A booster was a

motor–generator A motor–generator (an M–G set) is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electr ...

(MG) set used for

voltage regulation In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation desc ...

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...

(DC) electrical power circuits. The development of

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

and

solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their use ...

devices has rendered it obsolete. Boosters were made in various configurations to suit different applications.

Line booster

In the days of direct current mains,

voltage drop Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable ...

along the line was a problem so line boosters were used to correct it. Suppose that the mains

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

was 110 V. Houses near the

power station A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many p ...

would receive 110

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defi ...

s but those remote from the power station might receive only 100 V so a line booster would be inserted at an appropriate point to "boost" the voltage. It consisted of a motor, connected in

parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster of ...

with the mains, driving a generator, in

series Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used in ...

with the mains. The motor ran at the depleted mains voltage of 100 V and the generator added another 10 V to restore the voltage to 110 V. This was an inefficient system and was made obsolete by the development of alternating current mains, which allowed for high-voltage distribution and voltage regulation by

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

Milking booster

Again in the days of direct current mains,

s often had large lead-acid batteries for load balancing. These supplemented the steam-powered generators during peak periods and were re-charged off-peak. Sometimes one cell in the battery would become "sick" (faulty, reduced capacity) and a "milking booster" would be used to give it an additional charge and restore it to health. The milking booster was so-called because it "milked" the healthy cells in the battery to give an extra charge to the faulty one. The motor side of the booster was connected across the whole battery but the generator side was connected only across the faulty cell. During discharge periods the booster supplemented the output of the faulty cell.

Reversible booster

Before solid-state technology became available, reversible boosters were sometimes used for speed control in DC

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or a supercapacitor. Locomotives with on-board fuelled prime movers, such as diesel engines or gas ...

s. The boosters were called reversible, because they could either increase or decrease the speed of the locomotive. The motor of the MG set was connected in parallel with the supply, usually at 600 volts, and was mechanically coupled, via a shaft with a heavy

flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...

, to the generator. The generator was connected in series with the supply and the

traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric vehicle, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles (ele ...

s, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

es and

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

s in the field circuit. This allowed the generator voltage to either oppose, or supplement, the line voltage. The net output voltage could therefore be varied smoothly between zero and 1,200 volts as follows: * Generator producing maximum opposing voltage, net output zero volts * Generator producing zero volts, net output 600 volts * Generator producing maximum supplementary voltage, net output 1,200 volts To match the 1,200 volt output, the locomotive would have three 400 volt traction motors connected in series. Later locomotives had two 600 volt motors in series. When the locomotive was working at full power, half the energy came through the MG set and the other half came directly from the supply. This meant that the power rating of the MG set needed to be only half the rating of the traction motors. Thus there was a saving in weight and cost compared to the Ward Leonard system, in which the MG set had to be equal in power rating to the traction motors. If the power supply to the locomotive was interrupted (e.g. because of a gap in the

third rail A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway t ...

at a junction) the flywheel would power the MG set for a short period to bridge the gap. During this period, the motor of the MG set would temporarily run as a generator. It was this system that was used in the design of British Rail classes 70, 71 and 74 ( Class 73 does not utilise booster equipment).

Metadyne

Some types of London Underground stock (e.g.

London Underground O Stock The London Underground O and P Stock electric multiple units were used on the London Underground from 1937 to 1981. O Stock trains were built for the Hammersmith & City line, using metadyne control equipment with regenerative braking, but the tra ...

) were fitted with

Metadyne A metadyne is a direct current electrical machine with two pairs of brushes. It can be used as an amplifier or rotary transformer. It is similar to a third brush dynamo but has additional regulator or "variator" windings. It is also similar to an ...

s. These were four-brush electrical machines which differed from the reversible boosters described above.

Television receivers

When

cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...

s were the standard for television receivers, after many years of service the tube would lose brightness, due to low electron emission in the electron gun assembly of each tube. A small "booster" transformer could be added to a set experiencing such symptoms; it would raise the voltage applied to the filament slightly, which would increase emission and restore brightness. Sometimes this step would extend the life of the expensive CRT by years, making it more economical than a replacement. R.R. Gulati,''Monochrome And Colour Television'', New Age International, 2006 , page 582

References