Abbreviation for 'alternating current'. Indicates the sinusoidal voltage which is available through power sockets all over the world to power electrical devices.


Rate of increase in velocity with respect to time; equal to net torque divided by inertia.


Difference between the actual value and the measured or expected value.


A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.

Alternating Current (AC)

The standard power supply available from electric utilities.

Ambient temperature

Temperature of the surroundings. The standard NEMA rating for ambient temperature is not to exceed 40 degrees C.

Ampere (Amp)

The standard unit of electric current. The current produced by a pressure of one volt in a circuit having a resistance of one ohm.


Electronics that convert low level inputs to high level outputs.


The rotating part of a DC or universal motor.

Armature Current

The DC current required by a DC motor to produce torque and drive a load. The maximum safe, continuous current is stamped on the motor nameplate. This can only be exceeded for initial acceleration, and for short periods of time. Armature current is proportional to the amount of torque being produced, therefore, it rises and falls as the torque demand rises and falls.

Armature Reaction

The current that flows in the armature winding of a DC motor tends to produce magnetic flux in addition to that produced by the field current. This effect, which reduces the torque capacity, is called armature reaction and can effect the commutation and the magnitude of the motor’s generated voltage.

Axial Movement

Often called 'end play'. The end movement of motor or gear shafts. Usually expressed in thousandths of an inch.

Axial Thrust

The force or loads that are applied to the motor shaft in a direction parallel to the axis of the shaft (such as from a fan or pump).

Back EMF constant

It is 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.

Back of a Motor

The back of a motor is the end which carries the coupling or driving pulley (NEMA). This is sometimes called the drive end (D.E.) or pulley end (P.E.)


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 refers 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.


This is the typically undesirable quality of 'play' or 'slop' in a mechanical system. Gearboxes, depending on the level of the precision of the parts and the type of gearing system involved can have varying degrees of backlash internally. Usually expressed in thousandths of an inch and measured at a specific radius at the output shaft.

Base Speed

Base speed is the manufacturer’s nameplate rating where the motor will develop rated HP at rated load and voltage. With DC drives, it is commonly the point where full armature voltage is applied with full rated field excitation. With AC systems, it is commonly the point where 60 Hz is applied to the induction motor.


Bearings reduce friction and wear while supporting rotating elements. When used in a motor, they must provide a relatively rigid support, for the output shaft Bearings act as the connection point between the rotating and stationary elements of a motor. There are various types such as roller, ball, sleeve (journal) and needle. Ball bearings are used in virtually all types and sizes of electric motors. They exhibit low friction loss, are suited for high-speed operation and are compatible with a wide range of temperatures.

Bifilar winding

Indicates two distinct windings in the same physical arrangement; these windings are usually wired together, either in series or in parallel, to form one phase.

Bipolar chopper drive

Drive that uses the switch mode method to control motor current and polarity.


Braking provides a means of stopping an AC or DC motor and can be accomplished in several ways. A. Dynamic Braking slows the motor by applying a resistive load across the armature leads after disconnection from the DC supply. This must be done while the motor field is energised. The motor then acts as a generator until the energy of the rotating armature is dissipated. This is not a holding brake. B. Regenerative Braking is similar to Dynamic Braking, but is accomplished electronically. The generated power is returned to the line through the power converter. It may also be just dissipated as losses in the converter (within its limitations).

Breakaway Torque

The torque required to start a machine from standstill. It is always greater than the torque needed to maintain motion.

Breakdown Torque

The maximum torque a motor can achieve with rated voltage applied at rated frequency, without a sudden drop in speed or stalling.

Bridge Rectifier

A full-wave rectifier that conducts current in only one direction of the input current. AC applied to the input results in approximate DC at the output.

Bridge Rectifier (Diode, SCR)

A diode bridge rectifier is a non-controlled full wave rectifier that produces a constant rectifier DC voltage. An SCR bridge rectifier is a full wave rectifier with an output that can be controlled by switching on the gate control element.


