Permanent Magnet DC Motor: Types, Application, Selection Principle (2)

Figure 1: Permanent magnet DC motor.

The following content is the second half of the previous article, and continues introducing the basic principles of permanent magnet brushed DC motors, the application and selection principles of permanent magnet DC motors.

1. Permanent Magnet Brushed DC Motor

1.1 Working Principle

The permanent magnet brushed DC motor is mainly composed of a stator made of permanent magnet material, a rotor (armature) with coil windings, a commutator and brushes. Fixed main poles and brushes are installed on the stator of the brushed motor, and armature windings and commutator are installed on the rotor.
The electric energy of the DC power supply enters the armature winding through the brush and the commutator to produce armature current. The magnetic field generated by the armature current interacts with the main magnetic field to generate electromagnetic torque, which makes the motor rotate to drive the load.

The working principle of the brushed DC motor is shown in the figure below. There are magnets in the fixed part of the brushed DC motor, here called the main pole; there are also brushes in the fixed part. The rotating part includes an annular iron core with windings around it.

Figure 2: Working principle of brushed DC motor.

The stationary part (stator) of the four-pole brushed DC motor shown in Figure 2 is equipped with a pair of main poles N and S, and the rotating part (rotor) is equipped with armature core. There is an air gap between the stator and the rotor. An armature coil is placed on the armature core. The head and end of the coil are respectively connected to two arc-shaped copper sheets, which are called commutator segments.

The commutator segments are insulated from each other, and the whole composed of the commutator segments is called a commutator. The commutator is fixed on the rotating shaft, and the commutator segment and the rotating shaft are also insulated from each other. A pair of stationary brushes are placed on the commutator. When the armature rotates, the armature coil is connected to the external circuit through the commutator and the brushes.

1.2 Types

1.Brushed disc winding motor. The brushed disc winding motor is made of rare earth material bonded to a cylinder body to form a stator, and a disc winding formed by enameled copper wire which is placed in the cylinder body to form a rotor.

2.Brushed printed winding motors. Brushed printed winding motor uses printed copper foil sheet as winding to reduce the weight of the motor.

3.Brush pressed winding motor. By pressing the wound copper wire into a new type of winding, the efficiency of this motor can be increased to 74% to 78%, but the existing noise and life defects are still problems that must be improved.

4.The hub-type brushed DC motor with gear transmission is composed of two parts: a disc-shaped armature brushed motor and a gear reduction and transmission system. A disc armature is a rotor that rotates at high speed. The structure of the hub-type geared brushed DC motor is shown in Figure 3. The torque of the motor is transmitted to the first-stage gear through the shaft, and the hub shell is driven to rotate by the gear reduction.

Figure 3: The hub-type brushed DC motor with gear drive.

1.3 Advantages and Disadvantages

The brushed DC motor is famous for its advantages of good response speed, good braking effect and high control accuracy. The brushed DC motor belonging to the DC motor adopts a mechanical commutator, which makes the driving method simple.
As long as a certain DC current is passed through the A and B ends of the brush, the commutator of the motor will automatically change the magnetic field direction of the motor rotor, so that the rotor of the DC motor will continue to run.

Disadvantages of brushed DC motors: commutators and brushes play an important role in DC motors. Although they can simplify the structure of the motor controller, certain disadvantages exist:
1) The structure is relatively complex, which increases the manufacturing cost;
2) It is easily affected by the environment (such as dust, etc.), which reduces the reliability of work;
3) There will be sparks when reversing, which limits the scope of use;
4) It is easy to be damaged, which increases the maintenance cost, etc.

But in general, the economic cost of brushed DC motors is still relatively low, because the hardware structure of the motor is relatively simple, on the other hand, the production technology is relatively matured, and the processing cost is low. The economic cost will be relatively lower if mass production.

2. Application of Permanent Magnet DC Motor

1. In terms of stage lighting, permanent magnet DC motors, especially small permanent magnet DC gear motors, are used in a large amount.
2. Printers, scanners, hard drives, CD drives, recorders, cooling fans, etc., in the computer industry all use a large number of permanent magnet DC motors.
3. Various fans, wipers, spray pumps, flame extinguishers, and air pumps in the automotive industry use various permanent magnet DC motors.

Figure 4: Air pump.

4. Permanent magnet DC motors are used in automatic doors, automatic door locks, automatic curtains, automatic water supply systems, and towel machines in hotels.
5. In weapons and equipment, permanent magnet DC motors are widely used in missiles, artillery, satellites, spacecraft, ships, aircraft, tanks, rockets, radar, combat vehicles and other occasions.

6. In industry and agriculture, permanent magnet DC motors are also widely used in electrical and automation control instrumentation.
7. In the medical field, permanent magnet DC motors are even more useful, such as various medical instruments and surgical tools (like electric bone saws in brain surgery). Especially various instruments in field surgery basically use permanent magnet DC motor.

8. Permanent magnet DC motors are used in handicapped products, such as manipulators and handicapped vehicles.
9. In terms of life, it is more useful. Even toothbrushes adopt permanent magnet DC motors to become electric toothbrushes.

With the development of the times, the application of permanent magnet DC motor will be more, and many occasions where AC motors were originally used will be replaced by permanent magnet DC motors. Especially after the emergence of permanent magnet brushless motors, the production quantity of permanent magnet DC motors is constantly increasing.

3. Selection Principle of Permanent Magnet DC Motor

Figure 5: DC motor.

3.1 Type Selection

It is advisable to give priority to ferrite permanent magnet DC motors with high efficiency, low price and low temperature rise. Alnico permanent magnet DC motors or rare earth permanent magnet DC motors are only considered under the occasions where the performance requirements are strict, the volume is small, and the ambient temperature is high.

3.2 Reasonable Selection of Motor Power

The maximum power output by the motor is limited. If the power selection of the motor is too small and the load exceeds the rated output power of the motor, the motor will be overloaded. When overloaded, the motor will heat up, vibrate, decrease in speed, and produce abnormal sound. Severe overload will burn the motor.
If the power is too large, it will cause economic waste. Therefore, it is very important to choose the power of the motor reasonably.

3.3 Specification Selection

Often due to the fact that there are not many product specifications in actual production, it is difficult to select products. When selecting product specifications, it can be considered: in the case of adjustable power supply voltage, the specifications with torque and speed close to the corresponding rated value of the product can be selected according to actual needs, and get the required speed by changing the voltage.
In the occasion of fixed supply voltage, if there is no appropriate specification of the product available, you can first choose the appropriate one according to the torque, and the voltage and speed of the product can be adjusted appropriately.

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