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Application analysis of AC motor variable frequency governor

this paper summarizes some experience on the correct selection of capacity, optimal design of transmission system and external braking resistance of variable frequency governor in practical application

with the rapid development of power technology, AC motor variable frequency speed regulation technology has made breakthrough progress and entered the stage of popularization and application. In China, variable frequency governors are also being more and more widely used. At the same time, how to correctly select and use them has become a very prominent problem for the majority of users

1. About capacity selection

in the instructions of variable frequency governor, in order to help users choose capacity, there is a column of motor capacity. However, the meaning of this column is not accurate enough, which often leads to the wrong selection of frequency converter

in various production machines, the capacity of the motor is mainly selected according to the heating principle. That is, on the premise that the motor can be driven, as long as its temperature rise is within the allowable range, short-term overload is allowed. The overload capacity of the motor is generally set as 1% of the rated torque 2 times. The temperature rise of the motor is at least more than ten minutes in a so-called short time. The overload capacity of the frequency converter is 150%, 1 minute. This indicator is meaningful for the motor only in the starting process. In fact, the load is not allowed in the operation process

therefore, the exact meaning of the column "capacity of the equipped motor" is the actual maximum capacity of the equipped motor. When actually selecting the frequency converter, it can be selected according to the maximum current of the current increasing production enterprises of the motor in the working process, most of which are emerging micro enterprises. For the loads of blowers and pumps, because they are long-term constant loads, they can be directly selected according to the capacity of the motor

2. Transmission system optimization design

the range of effective torque and effective power of AC asynchronous motor after variable frequency speed regulation. When using variable frequency governor, the transmission system must be optimally designed according to the mechanical characteristics of production machinery and the requirements for speed regulation range. The main contents and approximate methods of optimal design are as follows:

2.1 determine the maximum operating frequency of the motor

(1) blower and pump loads. The resistance torque TL of this kind of load is directly proportional to the square of speed n, tl=ktn2, The output power pl is directly proportional to the power of the speed pl=kpn3, (KT and KP are constants), so it can be seen that if the speed exceeds the rated speed, the torque and power of the load will increase according to the square law and the cubic law respectively. Therefore, in general, when the company's tension machine transmission system adopts full arc synchronous belt deceleration, it is not allowed to operate above the rated frequency

(2) generally, the strength, vibration and wear resistance of various machines are designed on the premise that the motor speed does not exceed 3000r/min. Therefore, without redesigning the machinery, the maximum operating frequency of the class 2 motor should not exceed the rated frequency too much

(3) when the asynchronous motor operates above the rated frequency, because the power supply voltage is constant, when it is adjusted to FX, the electromagnetic torque TX is almost inversely proportional to the square of the frequency adjustment ratio KF, that is, t tn/kf2 (and TN is the torque at the rated frequency FN). Therefore, the maximum operating frequency should not exceed the rated frequency

(4) when the asynchronous motor operates at low frequency, torque compensation is often needed to obtain sufficient torque. Torque compensation will saturate the magnetic circuit of the motor, thus increasing additional losses and reducing efficiency. Therefore, as long as the situation permits, the upper limit of the operating frequency should be increased as much as possible

2.2 determine the transmission ratio of the transmission system and check the capacity of the motor

(1) the loads of blowers and pumps are generally direct drive, so it is unnecessary to consider the transmission ratio

(2) constant torque load. First, determine the highest and lowest frequency of motor operation according to the effective torque line and the required frequency adjustment range

assuming that the determined maximum operating frequency of the motor is Fmax and the minimum operating frequency is Fmin, the relative value of the corresponding torque is TTL, then the rated torque of the motor tn=tl/qtl (TL load torque). If there is no margin left for the original motor, the capacity of the motor should be increased by 1/ttl times after the frequency converter is equipped. The transmission ratio of the transmission system is equal to the speed ndmax of the motor at the highest operating frequency, and the ratio of the maximum speed nlmax required by the load

(3) constant power load: similar to constant torque load, first calculate the upper and lower limits of motor operating frequency according to the effective power line and frequency adjustment range

similarly, while calculating the highest and lowest operating frequencies, the corresponding relative value of power TPL is obtained, and the rated power of the motor PN PL/TPL (PL is the required power of the load)

when designing constant power load, two points should be paid attention to: (1) try to use the part above the rated frequency as much as possible; (2) When the adjustment range is large, try to use the two gear transmission ratio. Because when the transmission ratio is divided into two columns, the relationship between the frequency range f and the speed range n is. It can be seen that when the speed range is the same, the frequency range will be greatly reduced, which can reduce the capacity of the motor

the mechanical characteristics of the load are constant power loads, so the product of the abscissa and ordinate of any point on the curve is equal and proportional to the load power, that is, pl=kptlnl=kptlmaxlmin. The effective torque line when all rotating speeds are adjusted below the rated frequency. In this case, the required motor capacity pn=kptnnlmax kptlmaxlmax= NPL. This shows that the required motor capacity is larger than on times the load power, which is very uneconomical

⑴ when the maximum operating frequency is twice the rated frequency, it is said that the experimental machine of Jinan experimental machine factory is upgrading, and the dynamic ratio is only one gear. In this case, how to use the pressure testing machine with the required motor capacity pn=kptn1/2nlm? ax 1/2 nPL。 It can be seen that the required capacity is only greater than on/2 times the load power

⑵ when the maximum operating frequency is twice the rated frequency and the transmission ratio is two gears. At this time, the required motor capacity pn1/2 pl. It can be seen that this scheme is ideal for constant power load when n is 4

3. Self equipped external braking resistance

various variable frequency governors allow external braking resistance to speed up braking and connect external resistance. However, the supporting braking resistor is expensive and difficult to buy. During automatic configuration, its resistance value and power can be determined as follows:

voltage value of DC circuit up= 380=53v; The braking current is generally based on the principle of not exceeding the rated current IDN of the motor, i.e. is IDN, so the braking resistance Rs ud/is

because the time of passing current in RS is only a few seconds, its power PR can be selected according to (1//8) when it works, that is, pr= (0..125) ud2/rs

when RS is connected to the circuit, attention should be paid to cutting off the braking resistance inside the frequency converter. If it cannot be cut off, the value of RS should be appropriately increased to avoid excessive braking current

when connecting the braking circuit externally, in order to avoid burning the discharge high-power transistor (GTR) inside the frequency converter, sometimes the whole braking appliance (including the braking resistor and discharge transistor) can also be connected externally. At this time, the GTR should choose its vcex 700 V; ICN (1..5) IDN a

references

[1] Ma Xinmin, mining machinery, Xuzhou: China University of mining and Technology Press, 2002

[2] Li Ji, et al., analysis and prevention of electromechanical accidents in coal mines, Beijing: Coal Industry Press, 1997

[3] Chai Chang, et al., electromechanical safety technology, Beijing: Chemical Industry Press, 2006.1

[4] Liang Jie, electrical appliances and electronic control devices of construction machinery, Beijing: People's Communications Press, 1998

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