Radial Load: Radial load is the force applied to the encoder shaft perpendicular to the axis of rotation. Note: It is important to note where the force is being applied along the shaft, due to the cantilever effect.

Radial Load (Maximum): Maximum radial load is the maximum force which may be applied perpendicularly to the shaft axis at an exceeding manufacturer's specifications.

Radian: Standard Unit of Angle: The angle at which the arc of circle has the same length as the radius.

Random Wound: The standard type of stator winding used in motors under 1,000 volts. The coils are random wound with round wire as opposed to flat form wound coils.

Reactance (Inductive): The characteristic of a coil when connected to alternating current, which causes the current to lag the voltage in time phase. The current wave reaches its peak later than the voltage wave reaches its peak.

Reference Mark: Random graduation pattern which – when traversed over – produces a signal peak, which may be used to determine an absolute datum within an incremental measuring system.

Reference Pulse: Square-wave signal produced when the scale reference mark is traversed over; normally one measuring step wide; may be used to define an absolute datum within an incremental measuring system.

Reflective Wave: Reflective waves can occur in variable-speed motor applications when the drive and motor are placed a considerable distance apart. The combination of long lead (cable) lengths and the fast switching semiconductors in the drive can cause voltage spikes at the motor's terminals. These spikes can cause the motor's insulation to deteriorate.

Regeneration: The action during motor braking, in which the motor acts as a generator and takes kinetic energy from the load, converts it to electrical energy, and returns it to the amplifier.

Relay: A device that is operative by a variation in the conditions of one electric circuit to effect the operation of other devices in the same or another electric circuit.

Reluctance Synchronous Motor: A synchronous motor with a special rotor design which directly lines the rotor up with the rotating magnetic field of the stator, allowing for no slip under load. Reluctance motors have lower efficiencies, power factors and torques than their permanent magnet counterparts.

Reluctance: The characteristic of a magnetic material which resists the flow of magnetic lines of force through it.

Resilient Mounting: A suspension system or cushioned mounting designed to reduce the transmission of normal motor noise and vibration to the mounting surface. This type of mounting is typically used ina fractional horsepower motors for fans and blowers.

Resistance: The degree of obstacle presented by a material to the flow of electric current is known as resistance and is measure to Ohms.

Resistance, Hot (RH)(Ohms line-to-line): The motor's terminal resistance value specified at the hot winding temperature, which is at the motor's maximum rated temperature.

Resolution: The smallest increment into which a parameter can be broken down. For example, a 1000 line encoder has a resolution of 1/1000 of a revolution.

Resolver: An electromagnetic feedback device which converts angular shaft position into analog signals. These signals can be processed in various ways, such as with an RDC (resolver-to-digital converter) to produce digital position information. There are two basic types of resolvers; transmitter and receiver. A transmitter-type is designed for rotor primary excitation and stator secondary outputs. Position is determined by the ratio of the sine output amplitude to cosine output amplitude. A receiver-type is designed for stator primary excitations and rotor secondary output. Position is determined by the phase shift between the rotor output signal and one of the primary excitation signals.

Resonance: Oscillatory behavior caused by mechanical limitations.

Restart torque: The maximum friction load, at a particular inertial load, that can be applied to the shaft of a synchronous motor without causing it to lose synchronism when accelerating to a constant speed from standstill.

Reversing: Unless otherwise specified, a general-purpose DC motor is reversible. A DC motor can be reversed by changing the polarity of the field or the armature, but not both. When rapid reversing is necessary, the armature circuit is reversed. In some cases, it is advantageous to reverse the field connections of shunt motors, since the controls have to handle much less current, especially on large motors, than do armature-circuit contactors. An AC motor is reversed by reversing the connections of one l leg on three-phase power or by reversing the leads on single phase.

Reversal Error: Measuring error which results from approaching a position from different directions.

Ringing: Oscillation of a system following a sudden change in state.

RMS Current - Root Mean Square Current: In an intermittent duty cycle application, the RMS current is equal to the value of steady state current which would produce the equivalent motor heating over a period of time.

RMS Torque - Root Mean Square Torque: In an intermittent duty cycle application, the RMS torque is equal to the value of steady state torque which would produce the equivalent motor heating over a period of time.

Roller Bearing: A special bearing system with cylindrical rollers capable of handling belted applications too large for standard ball bearings rotates. This is a function of design and the power supply.

Rotating Magnetic Field: The force created by the stator once power is applied to it that causes the rotor to turn.

Rotor: The moving part of the motor, consisting of the shaft and magnets. These magnets are analogous to the field winding of a brush-type DC motor.

RPG (Rotary Pulse Generator): Rotary pulse generator is another term for incremental encoder.

RPM (Revolutions Per Minute): The number of times per minute the shaft of the motor (machine)

RTD (Resistance Thermal Detectors) Winding RTD: A resistance device used to measure temperature change in the motor windings to detect a possible over heating condition. These detectors are bedded into the winding slot and their resistance varies with temperature. Bearing RTD A probe used to measure bearing temperature to detect an overheating condition. The RTD's resistance varies with the temperature of the bearings.

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