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Alternating current. Both voltage and current vary in polarity at a pre-determined and fixed frequency - 50Hz in the U.K., 60Hz in the USA for instance. 
               Note: Hz = Hertz = cycles per second. 
               Diagrammatically, a sine wave a.c. supply is shown below. 

As can be seen, polarity changes every cycle, so in an a.c. system correct polarity need not be  observed. Any waveform shape is considered a.c. providing it changes polarity and repeats itself  every cycle. Examples can be square wave, saw tooth wave, sine wave with a large harmonic content as well as the pure sine wave shown. In a.c. power systems it is always the sine wave that is  


This unit is always the product of voltage and current in single phase a.c. systems. In the 
three phase case an extra constant is involved (√ 3 ). It is the normal way of quoting a rating for 
any a.c. generator. It is independent of power factor and is only used when considering  
a c systems. It is also normal to specify a kVA level at specific power factor therefore defining the  
level of real power capability of the machine and its prime mover:- 
          i.e. kW = kVA x power factor 
                      = real power capability 
This specific power factor is conventionally considered to be 0.8 p.f. lagging, although in practical 
installations power factors of nearer 0.9 p.f. lagging are usually measured. As systems get smaller,  
power factors approaching 0.95 p.f. lagging are obtainable in practice. Since quite large outputs are 
obtained from a.c. generators it is conventional to divide the product of voltage and current by one  
thousand to obtain the unit kVA. 
          i.e. 1kVA = 1000VA 

A.C. Generator

Also called “Alternator”. Provides electrical power output from the input of mechanical power; usually at a fixed voltage and frequency. It comprises a static part and a rotating part which needs to be driven by the prime mover. It is usually surrounded by a metal frame and the main materials used are copper wire (for the electrical windings) and lamination steel (for the magnetic field circuit).


Conventionally held to be the main output winding of an a.c. generator which incorporates a static magnetic field system and a rotating main output winding. These designs always have slip rings and brush gear in order to transfer the electrical output power from the main rotating winding to the external circuits.

Automatic Voltage Regulator (AVR)

An electronic unit which maintains the main machine output voltage at a fixed pre-set level irrespective of load or speed changes. It does this by comparing a reference or set voltage with the actual output voltage and automatically adjusting the excitation level as necessary. This is a closed loop voltage control system. This unit is sometimes called voltage control unit or VCU.


A design of a.c. generator without slip rings or brush gear. This design needs a static main output winding with a rotating magnetic field system. If the magnetic field is to be produced electrically an exciter is required.

Closed Loop

This defines a particular class of control system. In a.c. generator work it is referred particularly to voltage control. A closed loop system is one in which the system output is continually monitored and compared with the set requirement. The system input is then automatically and continuously corrected to ensure that no difference occurs between the actual system output and the set requirements. The voltage control unit performs this function in the a.c. generator. The alternative class of control system is called the open loop system.


The output current (or electrical flow) is determined by the nature of the applied load only. Before current can flow a load must be applied to the a.c. generator. Current is measured in ‘amperes’, abbreviation ‘A’. See also kVA.


Direct current. Both voltage and current are fixed polarity in d.c. systems. Positive and negative terminals need to be marked and in any wiring correct polarity is observed. Magnetic field systems which are electrically produced require a d.c. supply. Two examples of a d.c. supply are diagrammatically shown below.


The efficiency of any machine or process is a ratio of the amount of useful output energy against the required amount of input energy, usually expressed as a percentage. In the case of an a.c. generator the output is easily derived as the power (kW) figure from the standard output lists. The input is normally derived by summation of the output plus the machine losses. There are a number of machine losses which can be either calculated or measured.

    windage and friction loss

    iron loss

    copper loss

    stray loss

    excitation loss

From this it follows that the input power required from the prime mover is higher than the electrical output power of the a.c. generator by an amount corresponding to the machine losses, the actual factor being given by the efficiency figure. As the machine size increases, so efficiency improves. At 5kW output, typical efficiency figures are about 80% whereas at 500kW output, typically efficiency figures are about 93%.


An a.c. generator usually on the same shaft as the main machine. All the electrical power output produced by the exciter is rectified and used to establish the magnetic field of the main machine.


See Magnetic Field.


