1. Synchronous motor
A synchronous motor is distinguished by its rotor spinning
at the same rate as the oscillating field which drives it. Another way of
saying this is that it has zero slip under usual operating conditions. Contrast
this with an induction motor, which must slip in order to produce torque. Sometimes a synchronous motor is used, not to
drive a load, but to improve the power factor on the local grid it's connected
to. It does this by providing reactive power to, or consuming reactive power
from the grid. In this case the synchronous motor is called a Synchronous
condenser.
A synchronous motor may have either a revolving armature or
a revolving field, although most synchronous motors are of the revolving-field
type. The stationary armature is attached to the stator frame, while the field
magnets are attached to a frame that revolves with the shaft. The field coils are excited by direct
currents, either from a small DC generator (usually mounted on the same shaft
as the motor and called an exciter), or from some other source. Fig. 1
shows a directly connected exciter.
Electrical power plants almost always use synchronous
generators because it's very important to keep the frequency constant at which
the generator is connected. Low power applications include positioning
machines, where high precision is required, and robot actuators.
Advantages
Synchronous motors have the following advantages over
non-synchronous motors:
i.
Speed is independent of
the load, provided an adequate field current is applied.
ii.
Accurate control in
speed and position using open loop controls, eg. stepper motors.
iii.
They will hold their
position when a DC current is applied to both the stator and the rotor
windings.
iv.
Their power factor can
be adjusted to unity by using a proper field current relative to the load.
v.
Their construction
allows for increased electrical efficiency when a low speed is required (as in
ball mills and similar apparatus).
2. Squirrel-Cage Motor
The most
common form of induction motor is the squirrel-cage type. This motor has
derived its name from the fact that the rotor, or secondary, resembles the
wheel of a squirrel cage. Its universal use lies in its mechanical simplicity,
its ruggedness, and the fact that it can be manufactured with characteristics
to suit most industrial requirements.
Squirrel-cage motor consists essentially of two units, namely stator and
rotor. The stator (or primary) consists of
a laminated sheet-steel core with slots in which the insulated coils are
placed. The coils are so grouped and connected as to form a definite polar area
and to produce a rotating magnetic field when connected to a polyphase
alternating- current circuit.
The rotor
(or secondary) is also constructed of steel laminations, but the windings
consist of conductor bars placed approximately parallel to the shaft and close
to the rotor surface. These windings are short-circuited, or connected at each
end of the rotor, by a solid ring. The rotors of large motors have bars and
rings of copper connected at each end by a conducting end ring made of copper
or brass. The joints between the bars and end rings are usually electrically
welded into one unit, with blowers mounted on each end of the rotor. In small
squirrel-cage rotors, the bars, end rings, and blowers are of aluminium cast in
one piece instead of welded together.
The air
gap between the rotor and stator must be very small in order for the best power
factor to be obtained. The shaft must, therefore, be very rigid and be
furnished with the highest grade of bearings, usually of the sleeve or
ball-bearing type. A cutaway view of a typical squirrel-cage induction motor is
shown in Figure 2.
Advantages:
i.
Because of its
simplicity of construction and because it can be built with electrical
characteristics to suit almost any industrial requirement, has made it one of
the most widely used machines.
ii.
The speed of a
squirrel-cage motor is nearly constant under normal load and voltage conditions.
iii.
Suitable for medium or
low-starting-torque requirements.
Disadvantages:
Squirrel-cage motors as a rule are not suitable where a high
starting torque is required.
3. Wound
– Rotor Motor
A wound rotor motor is a
variation of the induction motor but does not use a squirrel cage winding. The
stator of the wound rotor motor is the same as the standard three-phase induction
motor, in that it produces a three-phase rotating magnetic field. The rotor is not
a squirrel cage winding. Squirrel cage
windings have cast conducting bars shorted together end rings and installed in
the laminated The rotor of the wound rotor motor act consists of conductors
(magnet wire) wound into coils on the rotor. (See Figure 3.)
The stator in the wound-rotor
motor is identical to the stator in the squirrel-cage motor. The basic
difference in the two motors lies in the rotor winding. In the squirrel-cage motor, the rotor winding
is nearly always self-contained; it is not connected either mechanically or
electrically to the outside power-supply or control circuit. However, in
wound-rotor motors, the rotor winding consists of insulated coils of wire that
are not permanently short-circuited, but are connected in regular succession to
form a definite polar area having the same number of poles as the stator. The
ends of these rotor windings are brought out to collector rings, or slip rings.
