D-Flange (Mounting Mounting): A type of motor mounting used when the motor is to be built as part of the machine. The mounting holds of the flange are not threaded. The bolts protrude through the flange from the motor side. Normally, D-flange motors are supplied without feet since the motor is mounted directly to the driven machine.
Damping: Damping is the reduction in amplitude of an oscillation in the system.
Dead Band: The range of values through which a system input can be changed without causing a corresponding change in system output.
Deceleration Time: The time required to stop a motor - whether - free running or with some braking means.
Definite Purpose Motor: A definite purpose motor is any motor design, listed and offered in standard ratings with standard operating characteristics and mechanical construction, for use under service conditions other than usual or for use on particular type of application (NEMA). An example would a vertical holloshaft motor.
Deviation: Difference between an instantaneous value of a controlled variable and the desired value of the controlled variable corresponding to the set point. Also called an error.
di/dt: The rate of change in current versus a rate of change in time. Line reactors and isolation transformers can be used to provide the impedance necessary to reduce the harmful effects that unlimited current sources can have on phase controlled rectifiers (SCR's).
Dimension Drawing: A dimension drawing or outline drawing (base plan or floor plan) is one which shows the physical space and mounting requirements of a piece of equipment. It may also indicate ventilation requirements and space provided for connections or the location to which connections are to be made.
Diode: A device that passes current in one direction, but blocks current in the reversed direction.
DC Contactor: A contactor specifically designed to establish or interrupt a direct current power circuit.
DC Motor - Compound Wound: Type of DC motor having both shunt and series field connections. This motor has good speed regulation and starting torque.
DC Motor - Permanent Magnet:Type of DC motor where the field poles and the armature poles are electromagnets. The only current used by the motor is that of the armature. Has high starting torque, good speed regulation and a definite maximum speed.
DC Motor - Series Wound: Type of DC motor which has its field winding connected in series with the armature. This motor has very high starting torque, but has a tendency to ‘run-away’ when lightly loaded or unloaded, and has poor speed regulation.
DC Motor - Shunt Wound: Type of DC motor which has its armature winding and field winding done in parallel circuits. This motor has very good speed regulation.
Distributed Pole: A motor has distributed poles when its stator or field windings are distributed in adjacent slots located within the arc of the pole.
Drain/Breather: A hole located in the lowest spot of the motor used to drain oil/grease when re-lubricating the motor.
Drift: Drift is the deviation from the initial set speed with no load change over a specific time period. Normally, the drive must be operated for a specified warm-up time at a specified ambient temperature before drift specifications apply. Drift is normally caused by random changes in operating characteristics of various controller components.
Drip Cover: A metal piece shaped like a pizza pan attached to the top of a motor mounted vertically shaft down to protect liquid from entering into the motor.
Drive Controller (Also variable speed drive): An electronic device that can control the speed, torque, horsepower and direction of an AC or DC motor.
Dual Voltage: A connection method with enough leads in the terminal box to permit simple reconnection to either of two voltages.
Duty Cycle: The relationship between the operating and rest time. 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.
dv/dt: The rate of change in voltage versus a rate of change in time. When a motor is operated under VFD power a high value of dv/dt will indicate voltage spikes and/or line disturbances.
Dwell: The time spent in one state before moving to the next. In motion control applications, for example, a dwell time may be programmed to allow for a tool change or part clamping operation.
Dynamic Braking: This is caused by current being applied to the windings after the power is shut off. This is accomplished by either excitation (D.C. motors) or by separate excitation, A.C. motors. It is seldomly used to hold a load, but it can be used as a retarding force to prevent over-running.
Dynamic Unbalance: A noise producing condition caused by non-symmetrical weight distribution of a rotating member. The lack of a uniform wire spacing in a wound armature or casting voids in a rotor or fan assembly can cause relatively high degrees of unbalance.
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