Inverters

Traditionally, the dc series connected motor has been extensively used in traction applications because of its characteristic high starting torque. The ac induction motor has a similar characteristic provided the frequency of the applied voltage is equal to the optimum slip frequency of the machine. As the machine accelerates, the supply frequency must be increased such that it maintains a difference, equal to the optimum slip frequency, above the motors rotational frequency. This, plus the control of the supply current, is the function of an inverter.