daisy chain
A number of series connections (A to B, B to C, etc.) as contrasted to parallel connections (A to B, A to C, etc).

DC
Direct current. Electrical current that flows only in one direction.

DC braking
A method of stopping an AC motor by supplying DC to the stator coils. This generates a significant amount of heat in the motor's stator coils. It can slow a motor, but it is not used to hold a motor's shaft in a single position.

DC bus
The direct current portion of the power circuit of an adjustable frequency drive. This may either be at a constant voltage or at a controlled voltage.

dead band
A range of values where a setpoint controller will not send any correction signal. For example, if the set point temperature was 21C with a dead band of + 2 C, the set point controller would send no correction signal until the temperature was higher than 23C or lower than 19C.

decibel
(dB) A unit that is used to measure the loudness (pressure) of a sound. A sound of 30 dB can just barely be heard by most people. Normal conversation is in the range of 60 to 70 dB. Continued exposure to sounds more than 96 dB can cause permanent hearing damage. While a change of 3 dB can just barely be distinguished by most people, it represents a doubling of the sound's power level. The decibel can also be used to measure the amplitude of electronic signals.

design outlet temperature
(of a cooling tower) The anticipated temperature of the water leaving the basin of the cooling tower and returning to the chiller or the process. This is the temperature that is measured and used to control the speed of the cooling towers fan when an adjustable speed drive is used.

design wet bulb temperature
(of a cooling tower) The atmospheric wet bulb temperature for which the cooling tower was designed. The air temperature that is registered by a thermometer bulb which is covered by a water-soaked wick is the wet bulb temperature. The evaporation of the water cools the wet bulb thermometer below the dry bulb temperature. The wet bulb temperature is the lowest possible outlet temperature that a cooling tower can provide and it is the only environmental information needed in sizing a cooling tower or calculating energy savings by using an adjustable speed drive. Wet bulb and dry bulb temperatures can be used together to determine relative humidity.

DIN
Deutsches Institut fr Normung, in English German Institute for Standardization, is a non-official registered association with its head office in Berlin. The work of standardization as undertaken by DIN is a service in the field of science and technology that is provided for the entire community.

DIN Standards are technical rules that promote rationalization, quality assurance, safety, and environmental protection as well as improving communication between industry, technology, science, government and the public domain.

diode
A device that allows electric current to flow in only one direction.

diode bridge rectifier
A device that converts alternating current to direct current through the use of diodes. This is one common type of input stage used in adjustable frequency drives.

direct acting
(contrast reverse acting) In reference to follower signals. An increase in the value of the input signal produces an increase in the output. Example: an increase from 4 mA to 10 mA in the input signal creates an increase in the speed of the drive.

displacement power factor
The ratio between the actual power and the apparent power in an AC circuit when undistorted voltage and current waves are shifted in phase with respect to each other. It is often written as cos(). This can range from 0 to 1. 1 is the ideal power factor. Displacement power factor describes only one aspect of total power factor. If the AC waves are not pure sine waves, the total power factor will be less than the displacement power factor.

distortion factor
Another name for total harmonic distortion.

distortion power factor
The ratio between the actual power and the apparent power in an AC circuit that results from harmonic distortion of the voltage and current waves. This can range from 0 to 1. 1 is the ideal distortion power factor.

drive disconnect switch
This door-interlocked, padlockable, manual switch disconnects input power from the drive. It can be padlocked in the off position. It also prevents entry into the drive enclosure when in the on position. This switch does not disconnect power from bypass circuitry.

dV/dt
The rate of change of voltage compared to time Usually expressed in volts per microsecond. Line disturbances, spikes and electric circuits can cause a high dV/dt which may damage electrical components and motors.

dynamic braking
When a load is being decelerated, the motor acts as a generator, converting the kinetic energy of the load into electrical energy. A dynamic braking circuit converts this electric energy into another form, usually heat, through the use of dynamic braking resistors. While dynamic braking can help slow a load quickly, it cannot act as a holding brake.