Protection of three phase induction motor has been done using the microcontroller, current transformer and step down transformer. This protection scheme protects the three phase induction motor from single phasing, under voltage, over voltage and over current. The overall process is monitored by ATmega32 microcontroller. Microcontroller sends signal through MOSFET to relay for operation. This microcontroller can sense 8 analog inputs up to 5V. Six analog inputs of microcontroller have been used for three phase voltages and currents individually for conversion to digital signal. The remaining inputs can be used for receiving the information from motor such as temperature, speed etc. The current transformer is an i-v converter (20A/20V), which gives output in terms of voltage and can be fed to the microcontroller directly. Step down transformer is (220V/6V). The output of these transformers will vary proportionately with respect to its input. The microcontroller senses the voltage, compares with the reference value and sends control signals to the respective protective relays. The overall system is cheap and reliable. It has been tested several times and gives the good results. A three phase inverter is designed to run the motor in emergency cases. In the event of single phasing the rectifier is connected to remaining two phases and the output is given to three phase inverter which converts the dc power into three phase ac supply. Six pulses are designed to give control signals to six MOSFETs of the inverter through the ATmega32 microcontroller. The output of the six gate pulses for the inverter have been checked on the CRO and the results are satisfactory. In this protection system, the limits for the voltages and currents have been taken as +10% and +30% respectively for 3 seconds. If any phase voltage or current goes beyond its limit, the microcontroller will send trip signal to all the three relays simultaneously so that the motor is disconnected from the power supply. In case of single phasing, the relays gives the supply to the rectifier and the power is given to the motor through inverter circuit. RCD snubber circuit is used to protect the MOSFET against voltage transients.

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