DC Motors | Principles of Operation of DC Motor, Advantages of DC Motors

Brushed DC motors

The classic DC motor design generates an oscillating current in a wound rotor, or armature, with a split ring commutator, and either a wound or permanent magnet stator. A rotor consists of one or more coils of wire wound around a core on a shaft; an electrical power source is connected to the rotor coil through the commutator and its brushes, causing current to flow in it, producing electromagnetism.

The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does) but rather keeps rotating indefinitely (as long as power is applied and is sufficient for the motor to overcome the shaft torque load and internal losses due to friction, etc.)
Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. (Sparks are also created inevitably by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections—and hence coil ends—momentarily while crossing the gaps.

Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes.) This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the output of the motor. The making and breaking of electric contact also cause electrical noise, and the sparks additionally cause RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance (on larger motors) or replacement (on small motors).

The commutator assembly on a large machine is a costly element, requiring the precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.
Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking (comparable to the problem of “valve float” in internal combustion engines). (Small brushes are also desirable for a lower cost.)

Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear.

There are four types of DC motors:

1. DC series motor
2. DC shunt motor
3, DC compound motor –

there are also two types:
(a) cumulative compound
(b) differentially

4. Permanent Magnet DC Motor

Principles of Operation of DC Motor

DC motors comprise four principal components a) field, b) armature, c) commutator, and d) brushes.
The field is the equivalent of a stator in an AC motor, and the armature functions as the rotor.

The brushes act as contacts between an external power source and the commutator. The design of these carbon brushes allows them to move up and down on a brush holder, to compensate for the irregularities of the commutator surface. Thus they are said to ride the commutator.

Each section of the commutator is connected to an armature coil, essentially a conductive loop of wire. A current induced in the armature coil, by way of the brushes and commutator, creates a magnetic field around the armature. Since the current flowing through the armature flows at a right angle to the field’s magnetic lines of flux, the two magnetic forces interact. This interaction creates a third magnetic field that tends to rotate counterclockwise.

The commutator regulates current flow in the armature coils, allowing it to flow in one direction only. Each segment of the commutator is directly connected to an armature coil, so the commutator rotates with the armature.

As it rotates, each segment of the commutator is constantly breaking contact with one brush, while simultaneously connecting with the other. Every time contact with a new brush occurs, the flow of current reverses in the armature coil.
The interaction of magnetic force from the armature and field poles is renewed each time the armature completes one-half of a rotation. This causes the armature to rotate for as long as the current is maintained in the coils.

Advantages of DC Motors

DC motors provide excellent speed control for acceleration and deceleration with effective and simple torque control. The fact that the power supply of a DC motor connects directly to the field of the motor allows for precise voltage control, which is necessary with speed and torque control applications.
DC motors perform better than AC motors on most traction equipment. They are also used for mobile equipment like golf carts, quarries, and mining equipment. DC motors are conveniently portable and well suited to special applications, such as industrial tools and machinery that is not easily run from remote power sources

Types of Motors

There are several kinds of DC motors commonly used in industrial applications. The motors have similar external appearances but are different in their internal construction and output performance. When selecting a DC motor for a given application, two factors must be taken into consideration:
1. The allowed variation in speed for a given change in load.
2 The allowed variation in torque for a given change in load.



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