Home / Comparative Analysis of AC and DC Servo Motors in Motion Control

Comparative Analysis of AC and DC Servo Motors in Motion Control

2024-01-26 15:11:34

The term "servo" originates from the Greek word for "slave". A servo motor can be understood as a motor that absolutely obeys control signal commands: before the control signal is issued, the rotor remains stationary; when the control signal is issued, the rotor immediately rotates; when the control signal disappears, the rotor can stop immediately.

A servo motor is a small special motor used as an actuator in automatic control devices. Its function is to convert electrical signals into angular displacement or angular velocity of the motor shaft. Servo motors are also called executive motors and are used as actuators in automatic control systems to convert electrical signals into angular displacement or angular velocity output of the motor shaft.

Classification of servo motors

Servo motors can be divided into two categories: AC servo and DC servo.

The basic structure of an AC servo motor is similar to that of an induction motor (asynchronous motor). It has two groups of windings on the stator, one of which is the excitation winding Wf, which is connected to a constant AC voltage. The other is the control winding Wco, which is used to control the motor's operation by applying an AC voltage or phase change to it.

AC servo motors have the following characteristics: stable operation, good controllability, fast response, high sensitivity, and strict machine and adjustment characteristics and nonlinearity indices (respectively less than 10% to 15% and less than 15% to 25%).

Advantages and disadvantages of DC servo motors

Advantages: accurate speed control, high torque-speed characteristics, simple control principle, easy to use, and low price.

Disadvantages: brush commutation, speed limit, additional resistance, production of wear particles (not suitable for dust-free and explosion-prone environments).

The basic structure of a DC servo motor is similar to that of a general DC motor. The motor speed n=E/K1j, where E is the counter-electromotive force, K is a constant, j is the magnetic flux per pole, Ua and Ia are the armature voltage and armature current, and Ra is the armature resistance. By changing Ua or the magnetic flux φ, the speed of the DC servo motor can be controlled. However, the commonly used method is to control the armature voltage. In a permanent magnet DC servo motor, the excitation winding is replaced by a permanent magnet, and the magnetic flux φ remains constant. DC servo motors have good linear regulation characteristics and fast response times.

Advantages and disadvantages of AC servo motors

Advantages: good speed control characteristics, smooth control throughout the entire speed range, almost no oscillation, high efficiency of over 90%, little heat generation, high-speed control, high-precision position control (depending on the accuracy of the encoder), can achieve constant torque within the rated running range, low inertia, low noise, no brush wear, maintenance-free (suitable for dust-free and explosion-prone environments).

Disadvantages: more complex control, need to adjust the parameters of the driver on site, more lines are required.

DC servo motors can be divided into brushed and brushless motors.

Advantages of brushed motors: low cost, simple structure, large starting torque, wide speed control range, easy control, and easy maintenance (replacing carbon brushes). However, they produce electromagnetic interference and have requirements for the operating environment, and are commonly used in cost-sensitive general industrial and civilian applications.

Advantages of brushless motors: small size and weight, large output power, fast response, high speed, low inertia, stable torque, smooth rotation, complex control, intelligent drive, flexible electronic commutation method, can be square wave or sine wave commutation, no maintenance, high efficiency, low electromagnetic radiation, low temperature rise, and long life, suitable for various environments.

AC servo motors are also brushless motors, which can be divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control, with a large power range and low maximum speed, and the speed decreases uniformly with increasing power. They are suitable for low-speed, stable running environments.

The rotor of a servo motor is made of permanent magnet material, and the controller controls the U/V/W three-phase electric shape to form an electromagnetic field. The rotor rotates under the action of this magnetic field, and at the same time, the encoder built into the motor feeds back a signal to the controller. By comparing the feedback value with the target value, the controller adjusts the angle of rotation of the rotor, and the accuracy of the servo motor is determined by the accuracy (number of lines) of the encoder.

Q: What are the differences in performance between AC servo motors and brushless DC servo motors?
A: The performance of AC servo motors is better because AC servo motors use sine wave control, and the torque pulsation is small; while brushless DC servo motors use square wave control. However, brushless DC servo motors are easier to control and cheaper.

