Synchronous Motor Working Principle
Electrical motor in general is an electro-mechanical
device that converts energy from electrical domain to mechanical domain. Based
on the type of input we have classified it into single phase and 3 phase
motors. Among 3 phase motors, induction motors and synchronous motors are more
widely used. When a 3 phase electric conductors are placed in a certain
geometrical positions (In certain angle from one another) then an electrical
field is generated. Now the rotating magnetic
field rotates at a certain speed, that speed is called
synchronous speed. Now if an electromagnet is present in this rotating magnetic
field, the electromagnet is magnetically locked with this rotating magnetic
field and rotates with same speed of rotating field.
Synchronous motors is called so because the speed of the rotor of this motor is
same as the rotating magnetic field. It is basically a fixed speed motor
because it has only one speed, which is synchronous speed and therefore no
intermediate speed is there or in other words it’s in synchronism with the
supply frequency. Synchronous speed is given by
where f
= supply frequency & p = no. of poles
Construction of Synchronous Motor
Normally it's construction
is almost similar to that of a 3 phase induction motor, except the fact that
the rotor is given DC supply, the reason of which is explained later. Now, let
us first go through the basic construction of this type of motor.
From the above picture, it is clear that how this type of motors are designed.
The stator is given is given three phase supply and the rotor is given dc supply.
Main Features of Synchronous Motors
1. Synchronous
motors are inherently not self-starting. They require some external means
to bring their speed close to synchronous speed to before they are synchronized.
2. The speed
of operation of is in synchronism with the supply frequency and hence for
constant supply frequency they behave as constant speed motor irrespective of
load condition
3. This motor
has the unique characteristics of operating under any electrical power
factor. This makes it being used in electrical power factor improvement.
Principle of Operation Synchronous Motor
Synchronous motor is a doubly excited machine i.e two
electrical inputs are provided to it. It’s stator winding
which consists of a 3 phase winding is provided with 3 phase supply and rotor
is provided with DC supply. The 3 phase stator winding carrying 3 phase
currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply
also produces a constant flux. Considering the frequency to be 50 Hz, from the
above relation we can see that the 3 phase rotating flux rotates about 3000
revolution in 1 min or 50 revolutions in 1 sec. At a particular instant rotor
and stator poles might be of same polarity (N-N or S-S) causing repulsive force
on rotor and the very next second it will be N-S causing attractive force. But
due to inertia of the rotor, it is unable to rotate in any direction due to
attractive or repulsive force and remain in standstill condition. Hence it is
not self
starting.
To overcome this inertia, rotor is initially fed some mechanical input which
rotates it in same direction as magnetic field to a speed very close to
synchronous speed. After some time magnetic locking occurs and the synchronous
motor rotates in synchronism with the frequency.
Methods of Starting of Synchronous Motor
1. Motor
starting with an external prime Mover: Synchronous motors are
mechanically coupled with another motor. It could be either 3 phase induction
motor or DC shunt motor. DC excitation is not fed initially. It is
rotated at speed very close to its synchronous speed and after that DC
excitation is given. After some time when magnetic locking takes place supply
to the external motor is cut off.
2. Damper
winding: In case, synchronous motor is of salient pole type, additional
winding is placed in rotor pole face. Initially when rotor is standstill,
relative speed between damper winding and rotating air gap flux in large and
an emf is
induced in it which produces the required starting torque. As speed approaches
synchronous speed, emf and
torque is reduced and finally when magnetic locking takes place, torque also
reduces to zero. Hence in this case synchronous is first run as three
phase induction motor using additional winding and finally it is synchronized
with the frequency.
Application of Synchronous Motor
1. Synchronous
motor having no load connected to its shaft is used for power
factor improvement. Owing to its characteristics to behave at any
electrical power factor, it is used in power system in situations where
static capacitors are expensive.
2. Synchronous
motor finds application where operating speed is less (around 500 rpm) and high
power is required. For power requirement from 35 kW to 2500 KW, the size,
weight and cost of the corresponding three phase induction motor is very high.
Hence these motors are preferably used. Ex- Reciprocating pump, compressor,
rolling mills etc.