5.1 SOUD STATE DEVICES

Ib) (b) Characteristics or a diode

...•-- LOO~ ... ~ u 300 0.... J 2005

u,

0 Hh"'S~

- Cathode

5

Solid State Motor Control: D. C Motor Speed Control

F i,. S.l (a) Semi-conductor diode

5.1.1 Diode

An ideal diode offers zero impedance to current flow in one direction and infinite impedance in the other . The action of the semi-conduetor device is based on the

127

(b) Characteristics of a power transister

1·'5 5'0 7·S VoIt~. "CE (V l

150

u -100

i.. ~

u

E emitter

(a)

E C

B

C collector

8 base

Fig. 5.2 (a) Transister (p-n.p)

'--+

Solid State Motor Control: D. C. Motor Speed Control

5.1.3 Thyrister

The thyrister is a three terminal. three junction, four layer semi-conductor device made up of alternate layers of p and n type silicon. The cathode connexion forms a pen junction, with the trigger electrode or gate connexion attached to the p layer. The outer p end is the anode and end n is the cathode. The anode to cathode is connected in series with the load. This device is basically a switch. It remains in off state when voltage blocking is effected and it has practically infinite impedance until the anode and . gate potentials are suitably positive with respect to the cathode. In the above .condition, when the thyrister switches on,

5.1.2 'Tnmsister

properties of p-and-"1-type materials in conjunction. In practice, solid state diode has very low voltage drop (0.5 to 1.5 V) due to its very low forward resistance; when blocking voltage is of the order of kV, the reverse current of the diode is of the order ofmA. Fig. 5.1 (a) shows the structure and symbol for a semi-conductor diode. Fig. 5.1 (b) shows the characteristics of diode.

Transister is a solid state device having three layer p-n-por (n-jrn) structure. In normal practice, the emitter base junction is forward biased and the collector base reversed biased. The collector current I; can be controlled by the .base current. Transisters have been developed, with ratings suitable for motor control. Trasisters are used in switching or on-off mode. The structure and symbol for a transister is shown hi Fig. 5.2 (a) and the characteristics of a power transister are shown in Fig. 5.2 (b).

Electric Motors: Applications and Control

Forward volt age

ModeC

1 For~ard 'break down

voltage ,latching . I -­ -- --­ -­.

(a)

Cathode

(b,

, Conduction

Fig. 5.3 (a) The Thyrister (silicon controlled rectifier) (b) The Thyrlster Characteristics

gate The structure of a thyrister and its +

symbol are shown in Fig. 5.3 (a); Fig. 5.3 (b) shows steady state thyrister charac­teristics;

(i) The Thyrister reverse biased

In this type of operation. the cathode is positive with respect lU the anode. .runctions 1 and 3 are reversed biased and junction 2 IS forward biased. As in the single jJ-n junction, only a.very small, leakage current flows. from cathoae to anode. The anode current is equal te the reverse saturation current of junction 1 plus a

128

current flows in the circuit and the con­dudion' continues without further signals from the gate. To switch off the thy­rister, the anode current should be reduc­ed to zero and there should not be any gate signal.

; .

i,. ., h

, ~

",

r:

Solid State Motor Control: D. C. Motor Spud Control 129

fraction of gate current; The mode of the characteristics is shown in Fig. 5.3 (b) as mode A.

(ii) The Thyrister forward biased and blocking

. In this mode of operation. junctions land 3 ~re forward biased but junction z is reverse biased. The anode current is small; this is equal to the saturation current at junction 2 plus a fraction of gate current; as the gate current increases, the anode current increases; this operation is shown in Fig. 5.3 (b) as mode B.

(iii) The Thyrister forward biased and conducting

At turn on voltage, the avalanche multiplication begins . and the current rapidly increases to a particular value depending on supply voltage and load impedance. This is the conducting mode. This is shown in" Fig. 5.3 (b) as mode C. The thyrister continues to operate in this mode until ' the forward current is reduced below the holding current level; at which point 'it. reverts to forward blocking state. The thyrister can be made to 'fire' at a given forward voltage by controlling the gate current. When the trigger pulse is applied to the gate of a thyrister having a positive anode, the device switches rapidly into conducting mode; there ill a brief delay of the orderof O.1.to 0.2 .Ii sec. The thyrister will remain in conducting mode if the load current has risen above.the latching current. The thyrister can be switched off by reducing the load current below the holding current, and reversed biased for minimum turn off time.

The thyristcr can be triggered by the following methods: (a) by electrical signals: gate turn on (b) break over voltage turn on . (c) rapid rate of rise of forward bias voltage: dv/dt turn on.

(a) Gate turn on

There is a minimum gate current below which the thyrister will not turn on. Tum on time is defined as the time from initiation of triggering to the time the device establishesa steady forward voltage drop. When the gate current is large enough, the thyrister. will switch on as soon as the anode becomes positive with respect to the cathode. The gate current will vary from a few rnA to 250 rnA depending on the size of die thyrister, The shape of the gate signal should have a sharp leading edge. In case of thyristers for general purpose industrial applications, a gate pulse with a rise time of 10 amps per Ii second may turn on the thyristor in O;I.1i second. For pulse modulation applications, thyristors may require gate signals-with rise time of 40 amps/ f' second.

(b) Break over voltage turn on

. An mC1"C8se of' anode to cathode forward voltage makes junction 2 forward biased. The anode current is limited by external load impedance only. At forward breakover voltage (V10), the thyrister changes its characteristics-high voltage with low ~eakagc;, ~Urtent to low vol~ge across the device with large forward current. The th~ster turns ~n at !'10. VIOlS greater ~ the reverse voltage rating; this method of tummg on thyrister IS 11Sed only for pen-pen diode.

