Operating speed and slip
In general, motors with higher efficiency have a higher operating speed i.e. a reduced slip compared to motors of lower efficiency. On average, the slip is reduced by some 20 to 30% per next higher efficiency class for motors of the same rated output power.
Copper rotor motors typically have a smaller slip and a higher operating speed compared to aluminium rotor motors.
Effects of Variation in Voltage and Frequency
Operation outside of the rated conditions of voltage and frequency may decrease both efficiency and power factor and may adversely affect other performance characteristics. The same condition is true when operating the motor on other than a sine wave of voltage. The effect of a variation in supply voltage, wave-form, or frequency on the motor's efficiency and power factor characteristics depends on the individual motor design.
Power P Current I Speed n cos ϕ Efficiency η
105 110 Voltage U
Typical variations of current, speed, power factor and efficiency with voltage for constant output power
For permissible voltage and frequency variations during operation see IEC 60034-1.
Effects of Voltage Unbalance
A balanced voltage of the three-phase power supply to the motor is essential to the efficient operation of the system. For example, a voltage unbalance of 3.5 percent can increase motor losses by approximately 20 percent. For this reason, single-phase loads taken from a three- phase power supply should be carefully allocated so that the voltage unbalance at the motor terminals will be kept as low as possible.
For details see IEC 60034-26.