Dc Motors

Brushed DC Motors

Brushed DC Motors consist of a motor and a controller, which permanently supplies the motor with direct current. Inside the DC motor, the so-called armature plate, a magnetic field is generated via windings. Due to the magnetic field of the housing magnet, the armature starts to rotate by attraction and repulsion between both magnetic fields.

This rotation continues until two pairs of magnets with the same polarity face each other. Now the polarity of the current is reversed in the armature winding via brushes, which serve as current supply, and the collector, which serves as a mechanical switch. This allows the armature to continue rotating by attracting and repelling the newly polarized magnetic fields.

During this process a lot of heat and sparks are generated by friction, which leads to wear and tear of the collector and brushes and finally to motor wear and tear. This in turn can cause the motor to overheat very quickly and lose power and torque.

Brushed DC motor have several advantages. For example, multi-motor systems require only one controller and switching effects run very smoothly.

Due to the simple construction of the motor, it is very cheap to buy and can be easily repaired and maintained.

Because of the friction of the brushes inside the motor, the motor also gives a very good sound. Due to the massive construction the higher weight often contributes to the optimal center of gravity of the RC.

Due to the constant friction of the DC motor brushes and the wear of the carbon brushes inside the motor, the brushed motors have a limited life span and speed, and have a lower performance level than brushless motors. The wear and tear of the motor must be counteracted by regular maintenance.

 Replacing old, worn collector carbons is very important. Short old carbons must be replaced immediately, as they can easily jam in the guide and in the worst case damage the collector. Cleaning the collector increases the conductivity and thus also the motor power. The heat development can also be reduced. Furthermore, it should be noted that driving in the wet will wear out the motor even faster and should be checked after every rain ride.

Brushless DC Motors

Brushless DC motors do not have carbon brushes which change the poles in the motor, but have a controller which takes over this task.

In a 3-pole motor, the controller generates a so-called rotating field by switching a DC voltage to two of three motor coils, whereby the third motor coil serves as a sensor line and determines the respective position of the rotating permanent magnet. Due to the magnetic field of the permanent magnet and the magnetic field of the motor coils, the permanent magnet begins to rotate by attraction and repulsion of the magnetic fields. This rotation continues until two pairs of magnets with the same polarity are almost opposite each other. Now the controller changes the voltage on the motor coils and the process starts all over again.

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In addition, brushless motors are differentiated between internal and external rotor DC motors:

• With internal rotor DC motors the inner part, the permanent magnet, of the motor rotates and the outer shell, the motor housing, is fixed. With internal rotor motors, the motor coils are mounted in the fixed motor housing. The permanent magnet is attached to the rotating rotor, which is also connected to the drive shaft. Due to this design, internal rotors achieve a higher speed, but have less torque than external rotor models.
• With external rotor models, the motor housing rotates and the inner part of the motor is fixed. Here the permanent magnets are attached to the rotating motor housing. The drive shaft is thus connected to the outer housing (motor bell) and rotates with it. Inside the motor, the so-called stator, the motor coils are attached around which the motor bell rotates.

Brushless DC motors have a rather lower heat development, because they do not have carbon bristles inside the motor and therefore no frictional energy is generated. Nevertheless, it is recommended to install a motor cooler, as the motor housing of brushless DC motors gets warm.

Most brushless DC motors also have a very high efficiency in terms of battery life and performance.

The maintenance requirements for brushless motors are minimal. The only thing to be considered is that when you buy the motor it is well sealed against water and dirt and the rotor is properly spaced in the housing. The ball bearings should be re-oiled regularly and checked for stones and sand.

Conclusion:

An engine that is in good shape and works flawlessly is a pleasure. Maintenance and care are essential for every DC model. For beginners and hobby riders, an inexpensive brushed motor is certainly a good option. For advanced model builder, the more expensive brushless motors offer a good possibility to equip their RC model due to better performance, higher speeds, easier maintenance, and longer life.

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Why choose DC motors?

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 has – due to its design – a particularly elastic and efficient ratio between torque and speed (rpm). It can therefore cope with high counter-torques and can easily cope with sudden changes in load. The motor speed adapts to the load.

Miniaturization:

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.

brushless direct-current motor

How does DC motors 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.

Note

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.