A liquid resistor is an electrical resistor in which the resistive element is a liquid. Fixed-value liquid resistors are typically used where very high power dissipation is required. They are used in the rotor circuits of large slip ring induction motors to control starting current, torque and to limit large electrical fault currents (while other protection systems operate to clear or isolate the fault). They typically have electrodes made of welded steel plate (galvanised to reduce corrosion), suspended by insulated connections in a conductive chemical solution held in a tank - which may be open or enclosed. The tank body is normally solidly grounded or earthed. A typical unit can be rated for continuous use, or for short periods when used for current limitation in protection systems.

Dynamic Braking Resistors (DBRs

When a motor is in an overhauling condition—that is, the load is moving faster than the designated motor speed—the motor acts as a generator and produces electrical energy from mechanical energy. This electrical energy, however, needs somewhere to go, and the most common way of dealing with its release is through dynamic braking.

With dynamic braking, the electrical energy generated during stopping is released as heat through a voltage regulated transistor and resistor. There are actually two versions of a dynamic braking circuit: a “chopper” and a “dynamic brake.” The dynamic brake circuit includes the switching device (an insulated-gate bipolar transistor, or IGBT), the control circuit, and the resistor.

A chopper, on the other hand, only includes the regulatory circuit and switching device, with the resistors being separate components. This allows the resistors to be appropriately sized and mounted remotely, which can be important, since the resistors generate a significant amount of heat. The combination of switching device and control circuit is generally referred to as the “chopper module,” while the resistor is referred to as a “dynamic brake resistor.” Dynamic brakes are typically rated for duty cycles in the 20 percent range, and choppers are often used in heavier-duty applications.

Switch

An electrical switch is any device used to interrupt the flow of electrons in a circuit.

Toggle Switches

Toggle switches are actuated by a lever angled in one of two or more positions. The common light switch used in household wiring is an example of a toggle switch. Most toggle switches will come to rest in any of their lever positions, while others have an internal spring mechanism returning the lever to a certain normal position, allowing for what is called “momentary” operation.

Pushbutton Switches

Pushbutton switches are two-position devices actuated with a button that is pressed and released. Most pushbutton switches have an internal spring mechanism returning the button to its “out,” or “unpressed,” position, for momentary operation. Some pushbutton switches will latch alternately on or off with every push of the button. Other pushbutton switches will stay in their “in,” or “pressed,” position until the button is pulled back out. This last type of pushbutton switches usually have a mushroom-shaped button for easy push-pull action.

40923 (162×75)

Selector Switches

Selector switches are actuated with a rotary knob or lever of some sort to select one of two or more positions. Like the toggle switch, selector switches can either rest in any of their positions or contain spring-return mechanisms for momentary operation.