Motor Terminology
D
"D"
Flange: A special end shield with holes for through bolts in the flange and
is primarily used for mounting the motor on gear boxes or bulkheads.
Standardized for frames 143T through 445T. "D" flanges are not
threaded and the bolt holes extend beyond the motor frame.
D.C. (Direct Current): A current that flows only in one
direction in an electric circuit. It may be continuous or discontinuous and it
may be constant or varying.
DC Motor: A motor using either generated or rectified D.C.
power (see Motor definition) . A DC motor is usually used
when variable speed operation is required.
Definite Purpose Motor: A definite purpose motor is any motor
design, listed and offered in standard ratings with standard operating
characteristics with special mechanical features for use under service
conditions other than usual or for use on a particular type of application. (NEMA)
Dimensions: NEMA has standard frame sizes and dimensions
designating the height of the shaft, the distance between mounting bolt holes
and various other measurements. The integral AC motor NEMA sizes run from 143T-
445T, and the center of the shaft height in inches can be figured by taking the
first two digits of the frame number and dividing it by 4.
The fractional horsepower motors, for which NEMA spells out dimensions, utilize
42, 48 and 56 frames whose shaft height in inches is figured by dividing the
frame number by 16.
Drip-Proof Guarded: A drip-proof machine with ventilating
openings guarded (with screens) as in a guarded motor.
Drip-Proof Motor: An open motor in which the ventilating
openings are so constructed that drops of liquid or solid particles falling on
it, at any angle not greater than 15 degrees from the vertical, cannot enter
either directly or by striking and running along a horizontal or inwardly
inclined surface.
Dual Voltage: Some motors can operate on two different
voltages, depending upon how it is built and connected. The voltages are either
multiples of two or the 3 of one another.
Duty Cycle: The relationship between the operating and rest
times or repeatable operation at different loads. A motor which can continue to
operate within the temperature limits of its insulation system, after it has
reached normal operating (equilibrium) temperature is considered to have a
continuous duty (CONT.) rating. One which never reaches equilibrium temperature,
but is permitted to cool down between operations is operating under intermittent
duty (INT.) conditions such as a crane and hoist motor which are often rated 15
or 30 min. duty.
Dynamometer: A device which loads the motor to measure output
torque and speed accurately by providing a calibrated dynamic load. Helpful in
testing motors for nameplate information and an effective device in measuring
efficiency.