When I was a lad toys had motors with magnets and real things had motors
with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

question: is a motor more efficient , does it have more 'umph' by having
permanent magnets or is it just a size saving.

is the 2.2 HP the output power or the input power?

I remember that motors were either series or parallel switched for the
field coils depending on what they were doing. trams had series to pull
away then parallel to run at cursing speed.

Gary

Gary wrote:
When I was a lad toys had motors with magnets and real things had motors
with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

question: is a motor more efficient , does it have more 'umph' by having
permanent magnets or is it just a size saving.


More a cost saving really. Efficiency is really related more to (mostly)
resistive losses and frictional losses. High efficiency is all about
lots of iron and copper for the power, and as good quality bearings as
you can use. And optimising RPM as well to balance all this. (high RPM
mens for a given torque and resistive losses you get more power, High
RPM leads to more frictional losses).


A small amiount of magnet replaces a lot of iron and copper.



is the 2.2 HP the output power or the input power?


Eiher. Its hard to get a production motor opeated in its limits worse
than about 50-60% eff. - the crappy can motors in electric tools are
probably in that region. Its vbery easy to get a decent motor up to
75%-80%, and quality ubnits are 90%-95%.


I remember that motors were either series or parallel switched for the
field coils depending on what they were doing. trams had series to pull
away then parallel to run at cursing speed.


ISTR the opposite. But perhaps that is a current limiting exercise.

Gary

On 30/06/2011 12:34, The Natural Philosopher wrote:
Gary wrote:
When I was a lad toys had motors with magnets and real things had
motors with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

question: is a motor more efficient , does it have more 'umph' by
having permanent magnets or is it just a size saving.


More a cost saving really. Efficiency is really related more to (mostly)
resistive losses and frictional losses. High efficiency is all about
lots of iron and copper for the power, and as good quality bearings as
you can use. And optimising RPM as well to balance all this. (high RPM
mens for a given torque and resistive losses you get more power, High
RPM leads to more frictional losses).


A small amiount of magnet replaces a lot of iron and copper.



is the 2.2 HP the output power or the input power?


Eiher. Its hard to get a production motor opeated in its limits worse
than about 50-60% eff. - the crappy can motors in electric tools are
probably in that region. Its vbery easy to get a decent motor up to
75%-80%, and quality ubnits are 90%-95%.


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.

A motor fed with DC is a different beast, especially one with permanent
magnets with no field power losses.


I remember that motors were either series or parallel switched for the
field coils depending on what they were doing. trams had series to
pull away then parallel to run at cursing speed.


ISTR the opposite. But perhaps that is a current limiting exercise.

Gary

Fredxx wrote:


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.
]
I really dont think that is correct. The fans are their to cool, not
limit power. And you need cooling if you try and push a lot of power
through a small package.


Say you have 1Kw motor on a vacuum cleaner. Now even at 90% there still
100W to get rid of. A 1KW motor is pretty powerful. At 70% there is 300W
to get rid of. Yup. 3 large bright very hot electric bulbs worth.

A motor fed with DC is a different beast, especially one with permanent
magnets with no field power losses.


There should be no field power losses Not on an AC motor, where
inductance will limit current and the current is all in quadrature to
the voltage.
..
Magnets are used more on DC motors because field windings wont current
limit by inductance..you need to put them in series so the back EMF then
limits the total current drawn. But that's a more expensive larger and
heavier motor than two chunks of ferrite magnet, a three pole armature
and a commutator and brushes.

You should compare a 700 watt mains motor with e.g. a 700 watt model
aircraft motor. probably 4:1 -5:1 lighter.

In message , The Natural Philosopher
writes
Gary wrote:





I remember that motors were either series or parallel switched for
the field coils depending on what they were doing. trams had series to
pull away then parallel to run at cursing speed.


ISTR the opposite. But perhaps that is a current limiting exercise.

That rings a bell from my student days, trying to learn about electrical
engineer.

I feel that the OP is correct. I seem to recall that a series motor has
high starting torque (the field current is high and, of course, equals
the armature current).

