Separation of D and L
The output equation gives the relation between D2L product and output of the machine. To separate D and L for this product a relation has to be assumed or established. Following are the various design considerations based on which a suitable ratio between gross length and pole pitch can be assumed.
i. To obtain minimum over all cost 1.5 to 2.0
ii. To obtain good efficiency 1.4 to 1.6
iii. To obtain good over all design 1.0 to 1.1
iv. To obtain good power factor 1.0 to 1.3
As power factor plays a very important role the performance of induction motors it is advisable to design an induction motor for best power factor unless specified. Hence to obtain the best power factor the following relation will be usually assumed for separation of D and L.
Pole pitch/ Core length = 0.18/pole pitch
or (πD/p) / L= 0.18/ (πD/p)
i.e D = 0.135P√L where D and L are in meter.
Using above relation D and L can be separated from D2L product. However the obtained values of D and L have to satisfy the condition imposed on the value of peripheral speed.
For the normal design of induction motors the calculated diameter of the motor should be such that the peripheral speed must be below 30 m/s. In case of specially designed rotor the peripheral speed can be 60 m/s.