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Basic knowledge of motors

Views: 240     Author: Site Editor     Publish Time: 2022-07-15      Origin: Site

学习一下电机知识的几个公式,给自己充充电!

电机,一般指电动机,也称马达,是现代化工业及生活中极为普遍的东西,也是将电能变为机械能的最主要设备。汽车、高铁、飞机、风机、机器人、自动门、水泵、硬盘甚至我们最普遍拥有的手机,都安装了电机。

   

 

很多初接触电机的或者刚学习电机拖动知识的,可能会觉得电机知识不好理解,甚至看到相关的课程就头大,有着“学分杀手”的称呼。下面通过零散式分享,可以让新手快速了解交流异步电机原理。

 

★电机的原理:电机的原理很简单,简单的说就是利用电能在线圈上产生旋转磁场,并推动转子转动的装置。学过电磁感应定律的都知道,通电的线圈在磁场中会受力转动,电机的基本原理就是如此,这是初中物理的知识。

 

★电机结构:拆开过电机的人都知道,电机主要是两部分组成,固定不动的定子部分以及转动的转子部分,具体如下:

 

1、定子(静止部分)定子铁心:电机磁路重要部分,并在其上放置定子绕组;定子绕组:就是线圈,电动机的电路部分,接电源,用于产生旋转磁场;机座:固定定子铁心及电机端盖,并起防护、散热等作用;

 

 

2、转子(旋转部分)转子铁心:电机磁路的重要部分,在铁心槽内放置转子绕组;
转子绕组:切割定子旋转磁场产生感应电动势及电流,并形成电磁转矩从而使电动机旋转;

 

★电机的几个计算公式:

1、电磁相关的

1)电动机的感应电动势公式:E=4.44*f*N*Φ,E为线圈电动势、 f为频率 、 S为环绕出的导体(比如铁芯)横截面积、 N为匝数、Φ是磁通。

 

公式是怎么推导来的,这些事情我们就不去钻研了,我们主要是看看怎么利用它。感应电动势是电磁感应的本质,有感应电动势的导体闭合后,就会产生感应电流。感应电流在磁场中就会受到安培力,产生磁矩,从而推动线圈转动。

 

从上面公式知道,电动势大小与电源频率、线圈匝数及磁通量成正比。

 

磁通量计算公式Φ=B*S*COSθ,当面积为S的平面与磁场方向垂直的时候,角θ为0,COSθ就等于1,公式就变成Φ=B*S。

将上面两个公式结合一下,就可以得到电机磁通强度计算公式为:B=E/(4.44*f*N*S)。

 

2)另外一个是安培力公式,我们要知道线圈受到的力是多少,就需要这个公式F=I*L*B*sinα,其中I为电流强度,L为导体长度,B为磁场强度,α是电流方向与磁场方向间的夹角。当导线垂直于磁场时候,公式就变成F=I*L*B了(如果是N匝线圈的话,磁通B就是N匝线圈的总磁通,而不需要再乘N了)。

 

知道了受力,就知道转矩,转矩等于扭力乘以作用半径,T=r*F=r*I*B*L(向量乘积)。通过功率=力*速度(P=F*V)以及线速度V=2πR*每秒转速(n秒)两个公式 ,可以与功率建立上关系,得到下面序号3的公式。不过要注意,这时候使用实际输出扭矩,所以计算出的功率是输出功率。

 

2、交流异步电机的转速计算公式:n=60f/P,这个很简单,转速与电源频率成正比,与电机极对子(记住是一对)数成反比,直接套用公式就好。不过这个公式实际计算出是同步转速(旋转磁场速度),异步电机实际转速会略低于同步转速,所以我们往往会看到4极电机一般是1400多转,达不到1500转。

 

3、电机转矩、功率计转速的关系:T=9550P/n(P是电机功率、n是电机转速),可以从上面序号1内容中推导出来,不过我们没必要学会推导,记住这个计算公式就可以。不过再次提醒,公式中功率P不是输入功率,而是输出功率,由于电机有损耗,输入功率不等于输出功率。但是书本上往往理想化,将输入功率等于输出功率了。

 

4、电机功率(输入功率):

1)单相电机功率计算公式:P=U*I*cosφ,如果功率因数为0.8,电压为220V,电流为2A,那么功率P=0.22×2×0.8=0.352KW。

 

2)三相电机功率计算公式:P=1.732*U*I*cosφ(cosφ为功率因素、U为负载线电压、I为负载线电流)。不过这类的U和I与电机的接法有关,星形接法的时候,由于三个相隔120°电压的线圈公共端连接一起,形成一个0点,所以加载在负载线圈的电压实际是相电压;而三角形接法时,每个线圈两端各连一根电源线,所以加载负载线圈上的电压就是线电压。如果使用的是我们常用的3相380V电压,星形接法时候线圈是220V,而三角形则是380V,P=U*I=U^2/R,所以三角形接法时功率是星形接法的3倍,这也就是为什么大功率电机采用星三角降压启动的原因。

 

掌握了上面的公式,理解透彻,电机的原理就不会在困惑了,也不会在怕学习电机拖动这种高挂科的课程。

 

★电机的其他部件 

1)风扇:一般安装在电机尾部,用于给电机散热;

 

2)接线盒:用于接入电源,如交流三相异步电机,还可以根据需要接星形或者三角形;

 

3)轴承:连接电机旋转和不动部分;

 

4、端盖:电机外面的前后盖子,起支撑作用。




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.