Booster (electric power)
Encyclopedia
A Booster was a motor-generator
(MG) set used for voltage regulation
in direct current
(DC) electrical power circuits. The development of alternating current
and solid-state
devices has rendered it obsolete. Boosters were made in various configurations to suit different applications.
, voltage drop
along the line was a problem so line boosters were used to correct it. Suppose that the mains voltage
was 110 V. Houses near the power station
would receive 110 volt
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 with the mains, driving a generator, in series 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
s.
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.
s. To avoid confusion, it should be explained that it is the electrical output of the booster that is reversible, not the direction of rotation.
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
, to the generator. The generator was connected in series with the supply and the traction motor
s, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting switch
es and resistor
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:
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
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).
) were fitted with Metadyne
s. These were four-brush electrical machines which differed from the reversible boosters described above.
Motor-generator
A motor-generator 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 electrical power supply line...
(MG) set used for voltage regulation
Voltage regulation
In electrical engineering, particularly power engineering, voltage regulation is the ability of a system to provide near constant voltage over a wide range of load conditions.-Electrical power systems:...
in direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) electrical power circuits. The development of alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
and solid-state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
devices has rendered it obsolete. Boosters were made in various configurations to suit different applications.
Line booster
In the days of direct current mainsMains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
, voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...
along the line was a problem so line boosters were used to correct it. Suppose that the mains voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
was 110 V. Houses near the power station
Power station
A power station is an industrial facility for the generation of electric energy....
would receive 110 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
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 with the mains, driving a generator, in series 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
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s.
Milking booster
Again in the days of direct current mains, power stationPower station
A power station is an industrial facility for the generation of electric energy....
s often had large lead-acid batteries
Lead-acid battery
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...
for load balancing
Load balancing (electrical power)
Load balancing refers to the use of various techniques by electrical power stations to store excess electrical power during low demand periods for release as demand rises....
. 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 locomotiveElectric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...
s. To avoid confusion, it should be explained that it is the electrical output of the booster that is reversible, not the direction of rotation.
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
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
, to the generator. The generator was connected in series with the supply and the traction motor
Traction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....
s, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es and resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
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
Third rail
A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...
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
British Rail Class 70
The British Rail Class 70 was a class of three 3rd rail Co-Co electric locomotives. The initial two were built by the Southern Railway at Ashford Works in 1941 and 1945 and were numbered CC1 and CC2. Electrical equipment was designed by Alfred Raworth and the body by Oliver Bulleid. CC2 was...
, 71
British Rail Class 71
The British Rail Class 71 was an electric locomotive used on the Southern Region of British Railways, unlike most other Southern Region electric locomotives they could not operate away from the electrified system.- History :As part of the BTC Modernisation Plan of 1955, twenty-four electric...
and 74
British Rail Class 74
British Rail Class 74 was an electro-diesel locomotive that operated on the Southern Region of British Railways, rebuilt from redundant Class 71 locomotives in the late 1960s...
(Class 73
British Rail Class 73
The British Rail Class 73 is a United Kingdom model of electro-diesel locomotive. The type is unusual in that it can operate from a 750 V DC third-rail or an on-board diesel engine to allow it to operate on non-electrified routes...
does not utilise booster equipment).
Metadyne
Some types of London Underground stock (e.g. London Underground O StockLondon Underground O Stock
The O and P Stock was built for the Metropolitan Line and Hammersmith & City Line in 1937–1940 by Gloucester RC&W and Birmingham RC&W. In all there were 262 driving motors and 103 trailers...
) were fitted with Metadyne
Metadyne
A Metadyne is an electrical machine with three, or more, brushes. It can be used as an amplifier or rotary transformer. It is similar to a third brush dynamo but much more complex, having additional regulator or "variator" windings. It is also similar to an amplidyne except that the latter has a...
s. These were four-brush electrical machines which differed from the reversible boosters described above.