Solid State Dc Drives Part1 PDF

SOLID STATE DRIVES AND APPLICATIONS

FIRST EDITION

J. Gopi ELECTRICAL AND ELECTRONICS ENGINEERING Specialization-Power Electronics Electronics and Drives

SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 1

The principal advantage of a d.c. motor is that its speed can be changed over a wide range by a variety of simple methods. Such a fine speed control is generally not possible with a.c. motors. In fact, fine speed control is one of the reasons for the strong competitive position of d.c. motors in the modem industrial applications

 Speed

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Control of D.C. Motors

Flux control method

by varying the flux , the motor speed (N  1/ 1/) can be changed and hence the name flux control method. In this method, a variable resistance (known as shunt field rheostat) is placed in series with shunt field winding .


The shunt field rheostat reduces the shunt field current I sh and hence the flux . Therefore, we can only raise the speed of the motor above the normal speed Generally, this method permits to increase the speed in the ratio 3:1. Wider speed ranges tend to produce instability and poor commutation. 

Advantages

(i) This is an easy and convenient method.


(ii) It is an inexpensive method since very little power is wasted in the shunt field rheostat due to relatively small value of Ish. (iii) The speed control exercised by this method is independent of load on the machine. 

Disadvantages (i) Only speeds higher than the normal speed can be obtained since the sh — the total field circuit resistance cannot be reduced below R sh the shunt

field winding resistance. (ii) There is a limit to the maximum speed obtainable by this method. It is because if the flux is too much weakened, commutation becomes poorer.

Note. The field of a shunt motor in operation should never be opened because its speed will increase to an extremely high value. 

Armature control method

This method is based on the fact that by varying the voltage available across the armature, the back e.m.f and hence the speed of the motor can be changed. This is done by inserting a variable resistance R C (known as controller resistance) in series with the armature.


N  V  IaR a  R C where R C = controller resistance

Due to voltage drop in the controller resistance, the back e.m.f. (E b) is decreased. Since N  E b, the speed of the motor is reduced. The highest speed obtainable is lhat corresponding to R C = 0 i.e., normal speed. Hence, this method can only provide speeds below the normal normal speed

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Disadvantages (i) A large amount of power is wasted in the controller resistance resistance since it carries full armature current Ia.

(ii) The speed varies widely with load since the speed depends upon the voltage drop in the controller resistance and hence on the armature current demanded by the load. (iii) The output and efficiency of the motor are reduced. (iv) This method results in poor speed regulation.


Due to above disadvantages, this method is seldom used to control tie speed of shunt motors. Note. The armature control method is a very common method for the speed

control of d.c. series motors. The disadvantage of poor speed regulation is not important in a series motor which is used only where varying speed service is required.

Voltage control method The voltage source supplying the field current is different from that which supplies the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method. is quite expensive. Therefore, this method of speed control is employed for large size motors where efficiency is of great importance.

Multiple voltage control

the shunt field of the motor is connected permanently across a-fixed voltage source. The armature can be connected across several different voltages through a suitable switchgear. In this way, voltage applied across the armature can be changed. The speed will be approximately proportional to the voltage applied across the armature. Intermediate speeds can be obtained by means of a shunt field regulator. Ward-Leonard system

the adjustable voltage for the armature is obtained from an adjustable-voltage generator while the field circuit is supplied from a separate source.


The armature of the shunt motor M (whose speed is to be controlled) is connected directly to a d.c. generator G driven by a constant-speed a.c.motor A. The field of the shunt motor is supplied from a constant-voltage exciter E. The field of the generator G is also supplied from the exciter E. The voltage of the generator G can be varied by means of of its field regulator. regulator. By reversing reversing the field current current of generator G by controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regulator is included in the field circuit of shunt motor M for additional speed adjustment. With this method, the motor may be operated at any speed upto its maximum speed.

Advantages

(a) The speed of the motor can be adjusted through a wide range without resistance losses which results in high efficiency. (b) The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of generator G. When the generator voltage is reduced below the back e.m.f. of the motor, this back e.m.f. sends current through the generator armature,


establishing dynamic braking. While this takes place, the generator G operates as a motor driving motor A which returns power to the line. (c) This method is used for the speed control of large motors when a d.c. supply is not available The disadvantage of the method is that a special motor-generator set is required for each motor and the losses in this set are high if the motor is operating under light loads for long periods.

Speed Control of D.C. Series Motors The speed control of d.c. series motors can be obtained by (i) flux control method (ii) armature-resistance control method.

Flux control method In this method, the flux produced by the t he series motor is varied and hence the speed. The variation of flux can be achieved in the following ways: Field diverters.


