R.P.M.: (Revolutions Per Minute) The number of times per minute the shaft of the motor (machine) rotates. This is a function of design and the power supply.

 
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.

 
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 would be embedded into the winding slot and their resistance varies with the 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.

 
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.


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

 
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. The reluctance motors have lower efficiencies, power factors and torques than their permanent magnet counterparts.

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

 
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 in fractional motors for fans and blowers.

 
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 frequently more 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 leg on three-phase power or by reversing the leads on single phase.

 
Roller Bearing: A special bearing system with cylindrical rollers capable of handling belted applications, too large for standard ball bearings.

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

 
Rotor: The rotating member of an induction motor made up of stacked laminations. A shaft running through the center and a squirrel cage made in most cases of aluminum which holds the laminations together and act as a conductor for the induced magnetic field. The squirrel cage is made by casting molten aluminum into the slots cut into each lamination.