Thread Content
The power of the motor should be selected based on the power required by the manufacturing machinery, with an effort to operate the motor at its rated load. The following two points should be considered when making a choice: (1) If the motor power is chosen to be too low, it will result in a situation where a small motor has to drive a heavy load, causing the motor to be overloaded over time. This leads to damage to the motor’s insulation due to heat, and in severe cases, the motor may even burn out. (2) If the motor power is chosen to be too large, a situation of \"using a powerful engine for a small task\" occurs; the mechanical power output cannot be fully utilized, and both the power factor and efficiency are low (see table), which is detrimental to both the user and the power grid. It also leads to electricity waste. To correctly select the power of a motor, the following calculations or comparisons must be carried out: (1) For continuous operation under a constant load, if the power of the load (i.e., the power on the shaft of the manufacturing machine) Pl (kW) is known, the required motor power P (kW) can be calculated using the following formula: P = P1 / (n1n2), where n1 is the efficiency of the manufacturing machine ； n2 is the efficiency of the motor. That is, the transmission efficiency. The power calculated using the above formula is not necessarily the same as the product’s power. Therefore, the rated power of the selected motor should be equal to or slightly greater than the calculated power. Example: A machine for manufacturing machinery has a power of 3.95 kW, with an efficiency of 70%. If an electric motor with an efficiency of 0.8 is to be used, what should be its power in kW? Solution: P = P1 / (n1 * n2) = 3.95 / 0.7 * 0.8 = 7.1 kW. Since there is no motor available with a power of 7.1 kW, a 7.5 kW motor is chosen instead. (2) Motors with a short-time duty rating. Compared to motors with a continuous-duty rating and the same power, they have a higher maximum torque, are lighter in weight, and are less expensive. Therefore, whenever conditions permit, motors with a short-time duty rating should be preferred. (3) For motors with a intermittent duty rating, the selection of power should be based on the duty cycle, and motors specifically designed for intermittent operation should be used. The formula for calculating the load continuous series Fs% is FS% = tg/(tg+to)×100%, where tg represents the working time, and t. To stop at time min ； tg plus to is the cycle time in minutes. Additionally, the analogy method can also be used to select the motor power. The so-called analogy method. It is a comparison with the power of motors used in similar production machinery. The specific approach is to find out what power rating the motors used in similar production machinery at one’s own unit or at other nearby units have, and then select motors with a similar power rating for testing. The purpose of the trial run is to verify whether the selected motor is suitable for the production machinery. The verification method is as follows: drive the production machinery using the motor, measure the operating current of the motor with a clamp ammeter, and compare the measured current with the rated current indicated on the motor’s nameplate. If the actual operating current of the electric motor is not significantly different from the rated current indicated on the nameplate, 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 indicates that the motor’s power rating is too high (in other words, it’s like using a large horse to pull a small cart); in such cases, a motor with a lower power rating should be used. If the measured operating current of the motor is more than 40% higher than the rated current indicated on the nameplate, it indicates that the motor’s power rating is too low (in other words, using a motor with insufficient power for the task), and a motor with a higher power rating should be used instead.