Electric motors, both ac motors and dc motors, come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. Thus, the first and most important step in electric motor selection is determining load characteristics -- torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost.
Starting and running torque are the first parameters to consider when sizing electric motors. Starting torque requirements for electric motors can vary from a small percentage of full load to a value several times full-load torque. Starting torque varies because of a change in load conditions or the mechanical nature of the machine, which the electric motor is installed in. The latter could be caused by the lubricant, wear of moving parts, or other reasons.
Electric motors feature torque supplied to the driven machine, which must be more than that required from start to full speed. The greater the electric motor's reserve torque, the more rapid the acceleration.
Electric motor drive systems that use gear reducers have parts that rotate at different speeds. To calculate acceleration torque required for these electric motors, rotating components must be reduced to a common base. The part inertias are usually converted to their equivalent value at the drive shaft. Equivalent inertia W2K22 of the load only is found from:
W2K22 =(W1K12)(N1/N2)2
where W1K21 = load inertia in lb-ft2, N1 = load speed in rpm, and N2 = electric motor speed in rpm.
Electric motors have bodies, which have a straight-line motion are often connected to rotating driving units by rack-and-pinion, cable, or cam mechanisms. For these electric motor parts, the equivalent WK2 is found from:
WK2 = W(S/2ΠN)2
where W = load weight, S = translation speed in fpm, Π is pi, and N = rotational speed in rpm.
