In the field of motors, DC motors and AC motors are like twins often confused; both can drive equipment, but they have distinctly different characteristics. Today, we'll focus on DC motors. In fact, we all encounter DC motors daily; for example, our mobile phones vibrate when there's a message or call. This vibration is achieved using a DC motor. Of course, DC motors also have many applications in other fields.

  1. Classification of DC and AC Motors

Based on the operating current, motors are divided into DC motors and AC motors. A DC motor is driven by direct current (DC), while an AC motor is driven by alternating current (AC). From a performance perspective, the main difference between DC and AC motors lies in speed control. The speed of a DC motor is proportional to the voltage, while the speed of an AC motor is proportional to the frequency and the number of magnetic poles.

  2. DC Motor Structure

  Main Magnetic Pole:

  Includes the main magnetic pole core and the excitation winding wound around it. Passing current through the excitation winding generates the main magnetic field. The enlarged part below the magnetic pole is called the pole shoe. Its function is to optimize the distribution of magnetic flux through the air gap and to securely fix the excitation winding to the core.

  Brush Device: Includes brushes and brush holders, which are fixed on the stator. The brushes maintain sliding contact with the commutator to connect the armature winding to the external current.

  Motor Frame: On the one hand, it is used to fix the main magnetic poles, commutating poles, and end covers, and serves as the support for the entire motor, fixing the motor to the base with foundation bolts. On the other hand, it is also part of the motor's magnetic circuit, so it is made of cast steel or pressed steel plate.

  Rotor Core: Houses the armature winding and conducts magnetic flux. This is to reduce eddy current losses and hysteresis losses in the armature core during motor operation.

  Rotor Winding: Its function is to generate electromagnetic torque and induced electromotive force, thus performing energy conversion. Commutator: Also known as a rectifier, in a DC motor, its function is to convert the direct current from the brushes into alternating current in the armature winding, ensuring that the direction of the electromagnetic torque remains constant.

  Main shaft: Outputs rotational motion.

  3. Working Principle of DC Motors

Brushed DC Motor: The stator generates a fixed magnetic field. When the rotor coil is energized, it experiences an Ampere force in the magnetic field, generating torque. The commutator switches the direction of the coil current at the appropriate time to maintain unidirectional torque, causing the rotor coil to rotate continuously.

  4. Advantages of DC Motors:

  4.1Good starting and speed control performance, wide and smooth speed control range, strong overload capacity, and less affected by electromagnetic interference;

  4.2.DC motors have excellent starting and speed control characteristics;

  4.3 DC motors have relatively high torque.

  5. Disadvantages of DC Motors:

  5.1 DC motors are relatively expensive to manufacture;

  5.2 Compared with asynchronous motors, DC motors have a complex structure, are inconvenient to use and maintain, and require a DC power supply;

  5.3 The complex structure limits the further reduction of the size and weight of DC motors, especially the sliding contact of the brushes and commutator, which causes mechanical wear and sparks, resulting in more failures, lower reliability, shorter lifespan, and greater maintenance workload for DC motors.

  5.4 Commutation sparks not only cause electrical corrosion of the commutator but also act as a source of radio interference, which can have harmful effects on surrounding electrical equipment. The larger the motor capacity and the higher the speed, the more serious the problem. Therefore, the brushes and commutator of ordinary DC motors limit the development of DC motors towards high speed and large capacity.

  6. Will all future electric vehicles use DC motors?

  Will all future electric vehicles use DC motors?

  In fact, currently, hybrid electric vehicles are still dominated by brushless DC motors, while pure electric vehicles are dominated by induction motors.

  First, the efficiency of induction motors is greater than or equal to that of DC motors. When used with a smart inverter, induction motors can optimize magnetic and conduction losses, thereby optimizing efficiency.

  With brushless DC motors, magnetic losses increase proportionally with increasing machine size, leading to decreased efficiency. With induction motors, losses do not necessarily increase with increasing machine size.

  Although the peak efficiency of induction motors is slightly lower than that of brushless DC motors, the average efficiency may actually be better.

  Secondly, in terms of cost, induction motors are superior to brushless DC motors.

  Permanent magnets are expensive, costing approximately $50 per kilogram. Additionally, permanent magnet (PM) rotors are difficult to handle due to their strong magnetic field, which exerts a powerful attractive force on any ferromagnetic material nearby.

  The magnetic field of induction motors is adjustable, and although it requires an inverter and more complex control, the overall cost appears to be lower.