For procedures, see your electrical laboratory manual.

Title: DC Motor


Aim

To determine the characteristics of a DC motor excitation

Theory

A machine that converts DC electrical power into mechanical power is known as a Direct Current motor.

A DC motor is an electrical machine that converts electrical energy into mechanical energy.

DC motor working is based on the principle that when a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

It works on the principle of Lorentz Law, which states that “the current-carrying conductor placed in a magnetic and electric field experience a force”. And that force is called the Lorentz force.

The direction of this force is given by Fleming’s left-hand rule and magnitude is given by;
F = BIL Newtons

According to Fleming’s left-hand rule when an electric current passes through a coil in a magnetic field, the magnetic force produces a torque that turns the DC motor.
The direction of this force is perpendicular to both the wire and the magnetic field.

Answers to Questions:

1. The nature of the speed-torque characteristics of a DC motor with constant excitation is described by a linear –ve slope graph.
a. Ideal no-load speed: at the ideal no-load speed, the back emf is equal to the driving source voltage.
b. Change in V does not affect the speed-torque characteristics, increase in Ra shifts the slope to the right and vice-versa, lowering of the speed of the motor increases the field current If, but this is not usually reliable.

2. Losses include; Electrical/Copper losses (Armature loss, Field Losses), Magnetic Field Losses, and Mechanical Losses.

3. This makes the motor run below the base speed. Because the high armature current will flow through this resistor, considerable power is consumed, and the overall efficiency of the motor is reduced accordingly.