When selecting a motor, it's essential to match its power to the requirements of the production equipment. Ideally, the motor should operate under its rated load to ensure optimal performance and longevity. Here are two key considerations when choosing the right motor:
(1) If the motor's power is too small, it may lead to a situation known as "small horse pulling a cart," where the motor is overloaded for extended periods. This can cause overheating, insulation damage, and even motor failure.
(2) On the other hand, if the motor is oversized—commonly referred to as "big horse pulling a cart"—it may not utilize its full mechanical output. This results in lower power factor and efficiency, which can be inefficient for both the user and the power grid, leading to unnecessary energy waste.
To properly select the motor’s power, you need to perform calculations or comparisons based on the specific application. For example, in a constant load continuous operation mode, if the load power (Pl) is known, the required motor power (P) can be calculated using the formula: P = Pl / (n1 × n2), where n1 is the efficiency of the production machine, and n2 is the motor's efficiency. The selected motor's rated power should be equal to or slightly higher than this calculated value.
Example: A production machine has a power of 3.95 kW, with a mechanical efficiency of 70%. If a motor with an efficiency of 80% is chosen, the required motor power would be P = 3.95 / (0.7 × 0.8) = 7.1 kW. Since there's no 7.1 kW motor available, a 7.5 kW motor would be the best choice.
For short-term operation motors, they typically have higher torque, lighter weight, and lower cost compared to continuously rated motors. Therefore, whenever possible, it's better to use a short-time duty motor. For intermittent duty motors, the selection should be based on the load duration rate. A motor specifically designed for intermittent operations should be used, and the load cycle can be calculated using the formula: FS% = (tg / (tg + to)) × 100%, where tg is the working time, and to is the rest time.
In addition to calculations, motor power can also be selected by analogy. This involves comparing the motor power used in similar production machines. You can check how much power similar machines in your unit or nearby facilities use and then test with a similar power motor. The goal of the test is to verify whether the selected motor matches the production equipment. During the test, measure the motor’s operating current using a clamp meter and compare it to the rated current on the motor’s nameplate. If the actual current is close to the rated current, the motor power is appropriate. If the current is significantly lower (around 70% less), the motor is oversized ("big horse-drawn cart") and should be replaced with a smaller one. Conversely, if the current is more than 40% higher than the rated value, the motor is undersized ("small horse-drawn cart"), and a more powerful motor should be used.
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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