Keywords Series Wound DC Motor двигатель постоянного

Keywords Series Wound DC Motor – двигатель постоянного тока с последовательной обмоткой возбуждения Separately Excited DC Motor – двигатель постоянного тока независимого возбуждения permanent-magnet DC motor – двигатель постоянного тока на постоянных магнитах Self Excited DC Motor – двигатель с самовозбуждением Shunt wound DC motor - двигатель с параллельным возбуждением Compound Wound DC Motor – двигатель смешанного возбуждения

reluctance motor – реактивный индукторный двигатель Permanent Magnet Synchronous Motor (PMSM) – вентильный двигатель Brushless Direct Current Motor (BLDC) – бесколлекторный двигатель

MOTOR CLASSIFICATION An electric motor is a device which converts electrical energy into kinetic energy (i. e. motion). Most motors described in this guide [gaɪd] spin on an axis, but there also specialty motors that move linearly. All motors are either alternating current (AC) or direct current (DC), but a few can operate on both. The following lists the most common motors in use today. Each motor type has unique [juːˈniːk] characteristics that make it suitable to particular applications.

The direct current motor or the DC motor has a lot of application in today’s field of engineering and technology. Starting from an electric shaver to parts of automobiles, in all small or medium sized motoring applications DC motors come handy. And because of its wide range of application different functional types of DC motor are available in the market for specific requirements. The types of DC motor can be listed as follows-DC motor: - Permanent Magnet DC Motor - Separately Excited DC Motor - Self Excited DC Motor - Shunt Wound DC Motor - Series Wound DC Motor - Compound Wound DC Motor - Short shunt DC Motor - Long shunt DC Motor - Differential Compound DC Motor

Types of DC motors

DC motors are often used in applications where precise speed control is required. They are divided into three sub-categories: • series • shunt • compound Advanced motors have been developed in recent years, a number of which do not neatly fall within traditional motor classifications. They are typically used in OEM applications. Examples include: • electronically commutated motors • switched reluctance

OPERATING PRINCIPLES a) Major Parts All motors have two basic parts: • The STATOR (stationary part) • The ROTOR (rotating part) The design and fabrication of these two components determines the classification and characteristics of the motor. Additional components (e. g. brushes, slip rings, bearings, fans, capacitors, centrifugal switches, etc. ) may also be unique to a particular type of motor. b) Operation The motors described in this guide all operate on the principle of electromagnetism. Other motors do exist that operate on electrostatic and Piezoelectric principles, but they are less common.

Motor’s Electrical Equations The elevator is driven by a permanent-magnet DC motor. The equivalent circuit of the permanent-magnet DC motor is confined to the armature circuit which is illustrated in Figure

The motor voltage equation of the armature circuit is: where: is the electro-motive force developed in the armature’s winding; is the motor constant; is the angular speed of the motor’s shaft; is the voltage difference between the armature terminals; is the resistance of the armature circuit; is the current flowing through the armature circuit; is the inductance of the armature circuit. The electro-magnetic torque developed by the motor is expressed as:

Mechanical System’s Motion Equations The motion equation of the entire system from the motor’s perspective is: where: is the motor’s moment of inertia; is the angular speed of the rotor; is the friction coefficient of the motor; is the load torque placed on the motor’s shaft.

The load torque that is placed on the drive pulley which is mounted on the motor’s shaft is expressed as: (1) where: is the radius of the drive pulley; is the inertia of the motor pulley; is the force exerted on the drive pulley.

Eqn. 1 was derived for the elevator mechanical system in which the force exerted by the accelerating car is split into two parts. One half acts on the belt attached firmly to the top of the elevator shaft and the other half acts on belt which is looped over the drive pulley. In a similar way the force exerted by the counter-weight is split into two parts, one half acting on the belt firmly attached to the top of the elevator shaft and one half acting on the belt which is looped over drive pulley. The belt which is driven by the drive pulley is moving with the speed which is twice as great as the car speed . This is expressed as: The car speed expressed in terms of motor speed is expressed as: The mechanical power of the car moving upwards with constant speed is:

This power when written with respect to the pulling belt is: Replacing the belt speed with the car speed, the power is shown to be: (3) Eqn. 3 shows that the power of the moving car is equal to the power of the pulling belt. In Eqn. 2 the force is affected by the gravitational force and the inertia of the elevator car , both of which are expressed as: (4) (5)

Eqn. 4 and Eqn. 5 hold only when the car is moving upwards and the belt is flexible; under these conditions the motor is not affected by the counter-weight during acceleration. With this assumption Eqn. 1 can be inserted into Eqn. 2. The load torque is now expressed as: (6) As mentioned, Eqn. 6 was derived for the elevator car moving upwards. When the elevator car accelerates moving downwards the counter-weight’s mass should be considered while the elevator car’s mass should be ignored. The equation for load torque when the elevator car is moving downwards has the form:

Substituting the load torque equation for when the elevator car is moving upwards (Eqn. 6) into the motor’s motion equation (Eqn. 1), the motion equation of the entire system is obtained as: or: where the equivalent moment of inertia is: and the load torque due to gravity is:

DC motor modeling

DC motors possess characteristics that make them attractive for certain applications. For example, very high torque at low speeds makes the series DC motor attractive for traction and engine starting applications. Rotational speed can easily be controlled by varying the supply voltage. The rotating part (rotor) of a DC motor is called the armature, and consists of windings similar to those in a wound rotor induction motor The stationary part (stator) introduces a magnetic field by either permanent magnets or field windings which act on the armature.

Current flows through the armature windings via carbon brushes and a commutator assembly. The commutator assembly is easily recognizable as a ring of parallel diametrically opposite pairs of rectangular shaped copper contacts at one end of the armature. Each pair of contacts is connected to a coil wound on the armature. The carbon brushes maintain contact with the commutator assembly via springs. When the motor is turned on, current flows in through one brush via a commutator contact connected to a coil winding on the armature, and flows out the other carbon brush via a diametrically opposite commutator contact.

This causes the armature to appear as a magnet with which the stator field interacts. The armature field will attempt to align itself with the stator field. When this occurs, torque is produced and the armature will move slightly. At this time, connection with the first pair of commutator contacts is broken and the next pair lines up with the carbon brushes. This process repeats and the motor continues to turn.

Series motor – steady-state operating characteristics The series connection of armature and Weld windings means that the Weld Flux is directly proportional to the armature current, and the torque is therefore proportional to the square of the current. Series-connected DC motor and steady-state torque–speed curve

Reversing the direction of the applied voltage (and hence current) therefore leaves the direction of torque unchanged. This unusual property is put to good use in the universal motor, but is a handicap when negative (braking) torque is required, since either the Weld or armature connections must then be reversed. If the armature and Weld resistance volt-drops are neglected, and the applied voltage ( ) is constant, the current varies inversely with the speed, hence the torque ( ) and speed (n) are related by A typical torque–speed characteristic is shown in Figure (b). The torque at zero speed is not infinite of course, because of the effects of saturation and resistance, both of which are ignored in equation.

It is important to note that under normal running conditions the volt drop across the series Weld is only a small part of the applied voltage, most of the voltage being across the armature, in opposition to the back e. m. f. This is of course what we need to obtain an efficient energy conversion. Under starting conditions, however, the back e. m. f. is zero, and if the full voltage was applied the current would be excessive, being limited only by the armature and Weld resistances. Hence for all but small motors a starting resistance is required to limit the current to a safe value.

Returning to Figure (b), we note that the series motor differs from most other motors in having no clearly defined no-load speed, i. e. no speed (other than infinity) at which the torque produced by the motor falls to zero. This means that when running light, the speed of the motor depends on the windage and friction torques, equilibrium being reached when the motor torque equals the total mechanical resisting torque. In large motors, the windage and friction torque is often relatively small, and the no-load speed is then too high for mechanical safety. Large series motors should therefore never be run uncoupled from their loads. As with shunt motors, the connections to either the Weld or armature must be reversed in order to reverse the direction of rotation.

Large series motors have traditionally been used for traction. Often, books say this is because the series motor has a high starting torque, which is necessary to provide acceleration to the vehicle from rest. In fact any d. c. motor of the same frame size will give the same starting torque, there being nothing special about the series motor in this respect. The real reason for its widespread use is that under the simplest possible supply arrangement (i. e. constant voltage) the overall shape of the torque–speed curve fits well with what is needed in traction applications. This was particularly important in the days when it was simply not feasible to provide any control of the armature voltage.

