A servo motor converts an input voltage signal into angular displacement or angular velocity output on its axis. It is commonly used as an actuator in automatic control systems, hence the name "servo motor." Its key characteristic is that the rotor rotates immediately when control voltage is applied and stops immediately when no control voltage is applied. The direction and speed of the axis are determined by the direction and magnitude of the control voltage. Servo motors are available in AC and DC versions.
I. DC Servo Motor
1. Basic Structure
A traditional DC servo motor is a common small-capacity DC motor, which comes in two types: separately excited and permanent magnet. Its structure is basically the same as that of a regular DC motor.
The rotor of the cup-shaped armature DC servo motor is a non-magnetic hollow cup-shaped cylinder, which is lightweight, has low rotational inertia, and fast response. The rotor rotates between inner and outer stators made of soft magnetic material, with a large air gap.
Brushless DC servo motors use electronic commutation devices instead of traditional brushes and commutators, making them more reliable. Their stator core structure is basically the same as that of ordinary DC motors, with embedded multi-phase windings, and the rotor is made of permanent magnet material.
2. Basic Working Principle
The basic working principle of a traditional DC servo motor is exactly the same as that of a regular DC motor. It relies on the interaction between the armature current and the air gap flux to generate electromagnetic torque, thus causing the servo motor to rotate. Armature control is typically used, meaning that the speed is adjusted by changing the armature voltage while keeping the excitation voltage constant. The lower the armature voltage, the lower the speed; when the armature voltage is zero, the motor stops. This is because when the armature voltage is zero, the armature current is zero, the motor does not generate electromagnetic torque, and therefore does not "rotate."
II. AC servo motor
1. Basic Structure
An AC servo motor mainly consists of a stator and a rotor.
The stator core is typically made of laminated silicon steel sheets. Two-phase windings are embedded in the slots on the surface of the stator core: one phase is the excitation winding, and the other is the control winding. The two windings are spatially separated by 90° electrical degrees. During operation, the excitation winding f is connected to an AC excitation power supply, and the control winding k is supplied with a control signal voltage Uk.
2. Working Principle
Without control voltage, the air gap of an AC servo motor contains only a pulsating magnetic field generated by the excitation winding, and there is no starting torque on the rotor. When control voltage is present and the control winding current differs from the excitation winding current, a rotating magnetic field is generated in the air gap, producing electromagnetic torque that causes the rotor to rotate in the direction of the rotating magnetic field. However, a servo motor not only needs to start under control voltage but also needs to stop immediately after the voltage disappears. If the servo motor continues to rotate like a typical single-phase asynchronous motor after the control voltage disappears, a loss of control occurs; this phenomenon of self-rotation due to loss of control is called autorotation.
(Image from Industrial Control Forum)
III. Differences between AC and DC servo motors
Disadvantages of DC servo motors:
a. Complex structure, difficult to manufacture, and high cost
b. Brushes and commutator are prone to wear, generating sparks during commutation and limiting the rotational speed.
Advantages of AC servo motors:
a. Simple structure, low cost, and smaller rotor inertia than DC motors.
b. The capacity of an AC motor is greater than that of a DC motor.
