UNIVERSAL SERIES MOTORS

The construction and principle of working of a universal series motor are similar to a dc series motor. To enable the motor to work satisfactorily on ac supply also, some modifications are required in its construction. The important modification required are:

(i) Field structure should be completely laminated to avoid losses due to eddy currents:

(ii) To combat effects of armature reaction and resulting poor commutation the armature is to be designed to have lower voltage gradient between adjacent commutator segments than in an equivalent dc motor;
(iii) Poor commutation with ac (due to emf induced by the alternating main field flux in a coil undergoing commutation) is improved by using distributed field windings and compensating field windings that are placed in a slotted stator core. When a universal motor is used with ac supply the armature reactance drop exerts a speed lowering effect with increased loading. At the same time at increased loading the effective flux per ac ampere is less than that produced per dc ampere. This condition tends the motor to run faster on ac supply. Out of the above two factors i.e., of armature reactance drop, and effective flux, the factor which pre-dominates determines the speed of the motor on ac supply.