29-04-2014, 11:27 AM
The Synchronous Machine
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The Synchronous Machine
Synchronous ac machines find application as motors in constant speed
applications and, when interfaced to the power source with a variable-frequency
converter system, in variable-speed applications. High-performance variable-speed motor
drives are often constructed using a permanent-magnet synchronous motor. Another
common application is the alternator that generates the power for the automobile
electrical system. In addition, practically all electrical energy produced commercially is
generated in rotating synchronous machines that are connected to the utility or other
power systems. Most power systems are supplied by a number of such machines
operating in parallel, and the systems themselves are interconnected to form power grids
of tremendous energy capacity. In comparing the capability of any single generator to the
overall capacity of the system to which it is connected, it can be seen that the individual
machine is relatively insignificant, even though its own power rating may be in the
neighborhood of one million kilowatts. In other words, it can be thought of as being
connected to a voltage bus of infinite capacity, so that regardless of how much energy it
delivers to (or receives from) the system, the voltage and frequency remain constant.
An equivalent circuit model for the synchronous machine
A basic synchronous machine is sketched in Fig. 1. The stator contains a three-phase
armature winding. When a source of three-phase ac is connected to this winding, a
magnetic field of constant amplitude is produced within the machine; in a two-pole
machine, this field rotates at frequency equal to the frequency of the applied ac. The rotor
contains a field winding that is excited by dc; this winding behaves as an electromagnet,
producing a field of strength proportional to the applied field current that is aligned with
the axis of the field winding. Alternatively, the field winding may be replaced by a
permanent magnet.
Discussion
When the synchronous machine is connected to an infinite bus, the shaft speed is
determined by the frequency of the infinite bus and is independent of other quantities
such as field currents, load torque, etc. To the extent that the synchronous machine is
ideal, its mechanical input power must be equal to its electrical output power. To change
the electrical output power, the energy source supplying mechanical input power to the
machine must be changed. For example, if the energy source is a gasoline engine, then to
increase the electrical output power of the generator the engine throttle must be opened,
burning more gasoline. In our laboratory experiment, the mechanical energy source will
be a dc motor.
Synchronizing to an infinite bus
Let’s consider next how to connect a generator to an infinite bus. When preparing to
operate a generator (either ac or dc) in parallel with other generators or a power system, it
is always necessary to make certain that no potential difference exists across the
terminals of the paralleling switch when it is closed. It must be remembered that both the
machine and the ac system represent very low impedance circuits, and even a small
potential difference can result in very large circulating currents that may cause unwanted
circuit breaker openings and/or damaging shaft torques.