The wound-rotor motor is often
used in cranes, hoists, and elevators. These devices are operated
intermittently and for short periods of time, where exact speed regulation and
loss in efficiency are of little consequence.
Wound-rotor motors can be used to start extremely heavy loads. Hence,
they are suitable for: (1) driving various types of machinery that require
development of considerable starting torque to overcome friction; (2)
accelerating extremely heavy loads that have a flywheel effect or inertia; and
!3) overcoming back pressures set up by fluids and gases, as in reciprocating
pumps and compressors.
Advantages:
i.
The wound rotor motor has higher starting torque per line amps than do
most AC squirrel cage motors.
ii.
The wound-rotor motor can operate at any speed from its maximum full-load
speed down to almost standstill.
iii.
Wound-rotor motors have the ability to start extremely heavy loads. Hence they are suitable
for:
Ø Driving various types of machinery
that require development of considerable starting torque to overcome friction.
Ø Accelerating extremely heavy loads
that have a flywheel effect or inertia.
Ø Overcoming back pressures set up
by fluids and gases in the case of reciprocating pumps and compressors.
Disadvantages:
i.
Most motor starters used with wound rotor motors have a provision that
will not allow you to start the motor if all secondary resistance is shorted
out of the secondary.
ii.
The wound rotor motor is a higher maintenance motor because of the
windings that are fitted into the rotor.
iii.
If lowered speed is required over longer periods, poor speed regulation
and loss in efficiency may become prohibitive.
4. Split-phase induction motor
Another common single-phase AC motor is the split-phase
induction motor, commonly used in major appliances
such as washing machines and clothes dryers.
Compared to the shaded pole motor, these motors can generally provide much
greater starting torque by using a special startup winding
in conjunction with a centrifugal switch.
In the split-phase motor, the startup winding is designed
with a higher resistance than the running winding. This creates
an LR circuit
which slightly shifts the phase of the current in the startup winding. When the
motor is starting, the startup winding is connected to the power source via a
set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal
switch. The starting winding is wound with fewer turns of smaller wire than the
main winding, so it has a lower inductance (L) and higher resistance (R).
The lower L/R ratio creates a small phase shift, not more than
about 30 degrees, between the flux due to the main winding and the flux of the
starting winding. The starting direction of rotation may be reversed simply by
exchanging the connections of the startup winding relative to the running
winding.
The phase of the magnetic field in this startup winding
is shifted from the phase of the mains power, allowing the creation of a moving
magnetic field which starts the motor. Once the motor reaches near design
operating speed, the centrifugal switch activates, opening the contacts and
disconnecting the startup winding from the power source. The motor then
operates solely on the running winding. The starting winding must be
disconnected since it would increase the losses in the motor.
Repulsion motors are wound-rotor single-phase
AC motors that are similar to universal motors. In a repulsion motor, the
armature brushes are shorted together rather than connected in series with the
field. Several types of repulsion motors have been manufactured, but the repulsion-start
induction-run (RS-IR) motor has been used most frequently. The RS-IR motor
has a centrifugal switch that shorts all segments of the commutator so that the
motor operates as an induction motor once it has been accelerated to full
speed. RS-IR motors have been used to provide high starting torque per ampere
under conditions of cold operating temperatures and poor source voltage
regulation. Few repulsion motors of any type are sold as of 2005.
6. Shaded-pole motor
A common single-phase motor is the shaded-pole motor,
which is used in devices requiring low starting torque, such as electric fans
or other small household appliances. In this motor, small single-turn copper
"shading coils" create the moving magnetic field. Part of each pole
is encircled by a copper coil or strap; the induced current in the strap
opposes the change of flux through the coil (Lenz's Law),
so that the maximum field intensity moves across the pole face on each cycle,
thus producing a low level rotating magnetic field which is large enough to
turn both the rotor and its attached load. As the rotor accelerates the torque
builds up to its full level as the principal (rotationally stationary) magnetic
field is rotating relative to the rotating rotor. Such motors are difficult to
reverse without significant internal alterations.
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