Battery driven lawnmowers question

[robgraham]

My daughter's partner has tried a Bosch corded mower (Rotak 43) and it's equivalent battery version (Rotak 43-Li).

He finds that for average lawn cutting, and particularly wet grass, they are equal in capability and yet the corded version has a 1700w motor and the battery one is driven off a 36v 2.6Ah battery, which rather obviously can hardly deliver the same electrical power.

Why is the lower powered DC machine equal in capability with the much higher powered AC driven mower? Is it in the mower design or are modern DC motors (I'm presuming they are stepper motor type design) that much more efficient and powerful?

Rob

[robgraham]

My daughter's partner has tried a Bosch corded mower (Rotak 43) and it's equivalent battery version (Rotak 43-Li).

He finds that for average lawn cutting, and particularly wet grass, they are equal in capability and yet the corded version has a 1700w motor and the battery one is driven off a 36v 2.6Ah battery, which rather obviously can hardly deliver the same electrical power.

Why is the lower powered DC machine equal in capability with the much higher powered AC driven mower? Is it in the mower design or are modern DC motors (I'm presuming they are stepper motor type design) that much more efficient and powerful?

Rob

[Fredxx]

For the simple reason that the public believe the more power you (may) be able to put into a motor the better it is. They forget that it is power out that is important but is never specified. High power inputs go to reduce life expectancy through heat but hey!! Similarly we all believe the more volts into a battery powered motor, the better the product is as well!!

To be honest DC motors are the most efficient and they tend to use permanent field magnets, AC series brushed motors are less so for a number of reasons.

Induction motors are less efficient for a given size still but have the advantage of being brushless.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember robgraham saying something like:

The DC motor might be geared down and is running at its most effective speed.

[The Natural Philosopher]

all mortors will be geared.

But the thing is a leccy motor will draw what it has to until its reaches some limit.

Ypu probably only need a couple of hundred watts on a 12" or so rotary to cut a shortish grass. Its when you hit the rough stuff. Ive got a

3.5bhp briggsie and that will stall on rank grass. Even the 13bhp one will as well. But that's a 48" cut.

So no difference on light duties. The 1700 watter will be running at a couple of hundred same as the battery one. The mains will 'go up to'

1700 which the battery one probably won't, although its deeply possible on short bursts to pull 80A out of a 12v lead acid of mower type dimensions. which is well over a brake horse.

[John]

Not all motors are geared.

[geoff]

No, but mortors might be

[The Natural Philosopher]

All electric motors used in lawnmowers will be geared.

Otherwise the mower blade tips would be supersonic long before the motor was up to sensible efficiency.

[John]

Wrong! Universal motors are governed to some extent by the resistance of the fan. There are no gears in my mower.

[The Natural Philosopher]

Then its using a far bigger motor than it needs to and has optimised itself for dire efficiency for the sake of saving a few pence.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like:

Bullshit.

[John]

Cylinder mowers may have some reduction belt drive - Rotary ones are direct (in my experience)

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "John" saying something like:

Exactly.

[Tony Bryer]

The last electric Flymo I had - bought 2002ish - had a belt drive. I got the impression (perhaps wrongly) that this was some form of safety measure, with the drive pulley shearing if the blade rotation was impaired.