Glossary of Terms

Understand some of the common terms used in industrial motor control.

A

Acceleration

The action of changing speed from any speed to a higher speed, irrespective of direction. Depending on drive design, the control of acceleration time may be known as "Up time", "Up rate" or similar variant. It is important to remember that some drive designs apply a single rate control to positive going speed demand changes irrespective of polarity. For example, consider a drive running backwards at half speed being asked to run forwards at half speed. The speed demand can be considered to be positive going, but the period between the drive running at half speed backwards and zero speed is termed deceleration and the period between zero speed and running at half speed forwards is termed acceleration. See also Angular acceleration, Linear acceleration.

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Adaptive control

A general term for changing one parameter against another. For example an adaptive current loop may apply differing gain characteristics depending on whether the armature current is discontinuous or continuous.

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Angular acceleration

A positive rate of change of position about a fixed point or axis. Measured in radians/second/second.

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Angular deceleration

A negative rate of change of position about a fixed point or axis. Measured in radians/second/second.

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Angular velocity

A rate of rotation about a fixed point or axis. Measured in radians/second.

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Anti-parallel

A term describing the bridge configuration of a regenerative drive. Essentially means "back to back" with the positive end of one bridge connected to the negative end of the other and vice versa allowing four quadrant operation. Also known as Inverse parallel.

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Armature

The rotating part of a motor, constructed of a series of current carrying coils.

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Armature choke

Sometimes (unconventionally) called a reactor, this device is connected in series with the motor armature and adds inductance to the overall armature circuit in order to increase the armature time constant and improve form factor. Typical values tens of milli Henries.

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Armature time constant

Expressed in seconds and calculated from L (inductance in Henries) / R (resistance in Ohms). Minimum value recommended for a single phase drive is 10mS, 3.3mS for a three phase drive assuming the drives are used on a 50Hz supply although 20mS and 6.6mS would be preferable.

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Armature voltage feedback

A method for approximately measuring motor speed in constant field motors. See also Back E.M.F.

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Autotune

A term used to describe any process whereby the drive optimise itself to the motor. The most common application of autotune is the drive current loop, where the drive automatically disables field current then perturbates current demand at increasing frequency in order to establish the point of current discontinuity and set appropriate proportional gain and integral time constant.

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Auxilliary

Any secondary function for example an additional speed demand input. Also used to differentiate between various power supplies required by a drive, for example where the drive requires separate supplies for the thyristor bridge, coding, field etc.

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B

Back E.M.F.

The rotation producing voltage in a motor, which is the armature voltage applied to terminals of the motor less (or plus, if regenerating) the armature resistance voltage drop. See also I.R. drop.

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Bridge

A term used to describe the power circuit of the drive, consisting of thyristors only in a fully controlled bridge and a combination of thyristors and diodes in a half controlled bridge.

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Brushgear

The term used to describe that part of a motor which is responsible for transferring electrical power to the commutator. Encompasses brushes, brush holders , springs etc.

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Base speed

The motor speed produced when full rated armature voltage is presented to the armature terminals, given that the correct field is present. See also Field weakening.

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C

Choke

See Armature choke.

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Closed loop

A generic term used to describe any process where the action of a loop is monitored and the feedback thus derived is used to modify its action. For example, monitoring material tension by use of a load cell roller, comparing that with desired tension and using that information via a P.I.D. loop to modify the speed of a driven nip responsible for producing the material tension. Can be considered to be analytical rather than predictive. See also Open loop.

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Coding

A term applied to the method of ensuring the drive thyristors fire at the correct point in time against the main supply, essentially by detecting zero crossover points.

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Common

A term used to indicate the return path for a signal, or a fixed point against which measurements are taken. It is important to realize that the use of this term does not necessarily indicate that it is itself at a low potential. For example, the common on an un-isolated drive will be at mains potential and must not be grounded.

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Compound wound

A term used to describe a motor field configuration, meaning that the motor in question has both series and shunt wound flux producing fields. See also Shunt wound, Series wound.

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Constant power

This term is applied to field weakening motors and describes the region above base speed where armature voltage is kept approximately constant regardless of speed. If the armature current is allowed to be limited at the same level throughout the field weakening range, armature voltage X armature current is constant hence constant power. This term is not automatically applied to field weakening motors, as certain of these require armature current profiling in the field weakening region to aid commutation.

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Constant torque

This term is applied to both field weakening and constant field motors. As torque is proportional to flux and armature current, maximum torque is always available regardless of speed whenever the field is at full strength. The term therefore applies to constant field shunt wound motors all the time, and to field weakening motors below base speed.

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Contactor

The device used to switch main power to a drive. In the case of a three phase drive it is usually important that the drive controls the contactor - i.e. that emergency stop sequencing cannot interrupt the contactor coil supply.