A variable resistance (called field diverter) is connected in parallel with series field winding . Its effect is to shunt some portion of the line current from the series field winding, thus weakening the field and increasing the speed ( N  1/ 1/). The lowest speed obtainable is that corresponding to zero current in the diverter (i.e., diverter is open). Obviously, the lowest speed obtainable is the normal speed of the motor. Consequently, this method can only provide speeds above the normal speed. The series field field diverter method is often often employed in traction traction work.

Armature diverter.

In order to obtain speeds below the normal speed, a variable resistance (called armature diverter) is connected in parallel with the armature .The diverter shunts some of the line current, thus reducing the armature current. Now for a given load, if Ia is decreased, the flux  must increase (  T  I Ia). Since N  1/ 1/, the motor speed is decreased. By adjusting the armature diverter, any speed lower than the normal speed can be obtained.

Tapped field control .

the flux is reduced (and hence speed is increased) by decreasing the number of turns of the series field winding .The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than normal speed are achieved.


Paralleling field coils .

This method is usually employed in the case of fan motors. By regrouping the field coils, several fixed speeds can be obtained.

Armature-resistance Armature-resistance control


a variable resistance is directly connected in series with the supply to the complete motor. This reduces the voltage available across the armature and hence the speed falls. By changing the value of variable resistance, any speed below the normal speed can be obtained. This is the most common method employed to control the speed of d.c. series motors. Although this method has poor speed regulation, this has no significance for series motors because they are used in varying speed applications. The loss of power in the series resistance for many applications of series motors is not too serious since in these applications, the control is utilized for a large portion of the time for reducing the speed under light-load conditions and is only used intermittently intermittently when the motor is carrying full-load.

Series-Parallel Control Another method used for the speed control of d.c. series motors is the seriesparallel method. In this system which is widely used in traction system, two (or more) similar d.c. series motors are mechanically coupled to the same load.

When the motors are connected in series , each motor armature will receive onehalf the normal voltage. Therefore, the speed will be low. When the motors are connected in parallel, each motor armature receives the normal voltage and the


speed is high ,Thus we can obtain two speeds. Note that for the same load on the pair of motors, the system would run approximately approximately four times the speed when the machines are in parallel as when they are in series.

Series-parallel Series-parallel and resistance control In electric traction, series-parallel method is usually combined with resistance method of control. In the simplest case, two d.c. series motors are coupled mechanically and drive the same vehicle.

(i) At standstill, the motors are connected is series via a starting rheostat. The motors are started up in series with each other and starting resistance is cut out step by step to increase the speed. When When all the resistance is cut out, the voltage applied applied to each motor is about one-half of the line voltage. The speed is then about onehalf of what it would be if the full line voltage were applied to each motor.

(ii) To increase the speed further, the two motors are connected in parallel and


at the same time the starting resistance is connected in series with the combination. The starting resistance is again cut out step by step until full speed is attained. Then field control is introduced.


Speed Control of Compound Motors Speed control of compound motors may be obtained by an y one of the methods described for shunt motors. Speed control cannot be obtained through adjustment of the series field since such adjustment would radically change the performance characteristics characteristics of the motor.

Necessity of D.C. Motor Starter At starting, when the motor is stationary, there is no back e.m.f. in the armature. Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current (Ia = V/R a) because of small armature resistance. As an example, 5 H.P., 220 V shunt motor has a full-load current of 20 A and an armature resistance of about 0.5 . If this motor is directly switched on to supply, it would take an armature current of 220/0.5 = 440 A which is 22 times the fullload current. This high starting current may result in: (i) burning of armature due to excessive heating effect, (ii) damaging the commutator and brushes due to heavy sparking, (iii) excessive voltage drop in the line to which the motor is connected. The result is that the operation of other appliances connected to the line may be impaired and in particular cases, they may refuse to work. In order to avoid excessive current at starting, a variable resistance (known as starting resistance) is inserted in series with the armature circuit. This resistance is gradually reduced as the motor gains speed (and hence E b increases) and eventually it is cut out completely when the motor has attained full speed. The value of starting resistance is generally such that starting current is limited to SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 14

1.25 to 2 times the full-load current.

Types of D.C. Motor Starters The stalling operation of a d.c. motor consists in the insertion of external resistance into the armature circuit to limit the starting current taken by the motor and the removal of this resistance in steps as the motor accelerates. When the motor attains the normal speed, this resistance is totally cut out of the armature circuit. It is very important and desirable to provide the starter with protective devices to enable the starter arm to return to OFF position

1. when the supply fails, thus preventing the armature being directly across the mains when this voltage is restored. For this purpose, we use no-volt release coil. 2. when the motor becomes overloaded or develops a fault causing the motor to take an excessive current. For this purpose, we use overload release coil. There are two principal types of d.c. motor starters viz., three-point starter and four-point starter. As we shall see, the two types of starters differ only in the manner in which the no-volt no -volt release coil is connected.

