DC machines: the principle of operation

Electric machines are devices for converting electrical energy into mechanical energy (and vice versa). The work of the DC machine is based on the law of electromagnetic induction.

Typically, these units are used in industry for traction mechanisms, such as cranes and winches. A significant drawback of the engine is the formation of carbon deposits on the collector from the brushes. To avoid excessive arcing, it is necessary to periodically inspect and carry out preventive work. The design of DC machines is different from asynchronous and synchronous motors.

Between the poles creating a constant magnetic flux, there is an anchor, made in the form of a steel cylinder. In its grooves are laid turns of a copper conductor, and the ends of the conductor are connected with semirings, which are isolated from other parts of the machine - this is the collector on which the brushes slide. They are connected to an external circuit.

Since an electromotive force appears in the windings, the DC armature of the machine starts to rotate when the field crosses its turns.

Due to the fact that the magnetic induction is distributed unequally on the steel cylinder, the speed of the generated EMF depends on the current density in the gaps between the turns. Thus, the magnetic induction is maximal at the poles, and zero at the center of the armature (on the longitudinal axis).

When the armature of the DC machine rotates every half-turn, the conductors change their polarity, because they fall under the influence of opposite poles, hence the direction of the electromotive force is reversed, and if the EMF changes in time and direction, it must be assigned to a variable value. In order to obtain a constant component in the external circuit, a collector is connected to the DC machine. This is a kind of switch. Fixed brushes, which are connected to an external chain, slide along the semi-rings rigidly fixed to the anchor.

Rotating, the anchor touches only with that brush, which is under a particular polarity. At a time when the direction of the electromotive force is changing, there is a switching of the rings, that is, there is no change in the direction of the EMF for the external circuit. Thus, the collector is a kind of rectifier that does not allow the generated current to change.

To exclude the pulsation of the electromotive force, there are turns at the anchor that are attached to the pairs of collector plates. The turns are shifted from each other to an insignificant angle, this allows to compensate for distortions in the harmonics and the current enters the circuit without pulsations.

If the DC machines operate in the engine mode, then, on the contrary, the voltage is applied to the brushes. Thus, passing through the collector, a current appears in the turns, which creates its own magnetic field. Interacting with the field of poles, the armature starts to rotate, however, at the time when the direction of rotation during the passage of the conductors through the opposite pole should have changed, the collector still switches polarity. Thus, the direction of the current and, accordingly, its magnetic field changes. In this case, the collector is an inverter, a DC-to-AC converter .