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Continuous current

A term used to indicate the level of armature current at which the current waveform lifts off zero and becomes continuous. Adaptive current loops use knowledge of this point to determine where the current loop gain break point is to be set.

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Current control

Otherwise known as torque control, a method whereby the drive speed loop is bypassed or saturated and the drive current loop is either directly addressed or limited externally.

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Current limit

The method by which the maximum excursion of integrated speed error is limited. Usually involves a form of automatic profiling so that more than 100% current is available for acceleration or deceleration, for example.

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Current loop

A generic term used to describe that part of the drive circuitry which integrates the error between current demand and feedback and eventually produces firing phase angle demand. The "innermost" loop in a simple drive system.

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D

D.C.V.T.

An abbreviation for direct current voltage transformer, this device usually consists of a V.C.O. (voltage controlled oscillator) followed by a transformer followed by a rectifier. The device isolates and scales high level d.c. voltages and is used in older drive systems as a method for providing armature voltage feedback.

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Deceleration

The action of changing speed from any speed to a lower speed, irrespective of direction. Depending on drive design, the control of deceleration time may be known as "Down time", "Down rate" or similar variant. It is important to remember that some drive designs apply a single rate control to negative going speed demand changes irrespective of polarity. For example, consider a drive running forwards at half speed being asked to run backwards at half speed. The speed demand can be considered to be negative going, but the period between the drive running at half speed forwards and zero speed is termed deceleration and the period between zero speed and running at half speed backwards is termed acceleration. See also Angular deceleration, Linear deceleration.

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Demand

A generic term used to describe any request for action, such as current demand, speed demand.

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Discontinuous current

See Continuous Current.

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Dynamic response

A wide ranging term used to describe how the drive or motor reacts to any change in demand. Encompasses all loop time constants and gain components, for example speed loop dynamic response depends on speed loop error, integral time constant, feedforward, together with the dynamic response of the following current loop.

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Dynamometer

A common application for DC drives, this is a system for testing other devices such as automotive engines by applying controlled loading. Incorporates additional control loops around force measurement devices.

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Derivative

Sometimes known as feedforward, this is the "D" in the commonly used abbreviation P.I.D. A method for applying an extra gain component to the edge of a change in demand. Similar in action to the output of a capacitor when its input is subjected to a change in DC level.

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Duty cycle

A means of describing the cyclical work demand on a system. For example, a drive that is required to run for 10 seconds then stop for 10 seconds could be described as having a 50% duty cycle, one required to run for 5 seconds and stop for 15 seconds could be described as having a 25% duty cycle. (% term is probably the most precise way of describing duty cycle, although they are sometimes expressed in other ways such as 1/1. Using % term is less open to misinterpretation.)

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Dynamic braking

A means of braking a motor that does not rely on the drive having regenerative capability, also occasionally employed on drives that do have regenerative capacity for safety reasons. Essentially the method involves using a contactor to connect a low ohm value resistor directly across the motor armature whilst at the same time quenching the drive. In order for the system to work it is essential that the motor field is maintained during braking, otherwise it will not work as a generator.

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Dynamic losses

Mechanical or electrical losses in a system that are as a result of dynamic changes such as acceleration. They are not necessarily subtractive - for example, dynamic losses in a rewind mean that not all the torque produced by the drive/motor system is translated into material tension, whereas an unwind system having dynamic losses will produce more material tension then would be produced by drive/motor torque. See also Static losses.

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E

Efficiency

The method of describing how much of the power delivered to a system is available for use by the controlled device. For example, a motor/gearbox having a full working rating of 100A but requiring 10A to turn the gearbox would be said to be 90% efficient.

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Error

In a P.I.D system, error is the difference between demand and feedback having been multiplied by the Proportional term.

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E.M.C.

The abbreviation for Electro Magnetic Compatibility, a vast set of regulations pertaining to installations within the European Union. When pertaining to drives, the phrase is usually taken to imply electrical noise suppression and immunity requirements and it is the responsibility of the drive manufacturer to provide guidelines regarding installation, and the

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Enable

See Quench.

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Encoder

A digital device used to measure shaft speed, having an output in so many pulses per revolution and usually a marker pulse once per revolution. 1000 pulses/rev would be typical, with two sets of pulses 90 degrees apart together with their inverse.

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Excitation

A term commonly used to mean field on a motor.

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F

Field

The wound or permanently magnetised part of a motor that is responsible for producing magnetic flux in which the armature turns. See Compound wound, Series wound, Shunt wound.

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Field time constant

Expressed in seconds and calculated from L (inductance in Henries) / R (resistance in Ohms). Typically around 1 - 2 seconds, it is important to allow sufficient time for the field to decay after removing power before open circuiting the field, when employing field reversal techniques for example.

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Field weakening

The technique of extending a motor speed range by reducing field flux at speeds above base speed, thus maintaining a relatively constant armature voltage. See also Constant power.

