Page 78 - Electric Machinery Fundamentals

:tq t:Lc LTKIL MALHINt:KY .FUNDAMENTALS

Recently, the SI units have superseded the English system almost everywhere in
the world except in the United States, but rapid progress is being made even there.
Since SI is becoming almost universal, most (but not all) of the examples in this
book use this system of units for mechanical measurements. Electrical quantities
are always measured in SI units.
In the section on mechanics, the concepts of angular position, angular veloc-
ity, angular acceleration, torque, Newton's law, work, and power were explained
for the special case of rotation about a single axis. Some fundamental relationships
(such as the power and speed equations) were given in both SI and English units.
The production of a magnetic field by a current was explained, and the spe-
cial properties of ferromagnetic materials were explored in detail. The shape of the
magnetization curve and the concept of hysteresis were explained in terms of the
domain theory of ferromagnetic materials, and eddy current losses were discussed.
Fal'aday's law states that a voltage will be generated in a coil of wire that is
proportional to the time rate of change in the flux passing through it. Faraday's
law is the basis of transformer action, which is explored in detail in Chapter 3. (
A cun'ent-carrying wire present in a magnetic field, if it is oriented properly,
will have a force induced on it. This behavior is the basis of motor action in all
real machines.
A wire moving through a magnetic field with the proper orientation will
have a voltage induced in it. This behavior is the basis of generator action in all
real machines.
A simple linear de machine consisting of a bar moving in a magnetic field
illustrates many of the features of real motors and generators. When a load is at-
tached to it, it slows down and operates as a motor, converting electric energy into
mechanical energy. When a force pulls the bar faster than its no-load steady-state
speed, it acts as a generator, converting mechanical energy into electric energy.
In ac circuits, the real power P is the average power supplied by a source to
a load. The reactive power Q is the component of power that is exchanged back
and forth between a source and a load. By convention, positive reactive power is
consumed by inductive loads (+61) and negative reactive power is consumed (or
positive reactive power is supplied) by capacitive loads (- 8). The apparent power
S is the power that "appears" to be supplied to the load if only the magnitudes of
the voltages and currents are considered.

QUESTIONS

1-1. What is torque? What role does torque play in the rotational motion of machines?
1-2. What is Ampere's law?
1-3. What is magnetizing intensity? What is magnetic flux density? How are they related?
1-4. How does the magnetic circuit concept aid in the design of transfonner and machine
cores?
1-5. What is reluctance?
1-6. What is a ferromagnetic material? Why is the permeability of ferromagnetic mate-
rials so high?