Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

Rob

In article ,
robgraham writes:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

I think most electric mowers are universal motors (i.e. with
brushes and a commutator). Certainly they were 8 years ago
when I last bought one -- I had to go some way off the beaten
track to find an induction motor mower. I've bought two over
the years -- I think they're both German but different makes
(both still working fine, although I've had to replace the
run capacitor in the 15 year old one).

The torque profile of a universal and induction motor is quite
different. An induction motor doesn't have much starting torque,
but when it's nearly up to speed, there's quite a lot of torque
to maintain speed, which makes it good for a mower. A universal
motor has a high starting torque, but low torque when up to
speed, which means it easily slows when under load. That isn't
good for a lawn mower. Universal motors also spin too fast for
direct coupling to a mower blade. You will also find you need
around 30-50% more powerful universal motor mower to get the
same cutting power as an induction motor. Universal motors
seem to get much hotter, but that may just reflect cheap lower
quality motors rather than any inherent reason.

Can anyone help or point me at a site on this topic please.

How many AHrs is the 36v Li-polymer battery?

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Andrew Gabriel wrote:
In article
,
robgraham writes:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can
have the same capability as a 1700w ac motor. OK 1700w is a
comfortable 2hp but single phase induction motors are not too good
on torque, and if I remember that is where dc motors score but are
modern ones going to match a 1700w ac motor ?

I think most electric mowers are universal motors (i.e. with
brushes and a commutator). SNIP

Cetrainly all the 'hovver' mowers would be, an induction motor would be far
to heavy.

Cost is also a factor, all the el cheapo pressure washers have universal
motors because of the cost, commercial machines have induction.


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

"Andrew Gabriel" wrote in message
...


I think most electric mowers are universal motors (i.e. with
brushes and a commutator). Certainly they were 8 years ago
when I last bought one -- I had to go some way off the beaten
track to find an induction motor mower. I've bought two over
the years -- I think they're both German but different makes
(both still working fine, although I've had to replace the
run capacitor in the 15 year old one).

IIRC qualcast use induction motors even in the little rotary mowers.

dennis@home wrote:


"Andrew Gabriel" wrote in message
...


I think most electric mowers are universal motors (i.e. with
brushes and a commutator). Certainly they were 8 years ago
when I last bought one -- I had to go some way off the beaten
track to find an induction motor mower. I've bought two over
the years -- I think they're both German but different makes
(both still working fine, although I've had to replace the
run capacitor in the 15 year old one).

IIRC qualcast use induction motors even in the little rotary mowers.



My Honda electric mower - about 15 years old with a rust free plastic
deck has an induction motor. I want it to break as my wife that likes
it will let me buy a petrol mower, but it just keeps on going...........

On 25 Oct, 23:35, (Andrew Gabriel) wrote:
In article ,
* * * * robgraham writes:

how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. *

I think most electric mowers are universal motors (i.e. with
brushes and a commutator).

Something that has already adopted the cost and complexity of Li-Po
may well be using a brushless motor too. As the extra cost of going
brushless remains largely constant, while the cost of vaguely robust
brushgear goes up with power, then this makes even more sense for
powerful motors. Once you have brushless control and some vaguely
smart control gear, you can get a lot more initial starting torque for
little effort.

robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator to
generate the AC..

If you are sticking 1700W in, its got to come out as heat, noise or as
shaft power.

I doubt efficiencies are any less on a brushed PM versus induction
motor, ..


Rob

"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator to
generate the AC..

??? In a DC motor the field is DC. The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque. Not
sure where you get 3 phase from. From a standard single pole pair DC motor
I would call it "single phase". 2 pole pairs on a DC motor is common place
for starter motors. I haven't come across with 3 pole pairs with 3 sets of
brushes?


If you are sticking 1700W in, its got to come out as heat, noise or as
shaft power.

I remain unconvinced about power in and useable power out for mains motors.
At present I would have hoped green thinking would have had some meaningful
measure of real power out, not just turned into heat. There is still the
concept that the more power in the better the motor! Generally small
induction motors are very inefficient, DC motors are inherently very
efficient. AC (brushed) motors are less efficient than DC and the best
thing that could be done would be to rectify the mains first.


I doubt efficiencies are any less on a brushed PM versus induction motor,
..


Rob

Fred wrote:
"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator to
generate the AC..

??? In a DC motor the field is DC.

So what?

Its still a three phase motor with the poles switched on one (or two) at
a time via a 2 brush system on a Nx3 commutator element.



The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque. Not
sure where you get 3 phase from.

Each winding gets an AC signal. Each winding is switched at 120 degrees
phase to the next one: If I told you that and told you nothing else,
you would say 'ah, a 3 phase AC motor!!'




From a standard single pole pair DC motor
I would call it "single phase". 2 pole pairs on a DC motor is common place
for starter motors. I haven't come across with 3 pole pairs with 3 sets of
brushes?