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Filter

The name for any device that imposes a time constant on or removes harmonics or disturbances from a signal. Current common usage is for a device that is installed on the a.c. supply to a drive for the purposes of E.M.C. noise immunity and suppression.

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F.L.C.

Abbreviation for Full Load Current, being the maximum rated armature current expressed in Amperes.

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Flux

The magnetic field produced by either permanent magnet or electromagnetic wound fields in motors.

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Force venting

Otherwise known as force cooled, and as opposed to natural venting or natural cooling this term represents the use of a fan or blower to increase air flow through a motor for cooling purposes.

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Form factor

The ratio of the R.M.S. to the mean value of armature current - usually assumed to be around 1.5 for a single phase drive and 1.05 for a three phase drive. This needs to be taken into account when sizing fusing, contactors, wiring etc. in the a.c. supply to drives.

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Four quadrant

The ability of a drive to control torque and speed in all directions. In other words, ability to motor and brake in both forward and reverse directions of rotation. So called because of the common diagrammatic method of representing the effects.

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Fully controlled

When applied to a thyristor bridge, this term means that all the devices within the bridge are thyristors as opposed to a mix of thyristors and diodes. See also Half controlled.

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G

Gain

Strictly speaking, another way of describing Proportional terms. Also occasionally used in the same way as Stability to describe the overall P.I.D. performance of a drive speed and current loops. See also P.I.D.

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H

Half controlled

When applied to a thyristor bridge, this term means that half of the devices within the bridge are thyristors and half are diodes. See also Fully controlled.

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Healthy

An output from the drive indicating it is not in a fault condition. In the case of three phase drives having external contactors it is usually assumed that the drive cannot be other than healthy when the contactor is de-energised and power is not applied to the bridge. See also Ready.

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I

Inverse parallel

See Anti-parallel.

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Integral

Usually expressed as a time constant, this is the "I" in the commonly used abbreviation P.I.D. A method for applying a linearly increasing or decreasing output change in response to error between demand and feedback. Essentially, an Integrated output will move in one direction or the other whenever its input is not zero.

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Inertia

The tendency of a mass at rest to resist motion, or the tendency to resist change of motion.

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I.R. drop

A motor armature has resistance across which volts are dropped according to current flow. This means that not all the terminal voltage is translated into speed producing back E.M.F. and leads to a speed difference between full load and no load. This can be compensated for by the introduction of I.R. compensation, which takes a proportion of current feedback and introduces it as an additional speed demand.

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Isolated

The control electronics is electrically isolated from the high power electronics to allow safe interconnection to other systems. If a controller is isolated its model number will contain the suffix i.

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I²T

A measure of energy let through expressed in Amps2Seconds used by fuse manufacturers of semiconductor protective fuses.

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J

Jog

The term used to describe a short duration of rotation, usually implying that the drive is not latched into a running condition permanently.

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Jumper

A device, either hardware or software, that connects one point of a drive circuit to another in order to provide a specific function.

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L

Line reactor

Sometimes (unconventionally) called a choke, this is inductance added to the main bridge a.c. supply to the drive. Typical values tens of micro Henries. Reduces disturbance to the supply caused by commutation notches and also helps to protect the drive from supply disturbances.

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Linear acceleration

A positive rate of change of position. Measured in metres/second/second.

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Linear deceleration

A negative rate of change of position. Measured in metres/second/second.

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Linear velocity

A rate of displacement. Measured in metres/second.

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Load

A general term used to describe the opposing force causing the drive to produce torque.

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M

Misfire

A term used to describe a fault phenomena whereby one or more of the drive bridge thyristors does not fire, causing distortion of the armature current waveform. Can lead to heating of the motor armature. Usually protected against by more sophisticated drives.

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M.M.I.

Abbreviation for Man Machine Interface, a term that has become adopted by drives manufacturers meaning the device by which drives can be interrogated, programmed etc. For example, the keyboard and display on a digital drive.

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N

Naturally cooled

A term applied to those motors not fitted with force venting fans or blowers, these motors rely on the use of an internal fan which turns as the motor turns - these motors therefore rely on rotation for cooling and it is not good practice to use this type of motor in applications requiring high torque at low speed.

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Noise immunity

See E.M.C.

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Non Isolated

Non isolated drives are lower cost, but are only suitable for simple systems as they have their control electronics floating at a high potential. They cannot be connected to other equipment such as other drives, control systems etc.

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O

Open loop

A term used to describe a control system that does not employ a feedback device and thus can be considered to be predictive rather than analytical. A good example would be a winder system that computes diameter, and sets torque accordingly in an attempt to control material tension. See also Closed loop.

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P

Permanent magnet field

The type of motor in which the field is not an electromagnetic wound coil but a fixed magnet

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P.I.D.

