1. Definition

A ac motor is a rotating machine that converts direct current energy into mechanical energy (dc motor) or mechanical energy into direct current energy (dc generator). It is a motor which can realize the mutual conversion of direct current electrical energy and mechanical energy. When it operates as a motor, it is a direct current motor, which converts electrical energy into mechanical energy. When operating a generator, it is a dc generator, which converts mechanical energy into electrical energy.

2. Characteristics

The characteristics [n=f(T)] of the motor's speed n changing with the torque T are called mechanical characteristics. It is an important basis for selecting motor. All kinds of electric motors are suitable for different occasions due to their own mechanical characteristics.

From the perspective of the armature circuit of dc motor, the voltage drop on the armature circuit resistance Rd must be balanced between the supply voltage U and the reverse electromotive force Ed and the armature current Id of the motor. U = Ed + IdRd

3. Classification

The excitation mode of dc motor refers to the problem of how to supply power and generate excitation flux potential to establish the main magnetic field. According to different excitation modes, dc motors can be divided into the following types:

• Excitating Way

1. separately excited dc motor

There is no connection between the excitation winding and the armature winding, and the dc motor powered by other dc power sources to the excitation winding is called the separately excited dc motor. In the figure, M is the motor, or G is the generator. Permanent magnet dc motor can also be regarded as separately excited dc motor.

shunt dcmotor

The excitation winding of the shunt dc motor is in parallel with the armature winding.  As a shunt excited generator, it is the terminal voltage from the motor itself that provides power to the excitation winding. As a shunt motor, the excitation windings share the same power supply with the armature, and the performance is the same as that of a shunt dc motor.

series wound dcmotor

After the excitation winding of the dc motor is connected in series with the armature winding, it is connected to the dc power supply. The excitation current of the dc motor is the armature current.

1. compound dcmachine

Compound dc motor has two excitation windings, parallel excitation and series excitation. The flux potential generated by the series winding is in the same direction as that generated by the shunt winding.If the two flux potentials are in opposite directions, they are called differential compounding.

• There are/no brush classes

1. Brushless dc motor: brushless dc motor is the ordinary dc motor stator and rotor for the exchange.Its rotor is permanent magnet to generate air gap flux: the stator is armature, composed of multiphase windings.In structure, it is similar to permanent magnet synchronous motor.
2. A brush dc motor: there are two brush brush motor (copper brush or carbon brush) is directly by insulating seat fixed on the motor back cover will be introduced to the power supply is negative on the commutation of the rotor, and the commutation device connected on the rotor coil, 3 coil constant alternating polarity with fixed on shell form 2 magnet force and rolling. Since the commutator is fixed to the rotor and the brush to the housing (stator), the constant friction between the brush and the commutator as the motor rotates generates a great deal of resistance and heat. So the brush-motor has very low efficiency and very high loss. But it also has the advantage of simplicity and low cost.

4. Performance Related

The performance of dc motor is closely related to its excitation mode. Generally, there are four excitation modes of dc motor: dc separately excited motor, dc shunt excited motor, dc series excited motor and dc compound excited motor. Master the characteristics of each of the four ways.

(1). Dc separately excited motor: the field winding has no electrical connection with the armature, and the field circuit is supplied by an additional dc source. Therefore, excitation current is not affected by armature terminal voltage or armature current.

(2). Dc shunt motor: the voltage at both ends of the shunt winding is the voltage at both ends of the armature, but excited

The winding is made of thin wires with a large number of turns and hence a large resistance, making the field current through it small.

(3). Dc series motor: current series, partial voltage, excitation winding is in series with armature, so the internal magnetic field of this motor changes significantly with the change of armature current. In order to avoid large losses and voltage drops in the field winding, the smaller the resistance, the better. Therefore, dc series motors are usually wound with thicker wires with fewer turns.

(4). Dc compound motor: the flux of the motor is generated by the excitation current in the two windings.

5. Advantages of dc motor

Good starting and speed regulation performance, wide and smooth speed regulation range, strong overload capacity, and little impact by electromagnetic interference; Dc motor has good starting characteristics and speed regulation characteristics; dc motor torque is relatively large, Cheaper maintenance; Compared with ac, dc motor is energy-saving and environment-friendly.

6. Application

Due to the unique advantages of dc motor, more countries have carried out special research on dc motor, and more and more related high-quality products appear. It is believed that the linear motor will be widely used in various fields in the near future.

• Maglev trains and subways, which can be driven by linear motors in the transportation industry, have the advantages of high speed, comfort, safety and no pollution, and will play an important role in the emerging transportation vehicles.
• In industry, dc motor has unique advantages in dc transmission and material transportation, such as sorting and conveying line, lift, etc. In various industrial machine tools, dc motor can be widely used instead of rotary motor, mainly to take advantage of the characteristics of fast air speed and high precision, such as linear motor driven punching machine, die casting machine, edm machine, etc.
• In civilian respect a few things of daily use are like home appliance (air conditioning, freezer, etc.), drive door to wait to be able to go up dc motor.
• In the military it is also possible to apply gas to missiles, rockets and artillery using the very high speed of dc motors.

Dc motor has its unique characteristics, but also has its own shortcomings, in the practical application of research should be weighed advantages and disadvantages to choose to give full play to the advantages of dc motor occasions. Through the application of dc motor, promote the progress of various fields of technology.

At present, we are trying to apply the linear motor in the edm machine, through the dc servo system advanced technology into the mature edm technology, so as to greatly improve the edm performance.