Three-Point Starter This type of starter is widely used for starting shunt and compound motors. It is so called because it has three terminals L, Z and A. The starter consists of starting resistance divided into several sections and connected in series with the armature. The tapping points of the starting resistance are brought out to a number of studs. The three terminals L, Z and A of the starter are connected respectively to the positive line terminal, shunt field terminal and armature terminal. The other SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 15

terminals of the armature and shunt field windings are connected to the negative terminal of the supply. The no-volt release coil is connected in the shunt field circuit. One end of the handle is connected to the terminal L through the over-load release coil. The other end of the handle moves against a spiral spring and makes contact with each stud during starting operation, cutting out more and more starting resistance as it passes over each stud in clockwise direction.


Operation of Three-Point Starter

(i) To start with, the d.c. supply is switched on with handle in the OFF position. (ii) The handle is now moved clockwise to the first stud. As soon as it comes in


contact with the first stud, the shunt field winding is directly connected across the supply, while the whole starting resistance is inserted in series with the armature circuit. (iii) As the handle is gradually moved over to the final stud, the starting resistance is cut out of the armature circuit in steps. The handle is now held magnetically by the no-volt release coil which is energized by shunt field current. (iv) If the supply voltage is suddenly interrupted or if the field excitation is accidentally cut, the no-volt release coil is demagnetized and the handle goes back to the OFF position under the pull of the spring. If no-volt release coil were not used, then in case of failure of supply, the handle would remain on the final stud. If then supply is restored, the motor will be directly connected across the supply, resulting in an excessive armature current. (v) If the motor is over-loaded (or a fault occurs), it will draw excessive current from the supply. This current will increase the ampere-turns of the over-load release coil and pull the armature C, thus short-circuiting the novolt release coil. The no-volt coil is demagnetized and the handle is pulled to the OFF position by the spring. Thus, the motor is automatically disconnected from the supply.

Drawback In a three-point starter, the no-volt release coil is connected in series with the shunt field circuit so that it carries the shunt field current. While exercising speed control through field regulator, the field current may be weakened to such an extent that the no-volt release coil may not be able to keep the starter arm in the ON position. This may disconnect the motor from the supply when it is not desired. This drawback is overcome in the four point starter.


Four-Point Starter In a four-point starter, the no-volt release coil is connected directly across the supply line through a protective resistance R. Now the no-volt release coil circuit is independent of the shunt field circuit. Therefore, proper speed control can be exercised without affecting the operation of novolt release coil. Note that the only difference between a three-point starter and a four-point starter is the manner in which no-volt release coil is connected. However, the working of the two starters is the same. It may be noted that the threepoint starter also provides protection against an openfield circuit. This protection is not provided by the four-point starter.


Grading of Starting Resistance — Shunt Shunt Motors For starting the motor satisfactorily, satisfactorily, the starting resistance is divided into a number of sections in such a way that current fluctuates between maximum (I m) and minimum (I) values. The upper limit is that value established as the maximum permissible for the motor; it is generally 1.5 times the full-load current of the motor. The lower limit is the value set as a minimum for starting operation; it may be equal to full-load current of the motor or some predetermined value. Fig. shows shunt-wound motor with starting resistance divided into three sections between four studs. The resistances of these sections should be so selected that current during starting remains between Im and I

(i) When arm A is moved from OFF position to stud 1, field and armature circuits are energized and whole of the starting resistance is in series with the armature. The armature current jumps to maximum value given by;

where R 1 = Resistance of starter and armature SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 20

(ii) As the armature accelerates, the generated e.m.f. increases and the armature current decreases as indicated by curve ab. When the current has fallen to I, arm A is moved over to stud 2, cutting out sufficient resistance to allow the current to rise to Im again. This operation is repeated until the arm A is on stud 4 and the whole of the starting resistance is cut out of the armature circuit. (iii) Now the motor continues to accelerate and the current decreases until it settles down at some value IL such that torque due to this current is just sufficient to meet the load requirement.

