Three phase induction motor
NTRODUCTION
Three-PhaseSynchronous Machines
Synchronous machines are called "synchronous" as their speed is directly related to the line
frequency.
ns = 120 f/p
revlm s p
where ns is the synchronous speed in rpm,
p the number of magnetic poles designed into each
machine and
machine and
f is the frequency in Hz of the supply voltage. Thus when two or more synchronous
machines are connected to the same supply line, they will all run in synchronism because they
are all operating at the same supply frequency. Of course, if one machine has 2 poles (Turboalternators)
and another has 12 poles, the 12-pole machine will run at precisely �th the speed of
the 2-pole machine.
small. The steady state power input to a field winding is only about 1/2 to 2 per cent of the rated
power of the machine. The input power to de field winding is all consumed as I2R loss except
during transient period which lasts for almost a second or two during which energy is being
stored in magnetic field.
The winding conductors are either of copper or aluminium and that could be in the form of
coils of wire or heavy bars depending upon the current carrying capacity required. Also each
winding consists of several coils or bars in series or in series-parallel combination depending upon
the voltage and current requirements of the machine. The ends of certain of the windings are
brought out to terminals to allow easy connection to the electrical system.
In some machines, the windings of either the rotor or the stator may be placed around
projecting magnetic pole pieces, called salient poles (salient means prominent). When a core has
salient poles the coils of the winding are wound around the waists of the pole pieces. These
narrower parts of the salient poles are called the pole cores. The shaped ends of the poles are called
the pole shoes. Their purpose is to provide the correct flux density distribution in the air gap.
Salient poles are used on the stator cores of de machines and on the rotor cores of many synchronous
machines and in both cases they carry de field windings. The mechanical weakness and air
resistance of salient poles prohibit their use on rotors oflarge high speed synchronous generator
designed to he driven hy steam or gas turbines. Turhoalternators as these are usually called
always have cylindrical rotors with the field windings embedded in slots cut into the rotor surface.
A machine having both its rotor and stator windings in slots is known as round rotor machine or
non-salient pole machine.
The insulation system consists of (i) the conductor or wire insulation e.g. superenamelled
wires or conductors ; (ii) the coil insulation usually some kind of tape or several layers of tapes
depending upon operating voltage ; and (iii) the slot lines when the windings consist of coils that
are located in slots, the coils are held in place in the slots by slot wedges. The insulation system
thus prevents short circuits between turns of a given winding coil and insulate the winding from
the iron core which is always grounded for safety's sake. The insulation system must protect the
machine against damage due to overvoltages and overcurrents that may occur during the operation
of the machine.
machines are connected to the same supply line, they will all run in synchronism because they
are all operating at the same supply frequency. Of course, if one machine has 2 poles (Turboalternators)
and another has 12 poles, the 12-pole machine will run at precisely �th the speed of
the 2-pole machine.
CONSTRUCTION
Rotating machines have an outside (stationary) part called the stator. The inner (rotating) part is
called the rotor. The rotor is centred within the stator so that the rotor axis is concentric with
that of the stator. The space between the outside of the rotor and inside of the stator is called the
air gap.
The rotor is mounted on a stiff rod (usually steel) called a shaft. The shaft is supported in
bearings so that the rotor is free to rotate. The shaft extends through one or both of the bearings
to provide a means to connect the machine to the mechanical system. It is to be noted that the
rotor is solidly fastened to the shaft so that the rotor and the shaft rotate at the same speed.
Therefore, the terms 'rotor speed', 'shaft speed' or machine speed all mean the same thing and are
used interchangeably.
The rotor and stator each have three basic parts, the core, the windings and the insulation.
Thus, it is proper to speak of "the rotor core", 'the stator windings' 'the rotor insulation' and so on.
The purpose of the rotor and stator cores is to conduct the magnetic field through the coils of the
windings. The cores are almost always made of iron or steel.
The stator winding is known as armature winding as it carries the load current whereas
the rotor winding is known as field winding as this winding provides exciting current or magnetiscurrent. Since the armature winding carries load current, it has to handle all of the power being
converted or transformed by the device, however, the magnetising power requirement is relatively called the rotor. The rotor is centred within the stator so that the rotor axis is concentric with
that of the stator. The space between the outside of the rotor and inside of the stator is called the
air gap.
The rotor is mounted on a stiff rod (usually steel) called a shaft. The shaft is supported in
bearings so that the rotor is free to rotate. The shaft extends through one or both of the bearings
to provide a means to connect the machine to the mechanical system. It is to be noted that the
rotor is solidly fastened to the shaft so that the rotor and the shaft rotate at the same speed.
Therefore, the terms 'rotor speed', 'shaft speed' or machine speed all mean the same thing and are
used interchangeably.
The rotor and stator each have three basic parts, the core, the windings and the insulation.
Thus, it is proper to speak of "the rotor core", 'the stator windings' 'the rotor insulation' and so on.
The purpose of the rotor and stator cores is to conduct the magnetic field through the coils of the
windings. The cores are almost always made of iron or steel.
The stator winding is known as armature winding as it carries the load current whereas
the rotor winding is known as field winding as this winding provides exciting current or magnetiscurrent. Since the armature winding carries load current, it has to handle all of the power being
small. The steady state power input to a field winding is only about 1/2 to 2 per cent of the rated
power of the machine. The input power to de field winding is all consumed as I2R loss except
during transient period which lasts for almost a second or two during which energy is being
stored in magnetic field.
The winding conductors are either of copper or aluminium and that could be in the form of
coils of wire or heavy bars depending upon the current carrying capacity required. Also each
winding consists of several coils or bars in series or in series-parallel combination depending upon
the voltage and current requirements of the machine. The ends of certain of the windings are
brought out to terminals to allow easy connection to the electrical system.
In some machines, the windings of either the rotor or the stator may be placed around
projecting magnetic pole pieces, called salient poles (salient means prominent). When a core has
salient poles the coils of the winding are wound around the waists of the pole pieces. These
narrower parts of the salient poles are called the pole cores. The shaped ends of the poles are called
the pole shoes. Their purpose is to provide the correct flux density distribution in the air gap.
Salient poles are used on the stator cores of de machines and on the rotor cores of many synchronous
machines and in both cases they carry de field windings. The mechanical weakness and air
resistance of salient poles prohibit their use on rotors oflarge high speed synchronous generator
designed to he driven hy steam or gas turbines. Turhoalternators as these are usually called
always have cylindrical rotors with the field windings embedded in slots cut into the rotor surface.
A machine having both its rotor and stator windings in slots is known as round rotor machine or
non-salient pole machine.
The insulation system consists of (i) the conductor or wire insulation e.g. superenamelled
wires or conductors ; (ii) the coil insulation usually some kind of tape or several layers of tapes
depending upon operating voltage ; and (iii) the slot lines when the windings consist of coils that
are located in slots, the coils are held in place in the slots by slot wedges. The insulation system
thus prevents short circuits between turns of a given winding coil and insulate the winding from
the iron core which is always grounded for safety's sake. The insulation system must protect the
machine against damage due to overvoltages and overcurrents that may occur during the operation
of the machine.
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