According to foreign media reports, engineers typically choose between direct current (DC) motors and alternating current (AC) motors. Recently, electronically rectified (EC) motors have joined the fray, offering advantages such as improved energy output control and efficiency. This technologically advanced device is currently replacing both DC and AC motors, enabling efficiency regulation. Both EC motors and brushless DC motors are controlled by external electronic circuit boards, enhancing both control and efficiency.
Principles of DC and AC motors
DC motors use carbon brushes and rectifier rings to change the direction of current and magnetic field electrodes in the rotating armature. The interaction between the inner rotor and the fixed permanent magnet causes the motor to rotate.
According to Maxon Motors, DC motors are limited by their brush systems, resulting in a lifespan of 1000-1500 hours, which may be less than 100 hours under extreme loads. Under favorable operating conditions, some motors may have a lifespan of up to 15,000 hours. Furthermore, high-speed rotation is limited by rectification, typically reaching a maximum speed of close to 10,000 revolutions per minute (RPM).
DC motors have a high efficiency rate, but they have certain specific losses. Factors that lead to failure include: initial resistance of the windings, brush friction, and eddy-current losses.
AC induction motors employ a series of windings controlled by an AC input voltage, which generates a stator field, which in turn produces a rotor field. Synchronous motors, on the other hand, are another type of AC motor, operating with high precision in their supply frequency. A magnetic field is generated when current flows through slip rings or permanent magnets. Synchronous motors operate faster than induction motors, whose speed is limited by the slip rings of asynchronous motors.
An AC motor means that its operating values will correspond to a specific point on a performance curve, which will align with the motor's peak efficiency. If it doesn't operate according to the values corresponding to that point, the motor's efficiency will drop significantly. AC motors generate induced current in the rotor, which in turn produces a magnetic field, a process that results in additional energy consumption. Therefore, AC motors are less efficient than DC motors. In fact, DC motors are more than 30% more efficient than AC motors because the permanent magnets generate a secondary magnetic field.
The figure above compares the motor efficiencies of electronically rectified motors, three-phase induction AC motors, single-phase AC induction motors, and shaded-pole motors.
Introduction and advantages of electronic rectifier motors
An electronically rectified motor is a brushless DC motor controlled by external electronic devices, which may be electronic circuit boards or variable-frequency drives. Mechanical commutation relies on the electronic circuitry, which alters the phase of the fixed windings to ensure motor rotation and provide the appropriate armature current. High precision can be achieved by delivering current in the correct direction within precise timeframes. Because the rotational speed of the electrodes is controlled by external electronics, electronically rectified motors do not suffer from synchronous speed limitations.
Electronically rectified motors offer several advantages: Since they are brushless, they eliminate sparks and the brush-related issues that could shorten their lifespan. They also eliminate energy waste because the stator is controlled electronically, resulting in superior performance and controllability. Compared to induction motors, they operate at lower temperatures. Smaller motor sizes save space, and manufacturers can achieve even greater space savings when using external rotors.
Using an electronically rectified motor results in cleaner power distribution. Brushless DC motors are driven by a separate DC power supply, and AC motor power supplies incur additional costs and are more complex to design. Electronically rectified motors can be directly connected to the AC mains power supply, but they are not entirely dependent on voltage or frequency; small voltage fluctuations will not affect the motor output.
If we compare electronically rectified motors with AC shaded-pole motors or AC permanent-split-capacitor motors, we will find that the efficiency range of shaded-pole motors is 15%-25%, that of permanent-split-capacitor AC motors (PCS) is 30%-50%, and that of electronically rectified motors is 60%-75%.
Electronically rectified motors can be used in low-power output applications such as small fans, servo motors, and motion-control systems. New methods are currently being explored to apply these motors to high-power output devices such as conveyor belts and condenser units.
Regarding speed control, electronically rectified motors offer multi-speed control, with a speed controller as a standard accessory. AC motors also provide most control functions and offer an external speed controller as an option. External controllers can adjust the incoming voltage of the AC motor's transformer, altering the sine wave, which can affect the motor's lifespan and increase noise.
For electronically rectified motors, the rectifier circuit uses pulse-width modulation (PWM) of 4-20mA and a voltage of 0-10V to control the motor speed within the range of 10%-100%. Monitoring of electronically rectified motors can be accomplished using integrated circuits, allowing designers to provide corresponding feedback. Electronically rectified motors also offer soft-start capabilities and exhibit lower noise and motor temperature.