The inherent suitability of the series motor for traction is illustrated by the curves in Figure, which relate to a railway application. The solid line represents the motor characteristic, while the dotted line is the steady-state torque–speed curve for the train, i. e. the torque which the motor must provide to overcome the rolling resistance and keep the train running at each speed. At low speeds the rolling resistance is low, the motor torque is much higher, and therefore the nett torque ( ) is large and the train accelerates at a high rate. As the speed rises, the nett torque diminishes and the acceleration tapers off until the steady speed is reached at point A in Figure

Some form of speed control is obviously necessary in the example above if the speed of the train is not to vary when it encounters a gradient, which will result in the rolling resistance curve shifting up or down. There are basically three methods which can be used to vary the torque–speed characteristics, and they can be combined in various ways. Firstly, resistors can be placed in parallel with the Weld or armature, so that a specified fraction of the current bypasses one or the other. Field ‘divert’ resistors are usually preferred since their power rating is lower than armature divert resistors. For example, if a resistor with the same resistance as the Weld winding is switched in parallel with it, half of the armature current will now flow through the resistor and half will flow through the Weld. At a given speed and applied voltage, the armature current will increase substantially, so the flux will not fall as much as might be expected, and the torque will rise, as shown in Figure (a).

This method is inefficient because power is wasted in the resistors, but is simple and cheap to implement. A more efficient method is to provide ‘tappings’ on the Weld winding, which allow the number of turns to be varied, but of course this can only be done if the motor has the tappings brought out. Secondly, if a multicell battery is used to supply the motor, the cells may be switched progressively from parallel to series to give a range of discrete steps of motor voltage, and hence a series of torque–speed curves. Road vehicles with 12 V lead–acid batteries often use this approach to provide say 12, 24, and 36 V for the motor, thereby giving three discrete ‘speed’ settings, as shown in Figure (b). Finally, where several motors are used (e. g. in a multiple-unit railway train) and the supply voltage is fixed, the motors themselves can be switched in various series/parallel groupings to vary the voltage applied to each.

Four-quadrant operation and regenerative braking The beauty of the separately excited d. c. motor is the ease with which it can be controlled. Firstly, the steady-state speed is determined by the applied voltage, so we can make the motor run at any desired speed in either direction simply by applying the appropriate magnitude and polarity of the armature voltage. Secondly, the torque is directly proportional to the armature current, which in turn depends on the difference between the applied voltage V and the back e. m. f. E. We can therefore make the machine develop positive (motoring) or negative (generating) torque simply by controlling the extent to which the applied voltage is greater or less than the back e. m. f.

An armature voltage controlled d. c. machine is therefore inherently capable of what is known as ‘four-quadrant’ operation, with reference to the numbered quadrants of the torque–speed plane shown in Figure

Secondly, the supply voltage is shown by the old-fashioned battery symbol, as use of the more modern circle symbol for a voltage source would make it more difficult to differentiate between the source and the circle representing the machine armature. The relative magnitudes of applied voltage and motional e. m. f. are emphasised by the use of two battery cells when and one when . We have seen that in a d. c. machine speed is determined by applied voltage and torque is determined by current. Hence on the right-hand side of the diagram the supply voltage is positive (upwards), while on the left-hand side the supply voltage is negative (downwards). And in the upper half of the diagram current is positive (into the dot), while in the lower half it is negative (out of the dot). For the sake of convenience, each of the four operating conditions (A, B, C, D) have the same magnitude of speed and the same magnitude of torque: these translate to equal magnitudes of motional e. m. f. and current for each condition.

If, with the motor running at position A, we suddenly reduce the supply voltage to a value which is less than the back e. m. f. , the current (and hence torque) will reverse direction, shifting the operating point to B in Figure. There can be no sudden change in speed, so the e. m. f. will remain the same. If the new voltage is chosen so that , the new current will have the same amplitude as at position A, so the new (negative) torque will be the same as the original positive torque, as shown in Figure. But now power is supplied from the machine to the supply, i. e. the machine is acting as a generator, as shown by the shaded arrow. We should be quite clear that all that was necessary to accomplish this remarkable reversal of power was a modest reduction of the voltage applied to the machine. At position A, the applied voltage was , while at position B it is . Since IR will be small compared with , the change ( ) is also small.

