Why choose a DC motor?
Many applications require a high torque when starting. The DC motor has - due to its design - a particularly elastic and efficient relationship between torque and speed (rpm). It can therefore cope well with high counter-torques and can easily cope with sudden changes in load; the motor speed adapts to the load. Moreover, the DC motor is the ideal solution for the applications developed by highly desired miniaturization because it offers high efficiency compared to other technologies.
Advantages of solutions with DC motors:
High starting torque:
The DC motor offers higher efficiency than other solutions.
Safety in operation:
The low voltage provides a level of safety that the requirements of the machinery directives EN 60335-1 and IEC 335-1 "Safety of electrical equipment for household and similar purposes' are fully met.
How does a brushless DC motor work?
When extra-long durability without maintenance, no audible noise and no electrical interference is required, the brushless DC motor is the optimal solution.
Brushless DC motors consist of three essential elements:
- A fixed part, the stator, with three coils responsible for the three phases of the motor.These coils act as electromagnets. They generate differently oriented magnetic fields that are evenly distributed around the central motor shaft.
- The rotating part, the rotor, has permanent magnets. Like the needle of a compass, these magnets constantly force the rotor to align itself with the magnetic field of the stator. To ensure optimum durability, the rotor runs on ball bearings.
- Three "Hall effect" magnetic sensors. These sensors constantly transmit data on the position of the rotor magnets. The brushless DC motor requires an electronic control to activate the motor phases. With the progressive development of electronics, these controls have become smaller and smaller, they cost less and they provide many additional functions.
How does a DC motor with brushes work?
DC motors with brushes also consist of three essential elements:
- A stator, which consists of a metal housing to which magnets are attached that provide a permanent magnetic field.
- An internal rotor located on the rotating output shaft. The metal structure of the rotor with coils of copper wire acts electromagnetically as soon as the power supply is activated. By the attraction of opposing magnetic poles and the repulsion effect of equal poles produces a torque that makes the rotor turn.
- A collector (commutator), which is connected to the rotor coils by selective energy pulses generates a rotation. An assembly with segments of copper gives an electrical Impulse to the coils when touched by a conductive carbon brush.
Therefore, this type is called a DC motor WITH brushes (Brush DC Motor). However, the carbon brushes wear out over time, and this also applies to the collector (commutator), but slower.
The durability of a DC motor with brushes is limited. In addition, the contact of the brushes with the rotating collector generates a noise level:
Whereby this "noise" is not only audible, but also has an effect in the electrical field in the form of interference, or more precisely as "electromagnetic interference" due to the sparking when the brushes are applied and when contact is broken. Nevertheless, the DC motor with brushes is a reliable solution in many cases.
Brushless DC motor explained simply
A brushless DC motor, also called Brushless Motor or Brushless DC or EC (Electronical Commutated) motor, is less of a DC motor. It is more like a synchronous machine. The idea is to "commutate" the motor by means of an electrical circuit so that we no longer need brushes. The rotor is therefore permanently excited and the stator consists of coils. The coils are controlled in such a way that a rotating exciter field (magnetic rotating field) is created.
The reason for the creation of torque is the force that arises as an interaction between two magnetic fields. The permanently excited rotor is thus pulled along by the exciter field.
Design of the brushless DC motor
The excitation windings are usually connected to three phases, i.e. three current connections with different phase relationships. The excitation windings are housed in the stator of the motor, each offset by 120 degrees. A motor with three coils arranged in this way has a pole pair number of 1.
If three more coils were inserted between the existing ones, the motor would have a pole pair number of 2, and so on. In general, the construction is similar to that of a synchronous machine. Most motors have their rotor inside the stator. They are called internal rotors. A brushless DC motor can also have its rotor outside. This is called the external rotor.
Commutation of the brushless DC motor
Although the construction corresponds to that of a synchronous motor, a brushless DC motor can also be operated with direct current. The commutation, i.e. the energization and reversal of the current direction, is then carried out by an electronic control system. This switches the coils on and off one after the other. The voltage of the individual phases is, therefore, a kind of pulsating DC voltage.
One coil is always switched off, while two others are switched on. To ensure that the motor control unit knows where the rotor is located, three Hall sensors are distributed around the circumference. To switch the coils on and off, a circuit consisting of three half-bridges is used. The switches of the half-bridges are often either MOS-FETs or IGBTs.