One company purchased advanced equipment that integrates automation and intelligence. When the equipment malfunctions, the manufacturer can remotely check and troubleshoot the problem. The intelligent control overcomes the problem of physical distance.
Intelligent control of equipment has brought both opportunities and challenges to motor manufacturers, especially fully automated production lines. More actions require the coordination of the control system, and how to effectively integrate the motors with the control system is a critical issue.
To avoid incompatibility issues between motors and supporting equipment, many motor manufacturers conduct necessary tests to select suitable matching systems. Currently, the most common practice is to purchase or supply frequency converters and motors in tandem. Especially for the development of high-efficiency products, many motor manufacturers have begun to move from single-machine energy saving to system-wide energy saving. The further development of large-scale automated production lines has effectively promoted the intelligent and systematic development of motors.
Motor control refers to the control of a motor's starting, acceleration, operation, deceleration, and stopping. Different types of motors and their applications have different requirements and purposes. For electric motors, motor control aims to achieve rapid starting, fast response, high efficiency, high torque output, and high overload capacity.
1. Motor speed control
Motor speed control methods include series resistance speed control, frequency conversion speed control, pole changing speed control, vector control, and direct torque control.
Series resistance speed control is mainly used for asynchronous motors, and the speed range is limited by the maximum torque of the motor.
Variable frequency speed control is suitable for induction motors, and speed regulation is achieved by adjusting the synchronous speed.
Pole-changing speed regulation produces 1/2, 1/3, ... speeds by changing the number of poles in the motor.
Vector control technology decouples the field winding and armature winding of a motor, making the control of an induction motor similar to that of a DC motor. By adjusting the magnitude of the motor's field current and armature current separately, the motor's torque, speed, back electromotive force, and other parameters can be controlled.
Direct torque control directly controls the stator flux space vector and electromagnetic torque, and has the ability to respond quickly.
2. Motor starting control
Three-phase asynchronous motor starting methods include full-voltage direct starting, reduced-voltage starting, and starting with increased rotor circuit resistance.
● Reduced-voltage starting. This mainly includes autotransformer starting, star-delta starting, and variable-voltage starting. When an asynchronous motor starts, the rotor is stationary, and the starting current is relatively large. Reduced-voltage starting can reduce the starting current. Since the starting torque of an asynchronous motor is proportional to the square of the voltage, reduced-voltage starting requires ensuring that the motor has a certain starting capability.
●Increase rotor circuit for startup. This method is suitable for wound rotors, deep slot rotors, and double-cage rotors, but cannot be used for squirrel-cage rotors.
● Starting a single-phase asynchronous motor. This includes capacitor starting, resistor starting, PTC starting, and shaded-pole starting. Because a single-phase winding in an induction motor cannot generate a rotating magnetomotive force when the rotor is stationary, an asynchronous motor with only a single-phase winding cannot start itself. Therefore, an auxiliary winding forming a 90° angle with the main winding needs to be installed on the single-phase asynchronous motor. This winding is mainly used for starting the motor; after starting, it can be disconnected or used for motor operation.
●Synchronous motor starting. Because the rotor of a synchronous motor rotates at a synchronous speed, there is no slip. When the rotor speed differs significantly from the synchronous speed, a loss of synchronization will occur, thus preventing self-starting. Starting methods for synchronous motors include frequency converter starting, asynchronous motor-driven starting, and linear motor self-starting.
● Variable frequency drive (VFD) start-up. With VFD start-up, the rate of change of the starting voltage frequency is typically set. Once the motor reaches 60% to 80% of its rated speed, the rated frequency is applied to the motor to directly synchronize it. Starting an asynchronous motor is similar.
● Linear motor starting. Its rotor structure consists of a permanent magnet and a squirrel cage, with the squirrel cage used for the starting process. Once the motor reaches synchronous speed, the squirrel cage no longer generates electromagnetic torque.
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