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

Err no, it's *an* alternating current in *your* meaning of the term
"True" alternating current is that whis is assumed in AC theory and
engineering and in the design of AC motors and equipment and is
sinusoidally varying.


Er, no, sinusoidal is not mentioned anywhere in the *definition* of
AC.
Oh well, that's all AC current theory out the window then, so that's
OK!


Not at all. Sinusoidal merely makes it easy to do calculations based on
single frequency operation.

In te real world, its almost never seen ourtside of ultra low distortin
oscillators.

You have to distinguish between 'electrical theory for dummies' and
'electrical engineering, an advanced course'
Agreed!

To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which way to
go round.

What on earth are you talking about? Are you seriously suggesting that it
doesn't matter what the waveform is for each of the phases, that you can
still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round

Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
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.
Most motors have far more than 3 commutator segments!


Depends. Most cheap ones have three. Larger motors have more, but
generally at least 5 are in contact with the brushes at any time.

So the brushes are in contact with more commutator segments than the motor
actually has?


It as you that said it had more segments than three, not me.

"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
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.
Most motors have far more than 3 commutator segments!


Depends. Most cheap ones have three. Larger motors have more, but
generally at least 5 are in contact with the brushes at any time.

So the brushes are in contact with more commutator segments than the
motor actually has?


It as you that said it had more segments than three, not me.


It was you who said that "generally at least 5 are in contact with the
brushes at any one time"! You also said that most cheap motors had 3
segments.

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

To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which way
to go round.

What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they are
sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.

Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which way
to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they are
sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.


you could if they followed each other like a normal 3 phase wave form
take 3 pulses as below (if it works)

_[]____[]_____
___[]____[]___
_____[]____[]__

--

Kevin R
Reply address works

"Kevin" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.


you could if they followed each other like a normal 3 phase wave form
take 3 pulses as below (if it works)

_[]____[]_____
___[]____[]___
_____[]____[]__


I agree, the fundamental for each phase is a sinewave with odd and even
harmonics added to each to make the pulse like the waveforms you sketch.
They aren't arbitrary waveforms.

Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which way
to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they are
sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.


wrong.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.



as long as teh three wavefortms are poahse related, whether sinusoid or
nnot, it will turn.

There is a lot of difference between sinusoid, repetitive non sinusoid
with a common fundamental frequency harmonically and phase locked, and
'arbitrary'.

YOU have merely set up a straw man by saying arbitrary. I never claimed
the signals applied to the poles were random and arbitrary, merely that
they don't need to be sinusoidal.

Kevin wrote:
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting
that it doesn't matter what the waveform is for each of the phases,
that you can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in
practise there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I
can assure you will not be able to tell which way it will turn.


you could if they followed each other like a normal 3 phase wave form
take 3 pulses as below (if it works)

_[]____[]_____
___[]____[]___
_____[]____[]__

thank you.

Fred wrote:
"Kevin" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round

If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.


you could if they followed each other like a normal 3 phase wave form
take 3 pulses as below (if it works)

_[]____[]_____
___[]____[]___
_____[]____[]__


I agree, the fundamental for each phase is a sinewave with odd and even
harmonics added to each to make the pulse like the waveforms you sketch.
They aren't arbitrary waveforms.


NO, the fundamental for each phase is a square wave.

Stop weaselling.

The point at which you have to introduce an infinite Fourier series of
sinusoids is the point it ceases to be worth talking about them.

The Natural Philosopher wrote:
Kevin wrote:
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close
to sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows
which way to go round.
What on earth are you talking about? Are you seriously suggesting
that it doesn't matter what the waveform is for each of the phases,
that you can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether
they are sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in
practise there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I
can assure you will not be able to tell which way it will turn.


you could if they followed each other like a normal 3 phase wave form
take 3 pulses as below (if it works)

_[]____[]_____
___[]____[]___
_____[]____[]__

thank you.
I still dont think a simple dc motor is three phase though :-) despite
it having three wound poles and two magnets

--
Kevin R
Reply address works

"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round


If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.


wrong.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.



as long as teh three wavefortms are poahse related, whether sinusoid or
nnot, it will turn.

