A.C motors

In a properly disciplined A.C motor, there will be no back emf. If it is badly motivated, however, lots of static can be expected. Many sitations will support this.

Yes.

The starting current is greater than the no load running current therefor there must be a back EMF.

All electromagnetic motors exhibit back emf, at least internally.

You want someone on USENET to validate an answer? No, that's your job -- we're just here to dispense free advise (I guarantee it's worth every penny, though).

To see that this is true, get a book on electric machines, read it, and think about what you've read.

A university library or bookstore would be a good site to pick up such a book.

The answer is obvious from thermodynamics - AC motors can do useful work, so there has to be back emf.

The one time I had to prove it we were using a permanent magnet excited stepper motor (a synchronous AC motor), so we just clamped the shaft of the motor in the chuck of an electric drill, used the drill to spin the motor at the rate we wanted to achice, and measured the back emf excited in the (stationary) drive coils.

The voltage generated was higher than my boss had expected - he should have known better - and made it very clear why our motor was skipping steps at full speed. Increasing the supply voltage solved the problem.

=46rom thermodynamics, the back emf of an AC motor - in volts per radian per second - is numerically identical to the torque constant in newton metres per ampere.

-- Bill Sloman, Nijmegen

-- Bill Sloman, Nijmegen

Interesting. Could you show us how the thermodynamic argument goes?

Cheers,

Phil Hobbs

It's so easy even I can do it:

You start with conservation of energy, which says that energy in = energy out.

Then you hold your right hand over your left eye and announce that you will now do the analysis for a 100% efficient motor.

With a 100% efficient motor, all of the mechanical energy coming out of the motor has to be equaled by electrical energy going in -- so at steady state, the mechanical power out must equal the electrical power in.

Then you solve for the volt-amp and torque-speed relationships, and you're done.

Note that Bill should have said that for an _ideal_ motor the emf/speed characteristic is equal to the torque/current relationship. Real motors aren't 100% efficient, so measured values may come out different by a few percent -- data sheets for DC motors usually show this, as they are compiled from real measurements.

That's what I was wondering about. Normally when I hear "thermodynamic argument" I'm expecting something along the lines of "if it weren't true you could build a perpetual motion machine." For instance, the Johnson noise formula comes out of that sort of argument, and so does the 'cavity radiation == black body radiation' identity. So it sounds as though what you can really establish by thermodynamics is that back EMF >= torque constant.

Cheers,

Phil

Tim is absolutely right, but the inefficiency of real motors is mostly due resistive losses in the windings with extra losses from resistive heating from the induced currents circulating in the magnetic field path, and friction in the bearings.

The torque constant to back-emf equality can be screwed up if the temperature of the magnets or the magnetic path is differs differs between the times when you are measuring the torque - which people tend to do with the rotor stationary - and when you are mesuring the back emf which people tend to do when the motor is spinning close to flat out, which tends to stir the air in the motor.

-- Bill Sloman, Nijmegen

rgy

rs

And that there has to be back emf in the first place.

-- Bill Sloman, Nijmegen