3rd and 5th harmonic rotor loss

It's complicated.

First, the torque ripple at 5x your power-line frequency may or may not drive any dissipative processes in the mechanism.

Second, mechanical vibration isn't the only loss mechanism. Depending on the effective inductance of the motor at that 5th harmonic, the current working into the armature resistance could very well have greater loss electrically, before it even gets a chance to cause any mechanical losses.

You would need to know the electrical characteristics of the motor, and the mechanical characteristics of the motor as attached to its load. Knowing those, you could then calculate the losses (and any mechanical vibration that may be externally apparent).

On the mechanical side, as an experiment, rig up a toy DC motor with an offset weight. If you let it run just hanging by it's leads it'll go at some speed. Grasp it loosely, to maximize the absorption of it's vibration by your fingers, and you'll hear the motor slow down. Finally, hold it tightly, or better yet hold it tightly against a block of wood or something -- it should go faster than it did hanging by its leads. You've just demonstrated that vibrational energy only gets absorbed if it has somewhere to go.

Is this a real life question?

Without more information I would have thought the loss would be the resistive loss element as the motor would look largely reactive at any harmonic.

The rms current of the fundamental plus the 1% 5th harmonic can be compared to the fundamental alone. It may make a difference to whether the motor is of star or delta variety, as any star point will exhibit some of the 5th harmonic.

BICBW

Thanks. And yes, it seems it's complicated.

Some absolute figures here:

But, I think I will skip the calculation and make a test instead :-)

Cheers

Klaus

See tag-line. ...Jim Thompson

Interesting article, I forgot that a 5th field harmonic would try to rotate the rotor at 5 times normal speed therefore represents a nominal

According to the article, it takes about 12% 5th harmonic distortion to cause a 1% reduction in efficiency. It is difficult to measure a 1% difference in efficiency, so it becomes nearly impossible to detect a change due to 12 times less (1%).

Odd harmonics are components of a square wave, and the fundamental plus third and fifth harmonics forms a fairly good approximation of a square wave. I have driven single phase motors with a "modified" waveform from an automotive inverter, and also with a square wave using a full three leg bridge for a three phase motor. They worked adequately, but probably with "horrible" efficiency.

It's unlikely that an otherwise pure sine wave would contain 1% of only 5th harmonic distortion, at least not from an ordinary AC line source.

Paul

Yes, I deduced the same. But, I think the article makes some assumptions and maybe it's only a first order model. So I will measure to make sure

In my case I actually have 5% 5th harmonic, but that would still be low loss if the article is correct

Cheers

Klaus

Also note that 5th harmonic fields rotate in the wrong direction (if I recall correctly) and 3rd harmonic fields do not rotate at all.

Doesn't the inductance of the windings come into play ? Seems to me it would.

Sorry, skimming the thread I missed that part before posting.

Also, I am wondering if this is a question on a test and help on the homework or if he is actually trying to run a motor off of some sort of inverter or something that is not quite sinousodial.

I haven't done homework in 15 years, except for the pro-bona hours for my employer

I could have mentioned the exact THD of each harmonic, but I am not looking for absolute numbers, but rather the theory behind it

Cheers

Klaus