Basic Motor Theory (5)

DC Motor Theory

Preface

The intent of this paper is to provide one with an understanding of DC Motors in order that they can be applied with confidence. This paper contains basic information and specific information that applies to Reliance Medium HP and Large HP DC Motors. Due to the nature of Baldor Systems business, emphasis has been placed on the Large DC motor product line.

Section 1: Dynamo Development
The first generators and motors were called dynamos or dynamoelertric machines. Dynamo is from the Greek word dynamis which means power. Webster defines dynamoelectric as “relating to the conversion of mechanical energy into electrical energy or vice versa”. The word motor is from the Latin word motus which means one that imparts motion or prime mover. The dynamo was the result of the efforts of several people, in different countries, in the mid-nineteenth century, to make electricity work for them.
Definitions
Dynamo: From the Greek word dynamis, which means power
Dynamoelectric: Relating to the conversion by induction of mechanical energy into electrical energy or vice versa
Dynamoelectric machine: A dynamo or generator
Motor: From the Latin word motus, one that imparts motion, prime mover. A device that changes electrical energy into mechanical energy.
Generator: A device that changes mechanical energy into electrical energy. Although the terms AC and DC generator are in common usage, a generator is normally considered to be a device that provides DC current.
Alternator: A device that changes mechanical energy into an alternating current electrical energy, an AC generator.
Landmarks Of Electric Motor Development
1820 The discovery of electromagnetism Hans Christian Oersted, Danish
1827 The statement of the law of electric conduction, Ohm’s law George S. Ohm, German
1830 The discovery of electromagnetic induction Joseph Henry, American
1831 The discovery of electromagnetic induction Michael Faraday, English The first practical dynamo, about 1867
Section 2: Electric Motor And Generator Basics

Electrodynamic Principles

Faraday’s Law In order that current can be obtained from an electric circuit, an electromotive force (voltage) must be established and maintained between the two ends of the circuit. This electromotive force may be established in several ways, one of which is by means of an electromagnetic generator. Michael Faraday discovered that an electric potential can be established between the ends of a conductor in the following three ways:
By a conductor moving or cutting across a stationary magnetic field. (DC Generator)
By a moving magnetic field cutting across a stationary conductor. (AC Generator)
By a change in the number of magnetic lines enclosed by a stationary loop or coil. (Transformer)
Faraday’s law states that, “the EMF (electromotive force) induced between the ends of a loop or coil is proportional to the rate of change of magnetic flux enclosed by the coil; or the EMF induced between the ends of a bar conductor is proportional to the time rate at which magnetic flux is cut by the conductor.”
This law emphasizes rate of change or rate or flux cutting rather than density or extent of magnetic field.
Lenz’s Law Lenz’s Law states that, “A change in the magnetic flux passing through or linking with, a loop or coil causes EMF to be induced in a direction to oppose any change in circuit conditions, this opposition being produced magnetically when current flows in response to the induced EMF.”
Whenever there is a change in current in a magnetizing coil, which works to change the flux in the coil, a voltage is induced which tends to prevent the change. Thus, if we attempt to diminish the current flowing in a magnetizing coil, a voltage will be developed that will tend to keep the current unchanged. Likewise, if we attempt to establish a current in a magnetizing coil, a voltage will be developed that will tend to keep the current from increasing.
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