The power of the motor should be selected according to the power required by the production machine to maximize the operation of the motor at rated load.
First, pay attention to the following two points when selecting the power of the motor:
1. If the motor power is too small, there will be a “small horse-drawn carriage†phenomenon, causing the motor to be overloaded for a long time. The insulation will be damaged by heat. Even the motor will be burnt.
2. If the motor power is too large, there will be a “big horse trolley†phenomenon. The output mechanical power cannot be fully utilized, and the power factor and efficiency are not high (see table), which is not only bad for the user and the power grid. It also causes a waste of electricity.
Second, to correctly select the power of the motor, must be calculated or compared as follows:
1. For constant load continuous operation mode, if the power of the load (ie the power on the production machine shaft) Pl(kw) is known, the power P(kw) of the required motor can be calculated as follows: P=P1/n1n2 N1 is the efficiency of the production machine; n2 is the efficiency of the motor. That is, transmission efficiency. The power obtained by the above formula is not necessarily the same as the power of the product. Therefore, the rated power of the selected motor should be equal to or slightly greater than the calculated power.
Example: The power of a production machine is 3.95kw. The mechanical efficiency is 70%. If a motor with an efficiency of 0.8 is selected, how much kw should the power of the motor be? Solution: P=P1/n1n2=3.95/0.7*0.8= 7.1kw because there is no 7.1kw - specification. So choose 7.5kw motor. (2) Short-time working rated motor. Compared with the continuous working rated motor with the same power, the maximum torque is large, the weight is small, and the price is low. Therefore, when conditions permit, try to use a motor with a short-time working quota. (3) For the motor with intermittent working quota, the power selection should be based on the magnitude of the load duration, and the motor specially used for the intermittent operation mode should be selected. The calculation formula of the load continuous string Fs% is FS%=tg/(tg+to)×100% where tg is the working time, t. To stop the time min; tg ten to the work cycle
2. Time min. In addition, analogy can also be used to select the power of the motor. The so-called analogy. It is compared with the power of a motor similar to that used in production machinery. The specific method is to know how much power the similar production machinery uses in this unit or Other nearby units, and then use a similar power motor for the test. The purpose of the test run is to verify that the selected motor matches the production machine. The verification method is to make the motor drive the production machinery, measure the working current of the motor with a clamp-type ammeter, and compare the measured current with the rated current marked on the motor nameplate. If the actual working current of the electric machine is not much different from the rated current indicated on the spleen, it indicates that the power of the selected motor is appropriate. If the actual operating current of the motor is about 70% lower than the rated current indicated on the nameplate, it means that the power of the motor is too large (ie, the "large horse-drawn car" should be replaced with a motor with a lower power. If the measured motor works The current is more than 40% larger than the rated current marked on the nameplate. It indicates that the power of the motor is too small (ie "small horse-drawn cart"), and the motor with higher power should be exchanged. Table: Load condition no load 1/ 4 load 1/2 load 3/4 load full load power factor 0.2 0.5 0.77 0.85 0.89 efficiency 0 0.78 0.85 0.88 0.895
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