Drive manufacturers recognise that a braking resistor will be required for certain applications and appropriate connections to the dc link are provided to enable one to be incorporated into the system. The control electronics monitor the level of voltage within the dc link and when this rises above a certain limit, the braking resistor is switched into circuit via a FET or IGBT, otherwise known as a ‘chopper’. When the dc voltage drops to a safe level again, the load resistor is switched out of circuit.
Braking resistors sometimes have coils manufactured from copper-nickel or nickel-chromium, both of which have resistance values that are constant over a wide range of temperatures. The coils (the number depending upon the unit’s rating) are connected in parallel, which means that the load does not rely on a single winding.
For safety and reliability purposes, windings can be embedded in a filler material such as quartz, which gives mechanical stability together with short circuit protection plus self-extinguishing properties. Such fillers also assist with the efficient transfer of heat from the resistor windings to the inner surfaces of the aluminium housing. Further electrical isolation can be provided by insulation sheets inserted between each winding and between the resistance winding assembly and the sides, top and bottom of the extruded aluminium housing.
For still further isolation and to provide high ingress protection ratings (to IP64) the resistor assembly can be encapsulated within a silicone sealant.
The extruded aluminium housing acts as a heat-sink and quickly dissipates the thermal energy generated by the resistance windings to open air. In some cases, power ratings can be increased substantially by fitting a fan.
When there is restricted space or if the braking resistor has to be built into a control panel, which has inadequate cooling, then forced air-cooled units have limitations and the solution is to use water-cooling. These may be indirectly cooled via coolant pipe loops built into the back of a panel, but a more efficient method is to incorporate the cooling circuit within the body of the resistor itself. Where this is employed, water connections are best located at the opposite end of the resistor body to the electrical connections. For higher power ratings, several braking resistors can be connected in series/parallel depending upon the required resistance value.
Typically, standard resistors are designed to operate in an ambient temperature of around 40ºC with maximum coolant temperatures 25ºC in-flowing and 45ºC out-flowing. It is important that an industrial coolant or drinking water is used, and generally the water should be soft, with no mechanical, chemical or biological contamination. There is a standard to follow - VGB-R 455 P - which applies to suitable coolants.
Thermal sensors can be fitted to monitor the resistor temperature. When the temperature limits are exceeded the sensor’s contacts open and this is used as a fault signal so that the motor drive system can be brought to a controlled stop.
Applications
There are many examples where braking resistors are an essential component of a motor drive system. When a crane is being used to lower a heavy load, for instance, it has to contend with surplus potential energy, which could cause the motor to run away in an uncontrolled manner, thus creating a safety hazard. A braking resistor can be used to prevent this from happening.
Similarly, when elevators and lifts are descending and carrying people there is excess potential energy which tends to drive the lift motor in reverse, effectively turning it into an alternator. A braking resistor can be used to dissipate the unwanted electrical energy. There are some purpose-built units for this duty, engineered for room-less lifts and for fitting inside the framework of an escalator. These resistors have special EMC connection boxes, which are required in public buildings. Importantly, this type of resistor does not require an additional housing, thus they can be used in confined spaces and usually cost less than enclosed versions.
Dynamic braking resistors are often used in conjunction with airbrakes in electric locomotives to reduce the amount of wear on mechanical components. When it is switched into circuit the braking resistor causes the current to flow in the opposite direction to when the locomotive is motoring, thus exerting a torque that opposes the forward motion.
The braking of a wind turbine can be achieved by ‘dumping’ energy from the generator into a braking resistor. This technique is useful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit. And when higher wind speeds are experienced, it is possible to introduce cyclical braking which causes the turbine blades to slow down, increasing the stalling effect and thus reducing the efficiency of the blades. In this way the turbine’s rotation can be kept at a safe speed.
Other applications include automatic assembly machines, which have large masses that have to move and stop quickly, and power supplies containing capacitors that have to be discharged for electrical safety reasons.