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Electronic VFDs are speed control devices which vary the voltage and frequency to an induction motor using a technique called Pulse Width Modulation (PWM). Since they are inexpensive and reliable, VFD's have become the preferred way to achieve variable speed operation. The following is a glossary of commonly used terms when describing or specifying Variable Frequency Drives for electric motors.

Alternating Current (AC): A periodic flow of electricity which changes direction each cycle, reaching a maximum in one direction, decreasing to null, then reversing to reach a maximum in the opposite direction.

Air Gap: The space between the rotating and stationary parts of an electric motor. Magnetic energy is transferred across this gap.

Ambient: The air surrounding a motor.

Ampere: A measure of the rate of electron flow (current). It is often abbreviated as Amp.

Breakdown Torque: The maximum torque a motor can develop at a rated voltage without stalling or experiencing an abrupt drop in speed.

Brush: The material in contact with an armature or slip ring assembly that provides the electrical connection between rotating and stationary parts of a motor.

Capacitor: A device used to store electrical charge. The unit of capacitance is the Farad.

Commutator: An assembly mounted on the shaft of a DC motor that provides the switched connections between the power supply and armature coils.

Conductor: Material which offer little opposition to the flow of electricity..

Duty Cycle: The relationship between the operating time and the idle time of an electric motor.

Eddy Current: Losses and heating in metals resulting from localized currents caused by an alternating magnetic flux.

Efficiency: The ratio of mechanical output to the electrical input power of a motor.

Electromotive Force (EMF): Induced or generated voltages in an electric circuit.

Field: The stationary part of a DC motor that provides the magnetic flux which interacts with the armature.

Flux: The magnetic field established around a current carrying conductor or a permanent magnet.

Frequency: The rate at which alternating current reverses its direction of flow expressed in cycles per second or Hertz.

Full Load Current: The current that a motor draws at rated voltage, frequency and load.

Full Load Slip: a ratio of the synchronous speed to full load speed of a motor.

Full Load Speed: The speed of the motor at rated voltage, frequency and load.

Full Load Torque: The torque that is necessary to produce rated horsepower at full load speed.

Horsepower: A measurement of power. One horsepower is equal to 746 watts.

Impedance: The vector sum of resistance and reactance used to express the total opposition a circuit offers to the flow of alternating current.

Inductance: The property of an electrical circuit which opposes a change in current due to the magnetic field induced by that current.

Inertia: The resistance of an object to a change in its state of motion. Since rotating parts do not operate at the same speed, calculating the inertia for each moving part allows them to be modeled as a single unit. It is determined by the weight of the object multiplied by the square of the radius of gyration.

Line Voltage: The mains voltage supplied to the power input terminals of an electric device.

Magnetomotive Force (MMF): The magnetic energy supplied to create a magnetic flux.

Phase: The relationship (in electrical degrees) between voltages and currents in a circuit or the spatial relationship (in angular degrees) of windings in an electric motor.

Power Factor: A measurement of the difference in phase between voltage and current in an electric circuit.

Reactance (capacitive): The property of a capacitor in a circuit which causes the voltage to lead the current.

Reactance (inductive): The property of an inductor in a circuit which causes the voltage to lag the current.

Resistance: The property of an electrical conductor which opposes the flow of electricity.

Rotor: The rotating part of an electric motor.

Service Factor: A multiplier applied to the rated horsepower of an AC motor indicating the permissible loading which may be carried under a set of specified conditions.

Slip: The ratio between the synchronous and operating speeds of an induction motor.

Stator: The stationary part of an AC motor containing the housing, steel laminations and windings.

Temperature Rise: The difference between operating and ambient temperature in a motor winding.

Torque: The turning force applied to a shaft, expressed as pound-feet (English) or Newton-meters (metric).

Voltage (Volt): The standard unit of EMF which produces a flow of current in a conductor.

Watt: A measurement of power in an electrical circuit. It is equal to one joule of energy being expended in one second.

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