A brush is a conductor, usually composed of some element of carbon, serving to maintain an electrical connection between stationary and moving parts of a machine (commutator of a DC motor). The brush is mounted in a spring-loaded holder and positioned tangent to the commutator segments against which it 'brushes'. Pairs of brushes are equally spaced around the circumference of the commutator.

Brushless and brush-type motors

Electric motors develop power through the attraction of two magnetic fields. One of these magnetic fields is able to rotate, which means that when this magnetic field is activated, the motor turns. The greatest power is obtained from an electric motor if the two magnetic fields act on each other at right angles. In order to achieve this, one magnetic field must be switched appropriately. This is done with carbon brushes in the case of brush-type motors and via the motor electronics in the case of brushless motors.


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


A device that stores electrical energy. Used on single-phase motors, a capacitor can provide a starting 'boost' or allow lower current during operation.

Capacitor Motor

A single-phase induction motor with a main winding arranged for direct connection to the power source, and an auxiliary winding connected in series with a capacitor. There are three types of capacitor motors: capacitor start, in which the capacitor phase is in the circuit only during starting, permanent-split capacitor, which has the same capacitor and capacitor phase in the circuit for both starting and running; two-value capacitor motor, in which there are different values of capacitance for starting and running.

Capacitor Start

The capacitor start single-phase motor is basically the same as the split phase start, except that it has a capacitor in series with the starting winding. The addition of the capacitor provides better phase relation and results in greater starting torque with much less power input. As in the case of the split phase motor, this type can be reversed at rest, but not while running unless special starting and reversing switches are used. When properly equipped for reversing while running, the motor is much more suitable for this service than the split phase start since it provides greater reversing ability at less watts input.

Case temperature rating

Maximum temperature the motor case can reach without the inside of the motor exceeding its internal temperature rating.

Centre Distance

A basic measurement or size reference for worm gear reducers, measured from the centreline of the worm to the centreline of the worm wheel.

Centrifugal Cutout Switch

A centrifugally operated automatic mechanism used in conjunction with split phase and other types of single-phase induction motors. Centrifugal cutout switches will open or disconnect the starting winding when the rotor has reached a pre-determined speed and reconnect it when the motor speed falls below it. Without such a device, the starting winding would be susceptible to rapid overheating and subsequent burnout.

Closed-loop control

Closed-loop control means that a controlled variable is measured and an electronic controller reacts as appropriate to this measurement. A closed-loop controller automatically detects external influences and can react in such a way as to eliminate errors. Strictly speaking, closed-loop control is when the controlled variable is measured directly, as is the case, for example, with a displacement encoder on the carriage of a linear axis. We use the term 'semi-closed-loop control' for cases where there is an additional mechanism between the measuring system and the controlled variable, for example in the case of a toothed-belt axis with a servo motor, where the measuring system is located on the motor and the toothed belt is between this and the carriage. The opposite of closed-loop control is open-loop control, where there is no feedback of the controlled variable.


A condition in which a motor does not rotate smoothly but 'steps' or 'jerks' from one position to another during shaft revolution. Cogging is most pronounced at low motor speeds and can cause objectionable vibrations in the driven machine.


A term that refers to the action of steering currents or voltage to the proper motor phases so as to produce optimum motor torque. In brush type motors, commutation is done electro-mechanically via the brushes and commutator. In brushless motors, commutation is done by the switching electronics using rotor position information typically obtained by hall sensors, a tachsyn, a resolve, or an encoder.


The commutator is a mechanical device in a brushed DC or universal motor that passes current from the brushes to the windings and is fastened to the motor shaft and is considered part of the armature assembly. It consists of segments or 'bars' that are electrically connected to two ends of one (or more) armature coils. Current flows from the power supply through the brushes, to the commutator and hence through the armature coils. The arrangement of commutator segments is such that the magnetic polarity of each coil changes a number of times per revolution depending on the number of poles in the motor.

