05-09-2014, 11:41 AM
ELECTRIC MOTORS
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INTRODUCTION
This section describes the main features of the electric motors.
1.1Where motors are used
An electric motor is an electromechanical device that converts electrical energy to
mechanical energy. This mechanical energy is used for, for example, rotating a pump
impeller, fan or blower, driving a compressor, lifting materials etc. Electric motors are used at
home (mixer, drill, fan) and in industry. Electric motors are sometimes called the “work
horses” of industry because it is estimated that motors use about 70% of the total electrical
load in industry.
2. TYPES OF ELECTRIC MOTORS
This section describes the two main types of electric motors: DC and motors. A list of
suppliers of electric motors is available on www.directindustryfind/electric-motor.html.
Figure 3 shows the most common electric motors. These are categorized based on the input
supply, construction, and operation mechanism, and are further explained below
AC Motors
Alternating current (AC) motors use an electrical current, which reverses its direction at
regular intervals. An AC motor has two basic electrical parts: a "stator" and a "rotor" as
shown in Figure 7. The stator is in the stationary electrical component. The rotor is the
rotating electrical component, which in turn rotates the motor shaft.
The main advantage of DC motors over AC motors is that speed is more difficult to control
for AC motors. To compensate for this, AC motors can be equipped with variable frequency
drives but the improved speed control comes together with a reduced power quality.
Induction motors are the most popular motors in industry because of their ruggedness and
lower maintenance requirements. AC induction motors are inexpensive (half or less of the
cost of a DC motor) and also provide a high power to weight ratio (about twice that of a DC
motor).
Wound rotor AC motor drives (slip ring induction motors)
Wound rotor motor drives use a specially constructed motor to accomplish speed control. The
motor rotor is constructed with windings that are lifted out of the motor through slip rings on
the motor shaft. These windings are connected to a controller, which places variable resistors
in series with the windings. The torque performance of the motor can be controlled using
these variable resistors. Wound rotor motors are most common in the range of 300 HP and
above.