Electric Motors Seminar Report

1 INTRODUCTION

Electric motors are widely classified into two distinct categories: AC (Alternating Current) and DC (Direct Current). DC is distinguished from AC with its ability to flow in one direction. There are different kinds of DC motors but they all work on the same principle [1]. Electric motors are a means of converting energy. There is a wide range of DC applications ranging from small to big applications in the industry. It is of vital importance for engineers to know motors so that they can choose the right one for their requirement.

1.1 PRINCIPLES OF OPERATION OF THE DC MOTOR

In any electric motor, the basic theory by which all DC motors operate is electromagnetism. There are two states which are compulsory to produce a force

According to Lenz’s Law, the back EMF opposes the applied voltage [4,///]. The emf can be expressed as follows: E = K ϕ ω [1]

Where K is the armature constant related to the magnetic properties of the motor; ω - is the speed of rotation.

1.3.1 Resistance

The resistance of a conductor depends on:

L - the length of the conductor, measured in meters [m]; ρ - the resistivity of the conductor in ohm meter (Ω.m); A - the cross-sectional area of the conductor, measured in m2; A= πr2.

The resistance can be expressed as: R = ρ *L / A [8]

According to research, the resistivity ρ of copper wire at 20⁰ is equal to 1.72x10-8 Ω.m [8].

1.3.2 Current

Additional current results in a powerful magnetic field, thus more torque [6]. Because torque is inversely proportional to speed and voltage [5], and since a series circuit is a voltage divider, the two batteries will be connected in series to get maximum voltage and the current will be constant. The following expression gives current in ampere

The speed in rpm is given by N= ( ω *60) /2π [11]

Where ω – angular speed in radians per second.

1.3.8 Mechanical Output power

Output mechanical power is directly proportional to speed the speed of the motor [5], and can be calculated using the formula: POUT = ω Ϯ [5]

Where ω= 2πrpm / 60; rpm - rotational speed in Newton minute (N.min); 60 - number of seconds per minute.

1.3.9 Efficiency

Motor efficiency increase with speed [33]. Efficiency is expressed as follows: E = (POUT / PIN) x 100 [1]

Where, Pin – input power, measured in watts (W); Pout - output power measured in watts (W).

1.3.10 Power Factor

High power factor contributes to high efficiency and lower losses [12]. Power Factor is given by the expression as follows: PF = (PIN/ POUT) x100