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I assume you mean either BLDC or AC PM, also there is ECM motors.
The type used in servo applications is generally sensored, due to the accurate positioning required.
These use some form of commutation sensor, previously hall sensors, now hall effect equivalent such as commutation tracks on the motor encoder.
With sensorless, such as the modern ECM, (electronically commutated motor) used in HVAC system fans etc, at power up, the controller uses current feedback and excersises the motor in order to find the rotor position, after this , the rotor position is constantly recorded and the speed kept constant.
Max.
The sensors in a typical BLDC motor allow the electronics to sense the approximate rotor position, generally six states per rotation.
That allows the electronics to apply power to the appropriate stator coils to produce a predictable amount of torque in a known direction.
A sensorless motor has to either follow the stator field passively, or once it has started to move the back EMF from the windings can be used to calculate the position as Max says.
Sensorless are ok for rotational loads where exact position does not matter; eg. speed control.
BLDC with sensors allow for control at an speed and can still provide controlled torque at low or zero speeds.
They are commonly used in machine tools and robotics, just about anywhere that large DC servos were previously used.
They are also used for such as motor wheels, as used in some electric vehicles, scooters and down to balance boards..
eg. Note the wires on these - three power cables for the stator, plus a five-way signal connector with power, ground and three sensor outputs:
**broken link removed**
The one other I have come across is a servo motor with an positioning encoder but no commutation tracks or devices.
This exercises the motor at power up and then keeps track of the rotor position via the position encoder and from then on, executes the commutation dependent on encoder position.
Max.
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