Regeneration: it sounds spiritual, and when applied to motor drives, can result in an almost qualitative increase in performance, a great leap toward ontological perfection. From an engineering viewpoint, not only can the reactive devices of electrical circuits, inductance and capacitance, store energy. So also can the corresponding reactive mechanical elements, compliance ("springiness") and mass, or rotational compliance and inertia.

Power amplifiers that drive motors, like their audio counterparts, usually only source energy at their outputs, and are not designed to sink energy. Although bidirectional current drive is common, bidirectional power flow is not. But motors are only motors when they input electrical energy and output mechanical energy. As energy conversion devices, they can convert in either direction. Like a wavering believer, a spinning motor connected to a large inertial mass (such as a flywheel) blackslides into being a generator when no longer under the dominating influence of electrical drive from the power amplifier. Instead of inputting electrical energy, it proceeds to source it instead, drawing from its mechanical supply. The measurable result is a winding-induced voltage that can forward-bias the clamp diodes in the bridge of a power driver, causing current to flow back into the bridge supply. The supply output capacitor is charged (for a voltage supply), and if too much energy is dumped back, can overcharge, overvoltage and fail. A sufficiently large capacitor might be the answer, but it is an expensive way to save a supply.

This generating action, destructive of a power-driver output stage, need not be. The motor electrical model shown below is driven by the driver supply, V_S. As shown, the motor induced voltage, v_w  = L×w, is opposed by the supply due to field-oriented phase control of the power driver. (The driver output switches applying V_S to the motor are assumed to be in the V_S source.) This is the correct phasing for motoring. But if the motor-drive polarity is inverted (a phase inversion of 180° el), then the drive switches apply the drive supply voltage to the motor 180° out of phase with the induced voltage, v_w. In other words, instead of opposing v_w, the drive aids it, promoting the flow of current in-phase with v_w. Because power flow is then into the motor-drive output terminals, current flow into the drive will oppose its positive supply voltage and reduce or even reverse supply current, if possible.

Power-driver phasing is relatively easy to accommodate. It is usually an additional detail in the DSP motor-control code or some extra control circuitry. But once the motor-supplied power gets past the driver, what of the driver supply? Most supplies are not designed for bidirectional power flow. But again, they could be. Generally, transistors must replace diodes, to allow bidirectional current flow. And additional control considerations apply for two-quadrant operation. Common topologies can convert power bidirectionally, taking mechanical reactive power which passes through the driver and dumping it back into the wall socket from whence it came. For instance, a boost converter (common-active configuration) becomes a buck (common-passive) when operated in the reverse direction. This is the high-efficiency, ultra-green, and elegant approach that takes design effort to achieve.

The more mundane, less efficient and less edifying alternative is to dump the mechanical power into a dissipative resistor bank. All is lost – of energy, that is, but not of functionality. This is done to allow active braking of mechanical loads. The electrical analog is an open-collector driver transistor; it can actively sink but not source current. Similarly, a motor-drive that cannot sink power can accelerate but cannot actively decelerate a mechanical load. At least the resistor bank allows for deceleration, though not efficiently.

Efficient regeneration is a capability for a new age in motor-drive design. It requires essentially no additional cost than the difference between diodes and MOSFETs, a small difference indeed for low to medium-volume drive products. What it mainly takes is a new outlook – a converted designer mind – that understands bidirectional power-flow implementation for both power-driver and motor controller, and applies these principles with faith that the additional performance will provide a competitive edge in the drive business. As the world goes "green" the customer benefit of regenerative motor control will become increasingly evident. Visionary designers will recognize this and equip themselves with the ability to design regeneration into their motor-drives. It's the spiritual thing to do.

Ó Dennis L. Feucht, 2000