Needless to say the motor will not remain at point B if left to its own devices. The combined effect of the load torque and the negative machine torque will cause the speed to fall, so that the back e. m. f. again falls below the applied voltage VB, the current and torque become positive again, and the motor settles back into quadrant 1, at a lower speed corresponding to the new (lower) supply voltage. During the deceleration phase, kinetic energy from the motor and load inertias is returned to the supply. This is therefore an example of regenerative braking, and it occurs naturally every time we reduce the voltage in order to lower the speed.

If we want to operate continuously at position B, the machine will have to be driven by a mechanical source. We have seen above that the natural tendency of the machine is to run at a lower speed than that corresponding to point B, so we must force it to run faster, and create an e. m. f greater than , if we wish it to generate continuously. It should be obvious that similar arguments to those set out above apply when the motor is running in reverse (i. e. is negative). Motoring then takes place in quadrant 3 (point C), with brief excursions into quadrant 4 (point D, accompanied by regenerative braking), whenever the voltage is reduced in order to lower the speed.

Separately Excited DC Motor The field (or stator) coil contains a relatively large number of turns which minimizes the current required to produce a strong stator field (Figure). It is connected to a separate DC power supply, thus making field current independent of load or armature current.

Excellent speed regulation is characteristic of this design which lends itself well to speed control by variation of the field current. Separately excited DC motors can race to dangerously high speeds (theoretically infinity) if current to the field coil is lost. Because of this, applications should include some form of field current protection as an unprotected motor could literally fly apart.

Compound DC Motor The compound DC motor uses both series and shunt field windings, which are usually connected so that their fields add cumulatively (Figure). This two winding connection produces characteristics intermediate to the shunt field and series field motors. Speed regulation is better than that of the series field motor.

Permanent Magnet DC Motors These motors use permanent magnets in place of field windings to establish the stator magnetic field (Figure) Permanent magnets provide constant field strength, with motor characteristics similar to that of the shunt field DC motor.

Permanent magnet motors are often used in low horsepower applications, particularly those that are battery operated (e. g. windshield wiper motor). However, with recent developments in magnet technology, permanent magnet motors can be greater than 200 HP. New high strength magnetic materials and power electronics have been combined to produce high efficiency variable speed motors ranging from sub fractional to multiple horse power units. Generally these motors/controls are purpose built and are therefore incorporated into OEM products.

The equations to model the system are:

Brushed DC Motor In this type of motors, magnetic field is produced by passing current through a commutator and brush which are inside the rotor. Hence, they are called Brushed Motors. The brushes are made up of carbon. These can be separately excited or self-excited motors. The stator part of the motor consists of coils connected in a circular fashion in such a way that the required alternative north and south poles are formed. This coil setup can be in series or in parallel to the rotor coil winding forming series wound DC motors and shunt wound DC motors. The armature or the rotor part of the DC motor consists of Commutator which essentially a current carrying conductor connected at one end to copper segments which are electrically isolated. External power can be connected to commutator via the brushes as the armature rotates.

Electronically Commutated Motor (ECM) An ECM is an electronically commutated permanent magnet DC motor (Figure). Electronics provide precisely timed voltages to the coils, and use rotation position sensors for timing. The electronic controller can be programmed to vary the torque speed characteristics of the motor for a wide variety of OEM applications such as fans and drives.

Although presently more costly than alternative motor technologies, the higher efficiency and flexible operating characteristics of these motors make them attractive. An ECM is essentially a brushless DC motor with all speed. Typical applications include variable torque drives for fans and pumps, commercial refrigeration, and appliances. For furnace fans, efficiency can be 20 to 30 percentage points higher than a standard induction motor at full load. However, for constant air circulation ECM’s have a definite advantage over standard direct drive blower motors. At half speed, the ECM might consume as little as 10% of the energy of a multi speed blower motor. For appliances such as washing machines, the ECM can replace the mechanical transmission due to wide range of torque speed characteristics it can produce.

