DC motors

[Gary]

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

[The Natural Philosopher]

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.

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%.

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

[Fredxx]

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.

[The Natural Philosopher]

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.

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.

[Ian Jackson]

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.

[Fredxx]

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.

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

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.

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

[damduck-egg]

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

G.Harman

[PeterC]

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.

[Dave Plowman (News)]

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.

[The Natural Philosopher]

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

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

[tony sayer]

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

[Fredxx]

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.

[NT]

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.

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

Cursing speed? Damn.

NT