21-11-2012, 11:55 AM
MOTORS
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Most rotary motion applications today depend on motors.
Electric motors are dominant, though air and hydraulic
motors are preferred in some applications, and engines or
turbines drive others.Several types of electric motors turn
shafts: dc, ac, servo, and step motors. Also, a motor may be just
a motor or it may be a gearmotor; that is, it may have an integral geared speed reducer. Air motors are usually rotary vane or
piston type; hydraulic motors are rotary vane, piston, or
gear type. Each has operating characteristics inherent to its basic design. Thus, motor selection is often a process
of matching application requirements to performance parameters
of the various types, then selecting the most compatible.
DC MOTORS
Direct current motors are used in
many industrial applications that require
adjustable speed.
In uses requiring quick stops, a dc
motor can minimize the size of a mechanical
brake or make it unnecessary.
This is done by dynamic braking
(motor-generated energy fed to a resistor
grid), or by regenerative braking
(motor-generated energy returned
to the ac supply).
DC motor speed can be controlled
smoothly down to zero, followed
immediately by acceleration
in the opposite direction (without
power circuit switching). Also, due
to high torque-to-inertia ratio, dc
motors respond quickly to controlsignal
changes.
Motor types
DC motors in all but fractional
and low integral horsepower sizes
(generally below 15 hp), have
wound fields and are categorized
as shunt-wound, series-wound, or
compound-wound motors. In fractional
and low integral horsepower
sizes, permanent magnets
are used instead of wound fields in
many motor designs.
Selecting dc motors
Choosing a dc motor type and associated
equipment for a given application
requires consideration of several
factors.
Speed range — The minimum
and maximum speeds for an application
determine the motor base speed.
Allowable speed variation —
Applications requiring constant speed
at all torque values should use a
shunt-wound motor. If speed changes
with load and speed variation must be
minimized to less than 2%, a regulator
employing tachometer feedback
must be used.
Torque requirements — The
torque requirements at various operating
speeds should be determined.
Many applications are essentially
constant torque, such as conveyors.
Others, such as centrifugal blowers,
require torque to vary as the square of
the speed. In contrast, machine tools
and center winders are constant
horsepower, with torque decreasing
as speed increases. Thus, the speedtorque
relationship determines the
most economical motor.