The basics of regenerative braking

Regenerative braking is an inherently good idea, particularly in a world that still relies heavily on fossil fuels and is threatened by the inevitable planet-warming effects that are the result.

A moving vehicle, to take one example, possesses a certain amount of kinetic energy. If it is to be stopped or, while descending a grade, the speed is to be held constant, the conventional mechanism is dynamic braking. Hydraulic cylinders press brake pads against rotors (or brake shoes against drums) and the vehicle slows. Due to friction, the kinetic energy converts to heat, which dissipates. This energy is wasted.

There is a highly effective workaround known as regenerative braking. Properly configured, an over-speeding electric motor will absorb kinetic energy and instead of dissipating it in the form of heat, it will become a generator and the output current can be used to charge batteries or capacitors.

Regenerative braking is appropriate for electric vehicles where this reclaimed energy can be used to extend driving distance between charges or in hybrid vehicles to reduce gasoline consumption.

Some internet sources are way off point. They make it sound like highly creative cutting edge automotive engineers developed this high-tech innovation just in time to save the world from a heat death. In actuality, the strategy is only incremental and has been in use since the 1800s. It was widely used in cranes and elevators. These machines operate in a downward going mode half the time, so regenerative braking is a good fit. In an elevator precisely how this plays out is altered by the presence of a counter weight and the loading of the car, which is subject to variation.

An additional aspect in elevator regenerative braking is that in hot weather the heat generated by dynamic braking does not have to be offset by the building’s air-conditioning system. The overall benefit of regenerative breaking in an elevator is fully realized when the excess electrical energy is fed back into the grid, as it is in wind generator or solar array cogeneration with the utility.

An interesting example of regenerative braking that makes good sense is the Kiruna, Sweden to Narvik, Norway railway. It regularly hauls thousands of tons of iron ore downgrade and returns empty upgrade. Electricity produced by regenerative braking feeds into the grid and the railroad is a net exporter of energy.

Test setups for characterizing regeneration of electric motors and drivetrains must measure electrical signals on the inverter as well as on the motor itself. Tests typically take place with the drivetrain connected with a dyno. Besides measuring voltage, current, torque, and speed, such test setups also typically check temperature because motor behavior heavily depends on its winding temperature.

Performance tests on electric motor regen typically include multimeters to measure dc current and voltage on batteries, power meters to record ac current and voltage on the inverter, and data-acquisition systems to record the mechanical variables torque and speed. One problem with making these measurements is the difficulty of synchronizing the timing of the various measuring systems. Measurement data can potentially be stored in three different systems and data formats.

It should also be pointed out that currently, there is no standard test procedure for characterizing EV motor regen. Standards bodies are working on test procedures for EV components, but the emphasis has been on battery tests and on safety.

To gauge regen under conditions that mimic those of the real world, it is typical for tests to vary the motor load, vary the operating temperature, and vary other operating parameters such fast/slow decelerations. Test points would include the voltage and current from the inverter (for all phases) and the motor regen output (voltage and current).

Manufacturers have devised test setups designed specifically for characterizing EV motors and drivetrains as a means of showing how these systems are likely to perform in real conditions. They now provide full access to raw data, and handle synchronous measurement data acquisition for mechanical and electrical signals.

For example, a setup developed by HBM includes a T12 digital torque transducer that records torque and speed as measurement variables. It also includes a high-speed data-acquisition system that synchronously records electrical signals from the inverter and motor as well as torque transducer data. Finally, measurement-data software gives access to raw data and lets developers analyze it later as well.