How reliable are motor power ratings through time (old motors vs new motors)?
How reliable are motor power ratings through time...?

The published data and the nameplate markings have been very reliable for motors manufactured to international standards. Such motors meet or exceed the stated ratings. Older motors may have been more conservatively designed and tend to exceed the stated ratings. When the NEMA motor standards were established, the standard dimensions for a given rating were larger. At some point, insulation was improved, increasing the safe operating temperature and making it possible to meet the same performance standards with smaller dimensions. As designs and manufacturing methods improved, it is likely that designers became less conservative and motors less likely to exceed the stated ratings.
With regard to single phase motors, Puchstein, Lloyd, Conrad, Alternating Current Machines, 3rd Edition 1954, in introducing the section on single-phase induction motors, says &quot;The single-phase induction motor displays less satisfactory operating characteristics than the polyphase machine...&quot; In general, single-phase machines are designed for home appliances and three phase machines are designed for industrial use.