The commonly used abbreviation for Proportional, Integral and Derivative - the three terms in a typical control loop.

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Proportional

Sometimes known as gain, this is the "P" in the commonly used abbreviation P.I.D. A method for multiplying error between demand and feedback by a set amount.

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Q

Quench

Also described as Enable, a term used to describe the action of preventing a drive from producing armature current. Usually implies that the control loops are in a reset state. This is a more precise term than Stop, as that term does not necessarily mean the drive is unable to produce current, merely that it is at rest.

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Quiescent

Another way of describing Steady state, whereby the drive is not necessarily at rest but is in a condition where it is not subject to any change in demand.

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R

Ramp

A term used to describe the application of a controllable rate of change to a signal, such as an acceleration time.

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Ready

As distinct from Healthy, this output from the drive is usually taken to mean that the drive - particularly three phase drives with external contactor control - have power applied and are not in an alarm state.

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Reference

A term commonly used by drive manufacturers to mean either a signal used as a demand (such as a speed reference) or the point against which such a signal can be compared.

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Regenerative

Often shortened to regen, the ability of a drive to return current to the mains supply for example when braking a motor.

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S

Setpoint

An alternative term for Reference, but more specific - this term can only be applied to a demand not the point to which it can be compared.

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Semiconductor fusing

The only method by which a thyristor bridge can be reliably protected in the event of an external fault such as an armature short circuit. See also I²T.

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Series wound

A term used to describe the type of motor field that is arranged in series with the armature windings. Not ideal for use with d.c. drives as they have low torque capability at low speeds, torque being dependant on flux which is produced by the current flowing through the armature and field.

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Shunt wound

A term used to describe the type of motor field that is arranged separately from the armature windings and which therefore can be supplied with a fixed voltage. The ideal motor type for use with d.c. drives.

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Single ended

A term often used instead of single quadrant, meaning a drive that is capable of a only one direction of rotation and motoring current only.

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Single quadrant

See Single ended. See also Four quadrant.

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Snubbers

A term commonly used to describe suppression devices used on thyristor bridges.

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Speed loop

A generic term used to describe that part of the drive circuitry which integrates the error between speed demand and feedback and eventually produces current demand. The "outermost" loop in a simple drive system.

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Speed holding

The ability of a drive to maintain a desired speed, assuming that load is constant.

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Stability

The ability of a system to reach a desired state without continual overshoot and undershoot, and its ability to retain that state following an external disturbance. Also used to describe an adjustable control in simpler drives not having access to all speed and current loop P.I.D. settings.

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Static losses

Mechanical or electrical losses in a system that are as a result of friction, or any other opposition to a move away from rest. They are not necessarily subtractive - for example, static losses in a rewind mean that additional torque is required from the drive/motor system when moving away from rest if constant material tension is to be maintained, whereas an unwind system having static losses will produce more material tension when moving away from rest than is produced by drive/motor torque. See also Dynamic losses.

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Steady state

See Quiescent.

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T

Tachogenerator

Sometimes shortened to tacho or even tach, this is an analogue device - in effect a small d.c. generator - used to measure shaft speed as a feedback device. Typical output 60V/1000 r.p.m. There are also a.c. versions which require rectification before they can be used with d.c. drives, but these types are best avoided as they will not differentiate between forward and reverse rotation. They will also not perform well at low speeds because of the volt drops associated with rectification.

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Three quadrant

A term sometimes used to describe a drive normally used for motoring in either direction and having regenerative braking (but not continuous controlled regenerative) capability. See also Four quadrant.

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Thyristor

Otherwise known as an S.C.R. (silicon controlled rectifier) used as the power switching device in most d.c drive bridges.

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Torque control

See Current control.

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Two quadrant

The term used to describe a drive that is capable of operation either in both directions but a single direction of current only, or in a single direction with motoring and braking capability. See also Four quadrant.

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U

Universal (AC/DC) motors

A phrase applied to series field wound motors. These are known as universal because they may be driven by either an AC or DC supply. (AC supply is possible because the field flux reverses at the same time as the armature current as the two circuits are in series. Rotation direction is therefore maintained.) See also Series Wound.

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V

V.S.D.

An abbreviation for variable speed drives, this term is occasionally used to refer to d.c. drives, but is not limited to them as the family would also include inverters, vector drives, servos etc.

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V.D.R.

An abbreviation for Voltage Dependant Resistor, a device that is designed to be installed on the a.c supply to a drive. Normally having a high impedance, it rapidly drops its impedance value to absorb any spikes that may be present on the supply.

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W

Windage losses

One component of Dynamic losses, caused by air resistance.

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Z

Zero interlocking

This term describes two commonly used techniques, preventing the start up of a drive without a zero speed reference and the automatic quenching of a regenerative drive below a small speed demand to prevent creep.

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