This paper designs a hybrid speed controller in which a Sine Triangle Pulse Width Modulated (SPWM) inverter is used below the base speed and a square wave inverter is employed above the base speed. The two inversion techniques complement each other for their advantages and disadvantages below and above the base speed. This paper proposes a unique strategy for the transition between SPWM and square wave by simply setting the frequency of the carrier signal equal to zero. The proposed methodology in a way uses only one inversion technique and realizes a seamless transition from the SPWM to square wave compared to conventional method in which modes are simply switched from SPWM to square wave and vice versa when the speed changes above and below the base speed, respectively. Computer simulations show that the proposed technique has smoother torque transition and thus a better speed response compared to conventional approach of inverter mode switching around the base speed. The performance of proposed hybrid approach is also validated on a small prototype induction motor through experimental results. 1. Introduction The high speed, medium to high power induction motor drives have a wide variety of applications like traction drive of high speed railway, alternative energy vehicles, and many other industrial applications. However, in these high speeds, high power drives, the sine triangle PWM technique becomes impractical because of lower DC bus utilization, higher fundamental frequency at which the motor operates during high speed operation, and unavailability of high power devices with high switching frequencies. Thus, for all such applications, square wave inverters are being employed [1–5]. However, the square wave inverter suffers from the harmonics problems which are currently being controlled using adjustable filters for such drive systems [6, 7]. The harmonics problem becomes worse as motor speed becomes lower than the base speed. The low frequency operation reduces the motor leakage inductance which increases the harmonics current and thus reduces the motor efficiency badly [2] and also causes torque pulsations in the machine. Another problem at the low speed operation is that motor voltage needs to be proportionally adjusted to the motor speed. This voltage for a fixed DC bus voltage cannot be controlled using a square wave inverter, while with a Sine Triangle Pulse Width Modulated (SPWM) inverter the inverter output voltage can be controlled by controlling the amplitude of reference signal. To overcome these problems, this paper designs a hybrid
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