Air Gap Values while designing Machine

Air gap is a necessary evil. Which means you can't avoid air gap in motors as if there is no air gap motor will not start. So you can't avoid air gap instead air gap can be minimized to as low value as possible.
In electrical systems air gap is main cause of concern in designing electrical motors. In an electromagnetic devices there is uniform practice that material selected is such that it will offer low resistance to passage of magnetic flux. This will reduces the electrical energy demand for creating the required flux. But there is always air gap in rotating machines which is unavoidable. This air gap requires increase in magnetizing current as there is high resistance of air gap which will requires more electrical energy to generate required flux.
This air gap will leads to undesirable electrical losses.

There is rule of thumb that higher the motor speed, the larger the gap. 


Why Air Gap Should be Small?

Let's State by taking an example of an Induction motor when Power supply is given to stator than magnetic flux is developed in rotor. There is always an air gap between stator and rotor. Air gap is having very high reluctance.
This high "reluctance" of air means that for every unit length of magnetic flux path, the mmf required to drive flux through the air portion of the path will far exceed what's needed for the magnetic portion. Consequently, the machine's magnetizing current  will be determined largely by the size of the air gap. The larger that gap in an induction motor, the lower the power factor and also lower is the efficiency of motor Or Alternator. 
This air gap is the only reason behind high no load current in Induction motors. In Induction motors there is high no load current is of 30 to 40% of full load current. In Transformers there is no air gap that is why Transformers have low no load current for same rating of machines.

In  case of other machines other than Induction motors such as synchronous and d-c machines. There are two separate magnetic fields interact in the air gap. The alternating current magnetic field created by the armature which is stationary in case of synchronous machines and rotating in DC machines, distorts that supplied by the d-c field, reducing its effectiveness and degrading machine performance thus air gap is required to be increased to reduce the effect of that "armature reaction." 
This is reason why Synchronous and DC machines have air gaps several times larger than those in induction motors.

That creates an unavoidable contradiction in the design process. 

For mechanical reasons, we want to avoid too small an air gap; for electrical reasons, we don't want it too large

So we always have to make a compromise between the two, For a given rotor diameter, the slower an a-c machine the smaller the gap. Horsepower output will be lower, and the power factor lower as well, so that the electrical effect of a large gap is relatively less acceptable than for a high-speed, higher-horsepower machine having that same rotor diameter. As polarity decreases and both speed and horsepower go up, the trend reverses.

Below Formula will give us the required value for air gap in our machines:-

Air gap, inch = 0.005 0.0003D 0.001 L 0.003V

in which D = rotor O.D., inches

L = core stack length, inches
V = rotor peripheral velocity in thousands of ft/min.= D(RPM/12,000)
Air gap must be uniform.. A non-uniform air gap also tends to increase noise.

Usually Air gap is kept between 0.2 MM to 5 MM of motors rating from ¾ Kw to 750 KW.