The speed of a three-phase short-circuit rotor motor can only be meaningfully controlled via a change in the frequency of the supply voltage. To this effect, frequency converters (FC) are used whose original frequency can be controlled independently from the grid frequency. The AC of the grid is converted to DC in an intermediate circuit and then to DC with adjustable frequency.
For drives operating in the I. and III. quadrant of the n/M diagram, i.e. those drives whose moment of inertia is substantially smaller than the load torque, frequency converters can be used without requiring changes. Four-quadrant drives, however, require an additional device to take on the kinetic energy.
As long as a load is acting on the motor, its armature is following the frequency of the feed voltage (slip). When the FC reduces the frequency or the motor is driven by external operating conditions (lowering, driving downhill), the rotor frequency is higher (over-synchronous) than that of the feed FC. The motor turns into a generator. The generator energy leads to a voltage increase in the intermediate circuit of the FC and must be dissipated. Energy recovery, however, is not always an economical alternative.
Braking resistors are used for lifting and driving gears, elevator drives, conveyors, drives on manipulators as well as all drives where fast speed changes are to be controlled and the excess energy is not consumed by the loads or losses of the machine. Resistors for smaller ratings can be installed together with the other equipment in the switchgear unit. Reistor units within a small power range are available in IP65. Due to the heat formation, resistors for larger ratings often have their own casing and are installed separately. The possible protection degree are from IP00 - IP23.
RBR = resistance Ω
UZ = intermediate circuit voltage V