Mechanical Power Calculation Method Of Electric Motor
The power of the motor should be selected according to the power required by the production machinery, and try to make the motor run under the rated load. When choosing, you should pay attention to the following two points:
(1) If the motor power is too small. It will cause the motor to overload for a long time. Its insulation is damaged by heat. Even the motor was burned out.
(2) If the motor power is too large. Its output mechanical power cannot be fully utilized, and the power factor and efficiency are not high, which is not only unfavorable to users and the power grid. And it will also cause waste of electricity.
To correctly select the power of the motor, the following calculations or comparisons must be made:
(1) For the continuous working mode with constant load, if the power of the load (ie the power on the shaft of the production machine) is known, Pl (kw). The power P (kw) of the required motor can be calculated as follows: P=P1/n1n2 where n1 is the efficiency of the production machinery; n2 is the efficiency of the motor. That is the transmission efficiency.
The power obtained by the above formula is not necessarily the same as the product power. 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. If the mechanical efficiency is 70%, if a motor with an efficiency of 0.8 is selected, what is the power of the motor?
Solution: P=P1/n1n2=3.95/0.7*0.8=7.1kw Because there is no 7.1kw this-specification. So choose 7.5kw motor.
(2) A motor with a short-term work rating has a large maximum torque, a small weight and a low price compared with a motor with a continuous work rating of the same power. Therefore, when conditions permit, the motor with short-term work quota should be selected as far as possible.
(3) For a motor with a rated intermittent operation, the selection of its power should be based on the size of the load duration rate, and a motor specially used for intermittent operation 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. is the stop time min; tg ten to is the working cycle 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 the electric motor used in similar production machinery. The specific method is: to understand the power motor used by the similar production machinery of this unit or other nearby units, and then select a motor of similar power to conduct a test run. The purpose of the test run is to verify that the selected motor matches the production machine. The verification method is: make the motor drive the production machinery to run, measure the working current of the motor with a clamp ammeter, and compare the measured current with the rated current marked on the nameplate of the motor. If the actual working current of the electric power machine is not much different from the rated current marked on the spleen. It indicates that the power of the selected motor is suitable. If the actual working current of the motor is about 70% lower than the rated current marked on the nameplate. It means that the power of the motor is too large, and the motor with less power should be replaced. If the measured working current of the motor is more than 40% larger than the rated current marked on the nameplate. It indicates that the power of the motor is too small, and the motor with a larger power should be replaced.
