AF泡沫泵的機(jī)械特性
    在定子電壓、頻率和有關(guān)參數(shù)固定的條件下，電磁轉(zhuǎn)矩M。與轉(zhuǎn)速n(或轉(zhuǎn)差率s)之間的函數(shù)關(guān)系稱為三相異步電動機(jī)的固有機(jī)械特性,對應(yīng)的圖線如圖1-74所示。

三相異步電動機(jī)穩(wěn)定運行時的轉(zhuǎn)矩Me與阻轉(zhuǎn)矩(主要是機(jī)械負(fù)載轉(zhuǎn)矩和空載損耗轉(zhuǎn)矩)相平衡。若忽略空載損耗轉(zhuǎn)矩，已知電動機(jī)額定輸出的機(jī)械軸功率Pe(kW),則電動機(jī)額定輸出轉(zhuǎn)矩M.v、額定轉(zhuǎn)速IN與輸出機(jī)械軸功率P.的關(guān)系為:

從三相異步電動機(jī)的機(jī)械特性曲線可以看到，有三個重要的轉(zhuǎn)矩:

(1)額定轉(zhuǎn)矩M。N。它是當(dāng)電動機(jī)在額定電壓下，以額定轉(zhuǎn)速運行并輸出額定功率時，電動機(jī)轉(zhuǎn)軸上輸出的轉(zhuǎn)矩，可由電動機(jī)銘牌參數(shù)利用式(1-99)求得。
    (2)最大轉(zhuǎn)矩Memix.它反映了電動機(jī)帶動最大負(fù)載的能力。電動機(jī)的最大轉(zhuǎn)矩Mamx5與額定轉(zhuǎn)矩M、之比λ= Mx/MN,稱為過載系數(shù)，一般三相異步電動機(jī)的過載系數(shù)為1.8~2.2。在選用電動機(jī)時,必須考慮可能出現(xiàn)的最大負(fù)載轉(zhuǎn)矩，根據(jù)所選電動機(jī)的過載系數(shù)計算出電動機(jī)的最大轉(zhuǎn)矩，它必須大于最大負(fù)載轉(zhuǎn)矩。由于三相異步電動機(jī)的最大轉(zhuǎn)矩Mmax與電壓的平方成正比，所以Memx對電壓的波動很敏感，使用時要注意電壓的變化。
    (3)啟動轉(zhuǎn)矩Mesro它是電動機(jī)啟動時的轉(zhuǎn)矩，體現(xiàn)了電動機(jī)帶載啟動的能力，若啟動轉(zhuǎn)矩M.>M.(負(fù)載轉(zhuǎn)矩),電動機(jī)能啟動，否則將不能啟動。
    三相異步電動機(jī)的機(jī)械特性曲線同時也反映了電動機(jī)的自適應(yīng)負(fù)載能力，即電動機(jī)的電磁轉(zhuǎn)矩可以隨負(fù)載的變化而自動調(diào)整。例如，當(dāng)啟動時,Mx>M1.電動機(jī)啟動,轉(zhuǎn)速n逐漸升高，電磁轉(zhuǎn)矩M.也升高;到達(dá)b點后,M.達(dá)到最大;隨著n繼續(xù)升高,Me開始下降,沿ba線段進(jìn)行;當(dāng)轉(zhuǎn)速達(dá)到額定轉(zhuǎn)速nN時，電磁轉(zhuǎn)矩與負(fù)載轉(zhuǎn)矩相等,轉(zhuǎn)速不再升高，穩(wěn)定運行。如果外界負(fù)荷增大，即負(fù)載轉(zhuǎn)矩M.升高電磁轉(zhuǎn)矩M.會暫時小于M.，導(dǎo)致轉(zhuǎn)速n下降，而轉(zhuǎn)差率s則隨之升高，造成轉(zhuǎn)子線圈內(nèi)的電流升高，致使電磁轉(zhuǎn)矩升高，達(dá)到新的轉(zhuǎn)矩平衡后定運行。外界負(fù)荷降低的情況可類似分析。三相異步電動機(jī)正常工作在曲線ab段上。
    另外，通過改變電動機(jī)的有關(guān)參數(shù)可得到電動機(jī)的人為機(jī)械特性，例如,改變定子電壓U1、定子回路電阻或在定子回路中串入三相對稱電抗，可對三相異步電動機(jī)進(jìn)行調(diào)速。
4.三相異步電動機(jī)的調(diào)速
    由公式(1- 97)和式(1 - 98)得到三相異步電動機(jī)轉(zhuǎn)速n的表達(dá)式:
                             n=(1-s)60f1/p
式中 s----轉(zhuǎn)差率;
     f1---電源頻率;

