Commutator

The commutator is one of the core components of a DC motor, and its performance plays a decisive role in the motor's operation. Any abnormalities in the commutator can lead to poor commutation, which can, in severe cases, cause other accidents, damage the DC motor, and disrupt the operation of winches, turntables, and mud pumps, thereby delaying drilling production.

Overview of the Motor Commutator

The commutator, also known as the rectifier, consists of brushes and commutator rings. It is a component used in DC permanent magnet series-excited motors to enable the motor to keep rotating. In some motors, we can see that the rotor is equipped with a conductive ring made up of several copper segments, which is connected to the external power supply via a pair of brushes. In electric motors, we refer to this component as the commutator, while in generators, it is called a rectifier.

The commutator includes: mechanical commutators, semi-plastic commutators, and fully plastic commutators. The commutators used in automotive starters primarily include mechanical arched commutators and plastic commutators.

Structurally, the commutator consists of several contact pieces arranged in a circular pattern, each connected to a rotor contact. The two external electrodes are called brushes, which maintain contact with only two of the commutator segments at a time.

The principle is as follows: when a current flows through the coil, it rotates due to the attraction and repulsion from the permanent magnets. When it reaches a point where the coil is balanced with the magnets, the segment of the commutator previously connected to the energized coil will separate from the brush, and the brush will then connect to the segment of the commutator corresponding to the coil generating the driving force. This process repeats continuously, allowing the DC motor to rotate.

Without the commutator’s function, the motor would only rotate half a circle before it would seize up, effectively functioning as an electric brake. The centrifugal force generated by the motor’s rotation is used to control the starting resistance, reducing the motor’s starting current, increasing the starting torque, and enabling the wound asynchronous motor to achieve brushless self-controlled operation. This device mainly consists of a casing, starting liquid, moving pole plates, springs, terminals, safety valves, exhaust valves, etc. This liquid resistance start-up device is characterized by low starting current, high starting torque, automatic adaptation to changes in power supply and load, and protection of the motor.

The commutator is a high-speed rotating device, and its rotor windings are subject to electrodynamic and centrifugal forces. Although it typically operates under rated parameters, the connection between the rotor windings and the commutator riser is the weak point. Traditional soldering methods are used, and based on the results of DC resistance measurements between segments, it is judged that the weld between the rotor windings and the commutator riser is loosened. This causes the end wires to fatigue, increasing contact resistance, generating more heat, accelerating oxidation of the contact surface, and further increasing the contact resistance. This vicious cycle eventually leads to the insulation of the commutator rotor being damaged at high temperatures, causing arcing and discharging, which can damage the commutator.

Inspection results from commutator disassembly show that the test data and conclusions regarding the rotor windings are accurate.

In operation, the commutator not only transmits longitudinal current but also performs the task of current commutation in short-circuit armature coils. These currents are reverse currents and reactance voltages generated during the commutation of the main current through the motor’s commutator. This causes sparks and arcs on the brush's contact surface as it slides on the commutator.

The effect of the motor commutator on the motor’s performance depends on the commutation process that occurs when the brush slides at a relatively high speed over the commutator surface to complete the electrical circuit. Although this process is complex and theoretical research is ongoing, comparisons of micro-motor operating conditions both domestically and abroad confirm that wear is the key factor in changes in contact resistance.

Types of Commutators

Commutators are available in hook type, slot type, and flat type, and are made from carefully selected raw materials to meet international advanced standards. They are widely used in electric tools, household appliances, automotive and motorcycle motors, and other fields. The commutator ring, carbon brush frame, and terminal board are available in various specifications and are used in automotive generators, gasoline generators, and other applications.

The commutator plays a rectifying role by ensuring that the current direction in the armature winding alternates to maintain a constant electromagnetic torque direction. In a generator, the commutator converts the alternating voltage in the components to a direct current between the brushes. In a motor, it converts the external direct current into alternating current within the components, generating a constant directional torque.