Continuous Duty (CONT)

A motor that can continue to operate within the insulation temperature limits after it has reached normal operating (equilibrium) temperature.

Continuous stall current

Amount of current applied to the motor to achieve the continuous stall torque.

Continuous stall torque

Maximum amount of torque a motor can provide at zero speed without exceeding its thermal capacity.

Control circuit

The control circuit is normally used to provide a power supply for electronic devices, such as logic components, measuring systems, switches and displays. The control circuit should be routed separately from the load circuit.


A controller or programmable logic controller (PLC) is an electronic device that processes all its inputs within a defined period and sets its outputs accordingly.


The process of changing AC to DC. This is accomplished through use of a diode rectifier or thyristor rectifier circuit. 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, an Inverter and a Control Unit.


The mechanical connector joining the motor shaft to the equipment to be driven.


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 and 1 Amp is defined as the constant current produced between two straight infinitely long parallel conductors with negligible cross section diameter and spaced one metre apart in a vacuum.

Current at peak torque

Amount of current required to produce peak torque.

Current Constant

The constant corresponding to the relationship between motor current and motor output torque.

Current, AC

The standard power supply available from electric utilities or alternators.

Current, DC

The power supply available from batteries, generators (not alternators), or a rectified source used for special purpose applications.

DC (Direct Current)

Is the type of current where all electrons are flowing in the same direction continuously. If the flow of electrons reverses periodically, the current is called AC (Alternating Current).


Rate of decrease in velocity with respect to time.

Decibel (dB)

A logarithmic measurement of gain. If G is a system gain (ratio of output to input) then 20 log G = gain in decibels (dB).

Demagnetisation (Current)

When a permanent magnet DC motor is subjected to high current pulses at which the motors permanent magnets will be demagnetised. This is an irreversible effect that will alter the motor characteristics and degrade performance.

Detent torque

Torque that is present in a non-energised motor.

Direct drive

A direct drive is a special design of motor that can generate very high forces at low speeds. This gives very high acceleration and reduces cycle times. With a direct drive, the load to be moved is mounted directly on the motor without any transmission or other mechanical components. This technology is available in the form of rotary direct drives (torque motors) and linear direct drives (linear motors).


Amplifier that converts step and direction input to motor currents and voltages.

Drive Controller

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

Drive, PWM

A motor drive utilising Pulse-Width Modulation techniques to control power to the motor. Typically a high efficiency drive that can be used for high response applications.

Drive, SCR

A DC motor drive that utilises internal silicon controlled rectifiers as the power control elements. Usually used for low bandwidths, higher power applications.

Drive, Servo

A motor drive that utilises internal feedback loops for accurate control of motor current and/or velocity.

Drive, Stepper

Electronics that converts step and direction inputs to high power currents and voltages to drive a stepping motor. The stepping motor driver is analogous to the servo motor amplifier.

Duty Cycle

The relationship between the operating and rest times or repeatable operation at different loads. A motor that can continue to operate within the temperature limits of its insulation system after it has reached normal operating (equilibrium) temperature is considered to have a continuous duty (CONT) rating. A motor that never reaches equilibrium temperature but is permitted to cool down between operations, is operating under intermittent (INT) duty. Conditions such as a crane and hoist motor are often rated 15 or 30 minute intermittent duty.

Dynamic Braking

A passive technique for stopping a permanent magnet brush or brushless motor. The motor windings are shorted together through a resistor which results in motor braking with an exponential decrease in speed.

Eddy Current

Localised currents induced in an iron core by alternating magnetic flux. These currents translate into losses (heat) and their minimisation is an important factor in lamination design.


Ratio of mechanical output to electrical input indicated by a percent. In motors, it is the effectiveness with which a motor converts electrical energy into mechanical energy.

