Smooth speed control of single-phase induction motors by integral-cycle switching

AC phase controlled switching is used for the speed control of single-phase induction motors but it introduces large high-order harmonics. Alternatively, an integral-cycle control method is also available but it introduces sub-harmonics in the line and the output voltage is adjustable in steps only. To mitigate these situations, a discontinuous phase-controlled switching technique is proposed. The voltage control is done by a combination of the phase control and the integral-cycle switching. Fine voltage and step voltage are controlled by the former and the latter methods respectively. For fan-type loads, performance of the proposed controller improves when this technique is applied to control the main winding voltage only. In case of constant-torque loads, conventional voltage controllers including AC regulators offer a very limited speed control range. The strong sub-harmonics of the controlled voltage due to integral-cycle control is used for such loads. The rotor is forced to lock at any desired sub-synchronous speed. Different types of motors operate smoothly over a wide range for fan-type loads and near various subsynchronous speed at derated conditions for constant torque loads.