Alternating current. Electrical current that periodically reverses direction. This is the type of current which power utilities supply to buildings. Typically, the frequency of the alternating current is 50 cycles/second, or 50 Hz. In the U.S.A. the frequency is 60 Hz

AC Drive
See adjustable frequency drive

A device or switch which starts an electric motor by connecting it directly to the supply line. Also called full voltage controller. See motor starter.

actual power
The actual amount of electrical power that is being supplied in an AC circuit. This is always less than or, at most, equal to the apparent power, which is calculated by multiplying the voltage times the current. Actual power is measured in watts (W) or kilowatts (kW).

adjustable frequency drive
An electronic device that takes the constant voltage and frequency AC power from the power utility and converts it into AC power of a controlled frequency and voltage. This is used to control the speed of AC induction motors. The speed of the motor is proportional to the frequency provided by the adjustable frequency drive. ambient temperature The temperature of the medium, such as air, into which the heat of the equipment is dissipated. For self-ventilated enclosed equipment, the ambient temperature is the temperature of the air outside the enclosure in the immediate vicinity of the equipment.

Ampere (A). The unit used to measure electric current.

Device which produces an output voltage or current that is continuously proportional to input voltage or current. Its purpose is to obtain either control or enhancement of signals.

The peak value of a wave.

American National Standards Institute. Source of US design and safety standards. ANSI standards are often incorporated into OSHA (Occupational Safety and Health Administration) regulations.

apparent power
The simple product of voltage and current. This does not represent the actual power in an AC circuit because it does not take into account phase shift and harmonic distortion. Therefore, instead of being expressed in watts (W) or kilowatts (kW), it is expressed in kilovolt-amperes (kVA). Apparent power is never less than actual power. Transformers, wires, and other power distribution equipment must be sized for apparent power.

approach temperature
(of a cooling tower) The difference between the design outlet temperature and the design wet bulb temperature. The smaller the approach temperature, the larger the cooling tower must be to provide the designed amount of cooling.

audio frequency range
The range of frequencies from 20 Hz to 20,000 Hz.

automatic bypass
(see contactor bypass) For automatic transfer from drive to line, automatic bypass may be added to contactor bypass. With automatic bypass, if a drive fault occurs, a timer is activated. If the fault does not clear and the drive does not restart before the timer times out, circuitry automatically transfers the motor to full speed operation from the AC power line.

A transformer with only one coil. The entire coil acts as a primary winding while part of it also acts as a secondary winding; or the entire coil acts as a secondary winding while part of it also acts as a primary winding. It is simple and low cost, but does not provide electrical isolation.

auxiliary contacts
Auxiliary contacts of a switching device are contacts in addition to the main circuit contacts and operate with the movement of the main circuit contacts.

American Wire Gage. An American standard for wire sizes relating to their diameter. The larger the number, the smaller the wire diameter.