13.1 ELEMENTARY CONCEPTS OF ROTATING MACHINES 557Figure 13.1.4Sketch of damper bars located on salient-
pole shoes of a synchronous machine.Because a synchronous machine operates only at synchronous speed under steady-state
conditions, the machine cannot operate at synchronous speed during the load transition when the
load on the synchronous machine is to be changed; the readjustment process is in fact dynamic.
In the case of a generator connected to an infinite bus, an increase in the electric output power of
the generator is brought about by increasing the mechanical input power supplied by the prime
mover. The speed of the rotor increases momentarily during the process, and the axis of the rotor-
field mmf advances relative to the axes of both the armature mmf and the resultant air-gap mmf.
The increase in torque angle results in an increase in electric power output. The machine then
locks itself into synchronism and continues to rotate at synchronous speed until the load changes
further. The same general argument can be made for the operation of a synchronous motor, except
that an increase in mechanical load decreases the speed of the rotor during the transition period,
so that the axis of the rotor-field mmf falls behind that of the stator mmf or the resultant air-gap
mmf by the required value of the load angle.
When the load on a synchronous machine changes, the load angle changes from one steady
value to another. During this transition, oscillations in the load angle and consequent associated
mechanical oscillations (known ashunting) occur. To damp out these oscillations, it is common
practice to provide an additional short-circuited winding on the field structure, made out of copper
or brass bars located in pole-face slots on the pole shoes of a salient-pole machine and connected
together at the ends of the machine. This additional winding (known asdamperoramortisseur
winding) is shown in Figure 13.1.4. The winding is also useful in getting a synchronous motor
started, as explained in the subsection on elementary induction machines. When the machine
is operating under steady-state conditions at synchronous speed, however, this winding has no
effect. Because there is no rate of change of flux linkage, no voltage is induced in it.
As seen in Figure 13.1.3, whenδris±π/2 or 90°, the maximum torque or power (calledpull-
out torqueorpull-out power) is reached for a fixed terminal voltage and a given field current. If the
load requirements exceed this value, the motor slows down because of the excess shaft torque.
Synchronous-motor action is lost because the rotor and stator fields are no longer stationary
with respect to each other. Any load requiring a torque greater than the maximum torque results
in unstable operation of the machine; the machine pulls out of synchronism, known aspulling
out of stepor losing synchronism. The motor is usually disconnected from the electric supply by
automatic circuit breakers, and the machine comes to a standstill. Note that the pull-out torque can
be increased by increasing either the field current or the terminal voltage. In the case of a generator
connected to an infinite bus, synchronism will be lost if the torque applied by the prime mover
exceeds the maximum generator pull-out torque. The speed will then increase rapidly unless the
quick-response governor action comes into play on the prime mover to control the speed.