Motor principle and several important formulas

★ principle of motor: the principle of motor is very simple. In short, it is a device that uses electric energy to generate rotating magnetic field on the coil and drive the rotor to rotate. Anyone who has learned the law of electromagnetic induction knows that the energized coil will rotate under force in the magnetic field. This is the basic principle of the motor. This is the knowledge of junior high school physics.

★ motor structure: as anyone who has disassembled the motor knows, the motor is mainly composed of two parts, a fixed stator and a rotating rotor, as follows:

1. Stator (stationary part)

Stator core: an important part of the magnetic circuit of the motor, on which the stator winding is placed;

Stator winding: it is the coil, the circuit part of the motor, which is connected to the power supply to generate a rotating magnetic field;

Base: fix the stator core and motor end cover, and play the role of protection and heat dissipation;

2. Rotor (rotating part)

Rotor core: an important part of the magnetic circuit of the motor. The rotor winding is placed in the core slot;

Rotor winding: cutting the stator rotating magnetic field to generate induced electromotive force and current, and forming electromagnetic torque to make the motor rotate;

★ several calculation formulas of motor:

1. Electromagnetically related

1) Induction electromotive force formula of motor: e=4.44*f*n* Φ， E is the coil electromotive force, f is the frequency, s is the cross-sectional area of the surrounding conductor (such as iron core), n is the number of turns Φ It's magnetic flux.

We won't delve into how the formula is derived. We mainly look at how to use it. Induced electromotive force is the essence of electromagnetic induction. After the conductor with induced electromotive force is closed, induced current will be generated. The induced current will be subjected to ampere force in the magnetic field, producing magnetic moment, which will drive the coil to rotate.

According to the above formula, the electromotive force is directly proportional to the power frequency, coil turns and magnetic flux.

Magnetic flux calculation formula Φ= B*S*COS θ， When the plane with area s is perpendicular to the direction of the magnetic field, the angle θ 0, cos θ Equal to 1, the formula becomes Φ= B*S。

Combining the above two formulas, we can get the calculation formula of motor magnetic flux strength as follows:

B=E/（4.44*f*N*S）。

2) The other is the ampere force formula. If we want to know the force on the coil, we need this formula f=i*l*b*sin α， Where I is the current intensity, l is the conductor length, and B is the magnetic field intensity, α Is the angle between the current direction and the magnetic field direction. When the wire is perpendicular to the magnetic field, the formula becomes f=i*l*b (if it is an n-turn coil, the magnetic flux B is the total magnetic flux of the n-turn coil, and there is no need to multiply n).

When you know the force, you know the torque. The torque is equal to the torque multiplied by the radius of action, t=r*f=r*i*b*l (vector product). Through the two formulas of power = force * speed (p=f*v) and linear speed v=2 π r* speed per second (N seconds), the relationship can be established with power, and the formula of No. 3 below is obtained. However, it should be noted that the actual output torque is used at this time, so the calculated power is the output power.

2. Speed calculation formula of AC asynchronous motor: n=60f/p

This is very simple. The speed is directly proportional to the power frequency and inversely proportional to the number of motor pole pairs (remember it is a pair). Just apply the formula directly. However, this formula actually calculates the synchronous speed (rotating magnetic field speed), and the actual speed of asynchronous motors will be slightly lower than the synchronous speed, so we often see that 4-pole motors generally have more than 1400 revolutions, less than 1500 revolutions.

3. Relationship between motor torque, power and speed: t=9550p/n (P is motor power, n is motor speed)

It can be deduced from the content of No. 1 above, but we don't need to learn to deduce. Just remember this calculation formula. However, it is reminded again that the power P in the formula is not the input power, but the output power. Due to the loss of the motor, the input power is not equal to the output power. But books tend to idealize that the input power is equal to the output power.

4. Motor power (input power):

1) Single phase motor power calculation formula: p=u*i*cos φ， If the power factor is 0.8, the voltage is 220V, and the current is 2a, then the power p=0.22 × two × 0.8=0.352KW。

2) Three phase motor power calculation formula: p=1.732*u*i*cos φ （cos φ Is the power factor, u is the load line voltage, I is the load line current)

However, such u and I are related to the connection method of the motor. In the star connection method, since the common ends of three coils with a voltage of 120 ° apart are connected together to form a 0 point, the voltage loaded on the load coil is actually the phase voltage; In triangle connection, a power line is connected at both ends of each coil, so the voltage on the loaded coil is the line voltage. If we use the commonly used 3-phase 380V voltage, the coil is 220V in the star connection method, while the triangle is 380V, p=u*i=u^2/r, so the power in the triangle connection method is 3 times that in the star connection method, which is why the star delta step-down starting is used for high-power motors.