What is Operation Principle of AC Servo Motor?

The output torque of the servo motor is proportional to the current that flows in the motor. Because the servo amplifier normally detects the motor speed and controls in such a way that the current flow in response to speed deviations, the servo motor can operate from a low speed to a high speed with a fixed torque. The figure in the follow shows the torque characteristics of a servo motor.

Operating principles:
The operating principles of every small and large motor are the same with torque occurring according to Fleming's left-hand rule, which states that if a current flows through a conductor in a magnetic field, a force acts in the conductor. For SM-type (synchronous type) AC servo motors, a permanent magnet is provided in the rotor, a coil through which a current flows is provided in the magnet, and current that corresponds to the rotor operation (rotation speed/direction, output torque) flows through the magnet coil.
A current flows through the coil that bisects the magnetic flux from the rotor magnet by turning the amplifier transistor ON/OFF. The applied voltage is subjected to switching by several kHz, and the flowing current is smoothened by reactance of the winding wire and forms a sine-wave. The + - interval of the coil voltage is determined by the magnetic pole position detection signal from the detector directly connected to the motor shaft, and no phenomenon of loss in synchronism such as with a normally synchronous motor does not exist because it is normally controlled in such a way that the magnetic flux and the current bisect.

Principles of IM type (induced current electrical motor) motor (vector control inverter) Even for induced current electric motors, the principles of the occurrence of torque are the same as for synchronous electric motors. However, there is no permanent magnet on the rotor side, as can be seen in the cross-sectional figure shown in the figure on the right, and it is not possible to supply the current Ia and the magnetic flux Φ individually. Accordingly, the current flows through the coil, and torque occurs due to the current that flows in the rotor groove by the electromagnetic induction action and by the magnetic flux created by the motor coil current. In this way, both the torque current and the magnetic flux current flow through the magnet coil. In other words, it is necessary to control the two currents individually in IM-type motors. This is called vector control. IM-type motors have the same torque characteristics as servo motors using vector control.

Although the servo motor has been developed from an easily controllable DC servo motor, the complicated control resulting from the development of electronic devices substituted in the micro processor is high-speed, and with the transition to an AC servo motor that is maintenance-free and has good production characteristics with the possibility of being produced at a low cost, applications of 50W or more are currently substituted with an SM-type AC servo. Furthermore, an IM-type motor has a robust structure and is easily applicable to large form and high-speed conversions, and a larger capacity also results in improved efficiency. For this reason, this type is mainly used in cases requiring 7.5kW or more, and with higher accuracy of large-form line control, their use has widened into areas where DC motors were the norm. Recently, however, vector control inverters are being changed to IM-type motors.