Electronic brake

An electronic brake function is produced by a motor controller that switches a motor to act like a generator and thus generate electricity and convert the mechanical energy of rotary motion into electrical energy. This energy conversion process slows the motor down but cannot be used to block the motor and stop it completely.

Emergency-stop circuit

The purpose of an emergency-stop circuit is to bring a machine into a safe state if a hazard is detected. This generally means first braking all motions to a standstill and then shutting off the power supply. The motor controllers CMMS, SFC and SEC have an input that can be used to obtain the fastest possible braking. The power supply is shut off within the customer’s system.


The initials of electromotive force (EMF) which is another term for voltage or potential difference. In DC adjustable speed drives, the voltage applied to the motor armature from the power supply is the EMF and the voltage generated by the motor is the counter-EMF or CEMF.

EMI (Electro-Magnetic Interference)

EMI is noise which, when coupled into sensitive electronic circuits, may cause problems.


The term used to describe the motor housing. The most common industrial types are Open Drip Proof (ODP), Totally Enclosed Fan Cooled (TEFC), Totally Enclosed Non-Ventilated (TENV), and Totally Enclosed Air Over (TEAO).

Encoder (1)

A type of feedback device that converts mechanical motion into electrical signals to indicate actuator position. Typical encoders are designed with a printed disc and a light source. As the disc turns with the actuator shaft, the light source shines through the printed pattern onto a sensor. The light transmission is interrupted by the pattern on the disc. These interruptions are sensed and converted to electric pulses. By counting the pulses, the actuator shaft position is determined.

Encoder (2)

An encoder is an optical or magnetic measuring system that outputs a certain number of pulses after a defined motion. With electric motors, the encoders used usually output 1000 to 4000 pulses per revolution.

End play

The amount of axial displacement resulting from the application of a load equal to the stated maximum axial load.

End shield

The part of a motor that houses the bearing supporting the rotor and acts as a protective guard to the internal parts of the motor; sometimes called endbell, endplate or end bracket.


Difference between the set point signal and the feedback signal. An error is necessary before a correction can be made in a controlled system.


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 bus

Field buses offer serial interfaces to provide communication between the various types of electronic devices within a system. They are standardised with regard to technical data, software and reaction time, thus allowing electronic devices made by different manufacturers to be combined in any desired way. The field bus systems offered for motor controllers are CAN, Profibus, and DeviceNet.

Field Weakening

The action of reducing the current applied to a DC motor shunt field. This action weakens the strength of the magnetic field and thereby increases the motor speed.


A device that passes a signal or a range of signals and eliminates all others.

Floating Ground

A circuit whose electrical common point is not at earth ground potential or the same ground potential as circuitry it is associated with. A voltage difference can exist between the floating ground and earth ground.


The tendency to change the motion or position of an object with a push or pull. Force is measured in ounces or pounds.

Form Factor

A figure of merit that indicates how much rectified current deviates from pur (nonpulsating) DC. A large departure from unity form factor (pure DC) increases the heating effect of the motor. Mathematically, it is expressed as Irms/Iav (Motor heating current / Torque producing current).

Four-Quadrant Operation

The four combinations of forward and reverse rotation and forward and reverse torque of which a regenerative drive is capable. The four combinations are - 1. Forward rotation / forward torque (motoring). 2. Forward rotation / reverse torque (regeneration). 3. Reverse rotation / reverse torque (motoring). 4. Reverse rotation / forward torque (regeneration).


The supporting structure for the stator parts of an AC motor. In a DC motor, the frame usually forms a part of the magnetic coil. The frame also determines mounting.


Alternating electric current frequency is an expression of how often a complete cycle occurs. Cycles per second describe how many complete cycles occur in a given time increment. Hertz (Hz) has been adopted to describe cycles per second so that time as well as number of cycles is specified. The standard power supply in North America is 60 Hz. Most of the rest of the world has 50 Hz power.

Frequency converter

A frequency converter is an electronic device that is used to provide an adjustment facility for the speed of a three-phase motor. With this device, speed can be controlled but it is not possible to approach positions. This requires the addition of a master controller and a measuring system.