130 Electric Motors; Applications and Control

!, I, f

r I;

d•

T,

~:

(c) dvjdt turn on

A rapid rate of increase of forward anode to cathode voltage dv/dt, can turn on the thyrister by the transient 1!ate current caused by anode to gate and gate to cathode capacitances. This should be avoided. Generally thyristers are limited to 20 to 200 VIp. sec rate. The dv/dt for switching is increased by having low external gate to cathode resistance.

Thyrister turn off

Turn off means that all forward conduction has ceased. and without any gate signal reapplication of positive voltage to the cathode. it will not start conduction and cause current to flow. Commutation is the process of turning off the thyrister. The thyrister can be switched off in the following ways:

(i) Natural commutation The thyrister is turned off when the anode current is reduced below a minimum

value corresponding to the holding current. This ~ done by opening the line switch. or increasing the impedance of load or shunting part of the load current through circuit parallel to the thyrister.

(ii) Reverse bias turn off

A reverse anode to cathode voltage will tend to interrupt the anode current. Here cathode is positive with respect to anode. In a.c. circuit, the thyrister in the line would be reverse biased every negative cycle and would turn off. This is lie line commutation or phase commutation.

If the thyrister is in d.c, line, a capacitor can be used to create reverse biased voltage across a thyrister, and to interrupt the current. This is known as forced commutation. In this method the capacitor is connected in parallel with the thyrister and discharged to tum off the thyrister. Fig. 5.4 shows the principle of forced commu­tation by the use of capacitor.

+ O-......,r---....., Th

C1VC V "--"

R ___-I~r~

Tit". t Fig. 5.4 Forced commutation in a d.c. circuit

Initially the thyrister is conducting and has a small voltage drop. The capaeitor C. precharged to voltage V c with polarity as shown, is then switched on in parallel with the thyrister; thus it imposes reverse bias and the current in the thyristor js opposed and quenched. The capacitor With this connexion discharges through the load . Vc falls to zero and then rises with reverse polarity towards the supply voltage level. If

131 Solid State Motor Control: D. C. Motor Speed Control

the time taken for this '0 sec is greater than the turn off time. then the thyrister stops conducting.

Gattlng For proper triggering. the gate pulse should lie between specified upper and lower

limits of voltages and currents. Thyrister turn off time is the time during which the charge present in the silicon structure decays to a level near its equilibrium off state. The turn off times depend on the voltage rating of the thyristers. They are generally:

10 I-' sec for low voltage. low current thyr isters > 20 JL sec for 500 V ratings of thyristers > 35 JL sec for 800 V ratings of thyristers > 50 I-' sec for 1200 V ratings of thyristers 100 to 200 I-' sec for 1500 to 2000 V ratings of thyrister.

Thyrister ratings

Voltage rating, current rating and power rating should be considered in specify. ing a thyrister suitable for application in given operating conditions.

Voltage ratings

There are three types of voltage rat ings which should be considered in case of thyristers :

(i) The Peak forward voltage (PFV): This is the limiting positive anode voltage above which the thyrister may be dam aged.

(ii) The forward break down voltage (VBO) is the minimum anode to cathode voltage to cause turn on when no gate signal is applied. V80 is still a function of dv/dt, In general PFV is greater than V80.

(iii) The Peak reverse voltage (PRV) is the maximum repetit ive voltage tbat can be applied to the thyrister so that the cathode is positi ve with respect to anode. If PRY is exceeded there may occur avalanche break down and the thyrister may be damaged.

Current ratings

The current rating should define the cooling medium temperature and then the current at which the junction approaches the maximum permissible temperature. A typical specification of a thyrister for pulse application should state:

Peak current (average current); maximum repetitive di/dt ; current rise time; applied voltage V ; gate current A with 0.1 fl. sec rise time.

Power ratings

This is strictly with reference to currents con d uct icn arid the forward voltage drop. i.e.• power loss. A silicon device has maximum temperature 120°C to 180eC at the junction. The rating of a thyrister is reduced with increase in' ambient tempe­rature. Thyrister power losses are of the following types:

(i) Load current forward conduction loss

The mean anode current X forward voltage drop across the thyrister = average Flower dissipated in the thyrister, For large size thyristers, large heat sinks and efficient cooling is requirea, Water cooled heat exchangers are used.

i'fl:l ,, r:

I

,:.

. ' ! ..: U J~ ! 1: .;

I ;

.. "

i

5.2 THYRISTER CIRCUITS

c

c

number of they share

Fig. ~ .~ Thyristers in series

r..2

Electric Motors: Applications and Control ,

Voltage is shared equally across the two thyristers Tit I and Th 2 by suitably connecting a resistance or zenor diode across them. This takes care of steady state conditions. For : equal · voltage sharing - under transient conditions. a capacitor and a low voltage non-inductive resistance are connected in parallel with each thyrister. All thyristers which are connected in series must be turned on at the same time when signals are applied to their gates simultaneously.

When the thyrister rating is smaller than the supply voltage. a thyristers can be connected in series. Care should be taken that equally the forward as well as reverse voltage. Fig. 5.5 shows the arrangement of thyristers in series.

132

When the load current is greater ·than the current rating of the thyrister, a number of thyristers are connected in parallel to share the load currert. The thyris­rers should be matched ones. They may have about 20% derating to allow for thermal effects. . The gate signals should be applied until latching currents of all thyristers in parallel have been exceeded. Current is shared equally between the thyristers by using external balancing resistances or reactors.

