# h bridge How can I implement regenerative braking of a DC motor

Posted on Feb 6, 2014

Many electric cars are able to convert the momentum of the car into stored energy in batteries, rather than convert it to useless heat at the brake pads. How does this work How can I implement it myself Probably you already have it, and just didn`t know it. If you are driving a motor with a half-bridge or H-bridge and PWM or similar, you have regenerative braking. Let`s consider a half-bridge, since for this analysis

we will run the motor in only one direction: First, let`s consider non-regenerative braking. If the bridge output is high (S1 closed, S2 open), the motor will accelerate to full speed. If the bridge is now switched low, the motor won`t just gently coast to a stop. It will slam to a stop, as if someone but a brake on it. Why A motor can be modelled as a series inductor and voltage source. The motor torque is proportional to current. The voltage source is called back-EMF, and it`s proportional to the speed of the motor. This is why a motor draws more current when it is loaded (or worst, stalled): with the speed decreased, the back-EMF is decreased, and it opposes the supply voltage less, resulting in higher current. Let`s redraw our schematic with that model, with values as if our motor is spinning at high speed: This motor is running at full speed. We have a small current to overcome the friction in the motor, and the back-EMF is the supply voltage, less the voltage drop over R1. Not much current flows because the back-EMF cancels most of the supply voltage, so L1 and R1 see only 100mV. Now what happens when we switch the bridge to the low side At first, nothing. L1 prevents an immediate change in current. However, this doesn`t last long, and very soon (defined by the time constant of \$L_1 / R_1\$, not more than a couple \$ms\$ typically) the back-emf (V1) has reversed the current, and now it`s going in the other direction....

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