Not all electrical appliances draw more current in direct proportion with the reduction in voltage and hence the answer given last week is not complete. Broadly, electrical appliances used in homes can be classified into two groups – appliances such as electric irons and electrical heaters which convert electrical energy into heat energy. Inca descent bulbs also come under this group. The second category includes those which use electrical motors, in which electrical energy is converted into mechanical (rotational) energy.
In the first group, the current drawn is proportional to the square root of the voltage. Hence, when the voltage is 90 per cent of the rated voltage, the current drawn will be 95 percent of the current drawn at rated voltage, that is when the voltage is lower, the current also will be lower, though not in direct proportion.
In the case of appliances using motors, the capacity marked on the name plate (usually in kW or W) is the power available on the shaft for conversion into mechanical power. In such appliances, the current drawn is inversely proportional to the applied voltage, that is, when the applied voltage is lower; the current drawn will be proportionally higher, with the mechanical power remaining constant.
Motors operating at low voltages burn out because they tend to draw unduly large currents which cannot be carried by the wires wound inside them.
In the case of fluorescent lamps, their characteristics vary with input voltage, that is, the exact changes depend on the type of circuit used, and so manufacturers’ technical data may be consulted.