Starter Step Calculations for D.C. Shunt Motor Consider d.c. Consider d.c. shunt motor starter with n resistance sections and (n + 1) studs. Let R 1 = Total resistance in the armature circuit when the starter arm is on stud no. 1 R 2 = Total resistance in the armature circuit when the starter arm is on stud no. 2 and so on

Im = Upper current limit I = Lower current limit n = Number of sections in the starter resistance V = Applied voltage R a = Armature resistance On stud 1. When the starter arm-moves to stud 1, the total resistance in the

armature circuit is R 1 and the circuit current jumps to maximum values Im given By

Since torque   Ia, it follows that the maximum torque acts on the armature to


accelerate it. As the armature accelerates, the induced e.m.f. (back e.m.f.) increases and the armature current decreases When the current has fallen to the predetermined value I, the starter starter arm is moved moved over to stud stud 2. Let the value value of back e.m.f. be E b1 at the instant the starter arm leaves the stud 1. Then I is given by

On stud 2. As the starter arm moves over to stud 2, sufficient resistance is cut

out (now total circuit resistance is R 2) and current rises to maximum value Im once again given by


The acceleration continues and the back e.m.f. increases and the armature current decreases. When the current has fallen to the predetermined value I, the starter arm is moved over to stud 3. Let E b2 be the value of back e.m.f. at the instant the starter arm leaves the stud 2. Then,

On stud 3. As the starter arm moves to stud 3,

As the starter arm leaves stud 3,



Electric Braking Sometimes it is desirable to stop a d.c. motor quickly. This may be necessary in case of emergency or to save time if the motor is being used for frequently repeated operations. The motor and its load may be brought to rest by using either (i) mechanical (friction) braking or (ii) electric braking. In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat. Mechanical braking has several disadvantages including non-smooth stop and greater stopping time. In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy which is dissipated in a resistance as heat or alternativley, it is returned to the supply source (Regenerative braking). For d.c. shunt as well as series motors, the following three methods of electric braking are used: (i) Rheostatic or Dynamic braking (ii) Plugging (iii) Regenerative braking

It may be noted that electric braking cannot hold the motor stationary and mechanical braking is necessary. However, the main advantage of using electric braking is that it reduces the wear wear and tear of mechanical mechanical brakes and and cuts down the stopping time considerably due to high braking retardation.


Rheostatic or Dynamic braking In this method, the armature of the running motor is disconnected from the supply and is connected across a variable resistance R. However, the field winding is left connected to the supply. The armature, while slowing down, rotates in a strong magnetic field and, therefore, operates as a generator, sending a large current through resistance R. This causes the energy possessed by the rotating armature to be dissipated quickly as heat in the resistance. As a result, the motor is brought to standstill quickly. Consider dynamic braking of a shunt motor. The braking torque can be controlled by varying the resistance R. If the value of R is decreased as the motor speed decreases, the braking torque may be maintained at a high value. At a low value of speed, the braking torque becomes small and the final stopping of the motor is due to friction. This type of braking is used extensively in connection with the control of elevators and hoists and in other applications in which motors must be started, stopped and reversed frequently.


We now investigate how braking torque depends upon the speed of the motor.

Plugging In this method, connections to the armature are reversed so that motor tends to rotate in the opposite direction, thus providing the necessary braking effect. When the motor comes to rest, the supply must be cut off otherwise the motor will start rotating in the opposite direction.


Considering plugging of a d.c. shunt motor. Note that armature connections are reversed while the connections of the field winding are kept the same. As a result the current in the armature reverses. During the normal running of the motor ,the back e.m.f. E b opposes the applied voltage V. However, when armature connections are reversed, back e.m.f. E b and V act in the same direction around the circuit. Therefore, a voltage equal to V + E b is impressed across the armature circuit. Since E b ~ V, the impressed voltage is approximately 2V. In order 10 limit the current to safe value, a variable resistance R is inserted in the circuit at the time of changing armature connections.

Thus braking torque decreases as the motor slows down. Note that there is some braking torque (TB = k 5) even when the motor speed is zero.

Regenerative braking In the regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply. The two methods of regenerative braking for a shunt motor.


*In one method, field winding is disconnected from the supply and field current is increased by exciting exciting it from another source source (in fig i).As a result, result, induced e.m.f. e.m.f. E exceeds the supply voltage V and the machine feeds energy into the supply. Thus braking torque is provided upto the speed at which induced e.m.f. and supply voltage are equal. As the machine slows down, it is not possible to maintain induced e.m.f. at a higher value than the supply voltage. Therefore, this method is possible only for a limited range range of speed. *In a second method, the field excitation does not change but the load causes the motor to run above the normal speed (e.g., descending load on a crane). As a result, the induced e.m.f. E becomes greater than the supply voltage V (in fig ii) . The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered. Hence the torque is reversed and the speed falls until E becomes less than V.



The 90 kW fan has an inertia of 60 kgm2. The nominal operating point for the fan is 1000 rpm. The fan is required to be stopped within 20 seconds. The natural braking effect caused caused by the load characteristics characteristics is at its maximum at the beginning beginning of the braking. The maximum energy of inertia can be calculated from formula. The average braking power can be calculated by dividing this braking energy by time. This value is, of course, on the very safe side due to the fact that the fan load characteristics are not taken into account.

