BigPatents India

The invention concerns a method for speed controlling an electric motor by means of a frequency converter, and a frequency converter applying the method. The frequency converter can be connected to one of several different mains voltages and is a socalled multi-voltage unit. When connected to a lower nominal mains voltage than the one, for which the frequency converter is dimensioned, or when the mains voltage drops, the frequency converter will demand a larger current from the mains to meet the power demands of the motor. However, this may damage the input of the frequency converter due to a too large current. According to the invention, the problem is solved in limiting the maximum output power of the frequency converter, when the actual mains voltage is lower that the maximum nominal mains voltage, for which the frequency converter is designed, and that during the limitation the frequency converter controls the speed of the motor within a power range up to the limited maximum output power. This gives a genuine multi-voltage unit, which can be connected to a wide range of mains voltages. Further, an adaptive current limit is described, said limit adapting to either temperature, frequency or load profile.