Capacitor Start.
A capacitor start induction run motor is similar in some ways to the split phase
type. The main difference is that a capacitor is placed in series with the start
winding. The capacitor start motor produces considerably more starting torque
than the split phase motor. After the start winding is disconnected from the
circuit, the performance is nearly identical with the split phase motor.
Logically, capacitor start motors should be used where the load acceleration
requirements exceed the capacity of a split phase motor.
Permanent - Split Capacitor. The permanent-split capacitor
also has an auxiliary winding with a capacitor. However, the capacitor and
auxiliary windings are continuously energized and aid in producing a higher
power factor than other designs. Locked rotor torque is low compared to split
phase or capacitor start motors and are therefore restricted to direct fan
drives and blowers. They are not recommended for belt-driven loads. Since the
capacitor and start winding operate continuously there is no need for a
centrifugal switch mechanism thus making this motor shorter than the other split
phase type motors.
Shaded Pole. The torque characteristics and application of
the shaded pole motor is similar to the permanent-split capacitor motors. In
place of an auxiliary winding, shaded pole motors have a continuous solid copper
loop around a small portion of each stator pole. This loop called a
"Shading Pole", causes a reaction giving the motor its starting
torque. Because of its simple construction, it is usually the lowest priced
motor available. However its low starting torque, except when used in
conjunction with speed reduction drives makes it suitable in most cases only for
direct fan and blower applications. Power loss is nearly constant between no
load and load. Therefore light loading does not reduce motor losses
significantly.
Repulsion Start - Induction Motors. The repulsion start
induction motor is a single phase motor consisting of a stator winding
connnected to the power source and a rotor winding connected to a commutator.
The motor starts as a series motor and at a predetermined speed, all the
commutator bars are electrically shorted by a device called a
"necklace" to give the equivalent of a squirrel cage winding. This
type of motor starts as a repulsion motor but operated as an induction motor
with constant speed characteristics. It does require more maintenance and has
therefore been almost entirely replaced by capacitor start motors, although the
repulsion start induction motor can develop more locked-rotor torque with much
less locked rotor current.
Synchronous Reluctance Motors. Synchronous reluctance motors
are built In a variety of types and constructions. However, they all have one
feature in common with induction motors, namely a stator winding which when
energized sets up a revolving magnetic field, known as the synchronous speed. It
is exactly proportional to the frequency. Synchronous motors are built so that
they lock in step with the rotating field and rotate at the same speed as the
latter. Hence their main use is for timing devices and clocks.
Squirrel Cage Induction Motors (Polyphase). Fractional H.P.
induction motors are almost always single phase simply because in most
applications where fractional H.P. motors are used, polyphase A.C. Is not
available. Also single phase induction motors are cheaper than their polyphase
counterparts because of manufacturing volume. On the other hand, in many
military applications, polyphase motors are used in small sizes in order to gain
minimum weight and maximum reliability. In this type of motor the stator
windings are excited by polyphase currents which produce a rotating field which
causes the rotor to follow It and thereby act as a motor. The speed of this
rotating field is known as the synchronous speed. The motor always runs slower
than synchronous speed and the difference is known as "slip". The slip
is usually expressed as a percentage of the synchronous speed.
3.2 D.C. Motor Types and
Functional Descriptions
Direct current motors do not rely on a rotating magnetic field but rather on
the reaction of armature current in the presence of a stationary magnetic field
This stationary field is produced either with an electric coil or permanent
magnet. Armature windings together with
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