Basic knowledge of motor principle

1. Classification of low-power motors According to national standards, small-power motors refer to motors with a continuous rated power not exceeding 1.1kw when converted to 1500r/min (rev/min), also known as micro-motors or horsepower motors, which are classified as follows:

1. Low-power motors are divided into the following four types:

1) low power asynchronous motor;

1 three-phase asynchronous motor;

2 single-phase resistance starter motor;

3 single-phase capacitor starter motor;

4 single-phase capacitor running motor;

5 single-phase capacitor double-value motor;

6 shaded asynchronous motor.

2) low power synchronous motor;

1 permanent magnet synchronous motor;

2 reluctance synchronous motor;

3 hysteresis (zhi) synchronous motor;

3) low power DC motor;

1 brushed DC motor;

A, excitation DC motor; B, permanent magnet DC motor.

2 Brushless DC motor.

4) low power AC commutator motor;

1 single-phase series motor;

2 AC and DC motor;

3 Repel the motor. Second, the operating principle of the motor

1, Ampere's law of the law - the law of the whole current law (chan

The law of the current generating magnetic field is expressed by the formula: The above formula indicates that the loop L is closed along any one, and the line integral of the magnetic field strength is equal to the full current surrounded by the closed path.

Apply the full current law to the motor. Since the magnetic circuit of the motor can usually be divided into several sections according to different materials and geometrical dimensions, the magnetic field strength in each section is the same, so the above formula can be written as: Hi----i Magnetic field strength of segment magnetic circuit (A/m) Li----calculated length of magnetic path of i-th segment (m) Wi----magnetic potential (A) W---number of coil turns

The strength and direction of the magnetic field induced current are represented by the magnetic induction. The image depicts the magnetic field with a magnetic line. The magnetic field line is a closed curve. The direction of the magnetic field line and the current generating the magnetic field conform to the right-hand rule. The number of magnetic lines passing through the unit area is the induction intensity. B. The magnetic induction is not only related to the current, but also related to the surrounding medium. When the ferromagnetic material is placed around, the magnetic field is greatly enhanced because different media have different magnetic conductance. Rate, permeability is expressed by u0. The vacuum magnetic permeability magnetic field material u is several hundred to several thousand times of u0, and is related to the magnetic field strength, not constant. The relationship between the magnetic field strength and the magnetic induction intensity is H=B/u where B is the magnetic induction intensity (T). U - indicates magnetic permeability (H / m) H - represents magnetic field strength, also called magnetic potential (A / m) in a uniform magnetic field, the magnetic field line passing through the area S is defined as magnetic flux.

2. The law of electromagnetic force

It is stated that the current-carrying conductor in the magnetic field is subjected to electromagnetic force. When the magnetic field and the current-carrying conductor are perpendicular to each other, when the conductors are perpendicular to each other, the electromagnetic force acting on the conductor is f=BiL

F——electromagnetic force (N)

B———Magnetic induction (T)

i-----current of conductor (A)

L-----effective length of conductor (m)

The direction of the electromagnetic force f is determined by the left-hand rule: the magnetic flux points to the palm of the hand, the straight four finger refers to the current direction, and the vertical thumb refers to the electromagnetic direction.

3. The law of electromagnetic induction

Explain the law of the induced potential of the magnetic flux change.

(1) Inductive potential generated by varying magnetic flux - transformer potential

Where W - the number of turns of the coil intersecting the flux Φ - the flux of the link with the coil (Wb) t - time (s) e - the induced potential (V)

(2) Cutting potential, when the wire moves in the magnetic field and cuts the magnetic field line, the induced electromotive force is generated in the conductor: e=BL.V where B—magnetic induction intensity (T) L—the effective length of the wire V—the velocity of the wire perpendicular to the magnetic field

The direction of the induced potential of the upper type is determined by the right hand rule, the palm of the hand is facing the magnetic flux, the vertical thumb points to the direction of the conductor movement, and the parallel four fingers point to the direction of the induced potential.

4, the principle of conservation of energy

In a physical system of constant mass, energy is always conserved, and energy cannot be produced out of thin air, nor can it disappear in a vacuum, but only in a form that can be transformed. The balance of the energy of the motor is: the power input from the power supply = the increase of the magnetic field energy storage + the energy loss of the thermal energy + the output of the mechanical energy: the energy loss converted into thermal energy mainly includes three parts.

(1) The copper loss of the stator and rotor windings;

(2) the iron loss of the alternating magnetic flux in the iron core;

(3) Mechanical loss caused by ventilation and friction.

Third, the working principle of single-phase asynchronous motor

In the three points, when the stator three-phase winding is connected to the three-phase AC power supply, the symmetrical three-phase AC current is

The three-phase symmetrical winding circulates to generate a circular rotating magnetic potential and a magnetic field. The following figure shows that the current changes with time, and the magnetic potential and the magnetic field are rotated in space. The rotational speed is determined by the power supply frequency f and the number of motor poles p. n=2x60/p*f where n—rotating magnetic speed (r/min) p—number of motor poles

f—Power supply frequency (HZ) In a single-phase motor, the motor does not have a starting torque due to the pulsed magnetic field generated by the single-phase winding, and cannot be started, as shown in the following figure:

Therefore, two windings are embedded in the stator core of the single-phase motor and the axes thereof are separated by 90° in the space. The wire diameter and the number of turns of the two-phase winding are different. One of the windings is called the main winding (represented by M). ). The other winding is called the pay winding (indicated by A). The winding iron phase shifting component is connected to the power source. The structural principle is as follows:

In a single-phase motor, if the main and two-phase windings on the stator are completely symmetrical, and the two-phase winding is connected to two symmetrical power sources, the circular rotating magnetic potential and magnetic field rotating in space are generated as in the three-phase motor, as follows. Figure:

It can be seen that the rotational magnetic potential generated by the symmetrical windings entering the symmetrical two-phase current is the same as the rotating magnetic potential generated by the three-phase motor. Its rotational speed is related to the power supply frequency and the number of motor poles: n=120f/p

A plurality of squirrel-cage aluminum guide bars that are connected to each other are cast in the rotor of the single-phase motor.

When the magnetic field in the motor rotates at n speed, the rotor bar in the rotating magnetic field will cut the magnetic lines of force to generate the induced potential and the induced current, and the induced current generates electromagnetic force and electromagnetic torque under the action of the magnetic field, forming a certain rotational speed n' . In general, the motor speed n' is not equal to the rotating magnetic field speed n. Because n'=n, the rotor bar is stationary relative to the rotating magnetic field, the induced potential and the induced current are not generated in the bar, the motor will not have electromagnetic torque, and the motor's rotation speed will naturally drop. Since the rotor speed is always lower than the rotating magnetic field speed, the motor is a "single-phase asynchronous motor".

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