capacitance
The measure of the electrical effect of a capacitor. This is often expressed in microfarads (mF) or picofarads (pF).

capacitor
A device that is used in electrical circuits to store an electrical charge on sets of internal plates. Its value is often measured in microfarads (mF) or picofarads (pF), and is also rated by the maximum voltage that can be applied to it. Capacitors are often used to correct the phase-shift power factor which AC induction motors cause. A capacitor used in conjunction with a coil can produce a resonant or "tuned" circuit.

carrier frequency
In a PWM inverter, this is the fixed rate at which voltage pulses are provided to the motor. Carrier frequencies from 1 kHz through 20 kHz are common.

CENELEC
ComitÈ EuropÈen de Normalisation Electrotechnique. CENELEC is the European Committee for Electrotechnical Standardization. Founded 1973 as a non-profit-making organisation under Belgian Law. It has been officially recognised as the European Standards Organisation in its field by the European Commission. Its members have been working together in the interests

European harmonisation since the late fifties, developing alongside the European Economic Community.

chassis ground
A ground connection made to the metal chassis on which the components of a circuit are mounted.

choke
A coil used in a direct current circuit to smooth out a pulsating wave form to reduce ripple.

chopper
An internal circuit in some adjustable frequency drives which regulates output voltage by quickly turning on and off the power applied to the output section of the drive. Drives which use a chopper to control output voltage commonly use a diode bridge rectifier input section.

clock frequency
The frequency, measured in Hz, of an oscillator used to time or synchronize the operations of a computer or a microprocessor. The higher the clock frequency, the faster the operation of the computerized circuit. Several international boards regulates the radio-frequency emissions caused by oscillators with a frequency of 10 kHz or higher.

closed loop
A regulator circuit in which the actual value of the controlled variable (such as pressure, temperature or speed) is sensed and a signal proportional to this value (feedback signal) is compared with a signal proportional to the desired value (set point signal). The difference between these signals (error signal) causes the controlling device to change in the direction that will reduce the difference in signals to zero.

cogging
A condition in which the motor does not rotate smoothly, but steps from position to position while rotating. This may occur at very low speeds when the motor is driven by an adjustable frequency drive.

common mode voltage
Voltage between a wire and circuit common or earth ground.

commutation
In an SCR circuit, momentarily reversing the polarity to turn the SCR off.

conducted noise
Electrical noise that travels along the power wiring of a building. Also called EMI. constant horsepower/kW range
When operated above base speed with an adjustable frequency drive, the speed range where base speed horsepower/kW can be utilized.

constant torque load
A mechanical device which requires approximately the same torque at all operating speeds. Starting torque requirements may be significantly greater than running torque.

constant torque range
When operated below base speed on an adjustable frequency drive, the speed range where base speed torque can be utilized.

constant volts per Hertz
(V/Hz) A description of one way an adjustable frequency drive varies the output voltage to be directly proportional to the frequency at all times. This is necessary for the motor to produce full torque over the operating speed range.

contactor
A two-state (on or off) device for making and breaking an electric power circuit.

contactor bypass
A three position (drive, off, line) selector switch that electrically selects whether the motor is driven by the drive, the line or disconnected from both. Both the motor starter, supplied inside the drive enclosure, and the contactor for the drive are controlled by this selector switch. A light usually indicates if the motor is running from the line. Terminals are available to remotely control the bypass, and to remotely indicate drive or line operation. (see automatic bypass and fused power disconnect)

contactor motor selection
This selector switch electrically transfers the drive from one motor to another by opening and closing contactors mounted in the drive enclosure. This allows the drive to operate either one motor or the other at adjustable speed. A terminal strip is available, if specified, for remote actuation of the contactors.

contactor reversing
An electro-mechanical method of reversing motor rotation by using two contactors. One contactor produces rotation in one direction, the other produces rotation in the other direction. They are interlocked so that both cannot be energized at the same time.

continuous rating
The maximum constant load that can be carried indefinitely without exceeding temperature and/or other established limitations of the motor, adjustable frequency drive, or driven equipment.

control circuit
The circuit which carries the electric signals directing the performance of a controller, but not carrying the main circuit power.

converter
(Electronic) A device to change DC to AC. e.g., a rotary converter or a solid state converter. An adjustable frequency drive is in part a solid state converter.

cos(–)
Displacement (or phase shift) power factor. The angle – indicates the number of degrees of displacement that exists between the voltage and current waves. Technically, displacement power factor describes only one aspect of total power factor, but most commonly, the term "power factor" is assumed to mean "displacement power factor".

counter EMF
Counter electromotive force. Voltage induced in an inductor or the armature coil of a motor by the flow of changing current through the inductor or armature. The polarity of this voltage is opposite (counter) to that of the change of current. Also called back voltage.

critical speed step-over
A circuit in an adjustable speed drive that can be adjusted to assure that the resonant speed or speeds of a cooling tower, vane axial fan or other driven system can be avoided.

C-UL Canadian Underwriters Laboratories
A product safety certification and listing service available through Underwriters Laboratories, Inc. that is accepted by federal, provincial and local authorities in Canada.

current
Measure of the rate of flow of electrical charge. This is measured in amperes (A), or amps.

current limit
Electronic drives have circuitry that automatically controls the maximum output current of the drive. This is necessary to protect the current carrying components. Typically the drive’s rating is at 100% current. Constant torque drives typically have a maximum current limit of 150%, variable torque drives a maximum current limit of 100 to 115%.

current source drive
An adjustable frequency drive that directly controls the current supplied to the driven motor. Such drives use a large inductor to help control current. While current source drives had been common in the past, they are now mostly reserved for drives larger than 300 HP/225kW and for applications where fast load reversals will be present.