Some circuit arrangements are common for control of thyristers, e.g., with reference to increasing power capabilities , gate circuits, protection. etc.

(in Forward leakage power loss

When a thyrister is blocking and has the positive voltage applied to the anoce , a loss occurs corresponding to

J v x i. This loss is small.

(iii) Reverse leakage power loss and tum off loss

(iv) Gate Power loss

The loss = gate voltage X current forcontinuous signals. This loss is small if pulse signals are used to tum on the thyrister.

(v) Tum on loss

Thyristers have a short thermal time constant The ratings therefore should not .be exceeded. There is generally no difference between continuous and intermit­tent ratings.

o

o

T Fig. S.7 Pulse signal circuit (on-off)

(i) d. e.firing signal

A continuous gate signal is not generally required except in applications where the thyrister may turn off before the time requir­ +_----_ ed. Fig. 5.6 shows a typical d. c. firing signals circuit. When the anode is positive and the thyrister is on, the gate current is reduced as the . voltage to produce the gate current now eorres­ponds to the forward voltage drop of the fh thyrister, which is about 1 V. The diode D is connected between the gate and the cathode and prevents large reverse voltage across the gate and the cathode terminals when the current is inter­rupted.

Fig. 5.6 D. C. Firing signals circuit

Solid State Motor Control: D. C. Motor Speed Control 133

5.2.1 Circuits to Tum on Tbyrfsters

It is necessary to apply the gate voltage to provide gate to cathode signal to turn the thyrister on. This is generally 2 to 10 V for gate currents of 100 fL A to 1500mA~ The basic circuits for application of signals to the gate are ';

(i) d. c. firing signals (ii) pulse signals (iii) a. c. phase signals.

(ii) Pulse firing signals circuit

There are advantages in having pulse signals rather than continued d. c. signals. Pulse signals may be single or multiple pulses which can becontrolled in time ; other­wise alternatively they may be on-otr ' +_-------, pulses. Fig. 5.7 shows a simple circuit giving on-off pulse signals. This circuit uses saturable core transformer. When the anode is positive, . the thyrister wiJI tum on if the transformer Th is unsaturated. When the switch is closed, the transformer will get satu­rated and the thyrister will not fire.

Single or multiple pulse can also be obtained from uni-junction transis­ter circuits.

(iii) A. Ci firing signals

Phase control is used to alter a,c, power. to be supplied to load. The power is modulated by varying that part of the voltage cycle duting which the -currenti s allowed to conduct. To achieve control, the pulse to turn oil the thytister at the correct part of the cycle is synchronized with . the a. c. supply and its phase with respect to the supply voltage is controlled. The anode to cathode phase control

134 Electric Motors: Applications andControl

can have any desired angle for conduction e.g, 90o~ 120o~ 1500 etc. Fig. 5.8 shows a simple circuit to obtain 90° phase control.

A variable resistance and a diode D is connected between anode and the gate across the thyrister Th. Alternating current supply is connected to thyrister and load. By varying the resistance, the delay time is adjusted. Increasing resistance delays the time or the phase in the cycle so that the voltage is positive enough to drive sufficient gate current to turn the thyrister on.

The anode to cathode voltage during phase control of 90° is also shown.

(b)

Load (a)

Fig. 5.8 Phase control circuit for 900 phase;

5.2.2 Circatts to Tum off Tbyfisfers

In phase commutation, the, thyrister is reverse biased over one half cycle when supply is a. c. ; this will turn off' the thyrister. The thyrister switches off when the forward current reduces to zero. This depends on the load reactance. If the load is capacitive, the current reduces to zero before the voltage.. This suggests the method of forced commutation in case ·of supply from a d.c, source. Some of the methods of capacitor tum off.are as follows:

(i) Self commutation by resonance.

(ii)· Tum off by auxiliary resonant circuit.

(iii) Parallel capacitance turn off. Ii (iv) Series capacitance turn off.

[i) Self commutation 'by resonance

Fig. 5.9 shows circuits for self commutation. Fig. 5.9 (a) shows a parallel resonant circuit, for turning off the thyrister. Once the thyrister is on, capacitance C discharges through the resonant circuit of Th, C and L. The resonanting current reverses after one half cycle. The thyrister win turn off if the resonanting current is greater than the load current. Fig. 5.9 (b) shows series resonant circuit to tum off the thyrister. Reverse voltage is obtained from the resonance derived from induction in the circuit supplied by d.c, The conduction period is fixed by the values of L and C. When the capacitor is charged, the resonant circuit tries to reverse the current and the thyrister is turned off.

13S

c

Ib)

1-0-----.,

Fig. 5.11 Parallel capacitance tum off

c

c

Th

Load

+Oo-----r---....,

Fig.510 Control by resonant turn ofJ

Thi

Thl

SoUd State Motor Control : D. C. Motor Speed Control

Fig.5.9 Self Commutation (a) Turn off by parallel resonant circuit (b) Turn off by series resonant circuit

(ii) Auxiliary resonant turn off

In the circuit shown in Fig. 5.10. turn oft' is accomplished by controlling auxiliary thyrister and resonanting L C circuit. Thyrister 2 is fired first to charge the capacitor C. Thyrister Th 2 is turned oft' when the current falls below the holding current. Thyrister 1 can be turned on to carry resonanting current of Land C and load current. When C changes polarity. the voltage is nearly twice the supply voltage. At the instant when Th -2 is fired again, the capacitor .voltage reverse biases Th 1 and turns it off. This circuit is used for application where variable average d. c. current is required,

(iii) Parallel capacitance turn of!

