POLES AND FREQUENCY OF ALTERNATING CURRENT (AC) GENERATORS BASIC AND TUTORIALS

The rotor and stator (field and armature) of a synchronous machine must have the same number of poles, as the magnetic interaction is between a succession of north-south magnetic-field pole pairs. The number of pole pairs for a machine will be noted as p.

The relationship between electrical frequency fe and mechanical speed N is

fe = pN/60

where P is number of poles (not number of pole pairs).

Synchronous generators are built in two elementary forms:

• Round-rotor machines are constructed with a rotor consisting of a cylinder of magnetic steel. In modern generators, the cylinder is formed from a single forging of vacuum degassed steel. The field winding is contained in radial slots in the surface of the rotor.

Round-rotor machines usually have two or four poles as illustrated in Figs. 7-2 and 7-3 respectively. The diameter of the rotor of a typical 25-MW generator is about 700 mm. The diameter of a 2000-MW generator can approach 2 m.

Figure 7.2 Round-rotor generator with two poles.

• Salient-pole machines are constructed with a number of pole pieces mounted to a central rotor shaft. The rotor pole pieces can be solid steel or assemblies of steel plates that are bound together axially with bolts.

The diameter of the rotor can range from less than 1 m in smaller salient pole generators to nearly 20 m in the largest hydroelectric generators.

In both round-rotor and salient-pole generators, the magnetic flux passing through the rotors does not vary in time, and the magnetic flux passing through the stator core does vary periodically in time at the electrical line frequency.

Figure 7.3 4-pole generator (left is round rotor, right is salient pole.

Consequently, the rotors can be made of solid steel, but the stator cores must be made of thousands of thin layers of highly permeable electrical steel. Each layer of stator core steel is coated with a thin layer of electrical insulation.

For electric utility operation, in which generation takes place at 50 or 60 Hz, mechanical speed is inversely proportional to the number of poles. Thus, 2-pole machines, which turn at 3000 or 3600 r/min, are used for most fossil-(fuel)-fired steam turbine generators which require high shaft speeds.

Most nuclear steam turbine generators, which have a lower shaft speed requirement, employ 4-pole designs and therefore turn at 1500 or 1800 r/min. Turbine generators for both fossil and nuclear power plants are typically round-rotor designs.

Hydroelectric generators, which typically have much lower shaft speeds than turbine generators and consequently require a large number of poles, are generally built as salient-pole machines. This is true also for generators intended for operation with large reciprocating engines, such as medium-speed diesels.

WHAT IS FREQUENCY? BASIC DEFINITION AND TUTORIALS

It has been explained that one cycle of alternating current covers a period in which the current value increases from zero to maximum in one direction, returns to zero, increases to maximum in the opposite direction, and then returns to zero.

The number of cycles occurring per second is the frequency of the current and is measured in a unit called the Hertz, named for Heinrich Rudolph Hertz, a German physicist of the late nineteenth century who made a number of important discoveries and valuable contributions to electrical science.

The repetition rate of a periodic signal used to represent or process a communication signal. Frequency is expressed in units of hertz (Hz). 1 Hz represents one cycle per second, 1 MHz represents one million cycles per second, and 1 GHz represents one billion cycles per second.

City lighting and power systems in the United States generally operate at a frequency of 60 Hz.  In the U.S., this is the standard frequency for ac. In some places it is 50 Hz. (Some remote places even use dc, but they are definitely the exception, not the rule.)

In radio practice, higher frequencies are common, and you’ll hear about kilohertz (kHz), megahertz (MHz) and gigahertz (GHz). Alternating currents in airplane circuits usually have a frequency of 400 Hz.

This frequency is commonly used for modern jet aircraft as well as for a number of other applications. The word alternation is frequently  used in discussing alternating current, and it means one half cycle. It is apparent, therefore, that there i: are 120 alternations in a 60-Hz current.

Usually, but not always, the waveshapes are of the type shown in Fig. 2-8. This waveform is known as a sine wave or a sinusoidal waveform.

The frequency of an alternating current has considerable effect on the operation of a circuit, for many units of electrical equipment operate only on current of a certain frequency. Wherever such equipment is used, it is important to make sure it is designed for the frequency of the current in the circuit in which it is to be used.

Units such as synchronous motors operate at speeds proportional to the frequency of the current even though the voltage is somewhat lower or higher than the rated voltage of the machine.

It is also important to remember that a circuit designed for a given frequency may be easily overloaded by using a current of a different frequency, even though the voltage 65 may remain the same. This is because of effects of inductive and capacitive reactance.