Simple DC machines
Simple DC machines includes a DC motor with permanent magnet field and wound armature with commutator. The permanent magnet DC motor is a good case to study.
In simple DC machines, the difference between being a motor and generator is often simply a matter of rotational speed. The motor develops an induced voltage in its windings by virtue of its rotational speed, and current flows in the winding if that voltage is different to the terminal voltage… the direction of current determined by which voltage is higher and the direction of current determines whether the torque assists or resists the rotation.
The counter torque from reverse current is often referred to as regenerative braking as the retarding effect of the current driven by the induced emf of rotation slows the motor, and current flows into the source.
If a simple DC machine is powered from a simple rectifier circuit, the rectifier will block the flow of reverse current, and so there is no regenerative braking… the rotation induced emf simply raises the terminal voltage of the motor (possibly dangerously), but no current flows and there is no counter torque.
If a simple DC machine is powered from an electronic regulated power supply, the situation is a little different. The regulator will commonly block reverse current, and it may sense that output voltage is greater than desired and shut down, it may even be damaged by the excess terminal voltage.
Brushless DC motors
Brushless DC motors use some form of electronic driver to provide commutation of current in the coils, whether derived from sensors fitted to the motor or sensed from the undriven coil at any instant.
Some driver configurations provide a path for regenerative current to flow to the power source. If the power source blocks the regenerative current, the terminal voltage of the motor and power supply may increase, possibly to levels that may damage the motor driver and damage or disrupt the power supply. Electronic power supplies do not usually contain provision for regenerative current.
An example sensorless brushless DC motor used in UAVs
This example illustrates the nature of regenerative current in a particular application where rapid response of the drive is very important.
Above is a supply current graph for a test scenario that subjects the drive to a number of acceleration / deceleration scenarios. The current sensor does not measure negative current, its output is clipped at I=0.
Above, a zoomed in view of one of the current reversals. As mentioned, the current sensor does not show negative current, so we can reasonably assume that when the trace is clipped at zero, negative current flowed (though we cannot infer the magnitude). What we can do is measure the duration, and this is one of the longest reversals in that test, it is during rapid deceleration and the duration is less than 150ms.
Though the duration is short, even shorter durations have been observed to disrupt an electronic regulated power supply. Another experience was that a faulty battery connection resulted in disconnection of the battery on a quadcopter running at 50% speed and associated with that disconnection was damage to a flight controller regulator, likely to have been due to a voltage transient due to regenerative current.
A simple and common solution is to include a shunt regulator to prevent the terminal voltage of the power supply and motor drive increasing dangerously and to provide a path for regenerative current and hence permit braking. Invariably a pair of diodes is needed to steer regenerative currents and back emf around the power supply and through the shunt regulator as needed.
Implementations often set the shunt regulator to a couple of volts higher than the power supply, and it is obvious that this really needs a well regulated power supply to transition smoothly from motor to generator mode.
Such solutions might be adequate for a drive that slows down over a long period, or must simply act as a brake over longish periods, but the behaviour of the system is different to that of using a high capacity / low impedance battery which will absorb regenerative current with negligible increase in terminal voltage.
Whilst the shunt regulator might provide some protection against power source and drive disruption or damage, it is unlikely to provide the same dynamic braking response as a low impedance battery source, and for that reason cannot be used to properly assess dynamic braking on those systems.
The best emulation of a low impedance battery in UAV drive testing is a low impedance battery, other simple solutions are probably inadequate.