When the braking chopper is dimensioned for this 16.4 kW value and the motor braking capability at a higher speed is far more than 16.4 kW, the drive has to


include a supervision function for maximum regeneration power. This function is available in some drives.

If one wants to optimise the dimensioning of the brake chopper for a specific braking time ,The speed reduces quickly from 1000 to 500 rpm without any additional braking. The natural braking effect is at its maximum at the beginning of the braking. This clearly indicates that it is not necessary to start braking the motor with the aforementioned 16 kW power in the first instance. the speed comes down from 1000 rpm to 500 rpm without any additional braking within less than 10 seconds. At that point of time the load torque is only 25% of nominal and and the kinetic energy conserved in the fan is also only 25% of the energy at 1000 rpm. If the calculation done at 1000 rpm is repeated at 500 rpm, it can be seen that the braking power in order to achieve deceleration from 500 rpm to 0 rpm is appr. 8 kW. As stated in previous calculations this is also on the safe side because the natural braking curve caused by the load characteristics characteristics is not taken into account. To summarize, the target for a 20 second deceleration time from 1000 rpm down to 0 rpm is well achieved with a braking chopper and resistor dimensioned for 8.2 kW. Setting the drive regenerative power limit to 8.2 kW sets the level of braking power to an appropriate appropriate level.


Constant torque application:

– The load torque characteristic does not depend on the speed. The load torque remains approximately approximately the same over the whole speed area. – The The power increases linearly as the speed increases and vice versa. – Typical Typical constant torque applications: cranes and conveyors.

Quadratic torque application:

– The The load torque increases to speed to the power of two. – When When the speed increases, the power increases to speed to the power o f three. – Typical Typical quadratic torque applications: fans and pumps.

Braking power evaluation: – The quadratic load characteristics mean fast natural deceleration between 50100% of nominal speeds. That should be utilized when dimensioning the braking power needed. – The quadratic load torque means that at low speeds the natural deceleration is mainly due to friction. – The The constant load torque characteristic characteristic is constant natural deceleration. – The The b raking power is a function of torque and speed at that specified operating point. Dimensioning Dimensioning the braking chopper according to peak braking power typically leads to over dimensioning. – The The braking power is not a function of motor nominal current (torque) or power as such. SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 33

– If the load torque disappears when braking starts the natural braking effect is small. This affects the dimensioning of the braking chopper.


Principle of the braking chopper

The other possibility to limit DC bus voltage is to lead the braking energy to a resistor through a braking chopper. The braking chopper is an electrical switch that connects DC bus voltage to a resistor where the braking energy is converted to heat. The braking choppers are automatically activated when the actual DC bus voltage exceeds a specified level depending on the nominal voltage of the inverter.

The main benefits of the braking chopper and resistor solution solution are:

– Simple Simple electrical construction construction and well-known technology. – Low Low fundamental investment investment for chopper and resistor. – The chopper works even if AC supply is lost. Braking during main power loss may be required, eg, in elevator or other safety related applications.

The main drawbacks of the braking chopper and resistor are:

– The The braking energy is wasted if the heated air can not be utilised. – The The braking chopper and resistors require additional additional space.


– May May require extra investments in the cooling and heat recovery system. – Braking Braking choppers are typically dimensioned for a certain cycle, eg, 100% power 1/10 minutes, long braking times require more accurate dimensioning of the braking chopper. – Increased risk of fire due to hot resistor and possible dust and chemical components in the ambient air space. – The increased DC bus voltage level during braking causes additional voltage stress on motor insulation.

– The The braking cycle is needed occasionally. – The The amount of braking energy with respect to motoring energy is extremely small. – Braking Braking operation is needed during main power loss.


CHOPPER CIRCUITS FOR DC MOTOR CONTROL

DC motor drives are used for many many speed and position control systems systems where their excellent performance, ease of control and high efficiency are desirable characteristics. DCmotor speed control canbe achieved using switchmode DC-DC chopper circuits. For both mains-fed and battery supplied systems, power MOSFETs and FREDFETs are the ideal switching devices for the converter stage. PowerMOS devices includes devices suitable for most DC-DC converters for motor control applications. Additionally, due to the ease with which MOSFETs and FREDFETs can be parallelled, PowerMOS devices can easily be used in chopper circuits for both low power and high power DC motor drives for vehicle, industrial or domestic applications.

DC motor drives

In a DCmotor, the static field flux is established using either permanent magnets or a stator field winding. The armature winding, on the rotor of a dc machine, carries the main motor current. The armature winding is a series of coils, each connected to segments of a commutator. In order that the motor develops constant torque as the rotor moves, successive armature coils must be connected to the external dc circuit. This is achieved using a pair of stationary brushes held in contact with the commutator.