Fig. 5.I 1 shows this circuit, which is used for repetitive operations where it is required that switching on one thyrister should switch off the other thyrister, When thyrister Th 2 is off, tbyrister 1 conducts the load current. Capacitance C is charged via R.C. Th 1 path. If C has stored energy when Til 2 is fired, capacitance C discharges to reverse bias Th 1 for a longer period than its tum off time.

+----r-----...,

I

!: :r ,j i

iii I

f

..;

Fig. 5.12 Series capacitance turn off .

S.J D.C. MOTOR SPEED CONTROL

D.C. motors are more often supplied from a.c. supply througn diodes on rectified d.c. than on direct d:c; supply. For control purposes ' thyristers are used instead of diodes. Thyristers are used in various combinations for speed control by adjustment or tupplyvoltage to armature or of the field winding.

The basic methods of d.c. motor control-bysolid state devices are as follows:

Electric Motors: ApplicatiOns andControl

S.2.3 1byrister Prottctioa ' The protection of thyristers is mainly against over current and voltage surges.

Thyrister .CKcuitshave protection against over current by suitable fuses. The fuse must have the specification of high breaking capacity and rapid interruption of current.

For voltage surges. protection is given by immediate storage of the energy in L or C elements followed by slow dissipation of that energy. Protection against high forward dvjdt is required to avoid thyrister tum on when it is not required. Gate circuits also need protection because of low voltage and power.

__---A..--­

+---.,.....---,

(iv) Series capacitance turn off

Fig. S.12 shows the circuit for the series capacitance tum off method. The inverter circuit has, a square wave voltage output.. If T1J 1 is on and Th 2 is off, ~e current flows in the load. The load current flows in the reverse direction if Th 2 is on aad Th I is off. The filter is inserted in the circuit to obtain sine wave output. The elements L C resonate at fundamental frequency; they also act as a low pass filter to attenuate the unwanted frequencies. By C1 current leads the voltage and provides tum off facility. There are four modes of operation of the circuit:

(i) Th 2 off and Th 1 conducting (ii) Th I and Th 2 off; D I conducting

(iii) Th 1 off and Th 2 conducting (iv) Th I, Th 2 offand D 2 conducting '

thus completing the cycle.

136

137 SaNd State Motor Control: D. C. Motor Speed Control

(a) Phase control

When supply is a,c, the current is controlled to Bow from the supply to the motor for a fracti~ of each positive half cycle; this is repeated cyclically. This type of control

, can beapplied for all motor ratings.

(b) Integral cycle control

From a.c. supply the current is made to flow for a number of complete cycles and quenched for a further number of cycles and the sequence of operation is repeated. 'On', 'Off' durations ratio is adjusted by control. This method is useful for small motors of fractional kW size.

(c) Chopper control

A d,c, voltage can be switched on and off rapidly and repeatedly by means of a thyrister to a d.c, motor. The average d.c, voltage is controlled by varyin!bQn/ofJ time rat io. This type of control is used for traction motors.

There are two types of thyrister units. Thyrister converter for converting a.c, to d.c, (controlled rectifier converters) and the other' is a chopper unit for d.c. supplies. Sometimes uncontrolled rectifier converters are used to provide constant d.c, voltage and thyrister chopper can be used in the circuit to give controlled d.c. voltage output for motor control.

5.4 THYRISTER CONTROLLED RECTIFIER CONVERTERS

There are different arrangements of thyrister rectifier converters depending on the power to be handled and the voltage ripple that can be tolerated in the rectifier supply. Upto 20 kW power, single phase circuits can be used. Above this rating, three phase units will be necessary. The rectifier converters can be uncontrolled or controlled. The uncontrolled rectifiers give out d,c, voltage of constant average value.

The basic circuits for uncontrolled rectifiers are :

Single Phase: (i) half wave (ii) full wave centre tap (iii) full wave bridge

Three Phase: (i) half wave (ii) full wave bridge (iii) centre tap (iv)double star.

, For controlled converters, the basic circuit arrangements are :

(i) half wave controlled (ii) fulJ wave half controlled single phase bridge (iii) full wave fully controlled single phase bridge (iv) full wave half controlled, three phase bridge (v) full wave, fully controlled three phase bridge (vi) full wave, fully controlled, double bridge.

5.4.1 UDCODtroUed Rectifiers

.1"lg. :).13(a) shows single' phase un ntrolledreetifier half wave circuit and 5.13 (b) 'full wave bridge circuit.

For a half wave rectification eire ..it, the diode D offers zero impedance and conducts during the positive half cycle of a.c, supply voltage. Full supply voltage appears as varying d.c, voltage across the resistance load .

VeI,c' = 0.45 VnnS'

. Load

(bl

,..----r--""""T'"--, +

Electric Motors: Applications and Control

(b)

Fig. S.14 Three pbase uncontrolled rectifiers (8) half wave circuit (b) full wave bridge circuit

(IJ

°1

+

°3 Load

Secondary . three phase

wind ings

(al

+ °3

.,. Load JI

FiB. S.13 Single Phase Uncontrolled Rectifiers (a) half wave circuit (b) full wave bridge circuit

During the negative cycle part, there is no conduction and the current is blocked. When the load is inductive instead of resistive, a free wheel diode is connected across the inductance to provide a path to the load current during the negative half period. In practice the load may have some resistance and some inductance or it is RL circuit.

138

. !

. !