Friction Torque

The sum of torque losses independent of motor speed. These losses include those caused by static mechanical friction of the ball bearings and magnetic hysteresis of the stator.

Front of a Motor

The end opposite the coupling or driving pulley (NEMA). This is sometimes called the opposite pulley end (O.P.E.) or commutator end (C.E.).

Full Load Amperes

Line current (amperage) drawn by a motor when operating at rated load and voltage on motor nameplate. Important for proper wire size selection, and motor starter or drive selection. Also called full load current.

Full Load Torque

The torque a motor produces at its rated horsepower and full-load speed.


Any machine that converts mechanical energy into electrical energy.

Grounded Circuit

An electrical circuit coupled to earth ground to establish a reference point. An electric circuit malfunction caused by insulation breakdown, allowing current flow to ground rather than through the intended circuit.

Holding brake

The brakes used in handling systems are usually holding brakes, i.e. they can hold an axis at a standstill. It is usually possible for a holding brake to bring a motion axis to a standstill a number of times in the case of a power supply failure.


A measure of the amount of work that a motor can perform in a given period of time.

Hysteresis Loss

The resistance offered by materials to becoming magnetised (magnetic orientation of molecular structure) results in energy being expended and corresponding loss. Hysteresis loss in a magnetic circuit is the energy expended to magnetise and demagnetise the core.

Incremental measuring system

With an incremental measuring system, there is no fixed zero point. It is first necessary to execute a reference travel motion to define a zero point before it is possible to approach an absolute position. Cf. 'Absolute measuring system'.

Incremental positioning

With incremental positioning, travel is relative to the current position, e.g. you specify that an axis should travel on a further 10 mm. Cf. 'Absolute positioning'.


The characteristic of an electric circuit by which varying current in it produces a varying magnetic field which causes voltages in the same circuit or in a nearby circuit.

Induction Motor

The simplest and most rugged electric motor, it consists of a wound stator and a rotor assembly. The AC induction motor is named because the electric current flowing in its secondary member (the rotor) is induced by the alternating current flowing in its primary member (the stator). The power supply is connected only to the stator. The combined electromagnetic effects of the two currents produce the force to create rotation.


A measure of a body’s resistance to changes in velocity, whether the body is at rest or moving at a constant velocity. The velocity can be either linear or rotational. The moment of Inertia (WK2) is the product of the weight (W) of an object and the square of the radius of gyration (K2). The radius of gyration is a measure of how the mass of the object is distributed about the axis of rotation. WK2 is usually expressed in units of Ib-ft2.


In motors, classified by maximum allowable operating temperature. NEMA classifications include - Class A = 105 °C, Class B = 130 °C, Class F = 155 °C and Class H = 180 °C.

Integral Horsepower Motor

A motor built in a frame having a continuous rating of 1 HP or more.

Intermittent Duty (INT)

A motor that never reaches equilibrium temperature (equilibrium), but is permitted to cool down between operations. For example, a crane, hoist or machine tool motor is often rated for 15 or 30 duty.

International Electrotechnical Comm (IEC)

The worldwide organisation that promotes international unification of standards or norms. Its formal decisions on technical matters express, as nearly as possible, an international consensus.


An electronic device that converts fixed frequency and fixed voltages to variable frequency and voltage. Enables the user to electrically adjust the speed of an AC motor.

IR Compensation

A way to compensate for the voltage drop across resistance of the AC or DC motor circuit and the resultant reduction in speed. This compensation also provides a way to improve the speed regulation characteristics of the motor, especially at low speeds. Drives that use a tachometer-generator for speed feedback generally do not require an IR Compensation circuit because the tachometer will inherently compensate for the loss of speed.


The steel portion of the rotor and stator cores 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.

Linear motor

See 'Direct drive'.

