Technical Information Site of Power Supply Design

2022.01.12 motor

# Power Generation Principle

Brushed DC Motor

In succession to the previous article on the principle of motor rotation, here the principle of power generation is explained.

## Principles of Power Generation in a Brushed DC Motor

The basic principle of power generation was explained here, drawing on related laws and equations. In this article, actual principles of power generation are explained using a schematic diagram of a brushed DC motor.

In a state in which a power supply is not connected to the brushes, we suppose that the coils (rotor) are rotating counterclockwise. As a practical example, there is a state in which the rotor is rotating due to inertia after the power supply of a rotating motor has been shut off.

In state , coil A is located midway between magnets N and S. The magnetic field due to the magnets is directed from N to S, and because coil A approaches magnet N through counterclockwise movement, the change in magnetic flux toward the rotation axis is positive (+) and is maximum (pink arrow). As a result, an emf occurs in coil A causing current (purple arrows) to flow from the rotation axis toward the outside.

Because coil B recedes from magnet N while coil C approaches magnet S, the change in magnetic flux is negative (-) (pink arrows), and because the positions are close to magnets, the change in magnetic flux is smaller than the maximum values. As a result, emfs occur in coils B and C causing current (purple arrows) to flow from outside toward the rotation axis.

When the emfs in coils A, B, and C are combined at this time, a positive (+) voltage occurs in the left-side brush relative to the right-side brush.

When state occurs, coil B is located midway between magnets N and S, and approaches S, so that the change in magnetic flux is negative (-) and is maximum. As a result, an emf occurs in coil B causing current to flow from the outside toward the rotation axis.

Coil A approaches magnet N, and coil C recedes from magnet S, so that the changes in magnetic flux are positive (+), and because the positions are close to magnets, the values are smaller than the maximum values. As a result, emfs occur in coils B and C causing currents to flow from the rotation axis toward the outside.

When the emfs in coils A, B, and C are combined at this time, a positive voltage occurs in the left-side brush relative to the right-side brush.

In this way, when the coils (rotor) are rotating counterclockwise, a positive (+) voltage is always generated in the left-side brush relative to the right-side brush. When the coils are rotating in the clockwise direction, the reverse operations cause a positive (+) voltage to be generated in the right-side brush. The generated voltage is rectified by the commutator to produce a DC voltage, and the higher the rotation rate, the higher is the voltage generated. Of course, a generator (dynamo) is based on this principle.

#### Key Points:

・Electric power is generated by the rotation of coils in a magnetic field, so that a DC voltage is produced across the brushes.