Understanding Current Collection in Electrical Machines

Electrical machines, like DC generators and motors, have several key components working together. The question asks about the specific part responsible for collecting the electric current from the commutator and transferring it to the external circuit or vice versa. Let's look at the roles of the components mentioned in the options and how they relate to current collection from the commutator.

Key Components and Their Functions

Here's a brief overview of the components listed and their primary functions in a typical DC machine:

• Armature: This is the rotating part of the machine. It contains the armature winding where the voltage is induced in a generator or where current flows to produce torque in a motor. The armature is connected to the commutator.
• Magnet: This provides the magnetic field within the machine. This field can be created by permanent magnets or electromagnets (field windings). The magnetic field interacts with the armature current/windings.
• Yoke: Also known as the frame, the yoke is the outer structural part of the machine. It provides mechanical support for the poles and field windings and also serves as the return path for the magnetic flux.
• Brushes: These are stationary contacts, typically made of carbon, that rest on the rotating commutator. Their primary function is to make electrical connection between the stationary external circuit and the rotating armature windings via the commutator.

The Role of the Commutator and Brushes

The commutator is a crucial component in DC machines. It is a segmented cylinder made of copper, mounted on the armature shaft. In a DC generator, the commutator rectifies the alternating voltage generated in the armature windings into a direct voltage for the external circuit. In a DC motor, the commutator helps to reverse the direction of current in the armature coils at the right moment, allowing the motor to continue rotating in one direction.

The brushes are the stationary components that make physical contact with the rotating segments of the commutator. This contact allows the electric current to flow between the armature windings (via the commutator) and the external load circuit (in a generator) or the external power supply (in a motor). The brushes "collect" the current from the commutator segments as they rotate past or "supply" current to them.

Analyzing the Question and Options

The question specifically asks for the component used for "collecting current from the commutator and supply it to the external load". Based on the functions described:

• The Armature contains the windings where current originates (generator) or flows (motor), but it doesn't collect current *from* the commutator.
• The Magnet creates the magnetic field.
• The Yoke is the structural frame and magnetic path.
• The Brushes make contact with the commutator to transfer current between the rotating armature and the stationary external circuit. This perfectly matches the description of collecting current from the commutator and supplying it to the external load (in the case of a generator).

Therefore, the component that performs this function is the brushes.

Revision Table: DC Machine Components

Reviewing the main parts and their roles is essential for understanding DC machine operation.

• Stator: Stationary part, includes yoke, poles, field windings.
• Rotor: Rotating part, includes armature core, armature windings, commutator, shaft.
• Brushes: Carbon blocks that make contact with the commutator.

Additional Information: Brush Material and Wear

Brushes are typically made of carbon, graphite, or metal-graphite composites. Carbon is used because it has a relatively high resistance (which helps with commutation, reducing sparking) and provides lubrication against the commutator surface, minimizing wear. However, brushes do wear down over time due to friction and sparking and need periodic replacement. The contact pressure between the brushes and the commutator is also critical for efficient current transfer and minimal wear.