ELECTRICAL MACHINES

ELECTRICAL MACHINES

MOTOR

Electric machine (Electric Machine) is a general term for a conversion device between mechanical energy and electrical energy. The conversion is bidirectional, and the principle of electromagnetic induction is mostly applied. A motor that converts mechanical energy into electrical energy is usually called a "generator"; a motor that converts electrical energy into mechanical energy is called a "motor". Other new types of motors have emerged, such as ultrasonic motors (using the piezoelectric effect), without the principle of electromagnetic induction. However, stationary motors refer to transformers.

DC

The working principle of the DC generator is to convert the alternating electromotive force induced in the armature coil, the principle that the commutator cooperates with the commutation function of the brush to make it become a DC electromotive force when it is drawn from the brush end.

The direction of the induced electromotive force is determined according to the rule of the right hand (the magnetic induction line points to the palm of the hand, the thumb points to the direction of the conductor movement, and the other four fingers point to the direction of the induced electromotive force in the conductor.

WORKING PRINCIPLE

The direction of the conductor force is determined by the left-hand rule. This pair of electromagnetic force forms moment acting on the armature. This moment is called electromagnetic torque in the rotating electrical machine. The direction of the torque is counterclockwise, which attempts to rotate the armature counterclockwise. If this electromagnetic torque can overcome the resistance torque on the armature (such as resistance torque caused by friction and other load torque), the armature can rotate in a counterclockwise direction.

DC motors are motors that operate on DC operating voltage and are widely used in recorders, VCRs, DVD players, electric shavers, hair dryers, electronic watches, toys, etc.

ELECTROMAGNETIC

Electromagnetic DC motor is composed of stator poles, rotor (armature), commutator (commonly known as commutator), brushes, casing, bearings, etc.

The stator pole (main pole) of an electromagnetic DC motor is composed of an iron core and an excitation winding. According to the different ways of excitation (the old standard is called excitation), it can be divided into series excitation DC motor, parallel excitation DC motor, other excitation DC motor and compound excitation DC motor. Due to the different excitation methods, the law of the stator pole magnetic flux (generated after the stator pole excitation coil is energized) is also different.

The field winding and the rotor winding of the series-excited DC motor are connected in series through the brush and the commutator. The excitation current is proportional to the armature current. The magnetic flux of the stator increases with the increase of the excitation current. The torque is approximately The Square of the armature current is proportional to the speed, and the speed drops rapidly with the increase of torque or current. The starting torque can be more than 5 times the rated torque, the short-term overload torque can be more than 4 times the rated torque, the speed change rate is large, and the no-load speed is very high (generally it is not allowed to run under no-load). Speed   regulation can be achieved by using external resistors in series (or parallel) with the series winding or by connecting the series winding in parallel.

The excitation winding of the parallel-excited DC motor is connected in parallel with the rotor winding, the excitation current is relatively constant, the starting torque is proportional to the armature current, and the starting current is about 2.5 times the rated current. The speed decreases slightly with the increase of current and torque, and the short-term overload torque is 1.5 times the rated torque. The speed change rate is relatively small, 5%~15%. The speed can be adjusted by the constant power that weakens the magnetic field.

The excitation winding of the separately excited DC motor is connected to an independent excitation power supply, the excitation current is also relatively constant, and the starting torque is proportional to the armature current. The speed change is also 5%~15%. The speed can be increased by weakening the magnetic field constant power or by reducing the voltage of the rotor winding.

In addition to the parallel excitation winding, the stator pole of the compound excitation DC motor is also equipped with a series excitation winding (which has fewer turns) connected in series with the rotor winding.

The direction of the magnetic flux generated by the series winding is the same as the direction of the magnetic flux of the main winding, the starting torque is about 4 times the rated torque, and the short-term overload torque is about 3.5 times the rated torque. The speed change rate is 25%~30% (related to the series winding). The speed can be adjusted by weakening the magnetic field strength.

The commutator segments of the commutator use silver-copper, cadmium-copper and other alloy materials and are molded with high-strength plastic. The brush is in sliding contact with the commutator and provides armature current for the rotor winding. Electromagnetic DC motor brushes generally use metal graphite brushes or electrochemical graphite brushes.

The iron core of the rotor is formed by laminating silicon steel sheets. It is generally 12 slots, and 12 sets of armature windings are embedded. After the windings are connected in series, they are respectively connected with 12 commutating sheets.

DC

The excitation method of the DC motor refers to the problem of how to supply power to the excitation winding and generate the excitation magnetomotive force to establish the main magnetic field. According to the different excitation methods, DC motors can be divided into the following types.

HELI

The excitation winding has no connection with the armature winding, and the DC motor powered by the other DC power supply to the excitation winding is called a separately excited DC motor.

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