A simpler motor control with block-commutated voltage of low switching frequency is also commonly used in small-size and/or high-speed motors (“brushless-DC” or “electronically commutated (EC) motors)”). The main disadvantage is the additional losses due to parasitic harmonic voltages and currents. The improvement in efficiency over asynchronous motors is less compared to the improvement of PWM (pulse-width modulation) controlled permanent- magnet or reluctance synchronous motors.
Another synchronous motor design features both permanent-magnets and a cage. It can therefore be used for on-line starting (line-start, permanent-magnet, synchronous-motors “LSPM”). These motors do not necessarily need a frequency converter for operation. However their starting performance is rather poor with torque ripple and noise and considerable restrictions on the permissible load torque and load inertia. They need to be closely matched to the application and cannot be used as general-purpose machines.
NOTE It is envisaged to expand the scope of IEC 60034-30 and amend it with this Annex A (as normative) when more experience with synchronous motors in standard-applications becomes available.
Variations in motor losses
All manufactured products are subject to tolerances associated with materials and manufacturing methods. No two products will perform exactly the same, even though they are of the same design and produced on the same assembly line.
This is also true for electric motors. Product tolerances in materials, such as steel used for laminations in the stator and rotor cores, will lead to variations in magnetic properties and ultimately affect iron losses and therefore motor efficiency. Using a tested 7,5 kW motor as an example, a 10 percent increase in iron loss (300 to 330 watts), which is within the tolerance offered by steel suppliers, would increase total motor losses from 946 to 976 watts and reduce efficiency from 88,8 (IE2) to 88,5 (IE1) percent.
Variations also occur as the result of manufacturing process limitations. There is an economic limit to the practical dimensional tolerances on motor parts. Combinations of mating parts contribute to dimensional variations, such as the size of the air gap, which cause variations in additional load loss and hence motor efficiency.
In addition, there are uncertainties caused procedures.
Thus in forecasting the efficiency of a given motor, one can speak of the rated efficiency as defined by the manufacturer (which should be equivalent to the average efficiency of a large population of motors) and above or equal to the required nominal efficiency of the rated efficiency class (in accordance to IEC 60034-30).
The actual efficiency at rated load of any individual motor, when operating at rated voltage and frequency, can be lower than the rated efficiency but not less than rated efficiency minus the tolerance of the efficiency according to IEC 60034-1. This is the level reached when both raw materials and manufacturing processes are at the least favourable end of their specified tolerances.
The rated efficiency should be used in estimating the power required to supply a number of motors. The minimum efficiency (rated minus tolerance) permits the motor user the assurance of having received the specified level of performance.