A DC Machine is an electro-mechanical energy conversion device. There are two types of DC machines; one is a DC generator, and another one is known as DC motor. A DC generator converts mechanical power (ωT) into the DC electrical power (EI), whereas, a DC motor converts d.c electrical power into the mechanical power. The AC motor is invariably applied in the industry for the conversion of electrical power into mechanical power, but at the places where the wide range of speeds and good speed regulation is required, like in the electric traction system, a DC motor is used.
The construction of the DC motor and generator is nearly the same. The generator is employed in a very protected way. Hence there is an open construction type. But the motor is used in the location where they are exposed to dust and moisture, and hence it requires enclosures for example dirt proof, fireproof, etc. according to requirement.
Although the battery is an important source of the DC electric power, it can only supply limited power to any machines. There are some applications where large quantities of DC power are required, such as electroplating, electrolysis, etc. Hence, at such places, DC generators are the used to deliver power.
Basic Structure of Electrical Machines
The rotating electrical or DC machine has mainly two parts; one is the Stator, and another one is Rotar. The stator and rotor are separated from each other by an air gap. The stator is the outer frame of a machine and is immovable. The rotor is free to move and is the inner part of machine.
Both the stator and the rotor are made of ferromagnetic materials. Slots are cut on the inner periphery of the stator and outer periphery of the rotor. Conductors are placed in the slots of the stator or rotor. They are interconnected to form windings.
The windings in which voltage is induced is called the Armature windings. The winding through which a current is passed to produce the main flux is called the Field windings. To provide main flux in some of the machine permanent magnets are also used.
Equivalent Circuit of a DC Machine Armature
The armature of a DC generator can be the represented by an equivalent electrical circuit. It can be represented by 3 series-connected elements E, Ra and Vb.
The equivalent circuit of the armature of a DC generator is shown below in the figure.
The equivalent circuit of armature of a DC Motor is shown below in the figure.
The element E in the equivalent circuit diagrams is the generated voltage, Ra is the armature resistance, and Vb is the brush contact voltage drop.