Braking Methods of Induction Motors The induction motor is a rugged, reliable, low maintenance and less expensive machine. Braking is one of important questions in electrodynamics machine system. During production, in order to decrease auxiliary work time, to raise production efficiency, to realize halt exactly, to improve product quality, to ensure personal safety and so on, a great deal of machines require effective braking. The braking is the process of reducing the speed of an induction motor. In braking, the motor works as a generator developing a negative torque which opposes the motion of a motor. The braking of an induction motor is mainly classified into three types. They are; Regenerative Braking Plugging or reverse voltage braking Dynamic Braking Regenerative Braking With the growing concerns towards the environmental protection and energy conservation, the development of Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV) has been the acceptable challenges for research and development. Regenerative braking is an important aspect to improve the efficiency of EV and HEV. Regenerative braking used in addition with the traditional mechanical braking is more apt to the driver's braking requirement while improving the operating range of the vehicle. During regenerative braking, the electrical traction motor provides negative torque to the driven wheels and converts the kinetic energy into electrical energy for recharging the battery. The energy is being dissipated in plugging and dynamic braking whereas in the regenerative braking the energy is recovered back into the system. Thus many researchers have coined new strategies and designed numerous control algorithms so as to reduce the complicacy in the development of regenerative braking. The basic problem regarding incorporation of regenerative energy storage in case of ac drive systems is high dc link voltage at the inverter input side. This requires a dc-dc converter for charging the battery at a specific charging rate. In the applications of hybrid electric vehicle (HEV) user applies brakes repeatedly. This causes a lot of energy loss in friction and heating. However, this energy can be stored in ultra-capacitor and then battery can be charged with help of it. This increases the life span of the battery. . Plugging or Reverse Voltage Braking The deceleration stopping method requires careful selection of the deceleration time to achieve optimum stopping performance . It is sometimes necessary to stop quickly or decelerate the induction motor under controlled condition as when lowering a load in a crane or hoist. Such retardation is effected to by providing a braking torque. Plugging is one of the electrical braking methods applicable in the case of induction motor. The principle of traditional plug braking is that changing the direction of revolving magnetic field to oppose the direction of former magnetic field by changing the phase sequence of threephase voltage, then the motor will be braked by opposing torque in a short time. If the leads on the stator windings are reversed suddenly, the direction of rotation for the stator field is reversed. The resulting slip is larger than one. The motor will come to an abrupt stop. The motor must then disconnected from the voltage source before it starts to rotate in the reverse direction, this method of bringing motors to a quick stop is commonly known as plugging. Reverse-current braking (plugging) is very effective but consumes approximately three times the stored kinetic energy of the system in reducing the speed to zero, and would run up as a motor in reverse rotation unless prevented. Dynamic braking This braking method involves dissipation of the kinetic energy of the rotor as heat in resistors after cutting off supply. This method is widely used in industrial applications where running motors can be brought to a halt quickly without any mechanical tear. After the supply has been cut off, the rotor of the induction motor, having some residual magnetic flux, continues to rotate due to inertia. Hence, the motor acts as a generator supply electrical energy to the stator. This energy needs to be dissipated and different methods have been suggested to do this. Conventional Dynamic Braking Methods A. Capacitor self excitation braking In this method of dynamic braking, capacitors connected to the stator terminals are used to consume the kinetic energy of the rotor. When the supply is cut, the rotor continues to rotate due to the inertia of the load. The residual magnetic flux present in the rotor induces emf in the stator windings and the machine now acts as a generator. This emf induced excites the three phase capacitors connected to the stator windings. The capacitors store this energy coming from the rotor. The electric power generated is dissipated as heat in the winding resistances. Braking is enhanced if external resistors connected in parallel with the capacitors are used. from the DC supply. When the rotor is at a standstill, the emf induced in the stator becomes zero thereby allowing the full current from the DC supply to flow through the stator. D. Zero sequence braking Zero sequence braking is achieved by connecting the stator terminals of the induction motor in series and supplying either AC or DC voltage across the two terminals This produces a static field in the stator windings thereby opposing the rotation of the rotor. One advantage of this method over the DC injection method is that the heating is more uniform in zero sequence braking due to equal currents in the three windings. References B. Magnetic braking When two or all of the three terminals of the stator are shorted after switching off main power supply, magnetic braking is achieved. After the supply is cut off, the residual magnetic flux of the rotor rotating due to inertia induces currents in the short circuit . These induced currents oppose the motion of the rotor according to Len’s law. In this way, magnetic braking is achieved. C. DC injection braking DC injection braking is achieved when a DC supply is connected between two stator terminals with the third being kept open and the main supply being switched off. This results in zero frequency current in stator windings and the induction motor gets inverted into a synchronous generator with the stator as the field and rotor as the rotating armature. This stops the rotation of the rotor with the rotor current becoming zero at standstill condition. One disadvantage of this method is that a separate DC source is required. The analysis of DC injection method has been done well in where at first, both the DC supply voltage and speed is varied and then the speed is kept constant and the DC supply voltage is varied. The standstill current observed in the both the cases is found to be greater than the braking current. This can be attributed to the fact that initially during braking, the rotor rotates due to inertia and the residual flux present in it induces emf in the stator windings to oppose the current R. Singh, S. Umashankar, D. Vijaykumar and D. P. Kothari, "Dynamic braking of induction motor Analysis of conventional methods and an efficient multistage braking model," 2013 International Conference on Energy Efficient Technologies for Sustainability, Nagercoil, 2013, pp. 197-206, doi: 10.1109/ICEETS.2013.6533382. A. A. Mahapatra and S. Gopalakrishna, "Regenerative braking in induction motor drives in applications to Electric Vehicles," 2014 IEEE Students' Conference on Electrical, Electronics and Computer Science, Bhopal, 2014, pp. 1-5, doi: 10.1109/SCEECS.2014.6804425. S. K. Agrawal, V. Kumar, A. Alam and P. Thakura, "Regenerative braking for induction motor drive," 2014 6th IEEE Power India International Conference (PIICON), Delhi, 2014, pp. 1-6, doi: 10.1109/POWERI.2014.7117698. H. A. Hairik, R. H. Thejel and W. A. Kadhem, "Proposed scheme for plugging three-phase induction motor," Melecon 2010 - 2010 15th IEEE Mediterranean Electrotechnical Conference, Valletta, 2010, pp. 1-5, doi: 10.1109/MELCON.2010.5476349.
