I have worked around electric motors for a long time. It still blows my mind that we can’t seem to settle on common terms and definitions of things we work with on a daily basis. When is a motor AC Synchronous or DC Brushless? They are identical. Same motor.
So you have to start from the beginning of the motor family tree and work your way through it. What makes a motor AC versus DC. The only definition I know of that makes any sense is this:
An AC motor has only one magnetic field, in the stator, and induces a second magnetic field through induction, into the rotor. The rotor becomes magnetized and follows the circulating magnetic field in the stator by passive attraction. This is why most AC motors are referred to as induction motors, because the magnetic polarity in the rotor is induces. AC motors as a result are constant speed machine based on the frequency of the electrical excitation. Since that’s mostly 60 hertz in the USA, its usually some multiple of 60, most commonly 1800 RPM (minus some rotor slip). AC machines require an inverter to change AC into DC and then DC into variable frequency AC in order to achieve variable speed performance. Due to decreasing costs for the AC motor and recent cost breakthroughs in inverter technology, AC variable speed is the dominant solution.
The DC motor has two active magnetic fields, one of which may be permanent magnet and operates by repulsion and attraction at the polarity of one of the magnetic fields is reversed. Reversing polarity in DC machines is often done mechanically using brushes, but there are many branches of the DC motor family such as Stepping Motors which do not use brushes.
DC motors are intrinsically variable speed and have other characteristics that make them a viable choice in many situations. The cost of permanent magnets has made them more expensive in larger designs. So DC machines are in somewhat declining popularity.
So it becomes possible to create lots of special purpose motors to solve different problems. And as the industry has evolved, navigating the many technology choices has become very difficult.
We have standardized on a 3 phase brushless DC motor for servo systems. We could have any number of phases, but it seems that 3 is very popular, probably as a result of the many 3 phase AC winding machines available. So from an electrical power standpoint, the motor is 3 phase. And the magnetic field in the stator is circulating.
But the rotor is permanent magnet. There are some 3 phase AC motors with buried magnets to increase the flux of the rotor. They are called “salient pole” because of the enhanced magnetic strength of the rotor. And because the rotor follows the stator’s magnetic orientation without slipping, its called AC Synchronous.
So which is it going to be? AC Synchronous or DC Brushless. Call me “Old School”, but I think its DC Brushless every time. Anyone out there agree?