There is a lot of difference between sinusoid, repetitive non sinusoid
with a common fundamental frequency harmonically and phase locked, and
'arbitrary'.

YOU have merely set up a straw man by saying arbitrary. I never claimed
the signals applied to the poles were random and arbitrary, merely that
they don't need to be sinusoidal.


AC motors are motors designed to work on an "AC supply"

An "AC supply" is an electrical supply system supplied by an alternator

An alternator provides a sinusoidally varying voltage/current (it has to)

AC theory allows design of such alternators and motors to be effective and
takes as a given that the supply is sinusoidal

The fact that a sawtooth waveform alternates in direction does not make it
an "AC supply"

The fact that some AC motors might "go round" if supplied by a sawtooth
waveform is irrelevant

A "DC motor" is not an "AC motor"

OK?


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

Bob Mannix wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting that
it doesn't matter what the waveform is for each of the phases, that you
can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round

If we're measuring degrees then it implies a sinusoid, though in practise
there would be harmonics of the fundamental.

wrong.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can
assure you will not be able to tell which way it will turn.


as long as teh three wavefortms are poahse related, whether sinusoid or
nnot, it will turn.

There is a lot of difference between sinusoid, repetitive non sinusoid
with a common fundamental frequency harmonically and phase locked, and
'arbitrary'.

YOU have merely set up a straw man by saying arbitrary. I never claimed
the signals applied to the poles were random and arbitrary, merely that
they don't need to be sinusoidal.


AC motors are motors designed to work on an "AC supply"

An "AC supply" is an electrical supply system supplied by an alternator

An alternator provides a sinusoidally varying voltage/current (it has to)

AC theory allows design of such alternators and motors to be effective and
takes as a given that the supply is sinusoidal

The fact that a sawtooth waveform alternates in direction does not make it
an "AC supply"

The fact that some AC motors might "go round" if supplied by a sawtooth
waveform is irrelevant

A "DC motor" is not an "AC motor"

Thats semantics.

This arose out of the comparison of 'AC' and 'DC' motors. I was merely
pointing out that at the core of whats happening inside them, they aint
so very much different, and that for analytic purposes a DC motor is an
AC synchronous motor with a mechanical DC to AC convertor called a
commutator.

OK?





OK?

"The Natural Philosopher" wrote in message
...
Bob Mannix wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
"The Natural Philosopher" wrote in message
...
Fred wrote:
To get the advantages of 3 phase, each waveform has to be close to
sinusoidal.


No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which
way to go round.
What on earth are you talking about? Are you seriously suggesting
that it doesn't matter what the waveform is for each of the phases,
that you can still tell me which way the motor will turn? Wow.


As long as they are 120 degrees apart, it doesn;t matter whether they
are sine, square or spikes. It will go round

If we're measuring degrees then it implies a sinusoid, though in
practise there would be harmonics of the fundamental.

wrong.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I
can assure you will not be able to tell which way it will turn.


as long as teh three wavefortms are poahse related, whether sinusoid or
nnot, it will turn.

There is a lot of difference between sinusoid, repetitive non sinusoid
with a common fundamental frequency harmonically and phase locked, and
'arbitrary'.

YOU have merely set up a straw man by saying arbitrary. I never claimed
the signals applied to the poles were random and arbitrary, merely that
they don't need to be sinusoidal.


AC motors are motors designed to work on an "AC supply"

An "AC supply" is an electrical supply system supplied by an alternator

An alternator provides a sinusoidally varying voltage/current (it has to)

AC theory allows design of such alternators and motors to be effective
and takes as a given that the supply is sinusoidal

The fact that a sawtooth waveform alternates in direction does not make
it an "AC supply"

The fact that some AC motors might "go round" if supplied by a sawtooth
waveform is irrelevant

A "DC motor" is not an "AC motor"

Thats semantics.

This arose out of the comparison of 'AC' and 'DC' motors. I was merely
pointing out that at the core of whats happening inside them, they aint so
very much different, and that for analytic purposes a DC motor is an AC
synchronous motor with a mechanical DC to AC convertor called a
commutator.


No, what I said was engineering, what you said was semantics - pushing the
term "AC motor" far away from its accepted engineering definition to merely
make your own point!

OK?

I'm OK, if you are!


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not

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.