Back to Resources# Power Electronics Terminology

**AC (Alternating Current):** The commonly available electric power supplied by an AC generator and is distributed in single- or three-phase forms. AC current changes its direction of flow with respect to time (cycles).

**Ambient temperature:** Temperature of the surroundings.

**Ampere (Amp):** The standard unit of electric current. The current produced by an electromotive force of one volt in a circuit having a resistance of one ohm equals to one ampere.

**Back-EMF: **Electromotive force generated when a conductor passes through a magnetic field. In a motor, it is generated any time the armature is moving in the field whether the motor is under power or not. The term "back" or "counter" EMF is referring to the polarity of the voltage and the direction of the current flow as being opposed to the supply voltage and current to the motor under power.

**Back EMF constant (V/RPM):** The constant corresponding to the relationship between the induced voltage in the rotor and the speed of rotation. In brushless motors, the back-EMF constant is the constant corresponding to the relationship between the induced voltage in the motor phases and the rotational speed.

**Bridge Rectifier (Diode): **A diode bridge rectifier is a non-controlled full wave rectifier that produces a DC voltage at the output when supplied by alternating voltage at the input.

**Capacitance:** As the measure of electrical storage potential of a capacitor, the unit of capacitance is the farad, but typical values are expressed in microfarads.

**Capacitor:** A device that stores electrical energy.

**Closed-loop:** Describes a system where a measured output value is compared to a desired input value and corrected accordingly (e.g. a servomotor system).

**Controller:** Used to describe collective group of electronics that control an electrical device (e.g. motor, drive, indexer, etc.).

**Converter:** The process of changing AC to DC or DC to AC or DC to DC. The term “converter” may also refer to the process of changing AC to DC to AC (e.g. adjustable frequency drive). A “frequency converter,” such as that found in an adjustable frequency drive, consists of a Rectifier, a DC Intermediate Circuit, and Inverter and a Control Unit.

**Current:** The flow of electrons through a conducting material. By convention, current is considered to flow from positive to negative potential. The electrons, however, actually flow in the opposite direction. The unit of measurement is the Ampere.

**Direct Torque Control:** A method of using current regulation circuitry to regulate torque instead of speed.

**Drive:** (Also called a Variable Speed Drive) An electronic device that can control the speed, torque, horsepower and direction of an AC or DC motor.

**Eddy Current: **Localized currents induced in a conductive material by a changing magnetic field. These currents translate into losses (heat) and their minimization is an important factor in motor design.

**Efficiency:** Ratio of output power over input power expressed in percent. In motors, it is the ratio of mechanical output to electrical input indicated by a percent i.e. the effectiveness with which a motor converts electrical energy into mechanical energy.

**EMF:** A commonly used acronym in motor design for electromotive force which is another term for voltage or potential difference.

**Feedback:** The element of a control system that provides an actual operation signal for comparison with the set point to establish an error signal used by the regulator circuit.

**Field Weakening: **The action of weakening the strength of the magnetic field in a rotating machine and thereby decreasing the back EMF that is produced. Consequently, the speed that the machine can be operated at can be increased.

**Filter:** A device that passes signals within a desirable frequency range and attenuates all others.

**Frequency:** Frequency is the number of occurrences of a repeating event per unit time. The typical unit for frequency is Hertz (Hz). One Hz is defined as one cycle per second. The standard frequency that is used by North American power utilities is 60 Hz. Most of the rest of the world uses 50 Hz power.

**Inductance:** Inductance is the ratio of the voltage over the rate of change of current. The unit for inductance is the Henry (H).

**Inductor:** An inductor is a passive electrical component, which resists changes in electric current passing through it by setting up a time-varying magnetic field within its core. The varying magnetic field induces a voltage in the conductor which opposes the change in current that created it.

**Inverter:** Typically describes an electronic device that converts DC voltage into AC voltage.

**Laminations:** The steel portion of a transformer or stator core of a motor make up a series of thin laminations (sheets) which are stacked and fastened together by cleats, rivets or welds. Laminations are used instead of a solid piece in order to reduce eddy-current losses.

**Open Loop:** A control system that lacks feedback.

**Power: **Work done per unit of time. Measured in horsepower or watts usually.

**Power Factor:** Power factor is the ratio of real Power (W) over apparent power (VA). In undistorted AC systems, it is also a measurement of the phase difference between the voltage and current. It is represented by the cosine of the angle of this phase difference.

**PWM:** Pulse width modulation. An acronym which describes a switch-mode control technique used in drives to control motor voltage and current. This control technique is used in contrast to linear or step wave control and offers the advantages of greatly improved efficiency.

**Rectifier:** A device that transforms alternating-current to direct-current.

**Reluctance: **The characteristics of a magnetic field which resist the flow of magnetic lines of force through it.

**Resistance: **It is the measure of opposition to current flow through a given medium [Ohm]. Substances with high resistances are called insulators and those with low resistances are called conductors. Those in between are known as semiconductors. The unit is the Ohm.

**Resistor:** A device that resists the flow of electrical current for the purpose of operation, protection or control. There are two types of resistors - fixed and variable. A fixed resistor has a fixed value of ohms while a variable resistor is adjustable.

**Resolution: **The smallest distinguishable increment into which a quantity can be divided (e.g. position or shaft speed). For encoders, it is the number of unique electrically identified positions occurring in 360 degrees of input shaft rotation.

**RMS Current:** Root mean square current. Root-mean-square (RMS) refers to the most common mathematical method of defining the effective voltage or current of an AC wave. In an intermittent duty cycle application, the RMS current is equal to the value of steady state current which would produce the equivalent resistive heating over a long period of time.

**Speed constant:** Inverse of Back EMF constant. See Back EMF constant.

**Speed Range:** The minimum and maximum at which an electric machine must operate safely under constant or variable torque load conditions.

**Speed Regulation:** In adjustable speed drive systems, speed regulation measures the motor and control's ability to maintain a constant preset speed despite changes in load from zero to 100%. It is expressed as a percentage of the drive system's rated full load speed.

**Transducer:** A device that converts one energy form to another (e.g. mechanical to electrical). Also, a device that when actuated by signals from one or more systems or media, can supply related signals to one or more other systems or media.

**Transistor:** A solid-state three-terminal device that allows amplification of signals and can be used for switching and control.

**Total Harmonic Distortion (THD): **A measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. For details on total harmonic distortion, click here.

**Voltage:** The force that causes a current to flow in an electrical circuit. The unit is the Volt. 1. Volt is defined as the difference of electric potential between two points on a conductor that is carrying a constant current of one ampere when the power dissipated between those points is one watt.

**Watt:** The amount of power required to maintain a current of one ampere at a pressure of one volt when the two are in phase with each other.

**Work:** A force moving an object over a distance. Work = force x distance travelled.