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Sprint Electric - Motor index
Category: Electronics and Engineering > DC Motor control
Date & country: 15/12/2007, UK
Words: 98


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…

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.

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

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

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

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.

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

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.

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.

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

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 …

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.

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.

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.

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.

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.

Choke
See Armature choke.

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 pr…

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.

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.

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.

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 i…

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.

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.

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.

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.

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.

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.

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 polar…

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

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.

Discontinuous current
See Continuous current.

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 describin…

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. I…

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…

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.

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.

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.

Enable
See Quench.

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.

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

Excitation
A term commonly used to mean field on a motor.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

I²T
A measure of energy let through expressed in Amps²Seconds used by fuse manufacturers of semiconductor protective fuses.

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

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.

Inverse parallel
See Anti-parallel.

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

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

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.

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

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

Linear velocity
A rate of displacement. Measured in metres/second.

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

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.

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.

Noise immunity
See E.M.C.

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.

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.

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

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.

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.

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.

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

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.

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.

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

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.

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.

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.

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.

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.

Single quadrant
See Single ended. See also Four quadrant.

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

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

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.

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.

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,…

Steady state
See Quiescent.

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…

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.

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

Torque control
See Current control.

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.

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.

Windage losses
One component of Dynamic losses, caused by air resistance.

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.