, -., ,

i'f "I

139 Solid State Motor Control: D. C. Motor Speed Control

In a full wave bridge circuit. during the positive half cycle D, conducts and in the negative half cycle. D t conducts. D, and Ds complete the circuit through the load. in each half cycle respectively. In this connexion, Vd• C' = 0.9 VrmS '

For higher powers. three phase , supply is used and bridge circuits are used for rectification. ' Fig. 5.14 (a) shows a half wave three phase rectifier circuit and Fig. 5.14 (b) shows full wave bridge for the rectification circuit using three phase supply.

With three phase supply. the voltage ripple after rectification is much reduced in magnitude and increased in frequency.

For half wave. Vdc = 1.17 Vph/rms ;

for full wave, Vde = 2.34 Vph/rms. In (b), D1, Da• D6 have positive terminal of the load circuit common, but at a given instant. only that diode which has most positive voltage wiU conduct, the other two being automatically reverse biased ; similarly D2, D" De can conduct with most negative phase voltage.

5.5 THYRISTER CONTROLLED CONVERTERS

(i) Half wave controlled converter Fig.S.15 (a) shows a half wave controlled converter. The thyris ter circuit uses

phase commutation and is the single phase half wave rectifier. The gating pulse is applied during the positive half cycle of the anode voltage ; the thyrister immediately conducts and the supply voltage appears across the load for the remainder of the positive half cycle. At the end of positive half cycle, the thyrister current falls and the supply voltage reverses. This applies reverse bias across the thyrister and turns it off rapidly.

The angle by which the thyrister retards the starting of conduction is known as delay angle or firing angle IX. If the IX is varied from zero to n, it reduces the average d.c, output voltage from maximum to zero. The process is known as phase commutation.

If the load is inductive, the build of the current is slow at start and decaying inductive current continues conduction of current after w t = tt, For inductive loads. a free wheeling diode is placed across it in the circuit. At the end of the positive half cycle. current in inductive load decays by circulating through the free wheeling diode. The decay is effected at its natural time constant.

The waveforms of the supply voltage. voltage across the load and load current in an inductive circuit having R and L are also shown in the Fig. 5.15 (b).

(if) Half wave controlled bridge converter

Fig. 5.16 shows the circuit for a single phase half wave controlled bridge converter. The thyrister Th r conducts-during the positive half cycle completing the circuit through the diode lJ4, back to negative of the supply. In the negative half cycle the thyrister Th 2 'conducts. completing the circuit through diode Da. The rectified voltage across the load Vdc is also shown in the figure. The conduction starts at the firing angle lX ,controlled by the gate.

,

current

R

Supply Y9ltage

Load voltage

£P. ,'• . de DO . C

I · \ " oMt· -<. ~ , ~ . "' ...'

(b)

wt

I I I I I

I I

Vi load

Th

(a 1

(a) .