Surprisingly, a motor tries to run faster when you decrease the field
current (in an attempt to maintain the same back EMF). At high speed and
low torque, you can only reduce the field current independent of the
armature current, if it is in parallel.
--
Ian

On 30/06/2011 14:02, The Natural Philosopher wrote:
Fredxx wrote:


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.
]
I really dont think that is correct. The fans are their to cool, not
limit power. And you need cooling if you try and push a lot of power
through a small package.

AC motors are generally a series armature - field arrangement, so even
taking into account AC impedances, can you give any other reason why
such a motor won't speed up to the point it all lets go without some
form of brake?

Yes the fan is there to cool, you only have to feel the hot air coming
out of the motor.



Say you have 1Kw motor on a vacuum cleaner. Now even at 90% there still
100W to get rid of. A 1KW motor is pretty powerful. At 70% there is 300W
to get rid of. Yup. 3 large bright very hot electric bulbs worth.

What happens to the 90% (or 70%), it all comes out as heat.


A motor fed with DC is a different beast, especially one with
permanent magnets with no field power losses.


There should be no field power losses Not on an AC motor, where
inductance will limit current and the current is all in quadrature to
the voltage.
.
Magnets are used more on DC motors because field windings wont current
limit by inductance..you need to put them in series so the back EMF then
limits the total current drawn. But that's a more expensive larger and
heavier motor than two chunks of ferrite magnet, a three pole armature
and a commutator and brushes.

With respect, a DC field winding can be parallel or series. And with
less losses than an AC field winding due to the AC magnetisation losses,
and the induced transformer action on the armature. I do agree that
inductance goes a long way to reduce start-up currents for AC motors,
much in the way a fluorescent tube has a choke to limit current.


You should compare a 700 watt mains motor with e.g. a 700 watt model
aircraft motor. probably 4:1 -5:1 lighter.

I agree, one is built to a power in spec without any reference to
efficiency or true power output.

On Thu, 30 Jun 2011 12:17:04 +0100, Gary
wrote:

When I was a lad toys had motors with magnets and real things had motors
with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

field coils depending on what they were doing. trams had series to pull
away then parallel to run at cursing speed.

Is that the speed they were doing when you saw one pass a stop that
you were running towards?

G.Harman

On Thu, 30 Jun 2011 13:35:48 +0100, Fredxx wrote:

Eiher. Its hard to get a production motor opeated in its limits worse
than about 50-60% eff. - the crappy can motors in electric tools are
probably in that region. Its vbery easy to get a decent motor up to
75%-80%, and quality ubnits are 90%-95%.


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.

A motor fed with DC is a different beast, especially one with permanent
magnets with no field power losses.

My old Numatic George vac. has a 1000W motor, performs well and doesn't blow
out much hot air. The Nilfisk has a 1400W motor, the performance is, as far
as I can tell, the smae but there's far more heat coming out.

It's all marketing and just a waste of energy.
--
Peter.
The gods will stay away
whilst religions hold sway

In article ,
Gary wrote:
When I was a lad toys had motors with magnets and real things had motors
with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

question: is a motor more efficient , does it have more 'umph' by having
permanent magnets or is it just a size saving.

is the 2.2 HP the output power or the input power?

I remember that motors were either series or parallel switched for the
field coils depending on what they were doing. trams had series to pull
away then parallel to run at cursing speed.

This is the snag. The motor characteristics depend on how the field is
connected. For perhaps most common uses, a decent permanent magnet gives
better results. And with the latest high power ones developed from
computer stuff they can be very good indeed.

I'll give you an example. The original wiper motor on my old Rover was
Lucas with a 'steel' PM. My BMW has a similar sized one with a ceramic
magnet. It produces three times the torque at stall, but uses
approximately the same running current.

--
*"I am " is reportedly the shortest sentence in the English language. *

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

Fredxx wrote:
On 30/06/2011 14:02, The Natural Philosopher wrote:
Fredxx wrote:


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.
]
I really dont think that is correct. The fans are their to cool, not
limit power. And you need cooling if you try and push a lot of power
through a small package.

AC motors are generally a series armature - field arrangement, so even
taking into account AC impedances, can you give any other reason why
such a motor won't speed up to the point it all lets go without some
form of brake?


