Hello, Owen, Garage and Kerstein,
I don’t know where this discussion come from but I will offer some comments.
Direct on line (DOL) AC motors are the worldwide standard to produce mechanical work from electricity. No dispute. Its huge.
DOL AC motors provde fixed speed operation. They are dependable and last a long time if environmental conditions are maintained to standard.
DC motors are only used when variable speed is needed and a DC controller (DC drive) is required.
So any discussion about comparing DC and AC motors requires a discussion about the motor controllers also.
DC motors are more efficient than AC motors. Typically DC motors are 97-99% efficient. AC motors are not more than 94% efficient (check the manufacturers’ tables).
DC drives are 99% efficient. AC motor speed controls (PWM VFD) are at best 97-98% efficient. Typically about 97%.
But efficiency is only one part of the discussion. Maintainence is another issue. DC motors require maintenence for the brushes. AC motors need grease and they run forever.
DC drives are simple and less costly and easy to maintain. PWM VFD are more complex and do require more maintenece than DC drives.
DC motors are considered special and have relatively long deliveries. AC motors for DOL use are stocked and readily available. For a given size rating, the DC motor is larger physically.
AC motors to be used with PWM VFD require a different design than do standard DOL AC motors. NEMA and other bodies have defined how these differ. Needless to say the AC motor that operates on VFD should be special.
Trade offs abound.
Generally, PWM VFD with AC motors are used as a preference over DC dirve and motors – today.