Salt water rheostat
Encyclopedia
A liquid rheostat or salt water rheostat is a type of variable resistor. It consists of a tank containing brine
in which electrode
s are submerged to create an electrical load. The electrodes may be raised or lowered into the liquid to respectively decrease or increase the electrical resistance
of the load. To stabilize the load, the mixture must not be allowed to boil. The salt water rheostat operates at unity power factor
and was widely used by generator assemblers, until 20 years ago, as a matter of course. They are still constructed for the commissioning of large diesel generators in remote places, where discarded oil drums and scaffold tubes typically form the tank and electrodes.
), about 5 feet (1.5 m) in size, possibly two off, standing on insulators, in which was suspended a hollow steel cylinder. This acted as the positive electrode
and was supported by a steel rope and insulator from an adjustable pulley. The water pipe connection included an insulated section. The tank contained salt water, but not at the concentration that could be described as “brine”. The whole device was fenced off for safety.
Operation was very simple, as adding more salt, more water or varying the height of the centre electrode would vary the load. The load proved to be quite stable, varying only slightly as the water heated up; it never came to the boil. Power dissipation was about 1 megawatt, at a potential of about 700 volt
s and current of about 1,500 ampere
s.
Disadvantages include:
s. These later designs, rated for 4000 hp, currently cost in the region of 100,000 to 180,000 euro
. Hence, it is economically advantageous for railways to build their own salt-water type. Modern locomotives often have rheostatic brakes and it may be possible to use the locomotive's own braking rheostat as a load bank, eliminating the need for an external one.
Some electric locomotives also used liquid rheostats, in particular in Italy for early three-phase AC types such as the FS Class E550
. Some Direct-Current designs also used them as starting resistors.
They are no more dangerous than electrode heaters, which work on the same principle, but with plain water, or electrical immersion heaters, provided the correct precautions are used. This requires connecting the container to both ground and neutral and breaking all poles with a linked over-current circuit breaker
. If in the open, safety barriers are required.
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
in which electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s are submerged to create an electrical load. The electrodes may be raised or lowered into the liquid to respectively decrease or increase the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the load. To stabilize the load, the mixture must not be allowed to boil. The salt water rheostat operates at unity power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
and was widely used by generator assemblers, until 20 years ago, as a matter of course. They are still constructed for the commissioning of large diesel generators in remote places, where discarded oil drums and scaffold tubes typically form the tank and electrodes.
Description
Typically they would consist of a steel cylinder (the negativeCathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
), about 5 feet (1.5 m) in size, possibly two off, standing on insulators, in which was suspended a hollow steel cylinder. This acted as the positive electrode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and was supported by a steel rope and insulator from an adjustable pulley. The water pipe connection included an insulated section. The tank contained salt water, but not at the concentration that could be described as “brine”. The whole device was fenced off for safety.
Operation was very simple, as adding more salt, more water or varying the height of the centre electrode would vary the load. The load proved to be quite stable, varying only slightly as the water heated up; it never came to the boil. Power dissipation was about 1 megawatt, at a potential of about 700 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 and current of about 1,500 ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s.
Advantages and disadvantages
An advantage is silent operation, with none of the fan noise of current resistive grid designs.Disadvantages include:
- corrosion to the copper connection cables and to the wire rope
- lack of insulation from ground, as this type will definitely trip any modern generator ground detection systemResidual-current deviceA Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
fitted
Uses
Railways commonly used salt-water load banks in the 1950s to test the output power of diesel-electric locomotives. They were subsequently replaced by specially designed resistive load bankLoad bank
A load bank is a device which develops an electrical load, applies the load to an electrical power source and converts or dissipates the resultant power output of the source....
s. These later designs, rated for 4000 hp, currently cost in the region of 100,000 to 180,000 euro
Euro
The euro is the official currency of the eurozone: 17 of the 27 member states of the European Union. It is also the currency used by the Institutions of the European Union. The eurozone consists of Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg,...
. Hence, it is economically advantageous for railways to build their own salt-water type. Modern locomotives often have rheostatic brakes and it may be possible to use the locomotive's own braking rheostat as a load bank, eliminating the need for an external one.
Some electric locomotives also used liquid rheostats, in particular in Italy for early three-phase AC types such as the FS Class E550
FS Class E550
The FS Class E550 was a class of three-phase electric locomotive used in Italy, introduced in the 20th century, which remained in service until 1965.-History:...
. Some Direct-Current designs also used them as starting resistors.
Safety issues
The salt-water load bank dates from an earlier, less regulated and litigious era. To pass current safety legislation would require very careful operation.They are no more dangerous than electrode heaters, which work on the same principle, but with plain water, or electrical immersion heaters, provided the correct precautions are used. This requires connecting the container to both ground and neutral and breaking all poles with a linked over-current circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
. If in the open, safety barriers are required.