VOLTAGE PWM TYPE CURRENT REGULATION
The schematic arrangement of PWM type control circuit is shown in fig. 3.11
Principle of operation
Through transducer (tachogenerator) the mechanical signal (speed) is converted into electrical signal (current), which is fed from at the base of transistor T2. Thos base current combining with collector current flows the emitter of transistor T2 through CLR to the
negative of the supply. Based on the feedback signal, the voltage at phase A changes. This feedback voltage is given as one input to the operational amplifier where it is compared with the reference voltage, correspondingly the difference is amplified and fed to the mono stable circuit. This circuit modulates the pulse width of the incoming signal based on the requirement and the modulated signal is given at the base of T1.This signal combines with collector current of T1 and flows through phase A as modulated current based on the requirement. Thus the current is regulated or controlled using pulse width modulation and rotor feedback.
A desirable future of both control methods is that the current wave form tends to retain the same shape over a wide speed range.
When the PWM duty cycle reaches 100%, the motor speed can be increased by increasing the conduction period. These increases eventually reach maximum values after which the torque becomes inversely proportional to speed squared but they can typically double the speed range at constant torque. The speed range over which constant power can be maintained is also quite wide and very high maximum speeds can be achieved, as in the synchronous reluctance motor and induction motor, because there is not the limitation imposed by fixed as in PM motors.