radiated noise
Electrical noise, generally in the range of radio frequencies, which is radiated through the air from its source. In general, radiated noise is not a major concern since its strength drops dramatically as the distance from the source increases. Also called RFI.

rated speed
The speed at which all standard operating parameters are established. For a motor, rated speed is the output shaft RPM at rated load, rated input frequency, and rated input voltage.

reactance
A measure of a circuit’s opposition to the flow of AC electricity due to only the effects of capacitors and inductors, usually measured in ohms. No net power is transferred to the reactance of a load.

reactive load
Load consisting of only capacitance and/or inductance.

reactor
An electrical circuit that is made up of inductors and/or capacitors. This is often used to filter electrical noise from a circuit.

rectification
The process by which electric energy is transferred from an alternating current circuit to a direct current circuit.

rectifier
An electrical component that only allows electric current to flow through it in one direction. Also, a circuit that is used to convert alternating current to direct current. It can either control the DC voltage level or allow it to be determined simply by the input AC voltage.

reduced voltage starter
Also called reduced inrush starter. A motor starter that initially reduces the voltage applied to the motor. It may use resistors, reactors, auto transformers or semiconductor power control devices to reduce the voltage. Its purpose is to limit the current draw when the motor is started. An adjustable speed drive should not drive a motor though a reduced voltage starter and it should not be connected to the output of a reduced voltage starter.

regeneration
(AC motor) The characteristic of an AC motor to act as a generator when the speed of its shaft is greater than the synchronous speed for the applied frequency.

regenerative braking
see dynamic braking

regulation
The ability of a control system to hold a value once it has been set. Regulation is usually stated as a percentage of full value. Speed regulation is the percentage of speed change with a defined change in load, usually 0% to 100%. Voltage regulation of a transformer is the percentage of voltage increase that will occur as the transformer’s load is decreased from full load to no load.

relay
An electrically controlled device that causes electrical contacts to change status. Closed contacts will open and open contacts will close when rated voltage is applied to the coil of the relay.

resistance
The resistance in a circuit to the flow of electricity, measured in ohms. The resistive component of a circuit is responsible for the power that is delivered by the source and typically lost as heat within the circuit. see reactance

resonance
(electronic) The effect that occurs when a "tuned" electrical circuit is connected to a source whose frequency is near the resonant frequency of the circuit. The amplitude of the voltage and/or current can quickly increase to very high levels.

resonance
(mechanical) A situation where an object is vibrated at a specific rate (frequency) at which it naturally vibrates easily. The amount of vibration can quickly grow to destructive levels.

resonant frequency
The frequency at which a device will naturally oscillate. The device can be considered to be "tuned" to this frequency. The resonant frequency is normally considered unwanted and is to be damped or avoided.

resonant speed
The speed at which a mechanical system will naturally vibrate. Operation at this speed for an extended time may cause mechanical failure of the system. Systems run at a fixed speed will be designed not to run at a resonant speed, but systems converted to adjustable speed may encounter resonant speeds. Critical speed step-over circuits are used to prevent systems from running at resonant speeds.

reverse acting
(contrast direct acting) In reference to follower signals: a decrease in the value of the input signal produces an increase in the output. Example: an decrease from 10 mA to 4 mA in the input signal creates an increase in the speed of the drive.

RFI
Radio frequency interference. Defined by the FCC as radiated interference in the frequency range of 450 kHz to 30 MHz. RFI is used to describe interference which is radiated through the air. (compare with EMI)

ripple
(ripple current, ripple voltage) A series of small AC voltage or current peaks in a DC component as a by-product of rectifying AC to DC. Filtering can reduce ripple. Also, the peak-to-peak value of a ripple component.

rms
Root mean square. A method of determining a numeric value of an AC signal. This average is taken by squaring the amplitudes of the waveform at various points in a cycle, adding these together, dividing by the number of points used, and then taking the square root of this value. The rms value is important because it indicates the true power-producing capacity of the electricity in the circuit. As a result, most test meters are calibrated to display rms values. Unfortunately, some meters do not accurately display the rms values of distorted waves.

RS-232
A serial communication standard which is commonly used by personal computers. It can be used to connect two devices together over a distance of up to 50 feet.

RS-485
A serial communication standard which is commonly used in large applications. It can be used to connect up to 32 transmitters and 32 receivers over a distance of up to 4000 feet.