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" wrote in message
...
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?


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.

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

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?

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

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

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?

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

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.

"The Natural Philosopher" wrote in message
...
Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember robgraham
saying something like:

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?

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

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.


Not all motors are geared.

In message zPe9o.22696$hp4.8069@hurricane, John
writes
"The Natural Philosopher" wrote in message
...
Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember robgraham
saying something like:

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?

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

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.


Not all motors are geared.


No, but mortors might be


--
geoff

geoff wrote:
In message zPe9o.22696$hp4.8069@hurricane, John
writes
"The Natural Philosopher" wrote in message
...
Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember robgraham
saying something like:

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?

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

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.


Not all motors are geared.


No, but mortors might be


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.