The motor torque is produced by the interaction of the field flux and the armature current and is given by:

The back emf developed across the armature conductors increases with the motor speed:

Permanent magnet DC motors are limited in terms of power capability and control capability. For field wound DC motors the field current controls the flux and hence the motor torque and speed constants. The field winding can be connected in series with the armature winding, in shunt with it, or can be separately excited. For the separately excited dc motor, the field flux is controlled and the motor can be made to operate in two distinct modes: constant torque operation up to the rated speed of the motor, and then constant power operation above rated speed,

The steady state state operation of the motor is described by:


For normal motor operation Ea and Ia are positive and the motor is operating in its ’first quadrant’. The motor is said to be operating in its second quadrant, that is braking or regenerating, by reducing Va below Ea such that Ia is negative. If the polarity of the applied voltage is reversed then motoring and regenerating operation can occur with the direction of rotation reversed. Thus by controlling the armature voltage and current polarities, full four-quadrant operation, as can be achieved.


SINGLE QUADRANT (STEP DOWN) CONVERTER

For single quadrant operation the chopper,. The average voltage applied to the motor, and hence its speed, is controlled by varying the duty cycle of the switch, S. fig shows the switching waveforms for the circuit. During the on time, ton, the supply voltage, Vdc, is applied to the motor and the armature current starts to increase. Neglecting the on-state resistance of the switch and the armature winding resistance the voltage across the armature inductance is Vdc-Ea and so the rate of rise of armature current is given by:

When the switch turns off the energy stored in the armature inductance must be dissipated. The polarity of the voltage across La reverses, the diode D becomes forward biased and the armature current continues to flow. Assuming that the motor speed remains constant and neglecting the forward voltage drop of the freewheeling diode the inductor voltage is equal to -Ea. The rate of fall of armature

current is given by:



FOUR QUADRANT, FULL-BRIDGE CONVERTER

If motoring and regenerating operation are required with both directions of rota tion then the full full bridge converter is required. required. Using this this configuration configuration allows the polarity of the applied voltage to be reversed, thus reversing the direction of rotation of the motor. Thus in a full bridge converter the motor current and voltage can be controlled independently. The motor voltage Va is given by:

whereV12 is controlled by switchingS1 and S2 as described above, andV 34 by switching S3 and S4. The usual operating mode for a full bridge converter is to group the switching devices so that S1 and S3 are always on simultaneously and that S2 and S4 are on simultaneously. This type of control is then referred to as bipolar control.



DC- DRIVE SYSTEM SCHEMATIC


Four quadrant operation was implemented for DC motor with single controllable switch. The common regenerative braking methods include adding an extra converter, or adding an extra ultra-capacitor, or switching sequence change of power switches. But the method of adding a converter not only increases cost but also reduces conversion efficiency. The method of adding an ultra capacitor doesn’t require extra DC-DC DC-DC converter, but it needs a sensor to detect the ultracapacitor voltage. This makes the circuit very complex and hard to implement. It conserves energy in the regenerative braking mode. Relay circuits are employed to run the motor during the accelerating mode The position signals obtained from the proxy sensors of the motor are read by the I/O lines of the dsPIC controller. The sensor inputs give the position of the rotor which is fed to the controller. The controller compares it with the reference speed and generates an error signal. The required direction of rotation either clockwise or counter clockwise can also be fed to the digital controller.


The PWM module of the controller generates appropriate PWM signals, which are applied to the DC motor. appropriate voltage signals. The gate signals are generated by comparing the actual speed with the reference speed. Thus a closed loop speed control is achieved with the help of PI control, present in the controller block. when the duty cycle is varying the armature current and the speed of the motor is varying. Similarly when the duty cycle of the gating pulse becomes constant the armature current and the speed also becomes constant.






DESIGN AND ANALYSIS OF DC MOTOR WITH PID CONTROLLER DC motors, because of their simplicity, ease of application, reliability and favourable cost have been a backbone of industrial applications. In these applications, the motor should be precisely controlled to give the desired performance. Modelling of any system is an important task t ask in control applications because the electrical and mechanical components should be represented in mathematical form. Although several methods are available for representing DC Motor model, the state space approach is unique in the sense, accurate and precise control is possible. This paper presents the state space approach in modelling of DC Motor. Mathematical models are developed for simple closed loop control of


DC Motor and closed loop control with proportional Integral and Derivative (PID) controller. In most of the advanced control algorithms DC Motors are used because of stable and linear characteristics associated with it. Also various speed control methods are available for DC Motor to meet the desired performance. Hence modelling should be done in such a way that every control algorithm available can be implemented. Generally modelling of any system is to represent mechanical, electrical or any physical systems or components in mathematical form. The DC Motor can be modelled by using four basic equations. Regularly every physical system is represented in the form of transfer function which is a relation between input and output, but absence of initial conditions limits this form of modelling. DC Motor can also be modelled by using state space equations. The state space representation is relation between state variables, their derivatives, input and output. Feasibility of defining initial conditions and being simple first order differential equations the state space approach finds its application in modelling various physical systems.