The rapid development of permanent magnet AC servo drive technology has brought a crisis of elimination to DC servo systems.

Since the 1980s, with the development of integrated circuits, power electronics, and AC variable speed drive technology, permanent magnet AC servo drive technology has made significant progress. Numerous famous electrical manufacturers around the world have successively introduced new series of AC servo motors and servo drives. AC servo systems have become the main development direction of modern high-performance servo systems, making DC servo systems face the crisis of elimination.

Compared with DC servo motors, the main advantages of permanent magnet AC servo motors are:

(1) No brushes and commutators, more reliable and maintenance-free.

(2) Significant reduction in stator winding heat generation.

(3) Small inertia, fast system response.

(4) Good performance in high-speed, large torque working conditions.

(5) Smaller and lighter in size for the same power.

Principle of servo motors:

The stator structure of an AC servo motor is basically the same as that of a capacitor-start single-phase induction motor. There are two groups of windings on the stator, one of which is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, which is connected to the control signal voltage Uc. Therefore, AC servo motors are also called two-phase servo motors.

The rotor of an AC servo motor is generally made in a cage shape, but in order to give the servo motor a wide speed regulation range, linear mechanical characteristics, no self-rotation phenomenon, and fast response performance, it should have two characteristics compared to a general motor: high rotor resistance and low rotational inertia.

The commonly used rotor structures are currently of two types: one is a high-resistance cage rotor made of high-resistance conductive material, which is thin and long to reduce rotational inertia; the other is an aluminum alloy hollow cup-shaped rotor with a cup wall thickness of only 0.2~0.3mm. The rotational inertia of the hollow cup-shaped rotor is very small, and the response is fast and stable. Therefore, it is widely used.

When there is no control voltage, only the pulsating magnetic field generated by the excitation winding in the stator of the AC servo motor exists, and the rotor remains stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates along the direction of the rotating magnetic field. Under the condition of constant load, the speed of the motor changes with the size of the control voltage. When the phase of the control voltage is reversed, the servo motor will reverse.

The working principle of an AC servo motor is similar to that of a capacitor-start single-phase induction motor, but the rotor resistance of the former is much larger than that of the latter. Therefore, compared to capacitor-start single-phase induction motors, AC servo motors have two prominent features:

  1. Large starting torque: due to the large rotor resistance, the torque characteristic (mechanical characteristic) is closer to linear, and the starting torque is large. Therefore, when there is a control voltage on the stator, the rotor starts immediately, and the servo motor has the characteristics of fast start and high sensitivity.

  2. Wide operating range: stable operation, small noise, and no self-rotation phenomenon. When the rotor of an AC servo motor loses the control voltage during operation, the motor stops immediately.

Precision transmission micro-special motors:

Precision transmission micro-special motors can quickly and accurately execute frequent changes in instructions in the system and drive the servo mechanism to complete the expected work. Most of them can meet the following requirements:

  1. Able to start, stop, brake, reverse, and run at low speeds frequently, with high mechanical strength, high heat resistance level, and high insulation level.
  2. Good response ability, large torque, small rotational inertia, and small time constant.
  3. Equipped with a drive and controller (such as a servo motor or stepper motor), with good control performance.
  4. High reliability and accuracy.

Classification and structural and performance comparison of precision transmission micro-special motors:

AC servo motors:

(1) Thin and long cage type two-phase AC servo motor (mechanical characteristics close to linear, small volume and excitation current, low power servo, low-speed operation is not smooth enough).

(2) Non-magnetic cup-shaped rotor two-phase AC servo motor (hollow cup-shaped rotor, mechanical characteristics close to linear, volume and excitation current larger, low power servo, smooth low-speed operation).

(3) Iron-magnetic cup-type rotor two-phase AC servo motor (iron-magnetic material cup-type rotor, mechanical characteristics close to linear, rotor rotational inertia large, and small slot effect, running smoothly).

(4) Permanent magnet synchronous type AC servo motor (formed by a permanent magnet synchronous motor, speed sensor, and position detection element in a coaxial integrated structure, with 3-phase or 2-phase stator windings, magnetic material rotor, must be equipped with a driver; wide speed regulation range, mechanical characteristics are composed of constant torque zone and constant power zone, can be continuously blocked, fast response performance, large output power, small torque fluctuation; there are square wave drive and sine wave drive methods, good control performance, suitable for machine tool main shaft speed regulation systems).

