Basics of Regenerative Braking

Basics of Regenerative Braking: * In Regenerative braking, the motor operates as a generator, while it is still connected to the supply, here N>Ns. * Mechanical energy is converted into energy, part of which is returned to the supply & rest of the energy is last as heat in wdg & bearings. * Most of electrical m/c pass smoothing from motoring region to generating region, when over driven by the load. * Here electric motor is drawing a trotley bus in the uphill & downhill direction. The gravity force can be resolved into 2 components in the uphill direction. One is perpendicular to load surface F & another one is parallel to the road surface Fl. The parallel force pulls the motor towards the bottom of the hill. * If we neglect the rotational losses, the motor must produce a force Fm opposite to Fl, to move the bus in the uphill direction. This operation is in I quadrant. Here motor torque is motor speed in same direction. But TL is opp to Tm. The power flow from motor to mech. Load. * Now consider same bus to travelling downhill. The gravitational force does not change its torque direction, but load torque pushed the motor toward the bottom of the hill. The motor produces a torque in the reverse direction because of the direction of the motor torque always opposite to the direction of the load torque. * Here rotation of the motor is still in the same direction on both sides of the hill. This operation in the second quadrant. This is known as "regenerative braking". In this regenerative braking mode, motor torque & speed are in the opposite direction. TL is opposite to Tm. The energy is exchange under regenerative braking operation is power flows from mechanical load to source. Hence the load is driving the m/c and the m/c is generating electric power that is returned to the supply. Regenerative braking of Induction Motor:- An induction motor is subjected to regenerative braking, if the rotor rotates in same directions as that of stator magnetic field, by N>NS. * Such a state occurs during any one of the following processes. i) Downward motion of a loaded hoisting mechanism. ii) If variable freq is available or if the motor is of pole change type. * Under regenerative braking mode , the m / c acts as an induction generator and this power fed back to the supply. The m/c taking only the reactive power for excitation. * If active load is present, motor speed > Ns and regenerative braking may be obtained. In this case, slip and torque developed become -ve. * Here Regenerative max torque > max motor torque. Regenerative braking of DC Motor : In regenerative braking , generated energy is supplied to the source, the condition is E>V and –Ia ---(1) Fild flux increases not beyond rated because saturation. In series motor, as N increases Armature current , flux decreases so (1) not achieved. Thus braking possible.