawn Hog line) sound like they're running 4,000 rpm (pitch key: C or Cmaj). So my mind thinks the motor has a pinion gear and an idler gear which make a 3.75:1 ratio.

Twin-blade mower motors use belt pulleys instead of toothed gears. The 15,000 RPM motor on these is geared down to about 7,500 RPM or however much the circumferences of the two pulleys are, depending on the length of the blade. Black & Decker's very first electric mowers were twin-blade units, with cutting widths of 18" and 22".

~Ben

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Comments (9)

canguy
14 years ago
Typical single phase motors run at 3450rpm, not 15000. In an ideal world with no friction losses they would run at 3600 rpm as household current is 60 hz or 60 cycles per second which translates to 3600 cycles per minute.
As with gas mowers the blade speed is limited by goobermint rules so yes, the blade speed is reduced by gear reduction or belts and pulleys.
oldlawnmowerman81
Original Author
14 years ago
Here is an excerpt on electric lawn mower motors that I found in the February 1967 issue of Popular Science:

ELECTRICS HAVE THE POWER (written by Jackson Hand)
"If you're one of those who tend to think electric mowers are kind of puny, think again. Typically, a good electric has a universal motor rated at about one to 1.5 hp. Universal motors run at about 15,000 RPM or so, as is done in the Lawn Boy, the Thor, and the single-blade Sunbeams, and you get three things: a) Power. The high-load torque of such mowers equals that of at least three-hp gasoline burning engines.; b) Minimized slowdown under load; and c) Quick stop of the blade when you turn the switch off - an important safety feature.
Twin-blade mowers such as Black & Decker and two Sunbeam models have belts instead of gears to reduce motor speed and provide equivalent power. Black & Decker models have flat belts, fiberglass reinforced, that operate without overheating at high speeds. Sunbeam uses toothed timer belts, necessary because the side-by-side blades must be synchronized so they don't hit each other. (The Black & Decker blades are staggered.)
Since blade-tip speed is an important factor in safety, held to 21,000 (now 19,000) feet per minute by the industry's safety code, these gear-downs serve the double function of bringing lightweight electric motors into efficient power range and of reducing blade speed to a point where hard objects thrown by the blade are less likely to be lethal."

~Ben
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tomplum
14 years ago
Fess up- whatcha buildin'?
oldlawnmowerman81
Original Author
14 years ago
Black & Decker's first deluxe single-blade electric rotary lawn mower, model 8010, used a permanent-magnet motor.

~Ben
tomplum
14 years ago
I remember working on these 30 years ago. You sound like you have some cool project going...
oldlawnmowerman81
Original Author
14 years ago
I'm not building anything, I was just asking general questions about electric lawn mower motors.

~Ben
oldlawnmowerman81
Original Author
14 years ago
I now want to ask all of you this question: there used to be electric mowers back in the '70s and '80s that were power propelled, as manufactured, for one, by Sunbeam/Aircap/Mastercut. When the power drive was activated on these units, didn't the speed of the electric mower motor bog down some?

~Ben
rosemallow
14 years ago
I don't understand why the electric lawn mowers are not using 3 phase motors. The technology today can change single phase to 3phase and you get a better longer lasting motor.
Most problem on single phase is a defective start winding. Then they burn up,
3 phase converters are cheap now a day.
Mark McLoughlin
3 years ago
Looking at the battery powered lawn mowers. It's difficult to gauge the power against petrol mowers for instance I have a cordless lawn mower from Warrior Eco Power Equipment. It's the 60V version with 2.5Ah battery charge by a 4A charger. So I get around 40 minutes run time and it takes around the same time to fully charge the batteries. Its advertise at max 1500 watts output. So how do you figure out an approx comparison to a petrol lawn mower.

If we take that electrical horsepower is defined as 746 watts,

Then horsepower = (torque X RPM) / 5252

150cc - 6.25 ft-lbs gross TQ at 2600 rpm but 5.70 ft-lbs @ 3600 rpm = 3.5 HP net

So at 60V maybe around 2.5 horsepower.

Does that sound about right?

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