Thyristor d. c. drives – general For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used to supply the Weld of the motor: the power is much less than the armature power, so the supply is often single-phase, as shown in Figure

The arrangement shown in Figure is typical of the majority of d. c. drives and provides for closed-loop speed control. The function of the two control loops will be explored later, but readers who are not familiar with the basics of feedback and closed-loop systems may find it helpful to read through the Appendix at this point. The main power circuit consists of a six-thyristor bridge circuit, which rectifies the incoming a. c. supply to produce a d. c. supply to the motor armature. The assembly of thyristors, mounted on a heatsink, is usually referred to as the ‘stack’. By altering the Wring angle of the thyristors the mean value of the rectified voltage can be varied, thereby allowing the motor speed to be controlled.

Low power control circuits are used to monitor the principal variables of interest (usually motor current and speed), and to generate appropriate Wring pulses so that the motor maintains constant speed despite variations in the load. The ‘speed reference’ (Figure) is typically an analogue voltage varying from 0 to 10 V, and obtained from a manual speed-setting potentiometer or from elsewhere in the plant. The combination of power, control, and protective circuits constitutes the converter. Standard modular converters are available as off-the-shelf items in sizes from 0. 5 k. W up to several hundred k. W, while larger drives will be tailored to individual requirements. Individual converters may be mounted in enclosures with isolators, fuses etc. , or groups of converters may be mounted together to form a multi-motor drive.

DC motor, a view inside Simple, cheap. - Easy to control. - 1 W - 1 k. W - Can be overloaded. - Brushes wear. - Limited overloading on high speeds.

DC motor control - Controller + H-bridge (allows motor to be driven in both directions). -Pulse Width Modulation (PWM)control. -Speed control by controlling motor current=torque. -Efficient small components. - PID control.

Speed Control of DC Motors: the speed of a motor is given by the relation It is obvious that the speed can be controlled by varying 1. Flux/pole, Φ (Flux Control) 2. Resistance Ra of armature circuit (Rheostatic Control) and 3. Applied voltage V (Voltage Control).

Speed Control of Shunt motor: 1. Variation of Flux or Flux Control Method: By decreasing the flux, the speed can be increased and vice versa. The flux of a dc motor can be changed by changing Ish with help of a shunt field rheostat. Since Ish is relatively small, shunt field rheostat has to carry only a small current, which means I 2 sh. R loss is small, so that rheostat is small in size.

2. Armature or Rheostatic Control Method: This method is used when speeds below the no-load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat in series with the armature circuit. As controller resistance is increased, voltage across the armature is decreased, thereby decreasing the armature speed. For a load constant torque, speed is approximately proportional to the voltage across the armature. From the speed/armature current characteristic, it is seen that greater the resistance in the armature circuit, greater is the fall in the speed.

Voltage Control Method: (a) Multiple Voltage Control: In this method, the shunt field of the motor is connected permanently to a fixed exciting voltage, but the armature is supplied with different voltages by connecting it across one of the several different voltages by means of suitable switchgear. The armature speed will be approximately proportional to these different voltages. The intermediate speeds can be obtained by adjusting the shunt field regulator.

Speed Control of Series Motors: 1. Flux Control Method: Variations in the flux of a series motor can be brought about in any one of the following ways: (a) Field Diverters: The series winding are shunted by a variable resistance known as field diverter. Any desired amount of current can be passed through the diverter by adjusting its resistance. Hence the flux can be decreased and consequently, the speed of the motor increased.

(b) Armature Diverter: A diverter across the armature can be used for giving speeds lower than the normal speed. For a given constant load torque, if Ia is reduced due to armature diverter, the must increase This results in an increase in current taken from the supply (which increases the flux and a fall in speed ). The variation in speed can be controlled by varying the diverter resistance.

(c) Trapped Field Control Field: This method is often used in electric traction. The number of series filed turns in the circuit can be changed. With full field, the motor runs at its minimum speed which can be raised in steps by cutting out some of the series turns.

(d) Paralleling Field coils: this method used for fan motors, several speeds can be obtained by regrouping the field coils. It is seen that for a 4 -pole motor, three speeds can be obtained easily.

