Page 348 - Compression Machinery for Oil and Gas

Drivers Chapter 7 333

close to its synchronous speed, at which the DC excitation is turned on.
When magnetic locking between the stator and rotor takes place, the power
is no longer supplied to the external starter motor.
(b) Motor starting with damper winding.
5. The synchronous motor is of salient pole type, additional winding is placed
in rotor pole face. Initially, when rotor is at standstill, relative speed between
damper winding and rotating air gap flux is large and an electromagnetic
force is induced in it to produce the required starting torque, according to
the same principles as in the induction motor. As the motor approaches syn-
chronous speed, excitation on the rotor is turned on by applying DC power.
The rotor is locked in and synchronized with the stator field. Hence, in this
case, the synchronous motor is first run as an induction motor using addi-
tional winding and finally it is synchronized with the frequency.

Synchronous motors with direct current excitation are typically supplied with
excitation control equipment to apply the excitation in the correct moment
and control the power factor.
Once the motor is in operation, the speed of the motor is dependent only on
the supply frequency. When the motor load is increased beyond the breakdown
load, the motor falls out of synchronization and stalls, such as when the applied
load is large enough to pullout the field winding from following the rotating
magnetic field.

Why Use Synchronous Motors?
Synchronous motors need excitation and they are generally more complex and
more expensive than the induction motors. Why are they used?
The main advantages of synchronous motors are:
l Rotational speed is independent of the load. The motor operates at constant
RPM (revolutions per minute). This is important in process industries where
constant speed is important for the quality of the product, for example, paper
industry.
l Efficiency is higher than of an induction motor of the same output and volt-
age rating because there are neither losses related to slip nor the additional
losses due to magnetizing current. With synchronous motors, there is no dif-
ference of speed between air gap rotating magnetic field and rotor. With
induction motors, rotating magnetic field and rotor are not at the same speed,
so eddy losses are present and those losses introduced by the slip are mainly
responsible for reduced efficiency. In addition, with synchronous motor, the
excitation is applied directly on the rotor field winding, while with induction
motor, the power required for excitation is coming from the stator and
induced on the rotor, so additional losses due to magnetization are present
with the induction motor.