OT: motor currents

Even at no load, there is some in-phase current, real power. That is sometimes called "windage."

Are you writing a manual, or is the issue more general?

Yes, the stator current required for the magnetization of the iron (and the air gap) generate heat, i.e. real power, in the windings.

Then there are hysteretic losses from the alternating magnetization of the iron.

Then in order for the motor to be able to run at all, you need a field generated by the rotor, i.e. current and therefore also resistive losses.

This rotor current emanates from the in the short circuited rotor cage induced voltage as the rotor turns a bit slower than that the rotor's magnetic field runs around through the air gap. So the rotor current is transformed into a stator current component, also leading to additional copper losses in the stator windings.

Then there are the losses in the bearings.

And maybe the greatest loss under 'no load' is from the cooling fan.

joe

No manual-writing; just generally speaking. What I'm getting at is, would it be more accurate to say:

that "under no-load conditions in an *ideal* motor, the *total* motor current equals the magnetizing current?"

Whereas in the real world, AIUI, with windage and friction etc taken into consideration, the no-load current comprises about 30% of the total motor current.

Are both the above statements true?

I think yes.

joe

Hence the term "windage", which is sometimes used to describe all the no-load losses. Air shear between the rotor and stator are part of that.

I took two semisters of Electrical Machinery, with labs, which I think isn't done much any more. I hated it and learned a lot.

Depends on what "ideal motor" means. I'd vote that an ideal motor has zero current spinning with no mechanical load.

For practical motors, like things people would buy, most of that current is reactive, not true power. But motor data sheets would have the numbers.

I didn't hate it at all.

joe

Yes, mostly reactive at no-load and light loads, but as the load goes up, the phase lag reduces and the power factor improves along with increased load current so presumably at some point the magnetising current (which is constant irrespective of load) will become less than the load current. I'm no motor expert but that would be my guess, anyway.

You didn't have Professor Webb!

It was required when my brothers were in school but they'd dropped it by the time I got there. I didn't have to take "welding shop".

Nah, he was the chairman of the department. ;-)

Everyone should learn to actually weld. There's a private high school near here, tuition about $45K, where welding is a required course.

Our machinery course was more of a smokehouse.

Where do you get the idea magnetising current is constant?

Also, you never specified what type of AC induction motor?

Basically, the standard model of a single phase AC induction motor won't even start! it'll just sit there and hum and get hot as hell as currents are high. Until of course, you give it a spin.

I am just curious as to where you get this idea, that is all.

Jamie

Yeah, I'm aware of that - and the fact that there are various additional components to the basic model that get it up and running; centrifugal or time delay switched in/out capacitors and whatnot.

Same site, I believe; in particular this page here:

There are single-phase AC induction motors, mostly in small appliances like shavers, mixers, washing machine pumps and small electric fans, where a part of the magnetic circuit in the rotor is modified with a short circuited resistive coil, covering a part of the air gap. This causes a small difference in the left-turning versus the right-turning magnetic field components in the air gap, causing the motor to start, after which the co-rotating magnetic field component can do its job and produce torque.

This requires a careful adjustment in the design and often a 50 Hz motor (washing machine pump) following this principle doesn't start on a

I'm sorry that I can't recall the principle sufficiently accurate to explain it more in detail.

joe

Welding! I would have loved that. Instead I got lathing.

joe

Probably not. I had some bad, but luckily also some good professors.

joe

That's "shaded pole", and are generally small motors with low starting torque, like for fans.

Except that the welding was done with 4-0 wire and 100HP DC motor-generators.