p---磁極對數(shù)。

從式(1 - 100)可以看出，有三種途徑可以改變?nèi)喈惒诫妱訖C(jī)轉(zhuǎn)子的轉(zhuǎn)速:改變極對數(shù)p;改變電源頻率f.(變頻調(diào)速);改變轉(zhuǎn)差率s。下面重點介紹變頻調(diào)速技術(shù)。
    隨著交流變頻調(diào)速技術(shù)的發(fā)展,作為大容量傳動的高壓變頻調(diào)速技術(shù)也得到了廣泛的應(yīng)用。目前，高壓大功率電動機(jī)在電力、石化、供水、冶金、交通等行業(yè)得到了廣泛的應(yīng)用，其容量一般達(dá)幾百到幾千千瓦,其消耗的電功率已經(jīng)約占我國發(fā)電功率的30%，因此對它們進(jìn)行變頻調(diào)速節(jié)能改造,挖掘潛力巨大?，F(xiàn)場實驗表明,采用高壓變頻技術(shù),可有效地節(jié)約電能，便于AF泡沫泵電動機(jī)調(diào)速運行，提高管輸效益。因此，高壓大功率變頻器的研究和開發(fā)得到了廣泛重視。
    高壓大功率變頻器可以分為高一低一 高型變頻器和直接高壓型變頻器兩類。

Mechanical properties of AF foam pump

When the stator voltage, frequency and relevant parameters are fixed, the electromagnetic torque M. The function relationship with the speed n (or slip s) is called the inherent mechanical characteristics of three-phase asynchronous motor, and the corresponding diagram line is shown in figure 1-74.

The torque me of three-phase asynchronous motor in stable operation is balanced with the resistance torque (mainly mechanical load torque and no-load loss torque). If the no-load loss torque is ignored and the rated output mechanical shaft power PE (kw) of the motor is known, the relationship between the rated output torque M.V, rated speed in and the output mechanical shaft power P. is as follows:

From the mechanical characteristic curve of three-phase asynchronous motor, we can see that there are three important torques:

(1) Rated torque M. N。 It is the torque output on the motor shaft when the motor operates at rated speed and outputs rated power under rated voltage, which can be obtained from the motor nameplate parameters using equation (1-99).

(2) Maximum torque memix. It reflects the ability of the motor to drive the maximum load. The ratio of the maximum torque mamx5 of the motor to the rated torque M, λ = MX / Mn, is called the overload factor. Generally, the overload factor of three-phase asynchronous motor is 1.8 ~ 2.2. When selecting a motor, the possible maximum load torque must be considered. The maximum torque of the motor can be calculated according to the overload coefficient of the selected motor, which must be greater than the maximum load torque. Since the maximum torque mmax of three-phase asynchronous motor is directly proportional to the square of voltage, memx is very sensitive to the fluctuation of voltage. Pay attention to the change of voltage when using.

(3) Starting torque mesro is the starting torque of the motor, which reflects the starting ability of the motor with load. If the starting torque M. > M. (load torque), the motor can start, otherwise it will not start.

The mechanical characteristic curve of the three-phase asynchronous motor also reflects the adaptive load capacity of the motor, that is, the electromagnetic torque of the motor can be adjusted automatically with the change of load. For example, when starting, MX > M1. When the motor starts, the speed n gradually increases, and the electromagnetic torque M. also increases; when reaching point B, M. reaches the maximum; as n continues to increase, me begins to decline, along the Ba line segment; when the speed reaches the rated speed NN, the electromagnetic torque and the load torque are the same, and the speed no longer increases, stable operation. If the external load increases, i.e. the load torque M. increases, the electromagnetic torque M. will be temporarily smaller than M., resulting in the decrease of the speed n, and the slip s will increase, resulting in the increase of the current in the rotor coil, resulting in the increase of the electromagnetic torque, so as to achieve a new torque balance and then the constant operation. Similar analysis can be made for the reduction of external load. The three-phase asynchronous motor works normally on the curve AB section.

In addition, the artificial mechanical characteristics of the motor can be obtained by changing the relevant parameters of the motor, for example, changing the stator voltage U1, the resistance of the stator circuit or putting three-phase symmetrical reactance in series in the stator circuit can speed the three-phase asynchronous motor.

4. Speed regulation of three-phase asynchronous motor

According to formula (1-97) and formula (1-98), the e-xpression of speed n of three-phase asynchronous motor is obtained

n=(1-s)60f1/p

Where s - slip rate;

F1 --- power frequency;

P --- number of poles.

It can be seen from formula (1-100) that there are three ways to change the rotating speed of the three-phase asynchronous motor rotor: changing the pole pairs p; changing the power frequency f. (variable frequency speed regulation); changing the slip s. The following focuses on the frequency control technology.

With the development of AC frequency control technology, high-voltage frequency control technology, as a large capacity drive, has been widely used. At present, high-voltage high-power motors have been widely used in power, petrochemical, water supply, metallurgy, transportation and other industries. Their capacity is generally hundreds to thousands of kilowatts, and the electric power consumed by them has accounted for about 30% of the power generated in China. Therefore, their energy-saving transformation of frequency conversion and speed regulation has great potential. Field experiments show that the high voltage frequency conversion technology can effectively save energy, facilitate the speed regulation of AF foam pump motor and improve the efficiency of pipeline transportation. Therefore, the research and development of high-voltage and high-power inverter has been widely valued.

High voltage and high power inverter can be divided into two types: high low high type inverter and direct high voltage type inverter.