Automotive & Transportation

DC motors as generators

06 April 2020

Both brushed and brushless DC (BLDC) motors can be operated as generators. However, there are some important points to consider when designing the drive. Here are five rules for making a successful selection.

DC or AC voltage?

Rule No. 1: For generation of DC voltage, users should select a brushed DC motor or use a brushless electronically commutated (EC) motor with voltage rectifier. For the generation of AC voltage, users should select a brushless EC motor and connect two phases only. Notably, Hall sensors are not needed on brushless motors.

Speed constant kn

Many generators are operated at speeds of 1,000 rpm or below, which is low for small motors. Generating 10 V or more at 1,000 rpm requires a speed constant of only 100 rpm/V or less. Such windings are difficult to find in the maxon portfolio. There are only a few high resistance windings on larger motors that satisfy this requirement and smaller motors have higher speed constants.

Table 1: A selection of motors with low speed constant (or high generator constant = generated voltage per speed). Source: maxonTable 1: A selection of motors with low speed constant (or high generator constant = generated voltage per speed). Source: maxon

Rule No. 2: Without considering the load, the winding should have a speed constant of kn < n/u or smaller. As an alternative, the motor speed can be enhanced by using a gearhead.

Resistance

Rule No. 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 No. 3: For stable output voltage over a certain load range, users should instead select a larger motor where the resistance is lower even on motors with high generator constant. For such an application, maxon recommends its EC-i 40 high torque motors.

Power restrictions

Users should not select the motor-generator on power considerations alone. In order to fulfill the torque requirements, a user might need a motor with a much higher power rating than the generated power; specifically, if the generator speed is low compared to typical motor speeds.

Torque and speed limitations

The amount of torque on the generator defines the size and type of the motor-generator. As such, users should select a motor type with continuous torque higher than generator torque. When calculating the torque or current load, consider the type of operation. 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 continuous torque or current must be selected. Users should also account for the maximum speed of the motor type, which is generally low.

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), maxon requires a generated voltage of the winding (Ut) that is larger than U.

Without considering the load, users should select the speed constant according to Rule No. 2. Since the current capacity decreases with increasing resistance, verify that the continuous current is still large enough.

Ut = n/kn ­­­­­– Rmot · IL > U

Table 2 demonstrates the ambivalent effects of different windings. For instance, 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 (Rule No. 3).

Table 2: The voltage-current lines of the different windings of the RE 40 with precious metal brushes at 500 rpm. Source: maxonTable 2: The voltage-current lines of the different windings of the RE 40 with precious metal brushes at 500 rpm. Source: maxon

Gear-motor combinations

Rule No. 4: To increase very low speeds, users are encouraged to select gearheads. However, maxon gearheads are not ideal for being driven from the output. Instead, gearheads that can be back-driven (planetary gearheads up to two stages, spur gearheads or custom gearheads) should be selected.

The reason for using gear-motor combinations is the slow driving mechanism in generators, which are driven by wind or water turbines or even by hand.

Consequently, the gearheads need to be driven in reverse operation. However, maxon gearheads are not designed for reverse operation and efficiency is low. Likewise, high reduction gearheads (three stages and higher) are not back-drivable.

While stage one or two planetary gearheads can be operated from the output, maxon recommends using spur gears instead. Spur gearheads can be easily and efficiently back-driven.

Special case: DC motor as DC tachos

Rule No. 5: For DC tachometers, maxon recommends using DC motors with precious metal brushes for small currents. Windings should be selected according to required tachometer voltage and the speed range according to application, making sure that there is a load resistance of several kΩ to keep currents small.

For more on DC motors and microdrive systems, go to maxon’s website: www.maxongroup.us.



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