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Basic knowledge of motors

2022/09/22

Motors, generally referred to as electric motors, also known as motors, are extremely common in modern industry and life, and are also the most important equipment for converting electrical energy into mechanical energy. Motors are installed in cars, high-speed trains, airplanes, wind turbines, robots, automatic doors, water pumps, hard drives and even our most common cell phones.



Many people who are new to motors or who have just learned the knowledge of motor driving may feel that the knowledge of motors is difficult to understand, and even see the relevant courses, and they are called "credit killers". The following scattered sharing can let novices quickly understand the principle of AC asynchronous motor.


★ The principle of the motor: The principle of the motor is very simple. Simply put, it is a device that uses electric energy to generate a rotating magnetic field on the coil and pushes the rotor to rotate. Anyone who has studied the law of electromagnetic induction knows that an energized coil will be forced to rotate in a magnetic field. This is the basic principle of a motor. This 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 (static part) stator core: an important part of the magnetic circuit of the motor, and the stator winding is placed on it; the stator winding: is the coil, the circuit part of the motor, connected to the power supply, used to generate a rotating magnetic field; base: fixed stator core And the 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 slot;


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




★ Several calculation formulas of the motor:


1. Electromagnetic related


1) The formula of the induced electromotive force 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 Pass.


How the formula is derived, we will not delve into these things, we will mainly see how to use it. Induced electromotive force is the essence of electromagnetic induction. After the conductor with induced electromotive force is closed, an induced current will be generated. The induced current is subjected to an ampere force in the magnetic field, creating a magnetic moment that pushes the coil to turn.


It is known from the above formula that the magnitude of the electromotive force is proportional to the frequency of the power supply, the number of turns of the coil and the magnetic flux.


The magnetic flux calculation formula Φ=B*S*COSθ, when the plane with area S is perpendicular to the direction of the magnetic field, the angle θ is 0, COSθ is equal to 1, and the formula becomes Φ=B*S.



Combining the above two formulas, you can get the formula for calculating the magnetic flux intensity of the motor: B=E/(4.44*f*N*S).


2) The other is the Ampere force formula. To know how much force the coil is receiving, we need this formula F=I*L*B*sinα, where I is the current strength, L is the conductor length, B is the magnetic field strength, α is the angle between the direction of the current and the direction of the magnetic field. 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).


If you know the force, you will 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 = 2πR * speed per second (n seconds), the relationship with power can be established, and the formula of the following No. 3 can be 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. The calculation formula of the speed of the AC asynchronous motor: n=60f/P, this is very simple, the speed is proportional to the frequency of the power supply, and inversely proportional to the number of pole pairs (remember a pair) of the motor, 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 rpm, but less than 1500 rpm.


3. The relationship between motor torque and power meter speed: T=9550P/n (P is motor power, n is motor speed), which can be deduced from the content of No. 1 above, but we don’t need to learn to deduce, remember this calculation A formula will do. But remind again, 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 are often idealized, and 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) Three-phase motor power calculation formula: 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, U and I of this type are related to the connection of the motor. In star connection, since the common ends of the three coils separated by 120° voltage are connected together to form a 0 point, the voltage loaded on the load coil is actually phase-to-phase. When the delta connection method is used, a power line is connected to each end of each coil, so the voltage on the load coil is the line voltage. If the commonly used 3-phase 380V voltage is used, the coil is 220V in star connection, and the delta is 380V, P=U*I=U^2/R, so the power in delta connection is star connection 3 times, which is why the high-power motor uses star-delta step-down to start.


After mastering the above formula and understanding thoroughly, the principle of the motor will not be confused, nor will you be afraid of learning the high-level course of motor driving.


★ Other parts of the motor



1) Fan: generally installed at the tail of the motor to dissipate heat to the motor;

2) Junction box: used to connect to the power supply, such as AC three-phase asynchronous motor, it can also be connected to star or delta according to needs;

3) Bearing: connecting the rotating and stationary parts of the motor;

4. End cover: The front and rear covers outside the motor play a supporting role.



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