How to select a servo motor

Date:2022-06-02 09:26

A servo system is not just a motor. It is a closed-loop motion system control engineering network that contains a controller, driver, electrical instrument, and feedback device, usually with an optical or magnetic encoder.

The servo system can use permanent magnets (PM) technology to synchronize machinery, with brushed or brushless PM motors, or to build asynchronous mechanical systems on an AC induction motor.

Permanent magnet synchronous motors have high peaks, as well as continuous torque, and are suitable for driving servos in high accelerations and fast deceleration of precise displacement systems. Torque is directly proportional to the input current. The motor shaft speed is related to the voltage of the input. The higher the input voltage, the higher the speed of the motor. The curve of the ratio of torque to speed is linear.

The permanent magnet structure is related to the motor air gap. For example, the structure of brushless PM motor contains two interactive magnetic structures, and the moving rotor (connected to permanent magnet) and the stator coil generate electromagnetic reactions, resulting in the torque and speed of the motor.

The three-phase stator field generates energy sequentially, and the PM rotor completes synchronous motion with the rotor field. A specific electronic compensation system to check the rotor position and add energy to the stator coils. Brushless PM motors are the first choice for precision displacement systems in all other motors, except automotive applications and oversized motor systems. Brushless PM motors are the only servo motor systems that can be used in closed-loop torque, speed or displacement systems.


Different rotors

AC induction motor has the same stator as PM brushless motor, but its rotor structure is completely different. The induction motor of the squirrel cage structure contains a series of induction aluminum or copper strips, placed in the rotor structure and connected to the end coil in the central control.

These short rotor strips are inducted by electromagnetic coupling with the rotating magnetic field of the stator, creating a new rotor field and reacting with the stator field to form rotor motion.

There is a difference between the synchronous stator and the slower stator field, and the actual speed. This difference in speed is known as slippage. The frequency of the input determines the speed of the motor.

For example, a 60 Hz, two-pole AC induction motor has a speed of nearly 3,600 rpm without load, and a four-pole AC motor runs at less than 1,800 rpm, depending on the slip value. When the motor starts torque, the slip increases and the speed decreases.

AC induction motors output more torque and reduce the speed to zero as the load reaches the point of failure. An inherent AC motor performance feature is that the starting torque is small and the load must be removed at the beginning of the motor.

With the emergence of the electronic drive of the inverter at the end of the 80s of the 20th century, the torque-speed performance curve unique to the motor has also undergone great changes. The performance of the frequency converter is that the torque-speed curve can be reconstructed by changing the voltage and frequency at the same time, using adjustable or variable speed drive, and the AC induction motor is the main part of the speed system.


How to use

The continuous improvement of drive technology in performance has brought brushless PM and AC induction motors into the competition of the drive market, but brushless PM motors still dominate the control field. AC induction motor discomfort should be used at low and high speeds.

A brushless PM motor is used in servo displacement systems, typically with a power of 50 kW (67 hp) or higher. AC induction motors are usually in constant speed or variable speed systems. Mixed scheme systems are rare. Other motors can also be partially implemented, but there are fewer solutions that exceed AC induction motors or brushless PM motors in performance.

Brushless PM motors have a certain impact on the speed control or lower power application market of 1 kW (1.37 hp) DC brushed motors in speed control. AC induction motors, on the other hand, control most applications larger than 1MW.