Brushed DC motor is a type of DC motor, with fixed main magnetic poles and brushes installed on the stator, and armature windings and commutators installed on the rotor. The electrical energy from the DC power supply enters the armature winding through brushes and commutators, generating armature current. The magnetic field generated by the armature current interacts with the main magnetic field to produce electromagnetic torque, causing the motor to rotate and drive the load. Due to the presence of electric brushes and commutators, brushed motors have complex structures, poor reliability, multiple faults, heavy maintenance workload, short lifespan, and are prone to electromagnetic interference caused by commutation sparks.

Principle of DC Brush Motor, Brushed DC (BDC) Motors

The stator generates a static magnetic field. This static magnetic field surrounds the armature (or rotor). The external power supply excites the armature magnetic field. There are also two arc-shaped copper sheets on the motor shaft of the DC brush motor, which are called commutators. When the motor rotates, the carbon brush slides on the commutator. In this way, a rotating magnetic field attracted by the static magnetic field of the stator can be generated. The current in the armature and stator windings is supplied by batteries or other DC power sources (permanent magnet DC brushless motor has no stator winding). The battery (or DC power supply) provides a constant DC voltage. The voltage amplitude determines the speed of the motor, so the battery or DC power supply is a linear excitation source. The most effective way to change the speed of DC brushless motor is to use pulse width modulation (PWM) technology.

PWM technology switches constant voltage source with fixed frequency. The motor speed can be adjusted by changing the pulse width of PWM signal. The ratio between the high and low levels of the pulse is called the duty cycle of the PWM signal. The amplitude of DC battery level is equal to the average amplitude of PWM signal.

Types of DC brush motor

① Brush disc winding motor. The brushed disc winding motor is bonded to a cylinder body with rare earth materials, and the disc winding made of enameled copper wire is placed inside the cylinder body to form the rotor. The motor phase is adjusted by a mechanical inverter. Mechanical inverters adjust voltage phase by friction between fixed carbon brushes and rotating copper commutation surfaces. The electric brushes of this motor are constantly worn out during use, and the lifespan of the motor is difficult to exceed 2000 hours. At the same time, due to the high speed of the motor, two-stage gear reduction must be adopted, which brings two problems: first, high noise, and second, large efficiency loss. The rated efficiency of the motor after reduction often only reaches 68% to 72%. The capacity of the battery used in electric bicycles is limited, usually 36V/12Ah. If the motor efficiency is not high, it will increase power consumption and affect the driving range.

② Brush printed winding motor. The brushed printed winding motor uses copper foil board as the winding, which reduces the weight of the motor. Due to the fact that all of these motors are produced on automatic production lines with reliable processes, the lifespan of the motor has been extended to 3000 hours, noise has been significantly reduced, and efficiency has been increased to 72% to 76%. But this type of motor has a "buzzing" high-frequency noise, and the efficiency is still not ideal even after gear reduction. The use of brushed inverters makes it impossible to further improve the service life of the motor.

③ Brush pressed winding motor. This type of motor can increase its efficiency to 74% to 78% by compressing the wound copper wire into a new type of winding. This type of motor is still widely used by electric bicycle manufacturers, but its efficiency, noise, and lifespan defects are still issues that must be improved.

The wheel type brushed DC motor with gear transmission consists of a disc-shaped armature brushed motor and a gear reduction and transmission system. A disc-shaped armature is a high-speed rotating rotor. The structure of a brushed DC motor with wheel type gear transmission is shown in Figure 2-2. The torque of the motor is transmitted to the first stage gear through the shaft, and through gear reduction, it drives the hub shell to rotate.