Method for Protection of Induction Motor from Phase Imbalance and Over Temperature

Abstract

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 and over temperatures. The overall process is monitored by ATmega32 microcontroller. Microcontroller sends signal through MOSFET to relay for operation. Knowing the phase sequence of three phase motors is quite an important thing. Considering if the motor uses a pumping action, even a single phase reversal that results in the wrong sequence may lead the motor to run in the wrong the direction. Well it may also lead to fail permanently due to dry run of the motor.

Here in this system a direct 3-phase 50Hz Ac supply is at first transferred through zener diode stabilized voltage drop setup to an (OR – NAND) gate integrated logic circuit in order to recognize proper RYB sequence arrangement by fixed duration pulse series. In the process of transforming the RYB sequence to YBR the NAND OR gate integrated combination generates an output having a pulse missing during fixed durations of time. This pulse is now used in order to provide a trigger as input signal to a microcontroller of 8051 family.

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. In case of single phasing, the relays gives the supply to the rectifier and the power is given to the motor through inverter circuit.