An AC servo motor is a machine that converts electrical energy into mechanical energy. It mainly consists of an electromagnet winding or distributed stator windings to generate a magnetic field and a rotating armature or rotor. The motor utilizes the phenomenon that a energized coil rotates in a magnetic field. The AC servo motor primarily comprises a stator and a rotor. The stator structure is basically the same as that of a rotary transformer, with two phase windings (one for excitation and the other for control) spaced 90 degrees apart in the stator core. AC servo motors offer high control precision, good torque-frequency characteristics, and overload capacity, and are widely used in material metering, sealing devices, and fixed-length cutting machines.
Basic types of AC servo motors
Similar to ordinary AC motors, AC servo motors are also divided into asynchronous and synchronous types. A two-phase AC servo motor is essentially a two-phase asynchronous motor. Its stator has two orthogonally placed windings: one is the excitation winding, and the other is the control winding. The rotor generally has two structural forms: one is a squirrel-cage rotor, whose structure is the same as that of a typical squirrel-cage induction motor; the other is a non-magnetic hollow cup rotor, whose structure is shown in the figure.
Comparing squirrel-cage rotors and coreless rotors, the former has a larger output torque, simpler structure, smaller excitation current, and higher efficiency. Its only drawback is its larger rotor inertia, resulting in a slower dynamic response compared to coreless rotors. Coreless rotors, on the other hand, have lower inertia, are more responsive, and have a wider speed range; however, their larger excitation current leads to a lower power factor and efficiency.
During operation, a fixed single-phase AC voltage is typically applied to the excitation winding. The speed is adjusted by controlling the control voltage of the control winding. It should be noted that the phases are different.
AC servo motor control method
There are three control methods for AC servo motors:
(1) Amplitude control
Amplitude control controls the motor speed by changing the magnitude of the control voltage Uc. During this control, the phase difference between the control voltage Uc and the excitation voltage Uf remains at 90° electrical angle. When the control winding is at its rated voltage, the resulting air gap magnetomotive force is a circular rotating magnetomotive force, producing the maximum electromagnetic torque.
(2) Phase control
The motor speed and direction are controlled by changing the phase difference between the control voltage Uc and the excitation voltage Uf, while the amplitude of the control voltage remains constant.
The phase of Uc can be changed by a phase shifter, thereby changing the phase difference between the two.
(3) Amplitude and phase control
The excitation winding is connected in series with a capacitor c and then to an AC power supply. The control voltage Uc is in phase with the power supply, but its amplitude can be adjusted. When the amplitude of Uc can be changed, the coupling effect of the rotor winding causes the current If in the excitation winding to change, which in turn changes the voltage Uf on the excitation winding and the voltage uc on the capacitor. The phase difference between Uc and Uf also changes accordingly. That is, changing the magnitude of Uc changes the phase difference between Uc and Uf, thereby changing the speed of the motor.
Structure and characteristics of AC servo motors
The stator of an AC servo motor is basically similar in structure to that of a capacitor-split-phase single-phase asynchronous motor, as shown in the figure. Its stator has two windings positioned 90° apart: one is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, connected to the control signal voltage Uc. Therefore, an AC servo motor is also called a dual-servo motor.
The rotor of an AC servo motor is usually made into a squirrel-cage type. However, in order to make the servo motor have a wide speed range, linear mechanical characteristics, no "self-rotation" phenomenon and fast response performance, it should have the characteristics of high rotor resistance and low rotational inertia compared with ordinary motors.
Currently, there are two main types of rotor structures that are widely used:
One type is a squirrel-cage rotor made of high-resistivity conductive material with high-resistivity conductor bars. To reduce the rotor's moment of inertia, the rotor is made slender. The other type is a hollow cup-shaped rotor made of aluminum alloy with very thin cup walls, only 0.2-0.3mm. To reduce the magnetic resistance of the magnetic circuit, a fixed inner stator is placed inside the hollow cup-shaped rotor, as shown in the figure.
When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of an AC servo motor is similar to that of a split-phase single-phase asynchronous motor, the former has a much higher rotor resistance than the latter. Therefore, compared with a single-phase asynchronous motor, a servo motor has three significant characteristics:
1. High starting torque
Due to its high rotor resistance, its torque characteristic curve, as shown in curve 1 in the figure, is significantly different from the torque characteristic curve 2 of a typical asynchronous motor. This allows for a critical slip S0 > 1, making the torque characteristic (mechanical characteristic) closer to linear and providing a larger starting torque. Therefore, when a control voltage is applied to the stator, the rotor rotates immediately, exhibiting characteristics of fast starting and high sensitivity.
2. Wide operating range
As shown in the figure, the servo motor can operate stably within the range of 0 to 1 when the difference rate S is between 0 and 1.
3. No rotation phenomenon
A normally operating servo motor will immediately stop operating if the control voltage is lost. When the servo motor loses the control voltage, it operates in a single-phase state. Due to the high rotor resistance, the two torque characteristics (T1-S1 and T2-S2 curves) generated by the interaction of the two rotating magnetic fields rotating in opposite directions in the stator with the rotor, as well as the resultant torque characteristic (T-S curve), are shown in the figure. These characteristics differ from the torque characteristics of a typical single-phase asynchronous motor (T′-S curve in the figure). The resultant torque T at this time is the braking torque, which causes the motor to stop quickly.
The figure shows the mechanical characteristic curve of a servo motor during single-phase operation. With a constant load, the higher the control voltage Uc, the higher the speed; conversely, with a constant control voltage, an increase in load leads to a decrease in speed.
