This project is an attempt to develop a working model of a “SPEED CONTROL OF INDUCTION MOTOR IN A NEW APPROACH” which is popularly known as the work horse of the industry. It is aimed to control the speed of the supply system while maintaining the ratio of voltage to frequency a constant (V/F drive).
As a first step in this direction, this report deals with varying the speed of a single phase motor by varying the frequency of the supply system without considering the ratio of voltage to frequency. A single phase bridge inverter is used for this purpose in which the power circuit consists of four Metal Oxide Semi-conductor Field Effect Transistor (MOSFET) and control circuit is built with pulse width modulation scheme like in any commercially available VFDs in the market.
A sinusoidal wave generated by an IC 8083 is compared with a triangular wave generated pulses were used to fire MOSFETs operated in positive half cycle of the sine wave generated. Necessary care is taken while transferring these pulses to the gate circuit of the MOSFETs by providing isolation circuit using optocouplers. Each MOSFET is connected to a diode in anti parallel connection to enable the transfer of current from MOSFETs operated in positive half cycles to MOSFETs operating in negative half cycle and vice versa. Out of four MOSFETs which are part of inverter circuit, two are fired simultaneously in positive as well as in negative half cycles. The pulses for the second set of MOSFETs are obtained by inverting the reference sinusoidal wave thereby the generated pulses are fed to MOSFETs.
The reference sine wave used in the pulse generation circuit will decide the frequency of the output sine wave of the inverter. As the reference frequency changes, keeping the triangular wave unaltered, the output sine wave frequency can be changed thereby the speed of the motor changes. The range of frequency is between 20Hz to 50Hz. A free-wheeling facility is also provided across the connected load to improve the performance of the circuit whenever the pulse is dropped.