DC motors How DC motors help save energy

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If you want to save energy, brushless DC motors are a good choice: they are already significantly more efficient than comparable induction motors.

Enormous efforts are needed to achieve CO₂ neutrality in the economy. It is not enough to massively expand the capacity of renewable energy sources. Electricity consumption in industry must also be reduced: according to the International Energy Agency (IEA), more than 50% of the energy generated globally is used to run electric motors (World Energy Outlook 2016). There is therefore enormous potential for savings here, which companies can leverage by choosing the right drive.

Electric motors are divided into two types according to the type of magnetic field generation in the rotor: Synchronous and asynchronous drives, which also include brushless DC motors and induction motors. Induction motors work less efficiently than brushless DC motors - this is due to their design. The drives consist of a stator with windings made of copper wire, which are surrounded by a housing made of laminated electrical sheets, and a rotor. When an alternating voltage is applied, the resulting current generates a rotating magnetic field as it passes through the windings of the stator, which also penetrates the rotor through an air gap. The rotor has a core made of laminated electrical sheets and is integrated into an aluminum cage rotor. It is set in motion by the rotating magnetic field.

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More losses due to higher electrical power

However, the current induced in the rotor also causes secondary copper losses, which are largely caused by the ohmic resistance of the winding. These losses are the main reason for the lower efficiency of induction motors compared to brushless drives. In addition, a magnetic flux must be generated in both the stator and the rotor. The electrical power required to turn the rotor is therefore higher with an induction motor than with a brushless drive. Particularly at low speeds and torques, the efficiency of induction motors deteriorates compared to brushless DC motors.

Permanent magnet makes the difference

In brushless DC motors, permanent magnets are attached to the surface of the rotor. Here too, current is passed through the stator winding, creating a rotating magnetic field that sets the rotor in motion. However, there is one major difference to induction motors: No current is induced in the rotor of synchronous drives. As brushless DC motors have a permanent magnet, there are also no secondary copper losses as with induction motors. And finally, thanks to the permanent magnet, less electrical power is required to turn the rotor or output shaft than with induction motors. The aforementioned properties ensure that brushless DC motors from Oriental Motor work significantly more efficiently than comparable induction motors.

Efficiency improved once again

The efficiency of the brushless DC motors is further increased by two additional design measures: the integrated circuit with the Hall effect sensors, which detects the rotor position, is located directly opposite the face of the rotor. The position of the rotor poles is therefore detected even more accurately than with brushless DC motors, where the sensors are mounted elsewhere. In addition, the engineers at Oriental Motor have optimized the technology integrated into the driver to control the motor current. The use of low-loss electrical laminations, the increase in winding surface area and the use of thicker electrical wires have also contributed to an increase in the already high efficiency of brushless DC motors.

Measurement with different methods

The efficiency of an electric motor can be determined using various methods. One possibility is to compare the power consumption of the drives with the same output power. To do this, the motors are operated with a constant load while the power supplied to the drive at each operating point is measured. Comparing induction motors with a rated output power of 90 W with brushless DC motors, which also have an output power of 90 W, shows that the power consumption of the brushless drive is lower.

If, on the other hand, the motor is operated with a variable-speed drive, the efficiency must be compared across the entire operating range. The reason for this is that the efficiency of these motors changes depending on the load factor and speed. If you compare induction motors and brushless DC motors with a variable-speed drive, you will see that the brushless DC motor operates more efficiently over a wider range than the induction motor.

Speed changes have hardly any effect

The difference in efficiency between brushless DC motors and induction motors is best illustrated by specific applications - e.g. a conveyor belt. If a brushless DC motor and an induction motor drive it at a constant speed, the brushless drive consumes 37% less electricity than the induction motor over the course of a year.

The effect is even greater when operating with speed changes: if the conveyor belt transports a load at 100 min^-1 and is slowed down to 30 min^-1 for inspection, both the power consumption and the CO₂ emissions of the brushless DC motor are 69% lower than those of the induction motor. This is due to the fact that the efficiency of the brushless drive is only slightly reduced by the change in speed, whereas that of the induction motor is greatly reduced. Brushless drives therefore also work more efficiently than induction motors in applications with varying speeds. A similar picture emerges when brushless DC motors are used as drives for fans: their power consumption is also considerably lower than that of an asynchronous drive. They also have another major advantage over induction motors in this application: as their speed can be adjusted via PWM signals or external analog settings, the noise emissions of the fans can also be reduced.

Compact design reduces emissions

Oriental Motor offers users four different series of brushless DC motors for DC and AC operation with rated output powers between 15 W and 400 W. The drives are not only characterized by high efficiency, but are also very compact. While an induction motor with gearbox is around 200 mm long, the brushless drive is only 95.4 mm. And instead of 4.7 kg like an asynchronous motor, a brushless DC motor including driver weighs just 3.1 kg. Thanks to the small dimensions, the CO₂ emissions caused by transportation are also lower with brushless drives than with induction motors.

This article was first published on our sister website www.elektrotechnik.de (German language)