Comparing the indirect field-oriented control with a scalar method

Two methods of induction motor control are examined. The indirect field-oriented (IFO) and the slip-control (SC) methods are formally compared. Their block diagrams are derived, analyzed and their similarities shown. It is demonstrated that the difference between the two is just due to a feedforward block that computes the current phase to be supplied to the motor in an IFO controller. This proves that the implementation complexity of either controller is almost identical. To perform comparative tests, both control methods were implemented using a single hardware. The torque rise time and frequency response of the speed control are presented. Based on experimental results, it is shown that the speed sensor resolution and the sampling rate of the controller strongly influence the dynamic response in the IFO method. The SC method is less sensitive to these parameters although it always presented a worse dynamic response. The IFO speed control presented a flat frequency response whereas the SC method presented a peak which could lead to an oscillatory speed response. It is also shown that the variation of the rotor electrical time constant influences strongly and in a similar way both control methods. Finally, it can be concluded that the implementation complexity of both control strategies are almost identical, although the IFO control method has a much better performance than the SC control method.