Remember the principle of the motor and several important formulas, and figure out how easy it is to figure out the electric motor!
Electric Motor, generally refers to the motor, also known as the motor, is a very common thing in modern industry and life, and it is also the most important equipment to turn electrical energy into mechanical energy.
Electric Motors are installed in cars, high-speed rail, airplanes, wind turbines, robots, automatic doors, water pumps, hard drives, and even the mobile phones we most commonly have.
Many people who are new to motors or have just learned the knowledge of motor drag may feel that the knowledge of motors is not easy to understand, and even have a big head when they see related courses, and they are called "credit killers".
The following is a scattered sharing, which can allow novices to quickly understand the principle of AC asynchronous motors.
The principle of the motor
The principle of the motor is very simple, simply put, it is a device that uses electrical energy to generate a rotating magnetic field on the coil and push the rotor to rotate. Anyone who has studied the law of electromagnetic induction knows that an energized coil will rotate under force in a magnetic field, and the basic principle of the motor is like this, which is the knowledge of junior high school physics.
Motor structure
Anyone who has disassembled the motor knows that the motor is mainly composed of two parts, the fixed stator part and the rotating rotor part, as follows:
1. Stator (stationary part)
Stator core: an important part of the magnetic circuit of the motor, and the stator winding is placed on it;
Stator winding: that is, the coil, the circuit part of the motor, connected to the power supply, used to generate a rotating magnetic field;
Frame: fix the stator core and motor end cover, and play the role of protection, heat dissipation, etc.;
2. Rotor (rotating part)
Rotor core: an important part of the magnetic circuit of the motor, the rotor winding is placed in the core groove;
Rotor winding: cut the stator to rotate the magnetic field to generate induced electromotive force and current, and form electromagnetic torque to rotate the motor;
Several formulas for calculating the motor
1. Electromagnetic related
1) The induced electromotive force formula of the 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 the iron core), N is the number of turns, and Φ is the magnetic flux.
We won't delve into how the formula is derived, we'll just look at how to use it. Induced electromotive force is the essence of electromagnetic induction, and when a conductor with induced electromotive force is closed, an induced current is generated.
When the induced current is subjected to ampere force in the magnetic field, a magnetic moment is generated, which pushes the coil to rotate.
From the above formula, we know that the magnitude of the electromotive force is proportional to the frequency of the power supply, the number of coil turns and the magnetic flux.
The formula for calculating the magnetic flux is Φ=B*S*COSθ, when the plane with an area of S is perpendicular to the direction of the magnetic fi
eld, the angle θ is 0, and COSθ is equal to 1, and the formula becomes Φ=B*S
Combining the above two formulas, we can get the formula for calculating the magnetic flux intensity of the motor as: B=E/(4.44*f*N*S).
2) The other is the ampere force formula, we need to know how much force the coil is subjected to, we need this formula F=I*L*B*sinα, where I is the current intensity, L is the conductor length,
B is the magnetic field strength, and 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 (in the case of an N-turn coil, the magnetic flux B is the total magnetic flux of the N-turn coil, without multiplying by N). Knowing the force,
we know the torque, which 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 * velocity (P = F * V) and linear velocity V = 2πR * speed per second (n seconds), the relationship with power can be established, and the formula of sequence number 3 below can be obtained.
However, it should be noted that the actual output torque is used in this case, so the calculated power is the output power.
2. The speed calculation formula of AC asynchronous motor
n=60f/p, this is very simple, the speed is proportional to the frequency of the power supply, and the number of pole pairs of the motor (remember to be a pair) is inversely proportional, just apply the formula directly.
However, this formula actually calculates the synchronous speed (rotating magnetic field speed), and the actual speed of the asynchronous motor will be slightly lower than the synchronous speed, so we often see that the 4-pole motor is generally more than 1400 revolutions, and cannot reach 1500 revolutions.
3. The relationship between motor torque and power meter speed
T = 9550P/n (P is the motor power, n is the motor speed), which can be deduced from the content of serial number 1 above, but we don't need to learn to derive, just remember this calculation formula.
However, again, the power P in the formula is not the input power, but the output power, and the input power is not equal to the output power due to the loss of the motor. But books often 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*cosφ, if the power factor is 0.8, the voltage is 220V, and the current is 2A, then the power P=0.22×2×0.8=0.352KW.
2) The power calculation formula of three-phase motor: P=1.732*U*I*cosφ (cosφ is the power factor, U is the load line voltage, and I is the load line current).
However, this kind of you and I are related to the connection method of the motor, and when the star connection method, because the common end of the three coils separated by 120 ° voltage is connected together to form a 0 point, the voltage loaded in the load coil is actually the phase voltage;
In the triangle connection, a power cord is connected to each end of each coil, so the voltage on the loaded load coil is the line voltage.
If we use the commonly used 3-phase 380V voltage, the coil is 220V when the star connection is used, and the triangle is 380V, P=U*I=U^2/R, so the power of the triangle connection is 3 times that of the star connection, which is why the high-power motor uses the star delta step-down start.
If you master the above formula and understand it thoroughly, you will not be confused about the principle of the motor, and you will not be afraid to learn the high-hanging course of motor dragging.
Other parts of the motor
1. Fan
It is generally installed at the tail of the motor and is used to dissipate heat from the motor;
2. terminal box
It is used to access power supplies, such as AC three-phase asynchronous motors, and can also be connected to stars or triangles as needed;
3. Bearings
Connecting the rotating and immobile parts of the motor;
4. an end cap
The front and rear covers on the outside of the motor play a supporting role.
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