Mechanical (friction and windage) losses (essentially independent of load) – Mechanical losses occur in the bearings, fans, and seals of the motor. These losses are generally small in IP4x and IP5x slow speed motors, but may be appreciable in large, high-speed or totally- enclosed IP6x motors.
Additional load losses (stray load losses) –The additional fundamental and high-frequency losses in the iron; strand and circulating-current losses in the stator winding; and harmonic losses in the rotor conductors under load. These losses are assumed to be proportional to the torque squared.
Listed below (table 1) are the motor loss components, with the typical percent of the total motor losses they represent, and the design and construction factors which influence their magnitude.
Typical percent of losses
Factors affecting these losses
Stator losses Rotor losses Core losses Additional load losses Friction and windage
4-pole mot 30 to 50 20 to 25 20 to 25 5 to 15 5 to 10
Stator conductor size and material. Rotor conductor size and material. Type and quantity of magnetic material. Primarily manufacturing and design methods. Selection/design of fan and bearings.
Table 1 Loss distribution in three-phase, 4-pole, cage-induction electric motors
In general, by increasing the active material in the motor, i.e., the type and volume of conductors and magnetic materials, the losses can be reduced.
Additional motor-losses when operated on a frequency converter
Harmonics of voltage and current in a cage-induction motor supplied from a frequency- converter cause additional iron and I²R winding losses in the stator and the rotor. The total value of these additional losses is essentially independent of load. These additional losses decrease with increasing switching frequency in the converter.
In adverse circumstances the additional losses in the motor caused by the frequency converter can increase the total motor losses up to 15…20% compared to grid operation.
For details see IEC 60034-17 and IEC 60034-25.
Motors for higher efficiency classes
It is expected that advanced technologies will enable manufacturers to design motors for higher efficiencies than IE3 with mechanical dimensions (flanges, shaft heights etc.) compatible to existing motors of lower efficiency classes (for example EN 50347, NEMA MG1 and other local standards). These motors usually require power electronics (frequency converters) to operate.
Losses in the rotor are almost eliminated by using synchronous motors without field winding. In Annex A, this guide proposes a super-premium efficiency-class IE4 which is specifically targeted at such motors (although the efficiency class IE4 as such is not limited to specific motors).
Permanent-magnet (PMSM) and reluctance (RSM) synchronous-motors are already developed and to some extent commercially available. PMSM usually have some inherent reluctance torque and RSM can be PM enforced thus hybrids are possible. Depending on the amount of magnet material used, a PMSM can have a higher power factor than an induction motor thus improving efficiency in the distribution network and in the frequency converter. These motors however require a frequency converter and a rotor position sensor (encoder) (unless an encoder-less control algorithm is used in the converter) for proper operation.