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AC Servo Motor Theory (with Video)

By Cayla | Published on Nov 18,2015

Stator construction of AC servo motor is substantially similar to capacitive phase single-phase induction motor, positions difference 90 °between each of the stator windings are mounted on the stator, one field winding is Rf, it is always connected to the AC voltage Uf;. Another is a control winding L, it is always connected control signal power Uc. So AC servo motor, also known as two servo motors.

AC servo motor rotor is usually made into squirrel cage, but in order to make the servo motor have wide speed range, the mechanical properties of linear, non rotation performance and rapid response, which compared with ordinary motor, it should have big rotor resistance and small moment of inertia. Currently the most used rotor structure are that one is squirrel-cage rotor made of high resistivity bars with conductive material having high resistivity, in order to reduce the moment of inertia of the rotor, rotor is made slender and other one is hollow cup-shaped rotor made of aluminum alloy, cup wall is thin and only 0.2-0.3mm, in order to reduce Magnetic circuit of the magnetic resistance, a fixed stator to be placed inside the hollow cup-shaped rotor, hollow cup-shaped the moment of inertia of the rotor is very small, response is small and operation is smooth, so that is be widely used.

AC servo motor in the absence of a control voltage, there is only pulsating magnetic field generated by the field windings in the stator, When a control voltage, will produce a rotating magnetic field in the stator, the rotor is rotating along the direction of rotation magnetic field, at a constant load, the motor speed changes along the size of control voltage, when the phase position is opposite of control voltage, servo motors will be reversed.

Although AC servo motor's work is similar with split-phase single-phase induction motor, the former rotor resistance is much greater than the latter, so compared to the single servo motor asynchronous motor, there are three salient features:

1, the starting torque

As the rotor resistance, comparing its torque curve characteristic curve with the ordinary asynchronous motor torque characteristic curve, there is a clear distinction. It allows critical slip S0> 1, so that not only the torque characteristics (mechanical properties)is closer to linear, but also has a larger starting torque. Therefore, while there is a control voltage in the stator, the rotor is rotating immediately, namely with a starting fast, high sensitivity.

2, a wider operating range

3, no rotation phenomenon

A normal operation servo motor, as long as lose control voltage, the motor stops immediately. When the servo motor loses control voltage, it is in single-phase running, because the big rotor resistance, both the torque characteristics of the rotating magnetic field in the stator and the rotor role opposite direction of the rotation of the generated . and synthetic torque characteristic .

AC servo motor output power is generally 0.1-100W. When the power frequency is 50Hz, voltages are 36V, 110V, 220,380V; when the power frequency is 400Hz, voltages are 20V, 26V, 36V, 115V and other.

AC servo motor is running smoothly, low noise. But the control characteristic is non-linear, and the big rotor resistance, big loss, low efficiency, as compared with the same capacity DC servo motors, it is bulky, heavy, it only applies to 0.5-100W small power system.

The video below introduces the working principle of ac servo motor well, if you have more interests, plz click:

AC Servo Motor Theory (Part 1 of 5):


AC Servo Motor Theory (Part 2 of 5)


AC Servo Motor Theory (Part 3 of 5)


AC Servo Motor Theory (Part 4 of 5)


AC Servo Motor Theory (Part 5 of 5)



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