13-05-2014, 12:10 PM
Induction and Synchronous Motor Fundamentals
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Introduction
Motors are electromagnetic devices that are used to convert electrical energy into
mechanical work. There are three classes of AC motors – synchronous motors, induction
motors and series wound motors. The most common motor classes are synchronous and
induction.
Synchronous motors
A synchronous motor is a synchronous machine used for a motor. A synchronous motor
cannot start without being driven. They need a separate starting means.
There are several types of synchronous motors. These include direct current excited
synchronous motor (field poles are excited by direct current), a permanent magnet
synchronous motor (field excitation is provided by permanent magnets) and a reluctance
synchronous motor (starts as an induction motor, is normally provided with a squirrel-
cage winding, but operates at synchronous speed).
Synchronous motors have fixed stator windings electrically connected to the AC supply
with a separate source of excitation connected to a field winding on the rotating shaft. A
three-phase stator is similar to that of an induction motor. The rotating field has the same
number of poles as the stator, and is supplied by an external source of DC. Magnetic flux
links the rotor and stator windings causing the motor to operate at synchronous speed. A
synchronous motor starts as an induction motor, until the rotor speed is near synchronous
speed where it is locked in step with the stator by application of a field excitation. When
the synchronous motor is operating at synchronous speed, it is possible to alter the power
factor by varying the excitation supplied to the motor field.
Induction motors
Induction motors are simple and rugged and relatively cheap to construct. They consist
of a wound stator and a rotor assembly. They have fixed stator windings that are
electrically connected to an AC power source. Current is induced in the rotor circuit.
The resulting magnetic field interacts with the stator field for the “induction” to occur.
No separate power source is required to provide the rotor field. An induction motor can
be started and accelerated to steady state running conditions simply by applying AC
power to the fixed stator windings of the motor. They do not rely on brushes like a DC
motor does. Induction motors have a longer life than synchronous motors and are
common for applications above 1 kW.
There are a couple of types of induction motors – a squirrel-cage motor and a wound-
rotor motor. A squirrel-cage motor is one where the secondary circuit consists of a
number of conducting bars that have their end pieces connected by metal rings or plates
at each end. A wound-rotor motor in one where the secondary circuit has a polyphase
winding or coils whose terminals are either short circuited or closed through suitable
circuits.
Weather Protected Machine
There are two types of weather protected machines: a Type 1 which is guarded with its
ventilation constructed to minimize the entry of rain, snow and air-borne particles, or
Type 2 which is constructed similar to a Type 1, but its ventilation is constructed at the
intake and discharge such that storms or high winds cannot blow directly into the electric
parts of the machine itself.
Totally Enclosed Machine
A totally enclosed machine is enclosed to prevent the free exchange of air between the
inside and outside of the case, but not airtight or dust tight. A totally enclosed
nonventilated machine is a machine cooled by free convection. A totally enclosed fan-
cooled machine is equipped for cooling by fans integral to the machine but external to the
enclosing parts. A totally enclosed water cooled machine is a machine cooled by
circulating water in contact with the machine parts. A water proof machine is generally
constructed so that a stream of water from a hose will not enter the machine.
Explosion Proof Machine
An explosion proof machine is a machine where the enclosure is designed to withstand
the explosion of a specified gas or vapor which may occur within it, and to prevent the
ignition of the gas or vapor surrounding the machine due to sparks, flashes or explosions
that may occur within the machine casing.
Insulation systems
There are different insulating components used in the process of building a motor, such
as the enamel coating on the magnet wire and the insulation on the leads in the motor
box. Another important component is the dipping varnish which is used to seal scratches
that may have occurred and binds the winding together so that it does not vibrate or chafe
when subjected to the magnetic force that exists in the motor. Insulation systems are
divided into classes based on the thermal aging and failure. Four classes are commonly
used in motors – A, B, F, and H. Refer also to IEEE Std. 1. The temperature classes are
separated by 25 degree C increments. The temperature capability of each class is defined
as the maximum temperature at which the insulation can be operated to yield an average
life of 20,000 hours.