Can you pls tell always the smaller dimensions motors are in efficient can you pls explain, Also pls tell what can be the minimum airgap will be in the smaller motors.

The smallest airgap that I have designed for is 0.3mm (a 400W induction motor).

"Why are large electromagnetic devices more efficient than small ones?" This was a question asked by Professor Eric Laithwaite (best known for his linear motors). To answer the question, he developed the concept of a "goodness" factor for electrical machines. His work is well worth reading.
One of his conclusions was that it was a good thing that Nikola Tesla was a confident and ambitious man. If he had decided to make his experiments with induction motors on machines of smaller size, they would not have worked!
Laithwaite also used to refer to "paralysis by analysis" - meaning that you just had to get out into the laboratory and build it :~)

For 2P (3000 RPM) motors, the minimum airgap (mm) should be 0.2 + stator bore diameter(D) / 666 and for 4 or 6 Pole (1500/1000 RPM) motors , the minimum airgap should be 0.2+ D /1000. For larger core length (stack length) machine, the aitgap generally higher than the limit as specified above.

For general purpose 2P motors the air gap size is Rotor OD - 0.8. For other poles normally, it is Rotor OD - 0.6. It all varies depends upon the design condition like Slip, Efficiency of the motor , Starting Torque and Insulation classes . For Normal calculations Please, follow CYRIL G. VEINOTT methods.

Ganesh - Smaller motors are more inefficient due to the fact that the torque is proportional to square of the motor diameter and linearly to length (T = D^2L). Therefore as diameter or length is reduced, output power suffers. To increase the output power, one must increase speed. That may or may not be feasible. Smaller airgap can be tricky to implement. Although Induction motors can have as small of an airgap as 0.1mm, permanent magnet motors with exposed magnets may present significant challenges in manufacturing with that kind of an air gap. Permanent magnet motors with embedded magnets inside the lamination can be handled better with smaller air gaps during manufacturing.

I am tempted to ask my question here considering august gathering here of so many designers following--- Can u design for me a universal mechano type setup of 100W rating wherein u could replace both rotor and stator coils to construct either DC motor / Generator , 1 and 3 phase squirrel cage motor and Slip ring alternator, universal motor ,reluctance motor etcI had read a project report from a Japanese Institute ,I wish to recreate the same
My email = ansgynabtech@hotmail.com

Even if u can offer design for one or two motors ,that will do ,not necessary to undertake all mentioned

Rajeev - We love designing motors. Our specialty is Permanent Magnet motors and motor drives for Induction Motors and PM Brushless Motors. We have offices in US, UK and Pune, India.