D Flange Mounting: This type of mount has clearance holes on the flange, and the mounting bolts stick out through the flange from the motor side. This mount is common in cases where the motor is integral to the machine.

Damping: Refers to the rate of decay or settling time of a signal.

Damping Ratio: Ratio of actual damping to critical damping; if the damping ratio is less than one a system is said to be underdamped and if it is greater than one a system is said to be overdamped.

DC (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 DC power. A DC motor is often used when variable-speed operation is required.

Deceleration: Rate of decrease in velocity with respect to time.

Decibel: Logarithmic measure of gain equal to twenty times the log of the gain.

Definite Purpose Motor: A definite purpose motor is any motor design listed and offered in standard ratings with standard operating characteristics but with special mechanical features for use under service conditions other than usual or for use on a particular type of application (NA).

Demag Current: The current level at which the motor magnets will start to be demagnetized. This is an irreversible effect, which will alter the motor characteristics and degrade performance. Also known as peak current.

Design A, B, C, D – for AC Motors: NA has standard motor designs with various torque characteristics to meet specific requirements posed by different application loads. The design B is the most common design.

Detent Torque: The maximum torque that can be applied to an unenergized step motor without causing continuous rotating motion.

Differential Line Driver: Output stage of the encoder which produces two complementary pulse trains per output channel. These complementary outputs can be transmitted through long cables with minimal loss of signal integrity. Electrical noise effects are reduced when the signals are compared by the differential line receiver. Receiver input impedance should be matched to the line driver output and the transmission line for best noise immunity.

Dimensions: NA has standard frame sizes and dimensions designating the height of the shaft, the distance between mounting bolt holes and various other measurements. Integral AC motor NA 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. Fractional horsepower motors, for which NA spells out dimensions, utilize 42, 48 and 56 frames. The shaft height in inches can be established by dividing the frame number by 16.

Direction Discriminator: Part of a bi-directional counter which determines the counting direction.

Direction Sensing: Direction sensing is the technique for detecting the direction of angular or linear motion where the output levels are Logic 1 (high) for one direction of motion and Logic 0 (low) for the opposite direction of motion. This is typically done in the user's control.

Distance-Coded Reference Marks: Incremental measuring method, whereby the absolute position can be determined by evaluating the systematically varying distances between consecutive reference marks.

DPBV - Dripproof Blower Ventilated: Type of motor cooled by blowing air through the inside of the motor using an attached blower.

Dripproof Guarded: A Dripproof machine with ventilating openings guarded (with screens) as in a guarded motor.

Dripproof 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 of by striking and running along a horizontal or inwardly inclined surface.

Drive: An electronic device that controls torque, speed and/or position of an AC or brushless motor.

Driver: Electronics which convert step and direction inputs to high power currents and voltages to drive a step motor. The step motor driver is analogous to the servomotor amplifier's logic.

Dual Channel: A dual-channel encoder produces two incremental outputs. These two outputs are generally in quadrature (90 degree phase separation) relationship to each other. They are typically referred to as Channel An and Channel B.

Dual Torque: A dual speed motor with torque values that vary with speed (as the speed changes the horsepower rains constant).

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 (%) = [On time / (On time + Off time)] x 100%

Duty Cycle: For a repetitive cycle, the ratio of on time to total cycle time.

Dynamic: Refers to something that is in motion.

Dynamic Braking: A passive technique for stopping a permanent magnet brush or brushless motor. The motor windings are shorted together through a resistor which results in motor braking with an exponential decrease in speed.

Dynamic Scanning: Scanning method by which two alternating signals of constant amplitude and slightly different frequencies are generated and where the phase between the two signals represents the measuring value.

Dynamometer: A device which places a load on the motor to accurately measure its output torque and speed by providing a calibrated dynamic load. Helpful in testing motors for nameplate information and an effective device in measuring efficiency.

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