D
“D” FLANGE
A special end shield with holes for through bolts in the flange and is primarily used for mounting the motor on gearboxes 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)
DESIGN A, B, C, D – for AC MOTORS
NEMA has standard NEMA motor designs of various torque characteristics to meet the various requirements posed by different application loads. The design “B” is the most common design. (See chart for characteristics of each design.)
| NEMA DESIGN | STARTING TORQUE | STARTING CURRENT | BREAK- DOWN TORQUE | FULL LOAD SLIP | TYPICAL APPLICATIONS |
| A | Normal | High | High | Low | Mach. Tools, Fans |
| B | Normal | Normal | Normal | Normal | Same as Design “A” |
| C | High | Normal | Low | Normal | Loaded compressor Loaded conveyor |
| D | Very high | Low | ——- | High | High Punch Press |
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 multiple of two or the three of one another.
DUAL TORQUE
Dual speed motor whose torque varies with speed (as the speed changes the horsepower remains constant).
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 is often rated 15 or 30 min. duty.
DYNAMOMETER
A device that 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.