Key Concepts For Running a Brushed DC Motor as a Generator

Although design engineers know that both brush DC and brushless DC (BLDC) motors can operate as generators, many tend to avoid running a brushed DC motor in generating mode because they believe this strategy is inefficient compared with motor use. Unfortunately, by avoiding using brushed DC motors as a generator, engineers pass up a number of benefits.

For example, brushed DC motors are less expensive than miniature generators 

even though they both perform the same function. Brushed DC motors also offer a wide range of options to address specific application needs. And, brushed DC motors are particularly well-suited for applications that require voltage as an input without a power source. This blog post provides an overview of two of the key engineering concepts to remember when considering using a brushed DC motor as a generator.

Back EMF
Back-EMF is directly proportional to the angular speed of the motor shaft rotation. When a motor operates as a generator, its shaft is mechanically coupled and rotated by an external source, causing the coil segments in the rotor to rotate through a sinusoidally varying magnetic flux in the air gap. Each turn of the rotor winding is induced with sinusoidal voltage, with the speed of rotation and the magnetic flux linkage determining the voltage magnitude. For instance, if a rotor coil consists of one turn, the induced EMF is sinusoidal with a period equal to one electrical cycle.

By design, a brushed DC rotor is wound in an odd number of segments and supplies power to the coils through a pair of brushes. When the shaft is rotated in generation mode, the generated back-EMF voltage is measured at the output terminals. Based on the properties of the motor design — which includes the number of coil segments — a voltage ripple is typically present and generally represents less than 5% of the output voltage.

Driving Torque and Power Balance
When a motor is driven in generator mode with open terminals, no current flows through the circuit and mechanical friction creates losses in the driving unit.

The generator must be driven with a torque that will generate the required load current through the winding when the terminals are closed across the load resistor. Motor selection is limited by the maximum amount of torque that can be applied on the shaft in generator mode. The operation of a brushed DC motor is limited by the maximum continuous torque and maximum continuous speed available. Selecting a motor that can handle the generator torque on the shaft and manage the maximum current through its circuit is similar to the process of sizing a motor based on desired load points.

Ideally the maximum power should always be higher than the required electrical output power from the generators. Depending on the load current, the actual power output may be less than the maximum power. While selecting the right motor to be used as a generator, actual power should be taken into consideration instead of maximum power.

As you consider whether to deploy a brushed DC motor as a generator, keep in mind that you do not have to sacrifice efficiency. With careful consideration to back-EMF, driving torque and power balance along with load handling, operating speed and careful motor selection, you can achieve efficiencies that rival conventional DC motor operation.

To learn more, read the complete White Paper or contact an engineer.