Electric Motors: ApplicatiolU imdCo1Jlro/

F' S 16 S'lng)e Phase half controlled briel.' ~ converterIg, , -(a) Converter circuit (b) Voltage at the load

~~~ I ' I I I I I n

(bl

Pig, S.IS Halfwave controlled converter and free wheeling diode (a) Converter circuit (b) Voltaae and ClII'l'eJJt at load

140

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1

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- - - - - - - - --- - --

141 Solid State Motor Control: D; C. Motor Sped Control

(iii) Full wave fully controlled converter bridge

Four thyristers are used con': nected tn bridge form to -act as converter and supply d.c. voltage to the load. The circuit and the output voltage wave form are shown in

Fig. 5.17. Compared to the half wave

circuit, this circuit has a much improved form factor of the output voltage (rms voltage/average vol, tage). Ip this , ' circuit, inversion and so regenerative braking is possible for the motor load. If

dC C:'')/D'

Voltage ".,double bridge is not used, the jpfields of the motor should be rever­ \. - I ..,t sed. Half wave controlled' circuits ',-",/ are used where .regeneration is not (b) required . Fig.5.17 Full wave fully controlled converter witb

single phase bridge (a) converter bridge circuit (b) Voltage at the load

(;)0) Three phase half controlled bridge converter

Fig. 5.18 shows a three phase half Controlled bridge converter. The thyristers Th), Th 3 andTh 5 are connected in phase A, Band C respectively. The return path is through diodes b., D, and D. connected in phase A, Band C - respectively. The phase eurrents ion the' positive side of the load are delayed by an angle Cl but those on the negative side are not delayed. The thyrister and diode -of each phase Conduct simultaneously. Fig. 5.18 (a) shows the circuit arrangement and (b) shows the phase voltages ~s well as d. c. output voltage resulting from the bridge.

, Thl

A~__-----~ Motor B o-4---r----~--~ toad

C

(a)

Fig. 5.18 Three phase half controlled bridge conver ter (a) Converter circuit .9

142 Electric Motors: Applications and Control

(b)

fig. 5.18 Three phase half controlled bridge converter (b) Supply and output d.c. voltaic

(v) Three phase fully controlled bridge converter

Fig. 5.19 shows three phase fully controlled converter ; (a) shows the circuit arrangements and (b) shows the phase voltages and d.c, output voltage. The circuit

.has 6 thyristers Th 1, Th 2. Th 3. Th 4. Th 5. Th 6 connected as shown. Th I and Th 4 in phase A. Th 3. Th 6 in phase Band Th S. Th 2 in phase C. In this circuit. the phase currents on the positive side of the Ioad-as well as negative side of the load are delayed by an angle« each. At first the current flows from ·.A. to the 'positive side of the load circuit and'returns to B through Th 6. After Th 2 fires. the return current is from Th 2 to C. The corresponding sequence occurs for the remainder of the time .

. I

I ' I,; I I ' ; l

A + 8 Vdc C

Va IHO

Fig. 5.19 Three phase fullycontrollcd bridge converter j ,

(a) Converter circuit

. J

143

3$.~'

Transformor 5tcondar~s

inA

Wt

Fig. 5.20 Full wave, fully controlled, double bridge

Fig 5.19 Three phase fully controlled bridge converter

(b) Supply and output d.c, voltage

(b)

Transformer secondar ies in ).

(vi) Full wave.fully controlled, double bridge

For large motors, the 3 phase 6 pulse connexions of the last two methods may not produce acceptable input and output harmonics. A 6 phase 12 pulse configuration is achieved by the use of transformer with two sets of secondaries connected in delta and star and each feeding a separate 3 phase 6 pulse converter, There is 30° phase displacement between two secondary voltage phasors and the combination gives a 12 pulse output. The converter .units can be connected in parallel or in series. Fig. 5.20 shows full wave fully controlled double bridge.

Solid State Motor Control: D. C. Motor Speed Control

5.5.1 The d.c, voltage obtained by the use of various types of converters is given below:

The average output voltaao is controlled by usinga thyrister circuit with delayed commutation.. The wavefonn of the output voltage is shown in Fig. 5.21. If m phase half wave circuit is considered, each phase conducts for a period 2 1Clm radians per

(5.2)

(5.3)

(5.4)

(5.1)

Electric Motors : Applicatlotuand Control

Full wave fully controlled bridge :

2.J' 2Vd = J'ph cos II n

.../2Vd = -- Vph ( 1+cos ll)

7f:

I, I

I, I 1

-.JL. '&)tm

Fig. 5.11 Output voltage for m pbUe half wave rectifier : firing angle = «

Half-wave controlled bridge converter :

Three phase half controlled bridge converter:

3 .J3 -Vd = n X -2- X .J 2 V,,, ( 1+eos II )

~V":= ~ n IVp ll (l+cos ll)

Three phase fully controlled bridge converter :

3 ,; 3 -Vd ::::I n x 2 x.J 2 V,hXZ OO S II

V. = 3 .../6 Vphcos ot n

144

(,i!) Anti-parallel bridges

The back to backconnexion of two three phase anti parallel bridge converters is used where motors require repeated and rapid reversals of direction.

S.6 VOLTAGE RELATION IN THYRISTER CONVERTER CIllCUITS

145 Solid State Motor Control : D. C. Moto; Speed Control

cycleof supply voltage. If Vph is the r.m.s. value of voltage per phase of the trans­'former secondary supplying the thyrister circuit, the instantaneous phase" voltage

v = .r z Vph cos wt. the reference zero time being taken from the instant of peak voltage. If IX is the firing angle, the phase conducts from (-n!m +IX) to (rr!m+«). The average output voltage Vd is given by .

17 -..j... at m'

d= coswt;'17J .j2Vph d(wt)

--17 + IX m

- m 1tVd = .j 2 Vph - sin - cos IX (5.5)

1t m When delay angle or tiring angle is zero.

- m 17vda = .j 2 Vph -- sin (5.6)11: m

Thus with Phase control, Vel = Vda (,'08 or. (5.7)

5.6.1 Overlap

In the above analysis, it is assumed that the load current lei 'commutates instan­taneously from one thyrister to the next. In paractice, the supply transformer has leakage inductance and this results in an angle of overlap u when two thyristers conduct simultaneously and two transformer phases are shorted. Fig. 5.22 shows the effecton voltage and current. In a phase controlled rectifier, u decreases.as at. increases.

yR B

1. &""4-~r-,~-~n--t~-\--+_-...l.:-_-""",-" ~ . wI

(a)

c.. ~ u ... !! .~}L..-_L.\,..-L,.....:........r;..a... _

... (b\

Fig. 5.22 . Voltage and current waveforms for a phase controlled rectifier

146 Electric Motors :App/ications and Control

For a half wave rectifier, mldX cos IX-COS ( IX+ u) = ---;;~ (5.8) 1T Vd,()

where X = equivalent leakage reactance or commutating reactance per phase. The

. I . . b m Id X Th I ' f V ithvoltage drop due to over ap IS given y 2 1t e average va ue 0 d WI

