The DC or direct current motor is a member of the class of electric motors and is primarily used for transforming electrical energy into mechanical energy. In this context, the majority of structural forms in which the DC motor is encountered are based on the use of magnetic forces; they contain a number of internal mechanisms, some of which are electronic and others electromechanical. A characteristic feature of conventional DC motors is the commutator, the function of which is to intervallically alternate the direction of the current flow within the motor. The brushless DC motor represents a modified form of the classic DC motor; it is more powerful, making it more suitable in many application areas. It has no sliding contacts and its benefits include a longer service life.

Despite the increasing use of - and great rivalry with - AC or alternating current motors, which are another member of the class of electric motors, DC motors continue to be hugely important to this day. Thanks to their specific characteristics, including, in particular, the fact that their speed and torque can be precisely controlled over a wide range, they are still found in a wide range of industrial applications. An example of this type of motor construction is the PMA range of servo motors made by Harmonic Drive AG, which consist of a highly dynamic DC motor equipped with an incremental encoder together with a PMG gearbox. They are ideal for applications in the semiconductor industry as well as in measuring and testing machines. 

Structure and function of the DC motor

The design of the traditional DC motor is based on a simple structure and incorporates relatively few components. The primary supporting elements are the stator and the rotor. The stator is an immobile, fixed component that generally consists of an electromagnet or - often in the case of small units - a permanent magnet. The rotor, also sometimes referred to as the anchor, is mounted inside the stator; this is a revolving component which is also based on an electromagnet in the case of conventional DC motors. Direct current motors consisting of an arrangement of stator and rotor as described are referred to as internal rotor motors, while the opposite construction is known as an external rotor motor.

The windings of the rotor are connected with the aid of a commutator. This serves as a polarity changer and houses the sliding contacts in the form of brushes made from an electrically conductive material. Common materials used for this purpose include graphite and - depending on the specific use of the motor - substances enriched with metal. In operation, the brush contacts are of decisive importance to the functionality of the conventional DC motor. This is because when a direct current flows through the windings of the rotor or through the rotor itself, it becomes electromagnetised and generates magnetic forces based on the characteristics of the stator. Since like poles repel and opposite poles attract, this creates a rotation of the rotor, which in principle would soon peter out. So, in order to maintain continuous rotation, the direction of the current must be reversed periodically. The commutator of the DC motor, consisting of brushes, is the component responsible for the regular reversal of polarity.

Additionally, the construction of brush-type DC motors varies according to the way the anchor and stator windings are connected. In the series wound motor, both the field windings and the windings of the rotor are in series, or connected one after the other, which creates the basis for an alternating current feed. The counterpart is the shunt wound or parallel wound motor, in which both coils are connected in parallel. The compound motor is a combination of series and parallel wound designs. Depending on its dimensions, this type supports a variety of operating behaviours, and displays benefits of both types of construction.

The commutator function in DC motors

A design that is in competition with the brush DC Motor is the brushless DC motor or BLDC for short. As its name suggests, this design differs from the classic construction in one significant aspect  – it contains no mechanical sliding contacts or brushes. The role of the mechanical commutator is taken over by an electronic power circuit that tracks the position of the rotor with the aid of a sensor, behaving as a form of electronic commutator. By integrating, for example, control algorithms, it is possible in many applications to perform the commutator function without sensors. The construction of a brushless DC motor can therefore be compared with that of an undamped synchronous motor, but with more flexible possibilities of control; moreover, thanks to the incorporation of an inverter circuit, it can be fed with direct current. Brushless DC motors, or rather the magnetic coils of the integrated stator, are frequently of a three-phase construction.

Brushless and brush DC motors in comparison

As the performance features of these two constructions differ fundamentally, a basic choice has to be made as to whether to employ a brush or a brushless DC motor. This can have far-reaching consequences and is governed by the requirements profile of the application in question. Other factors, such as purchase price and maintenance costs, also need to be taken into account.

Great consideration must be given to the effects of, and differences between, brush and brushless commutator systems, with particular regard to the service life of the DC motor. As the sliding contacts or brushes are physical components that operate mechanically, they are subject to constant wear. Their useful life is therefore limited. High rotational speeds can also have a major effect on the service life of the brushes. The useful life of brushless DC motors, on the other hand, is limited only by the ball bearings integrated inside them and can therefore be calculated to a relatively reliable extent. The absence of mechanical friction prevents the danger of a brush fire through the formation of sparks on the commutator. Brushes also have the effect of limiting usage under specific ambient conditions; in high vacuum applications, for example, only brushless DC motors can be considered.

In direct comparison with brush motors, brushless DC motors can be seen to have several advantages in terms of performance. These may vary according to the application and manifest themselves in different ways, but as a rule, they comprise a higher starting torque, a maximum-precision controller, which is moreover more resistant to load fluctuations, and higher rotational speeds.

The brush construction offers several operational advantages – to set the rotor in motion, all that is required is to supply a voltage to two contacts. There are not many types of motor that can be put into operation with such ease. In contrast, brushless DC motors require an electronic commutator system, and their startup operation is considerably more complex. The price factor also has to be taken into consideration, as the aforementioned electronic module as well as the sensory system frequently incorporated in brushless direct current motors tend to be more costly.

Fields of application of brush and brushless DC motors

Due to the great variety that is possible in their technical constructions, DC motors form an integral part of a wide range of applications and are found throughout many sectors. In particular, the relative simplicity with which their rotational speed and torque can be controlled, coupled with their precise regulation and high dynamics, mean they have a broad spectrum of use.

As a conventional and very widespread motor design, brush DC motors require few or no external components, making them suitable for use in rough operating conditions. Typical applications include turning and grinding machines, conveyor belts, and vacuum cleaners. Moreover, compressors, rotary presses and lifts can be driven by direct current motors.

DC motors with an integrated permanent magnet are found in applications requiring extremely precise control and low torque, such as in the field of robotics. Thanks to the many performance advantages of the brushless variety of DC motor, it is found in an ever-expanding range of application fields. There are many industries and sub-fields in a wide variety of industrial sectors in which brushless motors have meanwhile replaced the more traditional design.

The use of brushless direct current motors in industry is primarily concentrated on applications in production engineering and industrial automation. It is primarily motion control systems, actuator systems and positioning systems that benefit from the characteristics of this type of motor. A prominent type of brushless DC motor is the stepper motor, which is frequently found in the position controller of open control loops. In many industrial environments, stepper motors support automated processes and are increasingly found in machines used in pick-and-place procedures.

Brushless direct current motors enjoy great popularity not least among model aeroplane and drone users. In this context, the main aspect is the relation between performance and weight and the wide range of construction sizes in which this type of motor is produced, because while the profiles of early combustion engines meant that the model aeroplanes they were installed in were relatively large and heavy, compact brushless DC motors have now made lighter models possible.