The following section describes the different characteristics of AC motors and DC servo motor systems in terms of power drive, performance, and protection circuits.
Power drive
For the power amplification section of the drive motor in DC servo systems frequently used in radar, when the antenna is lightweight, rotates slowly, and the drive power is low (typically tens of watts), the motor can be directly controlled by a DC power supply. When the drive power requirement is close to kilowatts or higher, selecting the drive scheme, i.e., amplifying the armature current of the DC motor, becomes a crucial part of servo system design. Commonly used high-power DC power supplies include transistor amplifiers, thyristor amplifiers, and motor amplifiers. Kilowatt-level transistor amplifiers are less common. Thyristor technology experienced rapid development and widespread application in the early 1960s and 70s, but due to various reasons at the time, such as reliability issues, many products abandoned thyristor control. Current integrated drive modules are generally manufactured using transistors or thyristors. Motor amplifiers are traditional power amplification devices for DC servo motors ; due to their simple control and robust durability, they are still used in newer radar models. The following mainly uses amplified motors as an example to compare their advantages and disadvantages with AC servo motors.
An amplifier motor, often called an amplifier, typically uses an AC asynchronous induction motor to drive a two-stage DC generator set connected in series to achieve DC control. It has two sets of control windings, each with an input impedance of several thousand ohms; if connected in series, the input impedance is approximately 10 thousand ohms. Servo motors generally have complementary, balanced, symmetrical inputs. When the system input is non-zero, this balance is broken, allowing the amplifier motor to output a signal. When the input current is tens to hundreds of milliamps, its output can reach over 100V DC voltage and several to tens of amps of current, directly connected to the armature winding of the DC servo motor. Its main disadvantages are its large size and weight, and its nonlinearity, especially near zero, which requires careful consideration for demanding systems.
AC servo motors are equipped with dedicated drivers, making them significantly smaller and lighter than amplifier motors of the same power. They rely on internal switching circuits composed of transistors or thyristors, using an internal photoelectric encoder or Hall effect device to determine the rotor's position and thus the a, b, and c phase output states of the drive motor. Therefore, they offer excellent efficiency and stability. Unlike amplifier motors, they do not require dedicated power amplifier circuits. These motors are generally permanent magnet type, with the a, b, and c phase currents generated by the driver controlling the motor's rotation; hence the name AC servo motor. The control signal input to the driver can be a pulse train or a DC voltage signal (typically ±10V), so they are also sometimes called DC brushless motors.
