A Critical Approach to the Iron Losses in Induction Motors

Abstract

Since induction motors represent the more consistent electrical motors used in industrial environment, the increase of the induction motor efficiency represents a crucial aspect to obtain a consistent energy saving. In the USA, from 1997 the electric motor producers have to build high efficiency motors in according to the protocol promulgated by the Energy Policy Act (EPACT). This protocol defines the minimum value of the efficiency for general purpose, 2 or 4 poles, 400V, 50÷60 Hz, three-phase induction motors with a rated power in the range 1÷200 HP [1]. In Europe, the European Committee of Manufacturers of Electric Machines and Power Electronics (CEMEP) has realized a similar protocol to which the electric motor producers can to agree in voluntary manner [2]. An increase of the motor efficiency means a reduction of the loss contributions. The total motor losses can be divided in the stator and rotor Joule losses, in the iron losses and in the mechanical losses. A viable way to improve the efficiency of an induction motor is the reduction of the iron loss contribution, because this choice does not require a complete change of the magnetic sheet shape. A possible solution for getting a strong reduction of the iron losses could be the use of good quality magnetic materials (with specific iron losses in the range 2.7÷3.5 W/kg at 1.5 T) instead of the low or medium quality magnetic materials typically used in general purpose induction motors (with specific iron losses from 5.3 W/kg up to 8.0 W/kg at 1.5 T). In a previous paper [3], the authors have shown that a reduction of the magnetic material specific iron losses does not produce the expected iron loss reduction.