overlap is thus below the value by instantaneous commutation. In half wave rectifier, the mean output voltage with firing angle and overlap is given by

m IdXVd = Vde cos IX- 2 n: (5.9)

_ IdThe current I a is given by (5.10).jm fa is the phase current,

5.6.2 Tbree Pbase. Phase Controlled Full Wa~e Bridge Rectifier

This may be regarded as two three phasebalf wave circuits in series. The average output voltage is twice that of a three phase half wave circuit. The mean output voltage with negligible overlap and zero firing angle is given by

3J()VdO = - Vph = 2.34 Vph (5.11)

n:

5.7 THE PHASE CONTROLLED INVERTER

The phase controlled inverter is a unidirectional current device. The reversal of power flow is obtained by reversing voltage polarity and maintaining the same current direction . Thyrister current flows from anode to cathode. The direction of energy flow .can be reversed only if a,c, supply delivers current during periods of negative anode voltage. The current is forced to flow in opposition to the negative phase voltage by delaying commutation. . To obtain. inversion, the delay angle should be more than 90°. T~en the mean output voltage Vd = VdO CQi IX becomes negative. Fig. 5.23 shows a basic circuit for a three phase inverter. It! is maintained constant by the d.c. inductor. D.C. supply exceeds average Vd• Commutation occurs because the incoming thyrister is triggered while its anode is more positive than the anode of the conducting thyrister. Commutation must be completed before the instant at which the phase voltages become equal. The angle fJ must always be large enough to allow sufficient time for overlap and tum off of the thyrister

fJ = n:-IX

5.8 REGENERATION BY PHASE CONTROL

Reversible speed control in one direction is applied to cranes, paper mills. hoists, etc., regeneration in both directions is applied to tract ion, rolling mills, etc. As conduc­tion through thyristers is unidirectional, motor armature or field current must be

wt

D.C.supply ...oltage. V

Vd average

counter I.mf.

B

(a)

y

I I

o(~ I I I I I I I I I I I I I I I I I .I IH---l

(e) Basic circuit for a three phase inverter (a) Circuit diagram (b) Wa\".:forms of voltage and current

R

Fig. 5.23

Th3

C Go C :::JU'-­ ..I....iL.J._..,......L-L _

Solid Stale Motor Control: D. C. Motor Speed Control J47

reversed or a separate thyrister bridge should be provided for each direction, During regeneration, the converter unit should be so controlled that the current flows during negative half periods of supply only. This can be arranged in two Ways:

(i) Anti Parallel Bridges

To effect drive in either direction with regeneration, the two bridges connected

back to back must operate in such \'.ay that one rectifies while the other inverts and

148 Electric Motor$: Applications and Control

these modes are interchanged when the direction of the machine reverses. The firing angles at1 and ot2 must be so related that ot1 +~ = 7T

(ii) Fullycontrolled bridges supplied from singlephase or three phase supply can be employed for regenerative braking. . For regeneration the armature connexions of the motor are reversed.

5.9 INTEGRAL CYCLE CONTROL

This method is also termed 'burst firing' . This is used for fractional kW d.c. motors receiving rectified supply from a single phase source. The circuit diagram and thecy'cle of operation is shown in Fig. 5.24.

Th'

Th2

(al

.....-------(---------1 (,

(b)

Fig. 5.24 Integral cycle control

(a) Arrangement of cjrc~it

(b) Cycle of operation

The supply is switched on for an integral number of" cycles C1 and . is made off for further number of integral cycles Ct. Thus one on/off cycle is for time C = cI + Ct .

This on/off cycle is repeated cyclically. The switching is effected by means of thyristers as shown in Fig. 5.24 (a), free wheeling is provided by rectifier bridge diodes.

The speed control is obtained by varying C1/C 2' Smooth operation is achieved when C is between)O and 20 and cI/c! is less than unity. In integral cycle control, the triggering of thyrister occurs at zero voltage.

5.10 CHOPPER CONTROL

v

I

.. -­ - - ~ ---­ - -_. ,

r-t 1 - -12 -

I--~. . . .-f-- . "-:" ~ ._ .- f­

te-I I I • Time; .

t

~ ; ~ u L.o.L......;_.J-_-.....--Io_-~--f--~---t--

Solid State Motor Control: D. C. Motor Speed Control 149

fbI . Fig. 5.25 Chopper Controlof d. e. motor

(a) Circuit arrangement , (b) Waveforms of voltage and current

The mean d.c. voltage over a full interval corresponding to c is given by

2~2Vd = -~ Vph ( cJc ) (5.12)

where Vph is r.m.s. phase voltage of the supply

A thyrister chopper can be used to obtain variable output voltage from a constant d,c, supply source e.g., battery or d,c, railway traction supply. The voltage control is obtained by chopping and this is an economical method of controlling speed and torque of a d.c, motor with battery supply or traction motors. Fig. 5.25 shows the simple circuit in (a) and voltage and current waveforms ofbattery and motor in (b).

5.10.2 Braking

For series machines or for separately excited machines, the basic circui ts for

braking operation are shown in Fig. 5.27. In (a) , the ma in motor 's norma l running with the use of thyrister is shown. Th I

is the main thyrister. In arranging the circuit the upper current limit of the thyrister should be kept in view. Commutating circuit for thyr ister is necessary. D is the free wheeling diode. In (b) dynamic braking circuit is shown. With Th I conducting, the

The thyrister Th is used to have an on/off switching cycle. This i ~ effected either by variable on time with switching repeated at constant frequency such as 100Hz ; or this can also be done by application of 2 ms pulses of variable number per second. This may be between 20 and 500. The rectangular pulse of battery voltage is applied t.o the motor when the thyrister is on. The motor current rises depending on the

resistance, inductance and change in E~. During off time the motor current continues to flow through the diode D . The mean current is approximately ' proportional to mean voltage and speed.

Till

o

c

Fig. 5.26 Chopper turn ofT

+

Electric Motors: Applications and Control

S.10.1 Turo off with Chopper

The chopping frequency used is limited by the thyrister characteristics. Turn off

requires forced commutation. One of the methods used for turn off ill chopper control circuits is shown in Fig: 5.26. The previously charged capacitor C, controlled by auxiJiary'thyrister Th 2, is connected in parallel with the main thyrister Th I and dis­charged into it in the opposite direction of the main output current. It thus provides'

reverse bias. Turn off circuits are based on re­

sonance. Th I is the main thyrister and D is the free wheeling diode across the motor armature. The steps in which turn off is effected are as follows :

