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Title: The three phase synchronous
motor.
Aim: To investigate the
principal operating characteristics of a three phase synchronous motor.
Apparatus: See your laboratory manual for the apparatus.
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THEORY:
A synchronous
electric motor is an AC motor in which, at steady state,[1] the
rotation of the shaft is synchronized with the frequency of the supply current;
the rotation period is exactly equal to an integral number of AC cycles.
Synchronous motors contain multiphase AC electromagnets on the stator of the
motor that create a magnetic field which rotates in time with the oscillations
of the line current. The rotor with permanent magnets or electromagnets turns
in step with the stator field at the same rate and as a result, provides the
second synchronized rotating magnet field of any AC motor. A synchronous motor
is only considered doubly-fed if is supplied with independently excited
multiphase AC electromagnets on both the rotor and stator.
The
synchronous motor and induction motor are the most widely used types of AC
motor. The difference between the two types is that the synchronous motor
rotates in exact synchronism with the line frequency. The synchronous motor
does not rely on current induction to produce the rotor's magnetic field. By
contrast, the induction motor requires "slip", the rotor must
rotate slightly slower than the AC current alternations, to induce current in
the rotor winding. Small synchronous motors are used in timing applications
such as in synchronous clocks, timers in appliances, tape recorders and precision
servomechanisms in which the motor must operate at a precise speed; speed
accuracy is that of the power line frequency, which is carefully controlled in
large interconnected grid systems.
Synchronous
motors are available in sub-fractional self-excited sizes[2]
to high-horsepower industrial sizes.[1] In the fractional horsepower
range, most synchronous motors are used where precise constant speed is
required. These machines are commonly used in analog electric clocks, timers
and other devices where correct time is required. In high-horsepower industrial
sizes, the synchronous motor provides two important functions. First, it is a
highly efficient means of converting AC energy to work. Second, it can operate
at leading or unity power factor and thereby provide power-factor correction.
Theory of
operation
When the motor is supplied
with a.c. power supply, the stator poles get energised. This in turn attracts
(opposite) the rotor poles, thus both the stator and rotor poles get
magnetically interlocked. It is this interlock which makes the rotor to rotate
at the same synchronous speed with the stator poles. The synchronous speed of
rotation is given by the expression Ns=120f/P.
When the load on the motor
is increased gradually, the rotor even though runs at same speed, tends to
progressively fall back in phase by some angle, “β”, called the Load Angle or
the Coupling Angle. This Load angle is dependent on the amount of load that the
motor is designed to handle. In other words, we can interpret as the torque developed
by the motor depends on the load angle, “β”.
The electrical working of a
Synchronous Motor can be compared to the transmission of power by a mechanical
shaft. In the figure are shown two pulleys, “A” & “B”. Pulley “A” and the
pulley “B” are assumed to be keyed on the same shaft. Pulley “A” transfers the
power from the drive through the shaft, in turn making the pulley “B” to
rotate, thus transferring power to the load.
The
two pulleys which are keyed to the same shaft can be compared to the interlock
between the stator & rotor poles.
If the load increases, the
pulley “B” transfers the increase in load to the shaft, which is exhibited by
the twisting of the shaft.
Thus
the twist of the shaft can be compared to the rotor falling back in phase with
the stator.
The twist angle can be
compared to the load angle “β”. Also when the load increases, the twisting
force and the twist angle increases, thus the load angle “β” also increases.
If the
load on the pulley “B” is increased to such an extent that it makes the shaft
to twist and break, then the transmission of power through the shaft stops as
the shaft is broken. This can be compared with the rotor going out of synchronism
with the stator poles.
Thus Synchronous motors can
run either at synchronous speed or they stop running.
Know how a synchronous motors differ from the
conventional Induction motors. Also know where are the Synchronous motors are
widely used and its Applications.
Starting
procedure
All the Synchronous Motors
are equipped with “Squirrel Cage winding”, consisting of Cu (copper) bars,
short-circuited at both ends. These windings also serve the purpose of
self-starting of the Synchronous motor. During starting, it readily starts and
acts as induction motors. For starting a Synchronous motor, the line voltage is
applied to the stator terminals with the field terminals (rotor) left
unexcited. It starts as an induction motor, and when it reaches a speed of about
95% of its synchronous speed, a weak d.c excitation is given to the rotor, thus
making the rotor to align in synchronism with the stator.( at this moment the
stator & rotor poles get interlocked with each other & hence pull the
motor into synchronism.
Hunting/Surging/Phase Swinging
Hunting or Surging or Phase
swinging of a synchronous motor is caused by either
1. Varying load.
2. Pulsating supply frequency.
When a Synchronous motor is
loaded (such as compressors, pumps, shears etc), as the load increases, its
rotor falls back by a coupling angle “β”. As the load is increased further,
this angle “β” further increases to cope up with the increased load. At this
situation, if the load suddenly decreases, the rotor overshoots and then it’s
pulled back to suite the new load on the motor. In this way, the rotor starts
oscillating like a pendulum, about its new position corresponding to its new
load, trying to regain equilibrium. If the time period of these oscillations
happen to coincide with the natural frequency of the machine, then a resonance
is set-up, thus may throw the machine out of synchronism. To dampen such
oscillations, “damper” or “damping grids” known as “squirrel-cage windings” are
employed.
Applications of Synchronous Motors:
·
These motors are used as prime movers (drives) for
centrifugal pumps, belt-driven reciprocating compressors, Air Blowers, Paper
Mills, rubber factories etc, because of their high efficiency & high speeds
(r.p.m above 600).
·
Low speed Synchronous motors (r.p.m below 600), are widely
used for driving many positive displacement pumps like screw & gear pumps,
vacuum pumps, chippers, metal rolling mills, aluminum foil rolling machines
etc.
·
These motors are also widely used onboard ships. The
ship’s navigational equipment like Gyro-compass use a special type of
synchronous motor. They are also used as prime movers for Visco-Therm or
Viscometer, a device for measuring/regulating the viscosity of the main
propulsion engine’s fuel oil.
·
Most of the factories & industries use infinite number
of inductive loads. These may range from the tube lights to high power
induction motors. Thus these inductive loads have a drastic lagging power
factor. An Over-excited Synchronous motor ( a Synchronous Capacitor), having a
leading power factor, is used to improve the power factor of these supply
systems.
·
These Motors are also used for Voltage-regulation, where
a heavy voltage dip/rise occurs when a heavy inductive load in put on/off at
the end of the long transmission lines.
·
Synchronous motors can be run at ultra-low speeds by
using high power electronic converters which generate very low frequencies.
Examples of these motors are a 10 MW range used for driving crushers, rotary
kilns & variable speed ball mills.
For Procedure and Circuit Diagram, See your Electrical
Laboratory Manual on page 3.
Fill Up Tables in Your Manual and Continue
To….
PRECAUTIONS:
For
Precautions, See General
Electrical Lab Precautions
Answers to Questions:
No Questions in this
experiment.
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