Motor power should be based on the requirement of eqipment and make the motor run  at rated load. pay attention to the following two points:

    If the motor power is too small. It  will result in long-term motor overload. The  insulation damage due to fever. Even the motor were destroyed.

    If the motor power is too high. The output mechanical power can not be fully utilized, the power factor and efficiency are not high, not only detrimental to users and power grids. But also result in wasted energy.

  In order to choose a suitable motor power , must be calculated as:

For Constant load, If you know the load of the power Pl (kw). Motor can be determined using the following equation 

   P=P1/n1n2

  Where n1 is the efficiency of production machinery; n2 for is the motor efficiency. That the transmission efficiency. Calculated according to the type of power,maybe not to the same power of products. Rated power of motor should be equal to or slightly larger than the calculated power.

   For example: the power of a production machinery is 3.95kw. Mechanical efficiency is 70%, if choose the motor with efficiency  0.8, what about the power of the motor?

Answer: P = P1 / n1n2 = 3.95/0.7 * 0.8 = 7.1kw  There is no motor of 7.1kw so we choose 7.5kw motor.