Load circuit

A load circuit is used to provide large currents for motors. These are fed to the power stages of the electronic controllers, which then supply the motors. In many cases, only the load circuitis switched off in the case of an emergency stop, leaving the control circuit switched on. In this way, the position of an axis is known at all times. This can be done with the motor controllers CMMS, SFC and SEC.

Locked Rotor Current

Measured current with the rotor locked and with rated voltage and frequency applied to the motor.

Locked Rotor Torque

Measured torque with the rotor locked and with rated voltage and frequency applied to the motor.

Mechanical Time Constant

The time required by the motor to reach a speed of 63% of its final no-load speed from standstill.

Meggar Test

A test used to measure an insulation system’s resistance. This is usually measured in Megohms and tested by passing a high voltage at low current through the motor windings and measuring the resistance of the various insulation systems.


A device that takes electrical energy and converts it into mechanical energy to turn a shaft.

Motor controller

A motor controller is a frequency converter fitted with an additional controller. This allows a selective approach to various positions. We offer the motor controllers CMMS, SFC andSEC.


The plate on the outside of the motor describing the motor horsepower, voltage, speed efficiency, design, enclosure, etc.


The National Electrical Manufacturers Association is a non-profit organisation organised and supported by manufacturers of electrical equipment and supplies. Some of the standards NEMA specifies are - HP ratings, speeds, frame sizes and dimensions, torques and enclosures.

No-load current - [A]

The current consumption of the motor at nominal voltage and under no-load conditions. This value varies proportionally to speed and is influenced by temperature.

No-load speed

The maximum speed the motor attains with no additional torque load at a given voltage. This value varies according to the voltage applied to the motor.

Nominal Voltage

The voltage applied to the armature at which the nominal motor specifications are measured or calculated.

Open-loop control

We speak of open-loop control when an electronic controller feeds a signal to a motor and does not receive a feedback signal to say whether the position in question has been reached. All stepper motors without encoders operate in this way, which offers an adequate degree of safety as long as the peak torque of motor is not reached.

Output Power - [W]

The mechanical power that the motor generates based on a given input power. Mechanical power can be calculated in a few different ways. For motors, one common way is the multiplication of the output speed and torque and conversion factor.


Proportional-Integral-Derivative. An acronym that describes the compensation structure that can be used in a closed-loop system.


A method of braking a motor that involves applying partial or full voltage in reverse in order to bring the motor to zero speed.

PMDC Motor

A motor consisting of a permanent magnet stator and a wound iron-core rotor. These are brush type motors and are operated by application of DC current.

PNP and NPN logic

In Europe, logic inputs and outputs are wired using PNP logic, i.e. switching is from + via the load to –. With NPN logic, switching is from – via the load to +. The reason for this is partly historical but also safety-related. With NPN logic, there is a large number of terminals connected directly to the + conductor. If there is a short circuit from one of these terminals to a housing or –, no output will then work. If the same thing happens with PNP logic, the output transistor concerned will be destroyed but all the other inputs and outputs will continue towork.


Work done per unit of time. Measured in horsepower or watts - 1 HP = 33,000 ft-lb / min. = 746 watts.

Power Factor

A measurement of the time phase difference between the voltage and current in an AC circuit. It is represented by the cosine of the angle of this phase difference. Power factor is the ratio of Real Power (kW) to total kVA or the ratio of actual power (W) to apparent power (volt-amperes).

Prime Mover

In industry, prime mover is most often an electric motor. Occasionally engines, hydraulic or air motors are used. Special application considerations are called for when other than an electric motor is the prime mover.

Pull Out Torque

Also called breakdown torque or maximum torque, this is the maximum torque a motor can deliver without stalling.

Pull Up Torque

The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM.


Pulse width modulation. An acronym which describes a switch-mode control technique used in amplifiers and drivers to control motor voltage and current. This control technique is used in contrast to linear control and offers the advantages of greatly improved efficiency.


The acceleration and deceleration of a motor. May also refer to the change in frequency of the applied step pulse signal.

