Principle of operation and construction of a commutator
– A commutator consists of contact bars connected to the armature windings.
– The commutator reverses the flow of current in a winding as the shaft rotates.
– In a motor, the armature current generates a rotational force.
– In a generator, the mechanical torque maintains the motion of the armature winding.
– The commutator ensures current flow in only one direction in the external circuit.
– Practical commutators have at least three contact segments.
– Brushes are wider than the insulated gap to ensure contact with an armature coil.
– Even if two commutator segments touch one brush, the motor can still function.
– The remaining rotor arms can produce sufficient torque to start spinning the rotor.
– A generator can provide useful power with the remaining rotor arms.
– A commutator consists of copper segments fixed around the rotating machine.
– Spring-loaded brushes connect to the external circuit.
– Each conducting segment is insulated from adjacent segments.
– Mica or other insulating materials are used for insulation.
– Large motors may have hundreds of segments.
– Commutators require periodic maintenance such as brush replacement.
– Commutated machines are declining in use due to inefficiency.
– Alternating current (AC) machines are replacing commutated machines.
– Brushless DC motors are becoming popular, using semiconductor switches.
– Refilling or replacing damaged segments is common for larger industrial commutators.
Brush construction and maintenance
– Early machines used copper wire brushes, which caused wear on the commutator.
– Fine copper wire mesh or gauze provided better surface contact with less wear.
– Carbon brushes, made of a softer material, wear faster than copper brushes.
– Carbon brushes can be easily replaced without dismantling the machine.
– Resurfacing the commutator may be necessary for larger industrial equipment.
Brush holders and contact angle
– Spring is used with the brush for constant contact with the commutator.
– Brush wears down and needs to be replaced when it becomes small and thin.
– Power cable is directly attached to the brush to prevent heating and loss of spring tension.
– Assembly of multiple brush holders is used when more power is needed.
– High power, high current commutated equipment is uncommon nowadays.
– Copper brushes make tangential contact with the commutator.
– Strip/laminate copper brushes only have tangential contact.
– Copper mesh and wire brushes use an inclined contact angle.
– Carbon brushes have softness that allows direct radial end-contact with the commutator.
– Reaction-type carbon brush holder has reversely inclined brushes for firm contact.
Field distortion compensation and self-induction compensation
– The rotor field needs to be at a right angle to the stator field for efficient operation.
– The position of the brushes needs to be adjusted to compensate for field distortion.
– Field effects are reversed when the direction of spin is reversed.
– A compensation winding can help mitigate field distortion effects.
– Brushes are permanently fixed for highest efficiency at a certain fixed speed.
– Self-induction creates a magnetic field that resists changes in current.
– Currents in the rotor coils continue to flow even after the brush has been reached.
– Brush advance is necessary to minimize sparking and wasted energy.
– Spurious resistance, proportional to the armature speed, causes lagging current.
– Advancing the brushes helps reverse the lagging self-inducting current.
Use of interpoles and limitations/alternatives
– Interpoles are small field coils and pole pieces positioned between the primary poles.
– Dynamic varying fields applied to interpoles balance out field distortions.
– Fixed brush position is maintained, minimizing sparking across segments.
– Interpoles help correct armature reaction and improve efficiency.
– Interpoles are used in modern motor and generator devices.
– Sliding friction between brushes and commutator consumes power.
– Brushes and commutator segments wear down, requiring regular replacement.
– Brush drop, caused by resistance, leads to power losses in low voltage, high current machines.
– Large direct current machines cannot be built with commutators.
– Alternating current motors are more efficient and widely used.
– Repulsion induction motors have higher starting torque.
– Wire-wound rotor with a conventional commutator induces torque by repulsion.
– Brushes connected to each other induce currents into the rotor.
– Some repulsion induction motors have adjustable speed.
– Once at speed, the rotor windings function like a conventional induction motor. Source: https://en.wikipedia.org/wiki/Commutator_(electric)
A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings (coils of wire) on the armature are connected to the commutator segments.
Commutators are used in direct current (DC) machines: dynamos (DC generators) and many DC motors as well as universal motors. In a motor the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced. In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit. The first direct current commutator-type machine, the dynamo, was built by Hippolyte Pixii in 1832, based on a suggestion by André-Marie Ampère.
Commutators are relatively inefficient, and also require periodic maintenance such as brush replacement. Therefore, commutated machines are declining in use, being replaced by alternating current (AC) machines, and in recent years by brushless DC motors which use semiconductor switches.
