Braking Resistor and VFD System
By definition, a braking resistor is an electric device which uses voltage and dissipates that power into heat. Different types of braking resistors are available in market in terms of rating (Ohms & Watt), application and compartment depending upon the use. Most of the time, the braking resistor is put in a dedicated compartment to separate it from remaining instruments. The reason behind this is when it comes in use; huge heat is produces and to evacuate such a massive heat, fans / water tubing or both systems are used at a time depending upon the size of braking resistors.
In technical terms braking resistor is called dynamic braking resistor as it is widely used in locomotives, cranes, elevators, play lands etc. This device is a part of such systems where quick brakes are required. In the whole system, electric motors are used as brakes, during operation the power supply is being supplied to motors which operates trains, conveyor belts, cranes, elevators etc. but when there is a need to stop, the power supply is cut off but due to fully charged capacitor installed in the circuit, capacitor voltage is discharged through motor stator winding which produces magnetic field because of this magnetic force, electromagnetic force (emf) produces in rotor winding as well. That emf will produce a current which will oppose the motion of motor and in seconds motor comes to stop. In the path of motor and capacitor, braking resistor is there which uses extra voltage into it and dissipates that power into heat.
In locomotives, braking pads were used in past when applied, due to friction with metal wheels, the train stops. But there were a lot of wear and tear in braking pads which were risky and operational cost was also high. Dynamic braking system makes it possible to stop such thousands of tones heavy trains and cranes in seconds. The design of braking resistor is very easy; it is made in the form of coil wounded on a ceramic (high heat absorbent) and the whole assembly is fixed in a compartment making it separate from other parts of braking system. In past braking of trains was a nightmare for designers and train drivers especially in hilly areas. The braking pads used to heat up and catch fires while moving down when brakes apply continuously and severe accidents were occurred. After invention of dynamic braking system, the impossible becomes reality.
Variable Frequency Drive famously called VFD is an electrical device used to control the speed of electric motor by control the supply voltage and frequency. For example, a motor installed at prilling tower at height of 80-100 meter tall concrete tower used in Urea production facility, needs to control the speed of motor to produce uniform droplet size. If the motor rotates at high speed, the size of droplets will be small and in case the speed is too slow, the droplet size will be large. Frequency directly proportional to revolution per minute (RPM), if any process needs more speed then VFD enhance the frequency of power supply to motor and vice versa. There is a converter in start by design to convert the sinusoidal waveform into rectangular which is more suitable for motors in term of controlling speed. Rectangular waveform is produced due to six diodes which allow current only in one direction. Inverters are used to switch on off the DC waveform into AC to use motor at different speeds as per requirements. In modern VFDs, there is a human interface which is used to set different parameters like speed, current etc. via manually or remotely depending upon the ground circumstances.
VFDs are valuable when it comes to save energy as its evident that less energy is required when less speed is desired. This device is favourable in frequent motor operations as in start, motor draws inrush current which is 8-10 times higher than the rated.
This high current is dangerous for winding and starting inertia may damage the assembly as well. VFD can save the inrush currents and heavy jerks experienced in start of motor starting. This device not only improves the process effectiveness but also increase the life of motor / material as well due to less stress, wear and tear and maintenance cost. VFDs are available in different ratings designated as voltage or current and can be figured out in ranges up to 6 MW.
Dynamic braking is an integral part of sophisticated VFDs where large induction motors are installed. During stoppage of motors, the excess energy is utilized by dynamic braking resistors and the energy is dissipated in the form of heat by saving the VFD (electronic circuit).
VFDs are also used in lifts, elevators, conveyor belts and widely in process / packaging industry where smooth, continuous and efficient operations are desired.