2. Variable Resistance in Series with Motor: By increasing the resistance in series with the armature the voltage applied across the armature terminals can be decreased. With reduced voltage across the armature, the speed is reduced. However, it will be noted that since full motor current passes through this resistance, there is a considerable loss of power in it.

Electric Braking: A motor and its load may be brought to rest quickly by using either (i) Friction Braking or (ii) Electric Braking. Mechanical brake has one drawback: it is difficult to achieve a smooth stop because it depends on the condition of the braking surface as well as on the skill of the operator. The excellent electric braking methods are available which eliminate the need of brake lining levers and other mechanical gadgets. Electric braking, both for shunt and series motors, is of the following three types: (i) Rheostatic or dynamic braking (ii) Plugging i. e. , reversal of torque so that armature tends to rotate in the opposite direction. (iii) Regenerative braking. Obviously, friction brake is necessary for holding the motor even after it has been brought to rest.

Electric Braking of Shunt Motors: (a) Rheostatic or Dynamic Braking: In this method, the armature of the shunt motor is disconnected from the supply and is connected across a variable resistance R. The field winding is left connected across the supply. The braking effect is controlled by varying the series resistance R. Obviously, this method makes use of generator action in a motor to bring it to rest.

(b) Plugging or Reverse Current Braking: This method is commonly used in controlling elevators, rolling mills, printing presses and machine tools etc. In this method, connections to the armature terminals are reversed so that motor tends to run in the opposite direction. Due to the reversal of armature connections, applied voltage V and E start acting in the same direction around the circuit. In order to limit the armature current to a reasonable value, it is necessary to insert a resistor in the circuit while reversing armature connections.

(c) Regenerative Braking: This method is used when the load on the motor has over-hauling characteristic as in the lowering of the cage of a hoist or the downgrade motion of an electric train. Regeneration takes place when Eb becomes grater than V. This happens when the overhauling load acts as a prime mover and so drives the machines as a generator. Consequently, direction of Ia and hence of armature torque is reversed and speed falls until E becomes lower than V. It is obvious that during the slowing down of the motor, power is returned to the line which may be used for supplying another train on an upgrade, thereby relieving the powerhouse of part of its load.

Electric Braking of Series Motor: (a) Rheostatic (or dynamic) Braking: The motor is disconnected from the supply, the field connections are reversed and the motor is connected in series with a variable resistance R. Obviously, now, the machine is running as a generator. The field connections are reversed to make sure that current through field winding flows in the same direction as before (i. e. , from M to N ) in order to assist residual magnetism. In practice, the variable resistance employed for starting purpose is itself used for braking purposes.

(b) Plugging or Reverse Current Braking: As in the case of shunt motors, in this case also the connections of the armature are reversed and a variable resistance R is put in series with the armature.

(c) Regenerative Braking: This type of braking of a series motor is not possible without modification because reversal of Ia would also mean reversal of the field and hence of Eb. However, this method is sometimes used with traction motors, special arrangements being necessary for the purpose.

Servo motors Although there is no sharp dividing line between servo motors and ordinary motors, the servo type will be intended for use in applications which require rapid acceleration and deceleration. The design of the motor will reflect this by catering for intermittent currents (and hence torques) of many times the continuously rated value. Because most servo motors are small, their armature resistances are relatively high: the short-circuit (locked-rotor) current at full armature voltage is therefore perhaps only five times the continuously rated current, and the drive amplifier will normally be selected so that it can cope with this condition without difficulty, giving the motor a very rapid acceleration from rest.

The even more arduous condition in which the full armature voltage is suddenly reversed with the motor running at full speed is also quite normal. (Both of these modes of operation would of course be quite unthinkable with a large d. c. motor, because of the huge currents which would flow as a result of the much lower per-unit armature resistance. ) Because the drive amplifier must have a high current capability to provide for the high accelerations demanded, it is not normally necessary to employ an inner current-loop of the type discussed earlier. In order to maximise acceleration, the rotor inertia must be minimised, and one obvious way to achieve this is to construct a motor in which only the electric circuit (conductors) on the rotor move, the magnetic part (either iron or permanent magnet) remaining stationary. This principle is adopted in ‘ironless rotor’ and ‘printed armature’ motors.

