Thank you very much takao21203. I will enjoy reading it.

 

If we assume the same geometry, is the air gap a constant rather than a variable?
- Same magnetic field in stator with different current flows.
- Same air gap.
- Same toque
So, with different current flows at the stator windings, we may have same torque.
Am I right? I mean no load case.

I think the more relevant question is the component for the air gap MMF which is embodied in the total magnetizing current. If the air gap MMF is significantly greater than the magnetic material MMF then the return on investment for using higher permeability + high saturation flux density material may not be realized.

I'd ask the question - why haven't induction machine manufacturers gone down that road already if the economics were justified.

 

I'd ask the question - why haven't induction machine manufacturers gone down that road already if the economics were justified.

I think this is because there is no soft magnetic material which will satisfy the high permeability and high saturation magnetiztion. We don't know how to achieve that particular magnetic properties. We know what is the best material, but we do not know how to make it. And my question is if such materials are developed, what will be the merits of that materials in motor? Especially in terms of efficiency?

In electrical steels, the highest saturation magnetization material is Iron itself. However, because of low resistivity (or high eddy current loss) Si is added. By doing this, people can reduce iron loss. But at the same time, we sacrifies magnetization. Therefore, we increase the copper loss by adding Si since it is difficult to magnetize to, say, 1.5 T. So, it is a kind of trade-off between iron loss and copper loss. To obtain high permeability, it is necessary to control, so called, texture, preferred orientation in atomic arrangement. But, upto now, no easy method is available commercially.

 

You haven't addressed what I believe to be the more important question from my previous post ...

"I think the more relevant question is the component for the air gap MMF which is embodied in the total magnetizing current. If the air gap MMF is significantly greater than the magnetic material MMF then the return on investment for using higher permeability + high saturation flux density material may not be realized."

 

I found this discussion paper on the issue of induction motor efficiency which may be relevant to your considerations.

 

"I think the more relevant question is the component for the air gap MMF which is embodied in the total magnetizing current. If the air gap MMF is significantly greater than the magnetic material MMF then the return on investment for using higher permeability + high saturation flux density material may not be realized."

Air gap, Hmmm, it's a difficult part for me to explain because I do not understand it clearly. Sorry, I am a mateirlas engineer!
However, I consistently say that the air gap is a constant, not a variable.
And the starting point of motor torque is in fact magnetization of stator core.
If we have the same magnetic field at stator tooth, we will have the same torque!
But this is an indirect answer.

The problem in air gap business is that it has a low ability to carry magnetic flux density. So it acts as a bottle neck for magnetic flux. So how well my stator core materials can be magnetized, such effect can be significaly mitigated by the air gap.
This is the point I'v heard from motor guyes.

However, because of this high reluctance in motors (actually two air gaps between stator and rotor) we need high magnetization materials. To obtain high magnetization in stator to overcome the high reluctance, we need to apply high current in stator windings. To do that, we have very high copper loss (I^2R at stator core).

With high permeability materials, we can get the same magnetization value(say 1.5T) with less current. Therefore, we can save significant amount of copper loss.

My point is that the first step is magnetization of stator core.
Does it make sense?

 

With high permeability materials, we can get the same magnetization value(say 1.5T) with less current. Therefore, we can save significant amount of copper loss.

My point is that the first step is magnetization of stator core.
Does it make sense?

There's the root of the problem which must be resolved. Suppose 60% [or more] of the magnetizing current goes into driving the air gap flux - for the same air gap structure the pay-off for improving the magnetic material properties would not necessarily be significant.

I'm not sure the issue is just about high permeability either - the magnetic losses will depend on other factors to do with the material used. The presence of harmonics is probably also a significant factor.

I'm not [nor ever was] a machine designer. You would need to explore these ideas with someone who is suitably experienced in this area to tease out the practical issues and limitations involved.

I believe the pdf attachment I provided earlier explores some of the practical issues and options for efficiency improvement.

 

With respect to low loss core materials I note there is some interest in amorphous metals for magnetic circuit design in transformers and motors. Hitachi [for instance] has made some contributions. This might be a useful option in your thinking.

 

Thanks t n k! It was very helpful discussion!

 

I believe there are two types of role of stator cores in motors.
One is inductor-store energy in a magnetic field.
Second part is load carrying ability.
As an inductor,what is important is energy loss- iron loss and copper loss during induction.
As a load carrier, it should have a high magnetization for higher torque.

With high permeability materials,
we can reduce copper loss as well as enhance torque.

Maybe, now I understand it.

 

I have one more question.
If the air gap is so important, why nobody mentions about the energy dissipation by air gap? I haven't seen any comment on relationship between air gap and energy loss? This is because it is a reactive element?