Real-time capability

Real-time capability indicates that the reaction times of an electronic device or field bus lie within a certain period. In order to state this capability clearly, it is necessary to know the required reaction time. A modern controller processes all its inputs and outputs within 10 to 20 ms and thus has a reaction time within this period.


A device that transforms alternating-current to direct-current.


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.


The characteristic of a motor to act as a generator when the CEMF is larger than the drive’s applied voltage (DC drives) or when the rotor synchronous frequency is greater than the applied frequency (AC drives).


The characteristics of a magnetic field that resist the flow of magnetic lines of force through it.

Resistance - [Ohm]

It is the measure of opposition to current flow through a given medium. 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. 1 Ohm is defined as the resistance between two points on a conductor when an electric potential difference of one volt applied between those points produces a current of one Amp and when that conductor is not the source of any electro motive force.


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.


The smallest distinguishable increment into which a quantity can be divided (e.g. position or shaft speed). It is also the degree to which nearly equal values of a quantity can be discriminated. For encoders, it is the number of unique electrically identified positions occurring in 360 degrees of input shaft rotation.


A resolver is an inductive measuring system. Its functional principle is like that of a generator, i.e. a sinusoidal voltage is generated with a frequency that is directly dependent on the speed. The resolver electronics can use this frequency to determine the speed, acceleration and position.


The effect of a periodic driving force that causes large amplitude increases at a particular frequency. (Resonance frequency.)


Acronym for Radio Frequency iInterference.

Rise Time

The time required for a signal to rise from 10% of its final value to 90% of its final value.

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 resistive heating over a long period of time.

RMS Torque

Root Mean Square torque. For an intermittent duty cycle application, the RMS torque is equal to the steady state torque which would produce the same amount of motor heating over long periods of time.


The rotating component of an induction AC motor. It is typically constructed of a laminated, cylindrical iron core with slots for cast-aluminium conductors. Short-circuiting end rings complete the 'squirrel cage', which rotates when the moving magnetic field induces a current in the shorted conductors.


The inability of a reducer to be driven backwards by its load. As a matter of safety, no LEESON reducer should be considered self-locking.

Service Factor

When used on a motor nameplate, a number which indicates how much above the nameplate rating a motor can be loaded without causing serious degradation (i.e. A motor with 1.15 S-F can produce 15% greater torque than one with 1.0 S-F). When used in applying motors or gear motors, it is a figure of merit which is used to adjust measured loads in an attempt to compensate for conditions which are difficult to measure or define.

Servo motor

Servo' indicates closed-loop control. A servo motor has a built-in measuring system, so that the motor position is known accurately at all times. This allows external forces to be detected and compensated for and makes it possible to achieve very high accuracy.

Servo System

An automatic feedback control system for mechanical motion in which the controlled or output quantity is position, velocity, or acceleration. Servo systems are closed loop systems.

Settling Time

The time required for a step response of a system parameter to stop oscillating or ringing and reach its final value.

Shock Load

The load seen by a clutch, brake or motor in a system that transmits high peak loads. This type of load is present in crushers, separators, grinders, conveyors, winches and cranes.

Short Circuit

A fault or defect in a winding causing part of the normal electrical circuit to be bypassed, frequently resulting in overheating of the winding and burnout.

Shunt Resistor

A device located in a servo amplifier for controlling regenerative energy generated when braking a motor. This device dissipates or 'dumps' the kinetic energy as heat.

Silicon Controlled Rectifier (SCR)

A solid-state switch, sometimes referred to as a thyristor. The SCR has an anode, cathode and control element called the gate. The device provides controlled rectification since it can be turned on at will. The SCR can rapidly switch large currents at high voltages. They are small in size and light in weight.


The arrangement of laminations on a rotor or armature to provide a slight angular pattern of their slots with respect to the shaft axis. This pattern helps to eliminate low speed cogging in an armature and minimise induced vibration in a rotor, as well as reduce associated noise.


