Calculating torque on an electric lawnmower

[Snagglefoot]

Recent posts on electric lawnmowers have questioned the amount of torque available on the motor. So far I have only seen two numbers printed on the mower and battery; Voltage and battery charge capacity in Amp Hrs.

Typical gas mowers are 6 to 7 ft lbs at about 2400 rpm. Is there any practical way of deriving the torque and rpm numbers from an electric mower and why do you think the industry doesn't require the numbers to be reported?

 

[skyactiv]

Interesting read here: http://www.stevepake.com/life/2018/5/25/the-truth-about-electric-lawn-mower-horsepower

 

[JHZR2]

No, you cannot get it from voltage and amp hours directly.

That said, you can estimate based upon runtime. The Ah rating is how many amperes it will put out continuously for one hour.

Let's assume that it says you can mow for 20 minutes on a charge, and the battery is 40V, 10Ah.

20 minutes is 1/3 hr, so the battery will roughly put out 3x current for 1/3 of the time. There will also be some conduction loss, so rough estimate is then 30A for 20 min at 30v.

30A x 30V = 900W.

900W = 1.2HP.

Let's assume direct drive and 3000rpm for the blade.

Torque = 5252 x HP/RPM

So, = 5252 x 1.2/3000

Torque = 2.1 ft-lb.

Now you just need to fill in numbers that I guessed, primarily runtime. I just made the numbers easy, but ai doubt they're that far off.

 

[CR94]

^ At best, that method would yield the average torque (as opposed to maximum, or maximum sustainable) if the motor were 100% efficient---which it is not.