ELECTRIC MOTORS
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.
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