The frequency of the voltage from an a.c. generator depends on the driven speed and the number of magnetic field poles, thus:-

Note: HZ = Hertz = cycles per second (see a.c.)

Therefore for standard frequencies, the most common combinations of speed and poles are given below:-

    50 Hz 3000 rev/min 2 pole

    60 Hz 3600 rev/min 2 pole

    50 Hz 1500 rev/min 4 pole

    60 Hz 1800 rev/min 4 pole

    50 Hz 1000 rev/min 6 pole

    60 Hz 1200 rev/min 6 pole


Horsepower, a measure of the rate of doing work, use by prime mover manufacturers, but now being replaced by the S.I. unit, kW - see power. A ‘metric horsepower’ C.V. (cheval vapeur) or PS (pferdestarke) is common in Europe. The relationships are:

    1 h.p. = 0.746kW = 33,000 ft lb/min

    1 C.V. or PS = 0.735kW


See power.

Magnetic Field

A force set up around a magnet. The best known magnetic field is that of the earth, established by the North and South magnetic poles. The existence of a strong magnetic field is a pre-requisite of an a.c. generator. The magnetic field can be produced by using a permanent magnet material or by electrical methods. A d.c. supply is necessary for setting up a magnetic field electrically, commonly called an excitation supply. The magnetic field strength can be varied by varying the d.c. excitation supply. The number of magnetic field poles must be multiple of 2, as each ‘magnet’ comprises a North pole and a South pole. The most common poleages for a.c. generators are 2 pole, 4 pole or 6 pole. See also Frequency.


A range of operating conditions or values within which the machine can be safely and successfully run. See also Rating.

Open Loop

This defines a particular class of control system. In a.c. generator work it is referred particularly to voltage control. Once an open loop system has been set up its performance cannot be automatically adjusted during operating. There is no check on the output and no continuous correction of the input. The alternative class of control system is called a closed loop system.

Permanent Magnet

A magnetic material which once magnetised, retains its magnetic properties. This can be used as the magnetic field of an a.c. generator, the most common application being the permanent magnet exciter


This term can be applied to d.c. supplies or to magnetic fields. For a d.c. supply the polarity is either positive or negative. In a.c. work the polarity changes every cycle. In a magnetic field the polarity is either North or South. There may be any number of magnetic poles providing the number of Norths and Souths are identical.


See Magnetic field; Polarity.


This is defined as the rate at which work is done. The mechanical prime mover power input to an a.c. generator shaft must be enough to sustain the nominal speed throughout all conditions of excitation power and machine losses. See also kVA and Power Factor. Below is a detailed description of electrical power.

In a d.c. circuit, the power in watts is the product of voltage and current, i.e. W = V x I. A diagram of d.c. voltage and current is shown below:-

In an a.c. circuit, the real power in watts is not necessarily the product of voltage and current due to the effect of power factor, i.e. W = V x I x p.f. This is best explained diagrammatically.

In this picture the current and voltage peaks and zeros coincide. They occur at the same instant in time. This is the only case when voltage and current are said to be “in phase!” and where the power factor is unity 1.0 In other words. It is the only case, in a.c. work, when power is the product Of voltage and current, i.e. W = V x I.

In this case the load applied causes the current waveform to lag behind the voltage waveform by a constant amount, i.e., the voltage peaks and zeros occur at constant time before the current peaks and zeros.

This is the normal situation in a.c. work where a phase or time difference exists between the voltage and current waveforms. The instantaneous real power in watts is always the voltage-current product for that instant in time. Since voltage and current are no longer in phase, the summation of allof the instantaneous powers over one cycle will be less than the summation for the in-phase case.

Hence the power factor figure will reduce from 1.0 to a figure proportional to the amount of phase or time displacement. Consequently, as the phase difference between the voltage and current waveforms increases, the power factor figure will reduce, as will the real power output figure.

Since quite large powers are obtained the unit kW is preferred to the watt (W) where 1000W = 1kW.

Power Factor (p.f)

The nature of the load in an a.c. circuit will determine if the current drawn is in phase or out of phase with the generated voltage. The load again determines if that current waveform ‘leads’ the voltage waveform or ‘lags’ the voltage waveform. For normal industrial loads (e.g. motors) the current will lag the voltage by some time interval or phase angle. See also kVA and Power.

