831eb72a_es_content_from_Machine_Design__Induction_motor
[0] EFFICIENCY [PMAC] More efficient than induction motors, so run more coolly under the same load conditions [Servomotor] Designed to operate over wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage [Induction motor] 
Even NEMA-premium efficiency units exhibit degraded efficiencies at low load
The difference between AC induction, permanent magnet, and servomotor technologies | Motors/Drives content from Machine Design
Induction motor