Synchronous Motor
Introduction:
Synchronous motors are like induction motors in that they both have stator
windings that produce a rotating magnetic field. Unlike an induction motor, the
synchronous motor is excited by an external DC source and, therefore, requires slip rings
and brushes to provide current to the rotor.
In the synchronous motor, the rotor locks into step with the rotating magnetic
field and rotates at synchronous speed. If the synchronous motor is loaded to the point
where the rotor is pulled out of step with the rotating magnetic field, no torque is
developed, and the motor will stop. A synchronous motor is not a self-starting motor
because torque is only developed when running at synchronous speed; therefore, the
motor needs some type of device to bring the rotor to synchronous speed.
Synchronous motors use a wound rotor. This type of rotor contains coils of wire
placed in the rotor slots. Slip rings and brushes are used to supply current to the rotor.
Method of Starting:
A synchronous motor may be started by a DC motor on a common shaft. When
the motor is brought to synchronous speed, AC current is applied to the stator windings.
The DC motor now acts as a DC generator and supplies DC field excitation to the rotor of
the synchronous motor. The load may now be placed on the synchronous motor.
Synchronous motors are more often started by means of a squirrel-cage winding
embedded in the face of the rotor poles. The motor is then started as an induction motor
and brought to 95% of synchronous speed, at which time direct current is applied, and the
motor begins to pull into synchronism. The torque required to pull the motor into
synchronism is called the pull-in torque.
Synchronous motor rotor is locked into step with the rotating magnetic field and
must continue to operate at synchronous speed for all loads. During no-load conditions,
the center lines of a pole of the rotating magnetic field and the DC field pole coincide
(Figure 1a). As load is applied to the motor, there is a backward shift of the rotor pole,
relative to the stator pole (Figure 1b). There is no change in speed. The angle between the
rotor and stator poles is called the torque angle.
If the mechanical load n the motor is increased to the point where the rotor is
pulled out of synchronism, the motor will stop. The maximum value of torque that a
motor can develop without losing synchronism is called its pull-out torque.
Synchronous Motor Field Excitation:
For a constant load, the power factor of a synchronous motor can be varied from a
leading value to a lagging value by adjusting the DC field excitation (Figure 1). Field
excitation can be adjusted so that PF = 1 (Figure 1a). With a constant load on the motor,
when the field excitation is increased, the counter EMF increases. The result is a change
in phase between stator current (I) and terminal voltage (Vt), so that the motor operates at
a leading power factor (Figure 1b). Vp in the figure is the voltage trop in the stator
winding’s due to the impedance of the windings and is 90° out of phase with the stator
current. If we reduce field excitation, the motor will operate at a lagging power factor
(Figure 1c). Note that torque angle, α also varies as field excitation is adjusted to change
power factor.
Applications:
Synchronous motors are used to accommodate large loads and to improve the power
factor of transformers in large industrial complexes.
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.
Overexcited synchronous motors having leading power factor are widely used for
improving power factor of those power systems which employ a large number of
induction motors.
Because of their high efficiency and high-speed, synchronous motors are well-suited
for loads where constant speed is required such as centrifugal pumps, belt-driven
reciprocating compressors, blowers, line shafts, rubber and paper mills etc.
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:
Speed is independent of the load, provided an adequate field current is applied.
Accurate control in speed and position using open loop controls, eg. stepper motors.
They will hold their position when a DC current is applied to both the stator and the
rotor windings.
Their power factor can be adjusted to unity by using a proper field current relative to
the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can
be obtained by increasing this current slightly, which can help achieve a better power
factor correction for the whole installation.
Their construction allows for increased electrical efficiency when a low speed is
required (as in ball mills and similar apparatus).
