Thursday, September 29, 2011

Types Of Electric Motors (1)

DC motors:

Direct-current motors, as the name implies, use direct-unidirectional current. DC motors are used in special applications where high torque starting or smooth acceleration over
a broad speed range is required.

A DC motor is shown  has three main components:

�� Field pole. Simply put, the interaction of two magnetic fields causes the rotation in a DC
motor. The DC motor has field poles that are
stationary and an armature that turns on
bearings in the space between the field poles. A simple DC motor has two field poles: a
north pole and a south pole. The magnetic lines of force extend across the opening
between the poles from north to south. For larger or more complex motors there are one
or more electromagnets. These electromagnets receive electricity from an outside power
source and serve as the field structure.

�� Armature. When current goes through the armature, it becomes an electromagnet. The
armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the
case of a small DC motor, the armature rotates in the magnetic field established by the
poles, until the north and south poles of the magnets change location with respect to the
armature. Once this happens, the current is reversed to switch the south and north poles
of the armature.

�� Commutator. This component is found mainly in DC motors. Its purpose is to overturn
the direction of the electric current in the armature. The commutator also aids in the
transmission of current between the armature and the power source.
power supply. It can be controlled by adjusting:

�� the armature voltage – increasing the armature voltage will increase the speed
�� the field current – reducing the field current will increase the speed.

DC motors are available in a wide range of sizes, but their use is generally restricted to a few
low speed, low-to-medium power applications like machine tools and rolling mills because of
problems with mechanical commutation at large sizes. Also, they are restricted for use only in
clean, non-hazardous areas because of the risk of sparking at the brushes. DC motors are also
expensive relative to AC motors.

The relationship between speed, field flux and armature voltage is shown in the following

Back electromagnetic force: E = KΦN
Torque: T = KΦIa


E = electromagnetic force developed at armature terminal (volt)
Φ = field flux which is directly proportional to field current
N = speed in RPM (revolutions per minute)
T = electromagnetic torque
Ia = armature current
K = an equation constant



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