ELECTRIC MOTORS

Electric Motors - Acceleration time:
Acceleration time for electric motors is directly proportional to total inertia and inversely proportional to the electric motor torque. For electric motors with constant acceleration torque, acceleration time is:

where WK² = rotational inertia in lb-ft², (N₂ - N₁) = the speed difference, and T_x = acceleration torque in lb-ft. For translating bodies, acceleration time is:

where W = weight of the load in lb, (S₂ - S₁) = the translation speed difference, and F_x = translation force in lb.

An approximation method is necessary to find the electric motor's acceleration time if acceleration torque is not linear during speed increase. The quickest method is to break up the speed versus torque curves of the electric motor and the driven machine into segments and calculate acceleration time for each segment. Accurate electric motor acceleration times usually result.

Electric Motors - Power rating:
Electric motors offer the horsepower required to drive a machine, which is typically referred to as electric motor load. The most common equation for power based electric motors on torque and rotational speed is: hp = (torque X rpm)/5,250.

If the electric motor's load is not constant and follows a definite cycle, a horsepower versus time curve for the driven machine is helpful. From this curve both peak and rms the electric motor's horsepower can be determined. Rms load horsepower indicates the necessary continuous electric motor rating. Peak load horsepower is not necessarily an indication of the required electric motor rating. However, when a peak load is maintained for a period of time, electric motors feature a rating, which usually should not be less than peak load horsepower.

Electric Motors: Introduction to Electric Motor Design Engineering

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