The optimum situation is where current and voltage are in phase. This makes the power factor unity (1.0) and hence the real power (kW) the same as the product of voltage and current (kVA). Conventionally, a.c. generator work considers a lagging power factor of 0.8. In this case the current will lag the voltage by an amount which causes the real power level supplied (kw) to fall below the kVA level by a factor of 0.8 times.

It is possible for a load to demand a current which is almost totally out of phase with the generated voltage. Also that current may be lagging the voltage (inductive or motor loads) or leading the voltage (capacitive loads). This, therefore, completes the range of power factors for a.c. generators from zero p.f. lagging through conventional 0.8 p.f lagging to unity (1.0 p.f). to zero p.f. leading. One aspect of this is that even though only a small real power (kW) is demanded by the load which is well within the machines capability, damage can easily result if the load is a very low power factor load demanding a very high kVA level.

The diagram shows different power factor-kVA loads all supplying the same real power (kW) level. For normal constant voltage operation the length of the kVA line is directly proportional to load current. The high current overloads at lower power factors can be easily seen from this diagram.

Prime Mover

The method by which mechanical rotational power is provided to the a.c. generator shaft to sustain the nominal speed throughout all conditions of rated electrical load. It must also supply the excitation power and machine losses. Examples of such prime movers are diesel engines or gas turbines.


A specific continuous operating condition or value at which the machine can safely and successfully function. When the term is used generally it implies the maximum specific continuous operating condition or value. See also nominal.

Rectification / Rectified

The conversion of a.c. to d.c. The device which carries out this conversion is a rectifier or Rectified diode. There are a variety of rectification systems, one of the most popular is called the full wave diode bridge rectifier. The single phase circuit for this is shown below:-

Rotating Armature

A type of a.c. generator in which the main output winding is the rotating member. See also Armature.

Rotating Field

A type of a.c. generator in which the magnetic field system is the rotating member. This occurs in all brushless machines.


Generally used to describe all the rotating parts of an a.c. generator. It often refers specifically to the main magnetic field winding of a brushless machine. When the main output winding is on the rotor it is frequently called an armature

Self Excited

A design of a.c. generator where the source of power for the electrically produced magnetic field is derived from the main output winding of the machine itself.

Separately Excited

A design of a.c. generator where the source of power for the electrically produced magnetic field is derived external to the main machine. It may be provided by a completely independent machine or by an extra machine winding on the same shaft as the main machine. See also Permanent Magnet and Exciter.

Single Phase

Applicable only to a.c. work. Defines a single a.c. voltage source providing a supply, usually at a fixed frequency


See Frequency


Refers specifically to the main output winding of a brushless machine. Can also mean the main magnetic field winding of a rotating armature machine.

Three Phase

This can be considered as three equal but independent single phase supplies with one end of each supply forming a common (neutral or earth) point. These three supplies also have a time or phase difference of 120°. A particular 3 phase connection is called the star connection, and is shown on page 7.

Note: The relationships between the phase and line voltage and the phase and line current are:-

    voltage:- √ 3 Vph = VL

    current:- Iph = IL

There is also an alternative way of connecting a 3 phase supply, the delta connection. This, however, does not have a common or neutral point.

Note: This relationship between the phase and line voltage and the phase and line current are:-

    Voltage:- Vph = VL

    Current:- √ 3 Iph = IL

Three phase cont’d

Whichever method is chosen the open circuit voltage waveforms will be identical, except in magnitude. Below is a waveform picture of a 3 phase sinusoidal a.c. supply:-

In view of the complex nature of the above waveform, any theoretical analyses are first reduced to ‘single phase models’.

Note it is only a.c. supplies that can be connected in this manner. Three d.c. batteries connected in “delta” merely short circuits all the batteries; and equally, in star, there is no benefit over a conventional single d.c. supply of 3 batteries in parallel.


The output voltage (or electrical pressure) of an a.c. generator depends on the speed of rotation, the number of turns of copper wire in the output winding and the strength of the main magnetic field. The required frequency limits the speed choice and the turns are fixed in the manufacture of the machine; so the only variable is the magnetic field strength. The AVR adjusts this field strength as required by changing the excitation power supplied to the field.

Voltage Control Unit (VCU)

See Automatic Voltage Regulator