 

The gap is where the torque is actually made. The biggest reason nothing is done about the gap is cost. To maximize power production it wold take precision machining of both the stator inside diameter and the rotor outside diameter. This would need to be done after the assembly of both items. This would mean much higher production cost over the normal as stamped rotor and stator stacks. The off the shelf motor is a best case average between output of power verses cost.

 

By reducing air gap, we can increase magnetization of the rotor.
So it draws more current at the stator windings.
Therefore we will have higher torque.
If the gap is wide, magnetization at rotor will be less.
So, it will draw less current at the stator windings.
Therefore we will have less torque.

So the air gap is a matter of torque, not energy efficiency???

 

Maybe you should do some more research on how the motor actually works. No disrespect meant by saying this. Higher magnetization is not always a cure for making a better motor. I'll give some good links for you-

http://hmin.tripod.com/als/ccs/docs/pdf/150-br008.pdf

http://www.enginuitysystems.com/files/p20pg095%20(Motor%20design).pdf

http://www.pdhonline.org/courses/e176/e176content.pdf

 

Thanks shortbus for your help.
As you see, I am studying induction motors as a magnetic materials engineer.
I found difficulties in understanding how magnetic materials is used.

To my understanding, the best magnetic material for induction motors is,
"materials with low iron loss 'and' high magnetization"
This has been a direction of magnetic materials research for last 50 years in the field of Fe-Si alloys.

Especially, almost all the people in this area concern mainly on iron loss.
This is because there is a way to improve Iron loss properties; increasing resistivity by alloying Si, Al, Mn.
However, by doing this, we are sacrificing the magnetization. This is because magentization in iron-base alloys is due to iron and the portion of iron is reduced by alloying.
The only way to improve magentization under a given alloy composition is improving texture, preferred crystal orientation. But upto now, we do not have a commercial process to accomplish this.
And my question is if magnetic materials with high magntization with comparable iron loss materials to the existing electrical steels come out, how does that materials affect motor efficiency, torque, loss, etc..

 

I'm just a hobbyist and not an engineer, but have been working toward making a SRM so have been reading all I can. It seems that, from my understanding, the new step in motor/transformer steels is adding cobalt to the alloy. But then that also adds expense.

And there has/was been interest in ceramics - ferrite's too. But you are probably more able to get the latest up to date information than I am. The internet stuff is several years behind what is going on in industry.

 

Yes, if we can use cobalt, we can enhance magnetization significantly.
In Fe-Si, we are using 1.5~1.7T of magnetization. However, if we use permandur (Fe-50%Co), we can use 2.2~2.4T. So we can make a strong motor with small size. However the materials price is about 50 times expensive than the conventional Fe-Si(electrical steesl).
Little bit cheaper one is Fe-17~27%Co, however, their prices is more than 20 times expensive than Fe-Si.

If we are using ferrites, there are various grades. However, its con is low magnetization, probably much less than 1.0T.

 

Another thing they have been doing research on and been successful with is, using copper instead of aluminum for the rotor bars. It is the reaction of the eddy currents in the shorted rotor bars that helps create the gap forces that makes power.

But again that increases cost. The induction motor has been around for long enough now that efficiency verses cost has been pretty much balanced out. Until some new materials are found.

 

jinksung, the way I understand this and was taught in school, the current in an induction motor at no load is almost completely reactive current, ie. almost 90 degrees out of phase. You're not paying for the electricity used in most cases since the utility company bills on in phase component. As the motor is loaded the in phase resistive component increases and so does your electrical bill. The two components of current add vectorily so that means the phase angle of the current will move in the direction of zero degrees as the motor is loaded. Another point to remember is the spinning rotor creates a back emf opposite to the line voltage which limits the current through the motor windings just to the point of overcoming the resistance to turn the rotor. As the motor is loaded the back emf is reduced which increases the current flowing in the windings. That will make the motor heat up and if loaded enough stall and burn out the windings.

 

jinksung, the way I understand this and was taught in school, the current in an induction motor at no load is almost completely reactive current, ie. almost 90 degrees out of phase. You're not paying for the electricity used in most cases since the utility company bills on in phase component. As the motor is loaded the in phase resistive component increases and so does your electrical bill. The two components of current add vectorily so that means the phase angle of the current will move in the direction of zero degrees as the motor is loaded. Another point to remember is the spinning rotor creates a back emf opposite to the line voltage which limits the current through the motor windings just to the point of overcoming the resistance to turn the rotor. As the motor is loaded the back emf is reduced which increases the current flowing in the windings. That will make the motor heat up and if loaded enough stall and burn out the windings.

Well I consider it very illustrative to start up a 700W hand drill having a weak grid supply. You will see lights do darker for a moment...

When you drill into walls, after a while it will turn hot...