The difference between RPM of the rotating magnetic field and RPM of the rotor in an induction motor. Slip is expressed as a percentage and may be calculated by the following formula - Slip = Synchronous Speed - Running Speed x 100/ Synchronous Speed

Speed constant - [rpm/V]

The speed variation per Volt applied to the motor phases at constant load.

Speed Range

The minimum and maximum speed at which a motor must operate 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.

Stall torque

The torque developed by the motor at zero speed and nominal voltage.

Starting Current

Amount of current drawn at the instant a motor is energised - in most cases much higher than that required for running. Same as locked rotor current.

Starting Torque

The torque or twisting force delivered by a motor at the instant it is energised. Starting torque is often higher than rated running or full load torque.


The non-rotating part of a magnetic structure. In a motor the stator usually contains the mounting surface, bearings, and non-rotating windings or permanent magnets

Stepper motor

A stepper motor is a specially designed motor that advances in individual steps. Due to this stepwise advance, operation is relatively loud, but the motor does not vibrate when at astandstill. Normal stepper motors are not fitted with measuring systems. The motor is openloop-controlled and not closedloop-controlled,with the result that it is relatively inaccurate.


The ability of a device to resist deviation due to load change

Terminal inductance, phase to phase - [µH]

The inductance measured between two phases at 1 kHz.

Terminal resistance, phase to phase

The resistance measured between two motor phases. The coil temperature directly affects the value.

Thermal Protector

A device, sensitive to current and heat, that protects the motor against overheating due to overload or failure to start. Basic types include automatic rest, manual reset and resistance temperature detectors.

Thermal resistance Rth 1 / Rth 2 - [K/W]

Rth 1 corresponds to the value between the coil and housing. Rth 2 corresponds to the value between the housing and the ambient air. Rth 2 can be reduced by enabling exchange of heat between the motor and the ambient air (for example using a heat sink or forced air cooling.

Three-phase current

Three-phase current is a special form of alternating current which is transmitted via three conductors plus a neutral conductor (return conductor). It is used to drive particularly powerful electric motors and machines.

Thrust Load

Force imposed on a shaft parallel to a shaft's axis. Thrust loads are often induced by the driven machine. Take care to be sure the thrust load rating of the reducer is sufficient enough that its shafts and bearings can absorb the load without premature failure.


A turning force applied to a shaft, tending to cause rotation.

Torque Constant (in-lbs)

This motor parameter provides a relationship between input current and output torque. For each ampere of current applied to the rotor, a fixed amount of torque will result.

Torque Control

A method of using current limit circuitry to regulate torque instead of speed.

Torque motor

See 'Direct drive'. We should not use this term, since it is also used for the main drives of machines. For our applications, the term 'rotary direct drive' is more suitable.

Totally Enclosed Enclosure

A motor enclosure, that prevents free exchange of air between the inside and the outside of the enclosure but is not airtight. Different methods of cooling can be used with this enclosure.

Totally Enclosed Fan Cooled (TEFC)

Same as the TENV except it has external fan as an integral part of the motor, to provide cooling by blowing air around the outside frame of the motor.

Totally Enclosed Non-Ventilated (TENV)

No vent openings, tightly enclosed to prevent the free exchange of air, but not airtight. Has no external cooling fan and relies on convection for cooling. Suitable for use where exposed to dirt or dampness, but not for hazardous (explosive) locations.


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.


A momentary deviation in an electrical or mechanical system.


A solid-state three-terminal device that allows amplification of signals and can be used for switching and control. The three terminals are called the emitter, base and collector.

Underwriters Laboratories (UL)

Independent United States testing organisation that sets safety standards for motors and other electrical equipment.


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 1 ampere when the power dissipated between those points is 1 watt.


The amount of power required to maintain a current of 1 ampere at a pressure of 1 volt when the two are in phase with each other. 1 horsepower is equal to 746 watts.