The simples motor is a single phase. But its not self starting.
Therefore the simplest that works is three phase with two magnet
poles.It ameks no dss as to whether the current is originally DC with
permanent magnets, or AC with a field winding. It will work.

Unless you actyally go for synchronuus motors, the actual AC that drives
the MOTOR as opposed to a high frequency ripple on the magnetic fields,
is generated by brushes and commutators or an electronic switch.






If you are sticking 1700W in, its got to come out as heat, noise or as
shaft power.

I remain unconvinced about power in and useable power out for mains motors.
At present I would have hoped green thinking would have had some meaningful
measure of real power out, not just turned into heat.

On a mains motor, the frequency of commutation is nor far off mains
frequency for typical motors speeds - about 3000 RPM say. So there is
not much extra iron loss using a mains motor or a permanent magnet motor
or an induction motor off DC.

All are capable of better than 90&% efficiency if built well, or as low
as 50% if stamped out in thousands in Taiwan, and operated well off
decent efficiency points.


There is still the
concept that the more power in the better the motor! Generally small
induction motors are very inefficient, DC motors are inherently very
efficient. AC (brushed) motors are less efficient than DC and the best
thing that could be done would be to rectify the mains first.


I dont see why they should be really.

I doubt efficiencies are any less on a brushed PM versus induction motor,
..


yup


Rob

The Natural Philosopher wrote:
Fred wrote:
"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator
to generate the AC..

??? In a DC motor the field is DC.

So what?

Its still a three phase motor with the poles switched on one (or two) at
a time via a 2 brush system on a Nx3 commutator element.



The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque.
Not sure where you get 3 phase from.

Each winding gets an AC signal. Each winding is switched at 120 degrees
phase to the next one: If I told you that and told you nothing else,
you would say 'ah, a 3 phase AC motor!!'


I think you are wrong a dc motor is not a 3 phase or ac , some motors
might have three poles but that only 1 configuration , is a five pole
motor 5 phase? and a 7 pole 7 phase? and you cant feed a permanent
magnet motor AC current so to call a dc motor ac is simplifiing things
to far


--
Kevin R
Reply address works

Kevin wrote:
The Natural Philosopher wrote:
Fred wrote:
"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator
to generate the AC..

??? In a DC motor the field is DC.

So what?

Its still a three phase motor with the poles switched on one (or two)
at a time via a 2 brush system on a Nx3 commutator element.



The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque.
Not sure where you get 3 phase from.

Each winding gets an AC signal. Each winding is switched at 120
degrees phase to the next one: If I told you that and told you
nothing else, you would say 'ah, a 3 phase AC motor!!'


I think you are wrong a dc motor is not a 3 phase or ac , some motors
might have three poles but that only 1 configuration , is a five pole
motor 5 phase? and a 7 pole 7 phase?

I cannot think of any motor that does not have a multiple of 3 actual
windings. But I suppose its possible. So OK, they are 5, 7 whatever phse
motors!



and you cant feed a permanent
magnet motor AC current

Yes you can.

I have about 9 of them myself.

so to call a dc motor ac is simplifiing things
to far



I think not. All electric motors have alternating current in the windings.

Realising that takes a lot of bull**** about the so called difference
between 'AC' and 'DC' motors away.

Really there are only two basic types of motor: those that are driven,
either electronically, or by means of commutators, with a phased AC that
synhronises to the motor speed, and those that are synchronous to a
fixed frequency AC source.

The only other variable is whether permanent magnets or electromagnets
provide the field. And if electrical whether they are series or shunt
connected.

Apart from eddy current motors, which are off the topic here.

In article ,
The Natural Philosopher wrote:
The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque.
Not sure where you get 3 phase from.

Each winding gets an AC signal.

No - it gets switched polarity DC. A different thing entirely.

Each winding is switched at 120 degrees
phase to the next one: If I told you that and told you nothing else,
you would say 'ah, a 3 phase AC motor!!'

--
*Two many clicks spoil the browse *

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque.
Not sure where you get 3 phase from.

Each winding gets an AC signal.

No - it gets switched polarity DC. A different thing entirely.


NO, it is true alternating current in the meaning of the term. Current
that has an average value of zero, but has at any given time, a value
that is not zero.


Switched polarity DC *IS* AC!!

"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...


The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque. Not
sure where you get 3 phase from.

Each winding gets an AC signal. Each winding is switched at 120 degrees
phase to the next one: If I told you that and told you nothing else, you
would say 'ah, a 3 phase AC motor!!'


With 3 brushes equally spaced I suppose?

In some respects you're right but for all the wrong reasons!

Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...

The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque. Not
sure where you get 3 phase from.
Each winding gets an AC signal. Each winding is switched at 120 degrees
phase to the next one: If I told you that and told you nothing else, you
would say 'ah, a 3 phase AC motor!!'


With 3 brushes equally spaced I suppose?

No, three commutator segments spaced equally.

In some respects you're right but for all the wrong reasons!