 Braking resistors
Function
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.
First, the supply grid must be able to take on energy and second, the energy must be adjusted such that it can be returned to the grid. This may under certain circumstances require substantial technical adjustments, which would increase the investment cost and complexity of the system. In particular where high braking energies occur in the short-term and the entire braking energy is low in relation to the energy demand of the drive it is more economical to convert it to heat in a braking resistor.
An electronic switch (chopper) will switch on the resistor again before the voltage in the intermediate circuit reaches a value that is dangerous for the components. The resistor loads the intermediate circuit voltage. As soon as it has reached a certain level that is smaller than that of the switching voltage but larger than the grid voltage, the chopper will switch off the resistor again. This procedure will be repeated as soon as the voltage rises again whereby – as a function of the time constant of the circuit – frequencies ranging between several hundred up to several thousand Hertz are reached. The resistor operated with pulsed DC removes the energy from the drive and brakes it down. The energy is converted to heat and released to the environment.
Application
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. Due to the heat formation, resistors for larger ratings often have their own casing and are installed separately.
RBR = resistance ω
UZ = intermediate circuit voltage V
PBR = braking power W
References
| Customer / Product |
System, others |
| ABB |
- Steel grid resistors BEG
- Encapsulated resistor VPR
- Wire-wound, cemented resistor
|
| Emotron |
- Steel grid resistors BEG
- Wire-wound, cemented resistor DEZ
|
| Lenze |
- Steel grid resistors BEG
- Encapsulated resistor VPR
|
| Maschinenfabrik Leimbach |
- Steel grid resistors BEG
- Wire-wound, cemented resistor DEZ
|
| Moeller |
- Steel grid resistors BEG
- Wire-wound, cemented resistor DEZ
|
| Omron |
- Steel grid resistors BEG
- Wire-wound, cemented resistor DEZ
|
| Rekers |
- Steel grid resistors BEG
- Encapsulated resistor
|
| Schneider Electric |
- Steel grid resistors BEG
- Encapsulated resistor VPR
- Wire-wound resistor VPR
|
| Siemens AG |
- Steel grid resistors BEG
- Encapsulated resistor VPR
|
Text © GINO AG |
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