Liquid rheostat

Liquid rheostats used as motor start switches, circa 1900

A liquid rheostat or water rheostat ^[1] or salt water rheostat is a type of variable resistor. It consists of a tank containing brine, in which electrodes 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.

Description

Typically a liquid rheostat consists of a steel cylinder (the negative), about 5 feet (1.5 m) in size, 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.^[2] 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 volts and current of about 1,500 amperes.

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 which may trip a ground detection system

Uses

Railways commonly used salt water load banks in the 1950s to test the output power of diesel-electric locomotives.^[3] They were subsequently replaced by specially designed resistive load banks. These later designs, rated for 4,000 horsepower (3,000 kW), 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. 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.

Liquid rheostats were sometimes used in large (thousands of kilowatts/horsepower) wound rotor motor drives, to control the rotor circuit resistance and so the speed of the motor. Electrode position could be adjusted with a small electrically operated winch or a pneumatic cylinder. A cooling pump and heat exchanger were provided to allow slip energy to be dissipated into process water or other water system.^[4]

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. If in the open, safety barriers are required.

References