Advances in Motor Torque Control by M. Ahmad

By M. Ahmad

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The induction motor voltage is controlled through a firing angle ramp in an open loop. This technique is simple, and it is used in low-cost commercial soft starters. It will always produce a starting quadratic torque curve that can be applied to small hydraulic pumps and small fans. Fig. 3. Basic block diagram of the voltage-ramp technique. Considering the starting motor using the voltage-ramp technique, the waveforms of electromagnetic torque and motor current are shown in Fig. 4. 36 Advances in Motor Torque Control Fig.

For an example of a load with quadratic torque characteristics (Tc), Fig. , with current limitation. The current is kept constant at a predefined value during the whole motor starting time, but the current control can only operate under a rated voltage variation of 10 %. From Fig. 6, it can be noted that through the current limitation (IMAX) the initial electromagnetic torque developed (Te) varies very little until 70% of the synchronous motor speed. However, during the final period of the motor starting process, one can verify the existence of a torque pulse, in which, for certain kinds of loads, an abrupt acceleration can be generated (Palma, 1999).

This technique also has problems with the starting of nonconstant loads, such quadratic loads, linear loads and loads with high resistive torque, because it is difficult to predict the torque profile produced in this type of technique. Fig. 5. Basic block diagram of the current control technique. Fig. 6. Electromagnetic torque and motor current waveforms using the current control technique. 38 Advances in Motor Torque Control From the electromagnetic torque characteristics shown in Figs. 4 and 6, it is noted that, using the voltage-ramp technique or the current control technique for limiting current, in both cases a torque profile is obtained that does not adequately match with the load profile.

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