(i) Before the motor starts running, Th 2 is fired ; this charges capacitance C; when charging is complete, the current ceases and Th 2 turns off.

(ii) Th 1 is fired, motor starts and ro­tates as current flows through it. C discharges simultaneously through Th I, diode D'I, and inductance L, but only during the first half cycle. Potential of lower plate of C IS

doubled. (iii) Reverse bias Th I by triggering Th 2, C discharges through the mot or ; motor

current is quenched. (iv) When Th I is turned off completely, C is recharged to its original polarity and

the operation of th e next on mode, etc., is repeated .

ISO

151 Solid State Motor Control: D. C. Motor Speed Control

+0----......, TPl t

D

(b)(a)

Motor normal running' Dynamic braking

DTh1

(C) (d)

Plugging Regenerative braking

Fig. 5.27 Braking of d.c, motors by chopper control (a) Connexions Cor normal running of motor (b) Connexions Cor dynamic braking (c) Connexions Cor plugging (d) Connexions for regenerative braking

stored kinetic energy is dissipated through the resistance R I . When the current is higher than the thyrister current"limit, Th 1 is turned off and the motor armature is con­nected across higher resistance Rb to dissipate the energy and effect braking. In (c), the connexions for plugging or reverse current braking operation are shown. The armature connexion is reversed. When Th 1 is fired. the braking current and braking torque increases. When Th 1 is turned off, the current decreases while passing through R1 and diode D. The regenerative braking connexions are shown in (d). The connexions are similar to those of dynamic braking for Th 1 triggering and passing current through RI and the motor armature. When current is above the limit. Th I is turned off. If in this condition the rotational e.m.f, e; is higher than the supply voltage V,

then the current is fed back to the supply and the operation is regeneration. When the current fails, Th 1 is again turned on. Thus there can be transfer arranged for either dynamic braking conditions or regenerative braking.

152, Electric Motors; Applications and Cont

5.]0.3 Chopper Control-Applications

Chopper control operations are mainly for battery vehicles which need to and stop repeatedly. The chopper control replaces resistors required for control. method gives maximum use of the capacity of the battery and lengthens the duty period between battery charging by the use of regenerative , braking.

The other application of chopper , is for multiple unit railway trains having. desupply, Chopper control replaces the series resistances or rheostats used for control of traction motors of th is type.

5.11 POSITION CONTROL BY D.C. MOTORS

, Position control is required in hoisting loads onto platforms. When de motors , ,

aft' used as the drive for hoisting loads. the position control is effectedeither by open loop manual operation or by automatic control using closedloop control. In manual control. the motor is allowed to rotate through a small distance at low speeds and in discrete steps, A resistance is included in series with the armature of the motor; . the value of the resistance is such that when the motor is switched on with the resistance .in series. the torque produced by the motor is .not much higher than the load torque. Starting and stopping of the motor is done by the operator quickly in succession so that the movement of the motor has very slow acceleration and moves over a fraction of an inch at a time. ' The operation is known as <inching'. A thyrister chopper circuit is sometimes used to eliminate the resistance. maintaining low average voltage across the armature, low current, low torque and low speed for the operation.

Fig. 5.28 shows automatic position control in its basic form.

+

Amplifier

S=Vj-Vo

Fig.,5.28 Autornauc position control

Potential divider is connected across the separate d.c, source and potential across the resistance is adjusted corresponding to the desired position angle of the shaft and forms the reference voltage." The second potential divider gives the voltage proportional to the actual angle of shaft position. The differencebetween the voltage

153 Solid State Motor Control: D. C. Motor 'Speed Control

VI and Vo is the error signal which is fed to the amplifier and then this provides current proportional to this difference to the ficJa winding of the mOt:,Of. Thus the motor will provide torque proportional to the' fieldcurrent or the errur voltage, the motor will rotate and change Vo till it becomes equal to' reference voltage VI' The blockdiagram for the position control circuit is shown in Fig. 5.29.

Const.Tlt power

Fig. 5.29 Block diagram for position control

Instead of control by the field, thyrister control can be used controlling tbe armature voltage using the same .principle. The inductance of the armature heina low, it offers faster response than the field control. This will need higher :~ntro1JiDa powers, but the thyristers with high gain permit this.

REFERENCES

~11 Raymond Ramshaw, Power Electronics, Chapman and Hall, London, 1973. 5.2 J. M. D. Murphy, Thyrister Control of A.C. Motors. Pergamon Press, 1975. 5.3 M. G. Say and E. O. Taylor, Direct. CUrrent Machine, the Englis~ Languqe

Book Society and Pitman, 1980.

PROBLEMS

5.1 Explain the characteristics of an SCR thyrister and its working with reference to­(i) forward biased (ii) reversed biased and blocking (iii) methods of turn on (iv) methods of tum off (v) Current. voltage and power rating.

5:2 State and explain the operation of thyristers in series and thyristers in parallel. State applications.

5.3 Describe circuits for turning on thyristers. 5.4 Describe circuits used for turning off thyristers. 5.5 Explain the principle and action of uncontrolled rectifiers (i) Single phase (ii)

three phase. 5.6 Explain the operation of thyrister controlled converters:

(a) half wave controlled. (b) Full wave controlled.

154 Electric Motors : Applications and Contr

5.7 Obtain the expression for output for (i) half wave rectifier (ii) full wave ree ti having m phases. What is the effect of angle of firing lit and overlap u.

5.8 Explain the methods of speed control of d.c, motors by (a) Phase control Integral cycle control (c) Chopper control.

5.9 Explain the principle of phase controlled inverter. 5.10 How is regeneration of d.c, motors obtained by phase control? 5.11 Derive an expression for the output voltage in integral cycle control. How is t

control effected ? 5.12 How is the braking of d.c, motors effected by chopper control?

diagram. 5.13 Explain the position control of d,c, motors. What is 'inching' ?

matic position control obtained?