Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

instantaneous
the range of 0.5 to 30 cycles of the supply frequency.
instantaneous contact
the contacts of a contactor or relay that open or close with no time delay.
instantaneous frequency
(1) the time rate of change of the angle of an angle-modulated wave.

(2) the frequency of radiation at some chosen instant of time; the rate of change of phase in radians per second, divided by 2π.
instantaneous overcurrent relay
an overcurrent relay that operates with no intentional delay following sensing of a power frequency overcurrent, i.e., a current above its set point.
instantaneous power
power in an AC or modulated signal at a given instant in time.
instantaneous-trip circuit breaker
essentially an inverse-time circuit breaker with the thermal element removed. It will only trip magnetically in response to short-circuit currents. Thus, it is often referred to by other names, such as magnetic circuit breaker and magnetic-only circuit breaker.
Institute of Electrical and Electronics Engineers (IEEE)
a professional organization of electrical engineers and computer scientists. The world's largest professional organization.
instrument transformer
a two-winding transformer designed and optimized for metering applications. The essential features are accurate input to output ratios for the measured parameter and minimal burden (or load) on the circuit being tested.
See current transformer
insulation class
specification of insulating material. Primarily determined by the maximum temperature that the material can withstand. The class of insulating system used in a transformer or electrical machine is a key factor in determining its rated load.
insulation coordination
the practice of selecting the insulation of a power system such that breakdowns from overvoltages (such as from a lightning strike) will occur at points where the least harm will result.
insulation resistance
measured value of electrical resistance of the insulation of a product or device.
insulator
a device designed to separate and prevent the flow of current between conductors. Properties of the dielectric (insulating) material and geometry of the insulator determine maximum voltage and temperature ratings.
insulator string
a chain of two or more strain insulators that are coupled together to increase the total insulation level of the assembly.
integral horsepower motor
a motor built in a frame as large as or larger than that of a motor of open construction having a continuous rating of one horsepower at 1700-1800 rpm.
interference
(1) a process in which two waves interact. Interference occurs when the complex properties of the wave are combined instead of simply their two amplitudes. The wave interference can be constructive when they both have the same phase, or destructive when they have opposite phases.

(2) a disturbance on a communication system caused by the presence of a signal, or small number of signals, of man made origin. The interference may be due to the presence of signals external (other systems) or internal (e.g., multiple access interference) to the system of interest. Compare with background noise.

(3) an interaction whereby a process that is supposed to be independent (and isolated) from another process does indeed perform some action that has an effect visible from the second process. Interference can be caused by direct writing to the state of the second process or by indirect effects, such as by monopolizing the use of a system module.

(4) any external electrical or electromagnetic disturbance which causes an undesired response or degradation of the desired signal.

(5) the mutual influence of two waves or vibrations of any kind, producing certain characteristic distorting phenomena, is
known as interference. For example, radio-frequency interference (RFI) or crosstalk and intersymbol interference (ISI) caused by interference between successive pulses in an optical fiber.
interference cancellation
a signal processing technique where an interfering signal is estimated, regenerated based on the estimate, and then canceled from the received signal, leaving a potential interference-free, desired signal. Examples are multiple access interference cancellation, co-channel interference cancellation, echo cancellation.
interference channel
a multiple transmitter, multiple receiver communications system in which each received signal is a (possibly non-deterministic) function of two or more transmitted signals. See also broadcast channel, multiple access channel.
interharmonic component
the frequency component that is not an integer value of the periodic supply waveform.
interior permanent magnet machine
a permanent magnet machine in which the permanent magnets are buried in the rotor iron. With this construction the airgap is small, allowing the airgap flux to be reduced by current control. This allows an operating mode in which the top speed of the motor can be as much as five times the base speed, and is one of the main features of this motor.
intermittent fault
a fault that appears, disappears, and then reappears in a repeated manner.
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