Back EMF.
Because I don't think the ARE series field coils.

Yes the fan is there to cool, you only have to feel the hot air coming
out of the motor.



Say you have 1Kw motor on a vacuum cleaner. Now even at 90% there still
100W to get rid of. A 1KW motor is pretty powerful. At 70% there is 300W
to get rid of. Yup. 3 large bright very hot electric bulbs worth.

What happens to the 90% (or 70%), it all comes out as heat.


No, it goes into mechanical power..initially anyway. So it comes out as
noise and increased air turbulence as well..



A motor fed with DC is a different beast, especially one with
permanent magnets with no field power losses.


There should be no field power losses Not on an AC motor, where
inductance will limit current and the current is all in quadrature to
the voltage.
.
Magnets are used more on DC motors because field windings wont current
limit by inductance..you need to put them in series so the back EMF then
limits the total current drawn. But that's a more expensive larger and
heavier motor than two chunks of ferrite magnet, a three pole armature
and a commutator and brushes.

With respect, a DC field winding can be parallel or series. And with
less losses than an AC field winding due to the AC magnetisation losses,
and the induced transformer action on the armature. I do agree that
inductance goes a long way to reduce start-up currents for AC motors,
much in the way a fluorescent tube has a choke to limit current.


You should compare a 700 watt mains motor with e.g. a 700 watt model
aircraft motor. probably 4:1 -5:1 lighter.

I agree, one is built to a power in spec without any reference to
efficiency or true power output.

Say you have 1Kw motor on a vacuum cleaner. Now even at 90% there still
100W to get rid of. A 1KW motor is pretty powerful. At 70% there is 300W
to get rid of. Yup. 3 large bright very hot electric bulbs worth.

A motor fed with DC is a different beast, especially one with permanent
magnets with no field power losses.


There should be no field power losses Not on an AC motor, where
inductance will limit current and the current is all in quadrature to
the voltage.
.
Magnets are used more on DC motors because field windings wont current
limit by inductance..you need to put them in series so the back EMF then
limits the total current drawn. But that's a more expensive larger and
heavier motor than two chunks of ferrite magnet, a three pole armature
and a commutator and brushes.



Just FWIW good article here on motors for Railway traction might be of
interest to some....


http://www.railway-technical.com/tra...DCMotorControl
--
Tony Sayer

On 30/06/2011 18:45, The Natural Philosopher wrote:
Fredxx wrote:
On 30/06/2011 14:02, The Natural Philosopher wrote:
Fredxx wrote:


One of my bug bears with electric tools and vacuum cleaners is the
belief that the more powerful a motor the better it is. They are
generally brushed AC motors and are specc'd on input power. In short
they are designed to be inefficient to limit motor speed through and
often use cooling fan load to limit motor speed.
]
I really dont think that is correct. The fans are their to cool, not
limit power. And you need cooling if you try and push a lot of power
through a small package.

AC motors are generally a series armature - field arrangement, so even
taking into account AC impedances, can you give any other reason why
such a motor won't speed up to the point it all lets go without some
form of brake?


Back EMF.
Because I don't think the ARE series field coils.

I have yet to come across an AC motor which isn't series.

Back emf is a product of speed and field current. Increased back emf in
a series motor will then reduce field current, which then reduces the
back emf and so make the motor speed up all the more.

On Jun 30, 12:17*pm, Gary wrote:
When I was a lad toys had motors with magnets and real things had motors
with field coils.
Now I see things with 2.2 HP motors with permanent magnets.

question: is a motor more efficient , does it have more 'umph' by having
permanent magnets or is it just a size saving.

PMs consume no power, unlike field coils. OTOH its hard to get as much
mag field without going overboard on the magnets, and both factors
affect efficiency.

One reason field windings are preferred with ac motors is that its
then unnecessary to rectify the power, you jsut pass it through both
field and armature as ac.


is the 2.2 HP the output power or the input power?

input. With handheld tools where weight is important, motor efficiency
can be very poor.


I remember that motors were either series or parallel *switched *for the
field coils depending on what they were doing. trams had series to pull
away then parallel to run at cursing speed.

Gary

Cursing speed? Damn.


NT