DC Motors as Generators
Both brushed and brushless DC motors can be operated as generators. However, there are some important points to consider when designing the drive.
Every now and then, I’m asked about the suitability of small DC motors for use as a generator. The fundamental statement is that motors with low power losses are equally efficient when operated as generators. The basic calculations between speed and voltage as well as current and torque are very simple. In the following, a few rules for a successful selection.
DC or AC voltage?
Rule 1. For the generation of DC voltage select a brushed DC motor or use a brushless EC (BLDC) motor with voltage rectifier.
For the generation of AC voltage, select a brushless DC motor and connect two phases only.
Hall sensors are not needed on brushless motors.
Speed constant kn
Many generators are operated at speeds n of 1000 rpm or below. That’s quite a low speed for small motors. Generating 10 V or more at 1,000 rpm requires a speed constant of at most 100 rpm/V. Such windings are hard to find in standard motor portfolios. The smaller the motors, the larger typical speed constants because small motors generate power with speed and not torque. Hence, only larger motors of a certain size and minimum power rating exhibit sufficiently low speed constants. Still, it is usually the motor windings with the highest nominal voltage that exhibit in generator constants in excess of 10V/1000rpm.
Rule 2. Without considering the load, the winding should have a speed constant of
As an alternative, the motor speed n can be enhanced by the use of a gearhead (see below).
Rule 2 requires motors with high generator constant. Unfortunately, these windings have the highest resistance as well. High resistance reduces the output voltage under load and the output voltage becomes very sensitive to the load current.
Rule 3. For stable output voltage over a certain load range, select rather a larger motor where the resistance is lower even on motors with high generator constant.
Do not select the motor-generator on power considerations alone. In order to fulfill the torque requirements, you might need a motor with a much higher power rating than the generated power; in particular if the generator speed is rather low compared to typical motor speeds.
The amount of torque on the generator defines the size and type of the motor-generator. Select a motor type with a continuous torque higher than the generator torque. When calculating the torque or current load, consider the operation type. Will the generator run continuously for long periods of time, or in intermittent operation cycles, or during short intervals only? Accordingly, a motor size with sufficient continuous torque or current has to be chosen.
Also respect the maximum speed of the motor type. However, due to the generally low speeds this is hardly ever an issue.
Current and voltage limitations
The most appropriate winding of a given motor type follows from the current and generated voltage requirements. Select a winding that can generate the required voltage U even under load.
Assuming a fixed generator speed n, we require a generated voltage of the winding Ut that is larger than U
Without considering the load, select the speed constant according to Rule 2, i.e. a winding with a sufficiently high resistance. Since the current capacity decreases with increasing resistance, verify that the continuous current is still large enough.
Figure 1. The voltage-current lines of the different windings of a 150W brushed DC motor at 500 rpm. Observe the different slopes of each winding.
Figure 1 quite nicely shows the ambivalent effects of different windings. The higher the winding resistance, the higher the generated (no-load) voltage. However, the higher the winding resistance, the more sensitive the generated voltage becomes to load current changes. These contradictory effects can be eliminated to a certain extent by selecting larger motors that exhibit lower resistances for the same generator constant (according to Rule 3).
Rule 4. Use gearboxes to increase very low speeds. Use spur gears or specially designed gears that can be back-driven.
The reason for using gear-motor combinations is the very slow driving mechanism in generators; e.g. driven by a wind or water turbine or even by hand. A few observations and recommendations:
The gearheads need to be driven in reverse operation in these cases with relatively low efficiency. Many high reduction planetary gearheads (three stages and higher) are not back-drivable; i.e. they won’t turn when driven from the output with the maximum permissible torque. You may use one or two stage planetary gearheads; they can be operated from the output.
Rather use spur gears instead of planetary gearheads. Spur gearheads can more easily be back driven, and the back-driving efficiency is usually higher than with other gear designs.
Special case: DC motor as DC tacho
DC tachos use the generated voltage for speed evaluation. Current should be kept small to maintain the proportionality between voltage and speed.
Rule 5. For DC tachos, use DC motors with precious metal brushes that suit better the small currents. Select the winding according the required tacho voltage and the speed range in your application. Don’t worry about the winding resistance, just make sure that there is a load resistance of several kn to keep currents small.