How a DC Generator Works: Construction and Principles

A DC generator is defined as an electrical device that converts mechanical energy into direct current (DC) electricity. It works on the principle of electromagnetic induction, where an electric potential difference is created across a conductor when it cuts through a magnetic field, causing a current to flow if connected to a closed circuit.

A DC generator consists of several parts: a yoke, poles, field winding, armature, commutator, brushes and bearings. Each part has a job to do, and the sections below take them one at a time before covering how the machine works and where it fits.

What is a Yoke?

The yoke is the outer frame of a DC generator that serves two purposes:

• It holds the magnetic poles of the generator and acts as a protective cover for the machine.
• It carries the magnetic flux produced by the field winding.

The yoke is usually made of cast iron or cast steel, depending on the size and weight of the generator. For small generators, cast iron is cheaper but heavier than steel. For large generators, where weight is a concern, cast steel or rolled steel is preferred.

The yoke is shaped like a hollow cylinder with feet for mounting on a base, a terminal box for connecting to an external circuit, and hangers for supporting the armature and commutator.

What are Poles and Field Winding?

The poles and field winding form the stationary part of a DC generator, producing the main magnetic field inside the machine from their bolted position on the inner periphery of the yoke.

The poles are made of laminated sheet steel or solid cast iron or steel. The laminations reduce the eddy current losses in the poles. The poles are salient, meaning they project inward from the yoke.

The pole shoes are curved extensions of the poles that serve two purposes:

• They spread out the magnetic flux in the air gap between the poles and the armature.
• They support the field coils that are wound around them.

The field coils are made of insulated copper wire or strips that carry direct current from an external source or from a separate exciter. The field coils are connected in series or parallel with each other to produce alternate north and south poles along the direction of rotation.

What is an Armature?

The armature is the rotating part of a DC generator that carries the armature winding, where the magnetic field induces emf, mounted on a shaft turning between the poles.

The armature core is made of laminated sheet steel with slots on its outer surface. The slots hold the armature conductors that are insulated from each other and from the core. The laminations reduce the eddy current losses in the core.

The armature winding is formed by connecting several coils of insulated copper wire or strips in a specific pattern. There are two types of armature winding: lap winding and wave winding.

• Lap winding: In this type of winding, each coil end is connected to an adjacent commutator segment and to another coil end on the same side of the armature.
• Wave winding: In this type of winding, each coil end is connected to a commutator segment that is distant from it by one pole pitch and to another coil end on the opposite side of the armature.

The winding choice sets how many parallel paths carry current through the armature. Lap winding provides one path per pole, which suits high-current, low-voltage machines; wave winding provides just two paths whatever the pole count, which suits high-voltage, low-current machines.

What is a Commutator?

A commutator is a mechanical device that converts alternating emf induced in the armature winding into a direct voltage across the load terminals. It acts as a rectifier for DC generation.

A commutator consists of wedge-shaped segments of hard-drawn or drop-forged copper that are insulated from each other and from the shaft by mica sheets. Each segment is connected to an armature conductor through a riser or lug.

The commutator segments are arranged in a cylindrical shape on the shaft and rotate with it. The number of segments depends on the number of coils in the armature winding.

What are Brushes?

The brushes are made of carbon or graphite blocks that collect current from the commutator segments and transfer it to the external circuit. They also provide electrical contact between the stationary and rotating parts of the generator.

The brushes are housed in rectangular boxes called brush holders that are attached to the yoke or the bearing brackets. The brush holders have springs that press the brushes against the commutator with suitable pressure.

The brushes belong where the emf in the armature conductors reverses, the so-called neutral zones. Those zones sit on the magnetic neutral axis (MNA), which shifts slightly away from the geometrical neutral axis when the machine is loaded.

What are Bearings?

The bearings are used to support the rotating shaft of the generator and reduce friction between the shaft and the stationary parts. They also allow smooth and uniform rotation of the shaft.

For small generators, ball bearings are used because they have low friction and high efficiency. For large generators, roller bearings are used because they can withstand heavy loads and shocks.

The bearings must be lubricated properly for smooth operation and long life of the generator. The lubrication can be done by oil rings, oil baths, grease cups or forced lubrication systems.

How Does a DC Generator Work?

The working principle of a DC generator can be explained as follows:

• When the generator is driven by a prime mover, such as an engine or turbine, the shaft rotates along with the armature.
• As the armature rotates, the conductors cut through the magnetic field produced by the poles and field winding.
• According to Faraday’s law of electromagnetic induction, an emf is induced in each conductor proportional to the rate of change of flux linkage.
• The direction of induced emf in each conductor can be determined by Fleming’s right-hand rule.
• The induced emf in each conductor alternates as it passes under different poles.
• The induced emf in each coil is equal to the sum of EMFs induced in its conductors.
• The induced emf in each coil also alternates as it passes under different poles.
• The coil’s emf reverses as it passes from pole to pole, and the commutator segments swap their brush contact at exactly that moment.
• That timed swap keeps the current in the external circuit flowing one way, even though the coil current itself alternates.
• As a result, a unidirectional or direct current flows through the external circuit.
• The generated voltage rises with speed of rotation, number of turns per coil, number of coils and flux per pole, and it depends on the winding arrangement.

What are the Types and Applications of DC Generators?

There are mainly three types of DC generators based on their method of excitation:

• Separately excited DC generator: In this type, the field coils are excited by an independent external DC source such as a battery or another DC generator.
• Self-excited DC generator: In this type, the field coils are excited by their own generated voltage after initial magnetization by residual magnetism. There are three subtypes: series-wound, shunt-wound and compound-wound.
• Permanent magnet DC generator: In this type, there are no field coils but permanent magnets that provide constant magnetic flux.

DC generators have various applications, such as:

• Charging batteries for automobiles, inverters, and solar panels.
• Supplying power for traction motors in electric vehicles, trains and cranes.
• Supplying power for arc welding machines, electroplating equipment and electrolysis processes.
• Supplying power for remote areas where AC transmission is not feasible or economical.
• Supplying power for testing AC machines and circuits.

Conclusion

Yoke, poles, armature, commutator, brushes and bearings each play one part in the same trick: spinning a coil through a magnetic field and handing the result to the wires as direct current. How the field is powered separates separately excited, self-excited and permanent-magnet machines. That choice, plus the winding style, matches each generator to battery charging, traction, welding, electroplating or remote supply duty.