(5) Induction type three-phase AC servo motor (rotor similar to squirrel cage induction motor, must be equipped with a driver, using vector control, expanding the constant power speed regulation range, often used for machine tool main shaft speed regulation systems).

DC servo motors:

(1) Printed coil DC servo motor (disk-shaped rotor, disk-shaped stator axially bonded rod-shaped magnetic steel, small rotor rotational inertia, no slot effect, no saturation effect, large output torque).

(2) Disk-rotor DC servo motor (disk-shaped rotor, stator axially bonded rod-shaped magnetic steel, small rotor rotational inertia, control performance better than other DC servo motors, high efficiency, large output torque).

(3) Cup-type rotor permanent magnet DC motor (hollow cup-shaped rotor, small rotor rotational inertia, suitable for incremental motion servo systems).

(4) Brushless DC servo motor (stator with multiple-phase windings, rotor with permanent magnets, with rotor position sensor, no spark interference, long life, low noise).

Torque motors:

(1) DC torque motor (flat structure, multiple poles and slots, multiple conductors, and many commutator segments; large output torque, low speed or blocked operation can be continuously operated, good mechanical and regulation characteristics, small mechanical and electrical time constants).

(2) Brushless DC torque motor (structure similar to brushless DC servo motor, but flat, with multiple poles and slots, and multiple conductors; large output torque, good mechanical and regulation characteristics, long life, no sparks, low noise).

(3) Cage-rotor AC torque motor (cage-rotor, flat structure, multiple poles and slots, large starting torque, small mechanical and electrical time constants, can be continuously blocked for a long time, mechanical characteristics relatively soft).

(4) Solid-rotor AC torque motor (iron-magnetic material solid-rotor, flat structure, multiple poles and slots, can be continuously blocked for a long time, smooth operation, mechanical characteristics relatively soft).

Stepper motors:

(1) Reaction type stepper motor (rotor made of silicon steel sheets, evenly distributed with control windings, small step angle, high starting and running frequency, low step angle accuracy, no self-locking torque).

(2) Permanent magnet type stepper motor (permanent magnet rotor, radial magnetization pole; large step angle, low starting and running frequency, self-locking torque, low power consumption compared to reaction type, but requires positive and negative pulse current).

(3) Hybrid type stepper motor (permanent magnet rotor, axial magnetization pole; high step angle accuracy, self-locking torque, small input current, and the advantages of both reaction type and permanent magnet type).

Switched reluctance motor (rotor made of silicon steel sheets, all with salient poles, similar in structure to large step angle reaction type stepper motor, with rotor position sensor, torque direction independent of current direction, small speed regulation range, large noise, mechanical characteristics composed of constant torque zone, constant power zone, and series excitation characteristics).

Linear motors (simple structure, guide rails can be used as secondary conductors, suitable for linear reciprocating motion; good high-speed servo performance, high power factor and efficiency, good constant speed running performance).

In conclusion, servo motors are a critical component in precision control systems and play a crucial role in achieving high-performance motion control. The two main types of servo motors are AC and DC, each with their advantages and disadvantages. AC servo motors offer smooth control, high efficiency, and low maintenance, making them suitable for applications requiring high precision and speed. On the other hand, DC servo motors are simple in design, easy to control, and cost-effective, making them ideal for low-power applications.

The advancements in permanent magnet AC servo drive technology have led to the development of high-performance servo systems, posing a threat to DC servo systems. However, DC servo motors still have their place in the market due to their simplicity and cost-effectiveness.

When selecting a servo motor for a specific application, it is essential to consider factors such as accuracy, speed, torque, and cost. Additionally, the choice between AC and DC servo motors will depend on the specific requirements of the application.

Overall, servo motors have revolutionized the field of motion control, enabling precise and efficient control of machines in various industries, including manufacturing, robotics, and automation. As technology continues to advance, we can expect to see further improvements in servo motor performance, leading to even more sophisticated and high-performance motion control systems.

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