Voltage Stabilisers


Kedar Kulkarni

“Papa, why do we use stabilisers for the refrigerator and air conditioner?” Raju asked his father. He always wondered about the small box connected to the refrigerator.

Raju’s father said: “Good question, Raju. A voltage stabiliser is a device that stabilises the voltage going to the appliance so that it is supplied stable voltage and functions well. Every electrical appliance is designed to work efficiently at a specific range of voltage. We have seen sometimes that the lights suddenly become bright or begin to dim. This is due to fluctuations in the voltage. If it is too frequent, too high or too low, components in the circuits of the appliance may fail, thus making the appliance unusable. Sometimes the MCB trips which indicated there is high current flowing through the circuit. This may be because of short circuit or high voltage surge in the supply of electricity.”

The stabiliser acts as a safeguard for the appliance connected to it. It gives constant voltage in over-voltage and under-voltage conditions. It detects these conditions and brings the output end voltage in the required range of operating the voltage of the connected appliance. The stabiliser has an auto transformer, comparator, a switching circuit and relays.

An appliance maybe designed to operate within 230+/- 10 volts while the voltage supply may vary from 160 V to 270 V. An appliance subjected to these fluctuations could get damaged. A drop in voltage will reduce the torque and hence the power of a motor. A fan will run slower than normal in an under-voltage condition and run faster in case there is over-voltage. Whereas a bulb may fuse in case there is a sudden hike in voltage and glow very dimly in case of under-voltage. Some appliances like tube lights may not work at all at lower voltages. Refrigerators, air conditioners and television sets are high value appliances.  Refrigerators and air conditioners have motors for compressors and hence they need protection.

When the stabiliser senses a drop in the incoming voltage, it enables the electromagnetic relay so as to add more voltage from a transformer to boost the voltage to normal value. When the incoming voltage goes higher than the normal operating value, another electromagnetic relay is activated in order to bring down the voltage and maintain a normal value. In case of under-voltage condition, the stabiliser increases the voltage to a safe operating voltage. This is called boost operation and reduction in voltage in over voltage conditions is called buck operation. There are industrial 3 phase stabilisers. Today televisions, air conditioners and refrigerators come with built-in stabilisers, however one still sees stabilisers connected to these appliances as a safety measure.

(Writer is a mechanical engineer and runs a hands-on science activity centre at Margao)