Dear Sir.

Why the size of induction motor will affect the motor efficiency? i.e. Larger motor have higher efficiency.

Thanks

 

More effort is given in designing large motors because, at larger HP, the savings are greater. If you compare a 10hp at.92 eff, and a 1000hp at .96 eff, you can save much more $ if running at full load with the larger motor.

Plus, buiding a larger motor is easier (bigger parts). Small motors usually have wound rotors (easier to build), while large motors have solid squirrel cages which allow for better designs.

I'm sure there are more precise technical answers out there.

 

Magnetization flux for a larger motor, hence magnetization current of a larger motor is smaller as a percentage of total current required.

 

Responding to Accord.K's Tue, Oct 5 query:

An excellent question! Of course, tere are a myriad of factr to consider. And, although a motor designer could more adequately answer the question, I will try. My thesis is analogous to the theory of "Economy of Scale!" Although considered a myth in some circles... Big was Better!

In motor design 'Big is Better' has been well established, as Accord.K observed. Obviously, losses are decreased as machine size increases. Neglecting material advances, then, what is the reason?

The salient parameter is slip. Remember, in the energy conversion process, electrical power transferred across the air-gap is inversely proportional to slip. Thus, any thing that results in a lower slip will markedly increase power available for conversion to mechanical output. It can be shown that slip is a quadratic equation of the form:

A x S^2 + B x S + C = 0, having two solutions:

S1,2 = K1 +/- SQRT [K1^2 - K2], where,

K1 and K2 are constants involving motor impedances and losses. Most parameters vary inversely with size... some directly!

To corroborate my thesis I determined per-unit impedance and loss values from a large collection of motor data I acquired over the years. My data-base varied from 250 to 21,000 Hp, 2-pole, 50 and 60 Hz, induction (asynchronous) machines.

How, then does the above observation relate to size? Some of the improvement resulting from frame size increase follow:

o The length of the magnetic core increases, thus the ratio of active copper (in slot) to inactive copper (end-turns) increases.

o The stator-bore and rotor diameters increase, resulting in bigger slots, hence more copper.

o Gap-surface circumference increases, thereby allowing reduced specific electrical loading.

o Circumferential velocity increases resulting in higher coolant-air flow thru the gap.

o Mechanical losses are somewhat reduced because parts are larger, and specific mechanical loading is decreased.

Positive and/or negative critque are urged!!!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Some other factors may be the type of steel used in the magnetic circuit, the type and quality of the bearings used, and the diameter of the wire used. Low cost steel would have higher hysterisys losses. Low cost bearings would have higher frictional losses. Smaller gauge wire would have higher resistance losses.

Also with larger motors, the stator wire may lay in the slots better, resulting in less air gap between the windings and the stator iron. A further factor may be that producing a range of power ratings in a single physical motor size (for manufacturing reasons) likely means a less than optimum design for at least part of the product range.

I'm not a motor designer, but there are a lot of cost versus efficiency trade-offs to be made in a motor. With small motors customers are not normally as interested in efficiency as they would be with larger motors. If efficiency is not a factor in making sales, then the design criteria will be to meet the rated life at the lowest possible cost.

On October 8, 2004 01:29, Phil Corso wrote: <clip>
> An excellent question!
<clip>
> How, then does the above observation relate to size? Some of the
> improvement resulting from frame size increase follow:
>
> o The length of the magnetic core increases, thus the ratio of active
> copper (in slot) to inactive copper (end-turns) increases.
>
> o The stator-bore and rotor diameters increase, resulting in bigger
> slots, hence more copper.
>
> o Gap-surface circumference increases, thereby allowing reduced
> specific electrical loading.
>
> o Circumferential velocity increases resulting in higher coolant-air
> flow thru the gap.
>
> o Mechanical losses are somewhat reduced because parts are larger, and
> specific mechanical loading is decreased.
>
> Positive and/or negative critque are urged!!!
<clip>