DC MOTOR MODELLING.

The DC motor modeling is done summing the torques acting on the rotor inertia and integrating the acceleration to the velocity and also Kirchhoff s laws to ‟

armature circuit.

The mathematical model of DC motor can be constructed by suing four basic equations of motor. ---- (1) ---- (2) --- (3) --- (4) TRANSFER FUNCTION OF DC MOTOR

The transfer function of DC Motor is derived by simplifying four basic equations and applying Laplace transform. Here we will take that the input of the system is the reference voltage (Va ) applied to the motor's armature, while the output is the rotational speed of the shaft (Wr).



(7)

(8)

In closed loop network the difference between the reference input and feedback input is fed to the th e armature of DC Motor.

STATE SPACE EQUTATIONS FOR PID CONTROLLED DC MOTOR


The speed of the DC Motor can be controlled by installing a PID controller in the closed loop network of DC motor. With PID controller we get three state space variables and the single input single output DC Motor network is converted into five input and single output network.

Therefore equations (5) (6) and (15) form the state space equations for PID Controlled DC Motor. From the equation (15) it is observed that the input to the


DC Motor or the armature voltage is becoming a state variable in the presence of the PID controller. The state space equations can be represented in the matrix form given below:

Example output





choppers •

Chopper is a static device.

•

A variable variable dc voltage is obtained from a constant dc voltage source.

•

Also known as dc-to-dc converter. Widely used for motor control.

•

Also used in regenerative braking.

Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control.

Types of Choppers  Step-down choppers.  Step-up choppers.  In step down chopper output voltage is less than input voltage.  In step up chopper output voltage is more than input voltage. Step-down choppers

Chopper i0

V

+

V0

R

 SOLID STATE DC DRIVES AND APPLICATIONS-J.GOPI-AP-EEE-MIE APP LICATIONS-J.GOPI-AP-EEE-MIET T [email protected] [email protected] Page 61

• • •

A step-down chopper with resistive load. The thyristor in the circuit acts as a switch. When thyristor is ON, supply voltage appears across the load When thyristor is OFF, the voltage across the load will be zero. v

0

V Vdc t tON

tOFF

i0

V/R Idc t T

Average Output Current I dc

I dc



V dc R

 t  V   ON   d R  T  R

VO

V



1

tON , vo

 V

Therefore RMS output voltage



VO



VO



t ON

 v dt T 2

o

t ON

1

V T

2

dt

0

RMS value of output voltage VO

But during during

V

2

T

tON



t ON T

.V

d .V

0

Output power PO  VO I O But

I O



V O R

 Output power P O

P O

 

2

V O R

dV

2

R


Principle Of Step-up Chopper I

L

D +



+

L O A D

C

V Chopper

VO

 •

•

• •

Step-up chopper is used to obtain a load voltage higher than the input voltage V . The values of L and C are are chosen depending upon the requirement of output voltage and current. When the chopper is ON, the inductor L is connected across the supply. The inductor current ‘I’ rises rises and the inductor stores energy during the ON time of the chopper, t . ON

•

When the chopper is off, the inductor current I is is forced to flow through the diode D and load for a period, t

.

OFF •

The current tends to decrease resulting in reversing the polarity of induced EMF in L.

•

Therefore voltage across load is given by:VO

V  L

dI dt

i.e., VO

V


Class A Chopper

i0

+

v0

Chopper V FWD

L O A D

v0

V



ig

i0

Thyristor gate pulse t

i0

Output current CH ON t FWD Conducts

v0

Output voltage

tON

t T


•

Class A Chopper is is a step-down chopper in which power always flows form

source to load. •

•

It is used to control the speed of dc motor. The output current equations obtained in step down chopper with R-L load Chopper. can be used to study the performance of Class A Chopper.

Class B Chopper

D

i0

v0

+ R L

V

v0

Chopper E ig



i0

Thyristor gate pulse t

i0

tOFF

tON T Output current

Imax Imin v0

t

D conducts Chopper conducts

Output voltage

t


•

When chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure.

•

During the ON period of the chopper, the inductance L stores energy.

•

When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply.

•

Average output voltage is positive.