In the ironless rotor or moving-coil type (Figure 2. 14) the armature conductors are formed as a thin-walled cylinder consisting essentially of nothing more than varnished wires wound in skewed form together with the disc-type commutator (not shown). Inside the armature sits a 2 -pole (upper N, lower S) permanent magnet, which provides the radial flux, and outside it is a steel cylindrical shell which completes the magnetic circuit.

Needless to say the absence of slots to support the armature winding results in a relatively fragile structure, which is therefore limited to diameters of not much over 1 cm. Because of their small size they are often known as micromotors, and are very widely used in cameras, video systems, card readers etc. The printed armature type is altogether more robust, and is made in sizes up to a few kilowatts. They are generally made in disc or pancake form, with the direction of flux axial and the armature current radial. The armature conductors resemble spokes on a wheel; the conductors themselves being formed on a lightweight disc.

Early versions were made by using printed-circuit techniques, but pressed fabrication is now more common. Since there are usually at least 100 armature conductors, the torque remains almost constant as the rotor turns, which allows them to produce very smooth rotation at low speed. Inertia and armature inductance are low, giving a good dynamic response, and the short and fat shape makes them suitable for applications such as machine tools and disc drives where axial space is at a premium.

DC servo drives The precise meaning of the term ‘servo’ in the context of motors and drives is difficult to pin down. Broadly speaking, if a drive incorporates ‘servo’ in its description, the implication is that it is intended specifically for closed-loop or feedback control, usually of shaft torque, speed, or position. Early servomechanisms were developed primarily for military applications, and it quickly became apparent that standard d. c. motors were not always suited to precision control. In particular high torque to inertia ratios were needed, together with smooth ripple-free torque.

Motors were therefore developed to meet these exacting requirements, and not surprisingly they were, and still are, much more expensive than their industrial counterparts. Whether the extra expense of a servo motor can be justified depends on the specification, but prospective users should always be on their guard to ensure they are not pressed into an expensive purchase when a conventional industrial drive could cope perfectly well.

The majority of servo drives are sold in modular form, consisting of a high-performance permanent magnet motor, often with an integral tachogenerator, and a chopper-type power amplifier module. The drive amplifier normally requires a separate regulated d. c. power supply, if, as is normally the case, the power is to be drawn from the a. c. mains. Continuous output powers range from a few watts up to perhaps 2– 3 k. W, with voltages of 12, 24, 48, and multiples of 50 V being standard.

Position control As mentioned earlier many servo motors are used in closed-loop position control applications, so it is appropriate to look briefly at how this is achieved.

In the example shown in Figure, the angular position of the output shaft is intended to follow the reference voltage ( ), but it should be clear that if the motor drives a toothed belt linear outputs can also be obtained. The potentiometer mounted on the output shaft provides a feedback voltage proportional to the actual position of the output shaft. The voltage from this potentiometer must be a linear function of angle, and must not vary with temperature, otherwise the accuracy of the system will be in doubt.

The feedback voltage (representing the actual angle of the shaft) is subtracted from the reference voltage (representing the desired position) and the resulting position error signal is amplified and used to drive the motor so as to rotate the output shaft in the desired direction. When the output shaft reaches the target position, the position error becomes zero, no voltage is applied to the motor, and the output shaft remains at rest. Any attempt to physically move the output shaft from its target position immediately creates a position error and a restoring torque is applied by the motor.

The dynamic performance of the simple scheme described above is very unsatisfactory as it stands. In order to achieve a fast response and to minimise position errors caused by static friction, the gain of the amplifier needs to be high, but this in turn leads to a highly oscillatory response which is usually unacceptable. For some fixed-load applications, matters can be improved by adding a compensating network at the input to the amplifier, but the best solution is to use ‘tacho’ (speed) feedback (shown dotted in Figure) in addition to the main position feedback loop.

Tacho feedback clearly has no effect on the static behaviour (since the voltage from the tacho is proportional to the speed of the motor), but has the effect of increasing the damping of the transient response. The gain of the amplifier can therefore be made high in order to give a fast response, and the degree of tacho feedback can then be adjusted to provide the required damping (Figure). Many servo motors have an integral tachogenerator for this purpose.