An AC Servo Motor is a specially designed two-phase or three-phase induction motor used in servo-mechanism and closed-loop control systems where precise control of position, speed, and acceleration is required. Unlike ordinary induction motors, an AC servo motor is optimized for fast dynamic response, low inertia, high starting torque, and accurate controllability, making it highly suitable for automation and motion-control applications.
Servo systems play a fundamental role in robotics, CNC machines, aerospace systems, industrial drives, robotics arms, medical instruments, antenna positioning systems, automation equipment, and precision control devices. AC servo motors provide smooth rotation, stable operation at very low speeds, and excellent torque-speed characteristics required in servo applications.
Definition of AC Servo Motor
An AC Servo Motor is defined as a two-phase or three-phase induction motor with specially designed rotor and winding characteristics, optimized for high torque at low speeds, rapid acceleration, quick reversal of rotation, and precise control of motion in servo-mechanism systems.
The stator carries two windings — a reference (fixed) winding and a control winding placed at a spatial phase difference of 90°. The control voltage determines the torque and direction of rotation, which allows accurate positioning and speed regulation.
Construction of AC Servo Motor
Fig. 1: Two Phase AC Servo Motor.
The stator of an AC servo motor contains two pole-pairs, identified as A-B and C-D, which are arranged 90° apart around the inner periphery. Each pole-pair carries a separate winding. The rotor consists of bars placed in slots and short-circuited through end rings, similar to a squirrel-cage rotor.
The rotor diameter is intentionally kept small to reduce inertia. This helps the motor achieve faster acceleration and better dynamic response — a key requirement in servo systems that undergo frequent speed changes and reversals.
The construction of an AC servo motor is similar to that of a two-phase induction motor, but its design is modified to achieve better torque-speed characteristics and improved control response. The main parts of an AC servo motor include:
- Stator – carries two pole pairs and windings placed 90° apart in space
- Reference Winding – supplied with constant AC voltage
- Control Winding – supplied with variable AC voltage from a servo amplifier
- Rotor – may be squirrel cage, drag cup, or solid iron type
- End Rings – short-circuit rotor bars
- Shaft – delivers mechanical output
- Bearings – support friction-free rotation
The diameter of the rotor is kept small to reduce inertia and enhance acceleration response. High rotor resistance ensures linear torque-speed characteristics, which is essential for servo control.
Excitation and Phase Relationship
The two stator windings must carry currents that differ in phase by approximately 90°. Under normal operating conditions:
- One winding is supplied with a fixed AC voltage – known as the reference phase.
- The second winding receives a voltage of variable magnitude and reversed polarity, applied through a servo amplifier.
This second winding is called the control phase. Since the voltages of both windings are intentionally unbalanced, the magnitude and direction of the developed torque depend directly on the control signal. Reversing the phase sequence reverses the direction of rotation.
Working Principle of AC Servo Motor
The AC servo motor operates on the principle of the rotating magnetic field. When two-phase AC supply is provided to the stator windings, the resulting flux induces voltage in the rotor conductors, which produces current and torque.
The reference phase receives a fixed AC voltage, while the control phase receives a variable voltage that is:
- 90° out of phase
- adjustable in magnitude
- controlled by the servo amplifier
The resulting torque depends on the phase relationship and magnitude of supplied voltages. When the input control signal changes, the torque direction and speed change accordingly, enabling precise motion control. Reversing the phase sequence reverses the direction of rotation.
Types of AC Servo Motors
| Type | Description | Characteristics | Applications |
|---|---|---|---|
| Two-Phase AC Servo Motor | Uses reference and control windings at 90° displacement | High precision, smooth operation | Instrumentation, light automation |
| Three-Phase AC Servo Motor | Higher torque and power capacity | Robust and efficient | CNC, robotics, industrial motion |
| Drag Cup Rotor Type | Very thin lightweight rotor | Extremely fast response | Measurement systems, servoinstruments |
| Squirrel Cage Rotor Type | Conventional cage construction | Reliable and rugged | General servo drive systems |
Torque-Speed Characteristics
The AC servo motor is designed with **high rotor resistance**, which results in a **linear torque-speed curve** suitable for servo operation. This allows stable control at different operating speeds and ensures consistent performance even under varying loads.
Advantages of AC Servo Motor
- High efficiency and excellent dynamic response
- Low cost and minimal maintenance
- No brushes or commutator
- Capable of precise position and speed control
- Lightweight rotor ensures fast acceleration
- Stable operation at low and high speeds
- Better reliability and long service life
- Smooth and quiet operation
- Highly suitable for automation systems
Disadvantages of AC Servo Motor
- Requires sophisticated servo amplifier
- Complex control circuitry
- Higher cost than normal induction motors
- Performance depends on feedback accuracy
- Speed control is sensitive to parameter variations
Applications of AC Servo Motor
- X-Y Recorders and plotters
- Disk drives and tape drive systems
- Printers and scanners
- Robotics arms and automated machines
- Machine tools and CNC systems
- Radar and antenna positioning systems
- Textile and packaging machines
- Medical imaging and diagnostic equipment
- Process control and instrumentation
Conclusion
The AC Servo Motor is a vital component of modern automation and motion control technology. Its superior torque-speed characteristics, rapid response, high efficiency, and accurate controllability make it ideal for precision-driven engineering applications. From robotics and CNC machining to aerospace systems and industrial automation, AC servo motors play a crucial role in delivering precise, smooth, and reliable motion control.
With continued advancements in power electronics and control systems, the importance and performance capabilities of AC servo motors will continue to expand in future engineering and industrial innovations.