•

Average output current is negative.

•

Therefore Class B Chopper operates operates in second quadrant.

•

In this chopper, power flows from load to source.

•

Class B Chopper is is used for regenerative braking of dc motor.

•

Class B Chopper is is a step-up chopper.

OUTPUT CURRENT During the interval diode 'D' conducts voltage equation is given by V



LdiO dt

 RiO  E

For the initial condition i.e.,

   I min at t  0

iO t

The solution of the above equation is obtained along similar lines as in s tep-down chopper with R-L load R R  t  t  E  L L  iO  t   1  e   I mine 0  t  tOFF R   At t  tOFF iO  t   I max

V

I max

R  t  E  L  1  e R 

V

OFF

R  t  L   I mine 

OFF

During the interval chopper is ON voltage equation is given by 0

LdiO dt

 RiO  E


Redefining the time origin, at

t

0

   I max

iO t

The solution for the stated initial condition is

   I max e

iO t

At



t



R

t

L



 tON

I min

 I max e



R L

E 

  R   iO  t   I min 1  e

tON



E 



R

t

0  t  t ON

L

1  e R 



R L

t ON

  

CLASS C CHOPPER

CH1

D1 i0

v0

+ R

V CH2

L

D2

v0

Chopper E • •

i0



Class C Chopper is is a combination co mbination of Class A and Class B Choppers . For first quadrant operation, CH is ON or D conducts. 1

2

•

For second quadrant operation, CH is ON or D conducts.

•

When CH is ON, the load current current is positive. positive.

2

1

1

•

•

The output voltage is equal to ‘V’ & the load receives power from the source. When CH is turned OFF, OFF, energy stored in inductance inductanc e L forces current to 1

flow through the diode D and the output voltage is zero. 2


ig1

Gate pulse of CH1 t

ig2

Gate pulse of CH2 t

i0 Output current t D1

CH1 ON

D2

CH2 ON

D1

CH1 ON

D2

V0

CH2 ON

Output voltage

t

• •

Current continues to flow in positive direction. When CH is triggered, the voltage E forces current to flow in opposite 2

direction through L and CH . 2

• •

The output voltage is zero. On turning OFF CH , the energy stored in the inductance drives current 2

through diode D and the supply 1

•

• • •

Output voltage is V , the input current becomes negative and power flows from load to source Average output voltage is positive Average output current can take both positive and negative values. Choppers CH & CH should not be turned ON simultaneously simultaneousl y as it would 1

•

•

2

result in short circuiting the supply. Class C Chopper can be used both for dc motor control and regenerative braking of dc motor motor.. Class C Chopper can can be used as a step-up or step-down chopper.


CLASS D CHOPPER v0

CH 1

D2 R

V

i0

L

E



v0

+ D1

i0

CH 2

•

Class D is a two quadrant chopper.

•

When both CH and CH are triggered simultaneously, the output voltage v 1

2

O

= V and and output current flows through through the load. load. •

When CH and CH are turned OFF, the load current continues to flow in 1

2

the same direction through load, D and D , due to the energy stored in the 1

2

inductor L. Output voltage v = - V . . O

•

Average load voltage is positive if chopper ON time is more than the OFF time

•

Average output voltage becomes negative if t ON

•

< t

.

OFF

Hence the direction of load current is always positive but load voltage can be positive or negative. negative.


ig1

Gate pulse of CH1 t

ig2

Gate pulse of CH2 t

i0 Output current

v0

CH1,CH2 ON

t D1,D2 Conducting Output voltage

V Average

v0

t


ig1

Gate pulse of CH1 t

ig2

Gate pulse of CH2 t

i0 Output current CH1 CH2

t D1, D2

v

0

Output voltage

V Average v0

t


CLASS E CHOPPER FOUR QUADRANT OPERATION • •

Class E is a four quadrant chopper When CH and CH are triggered, output current i flows in positive 1

4

O

direction through CH and CH , and with output voltage v = V . 1 • •

4

O

This gives the first quadrant operation. When both CH and CH are OFF, the energy stored in the inductor L drives 1

4

i through D and D in the same same direction, but output voltage v = -V . O

2

3

O

•

Therefore the chopper operates in the fourth quadrant.

•

When CH and CH are triggered, the load current i 2

3

O

flows in opposite

direction & output voltage v = -V . O

•

Since both i and v O

•

are negative, the chopper operates in third quadrant quadrant

O

When both CH and CH are OFF, the load current i continues to flow in 2

3

O

the same direction D and D and the output voltage 1 •

v = V .

4

O

Therefore the chopper operates in second quadrant as v

is positive but i is O

O

negative.


Solid State Dc Drives Part1 PDF

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