A thyristor driven pump is operated by varying the DC input signal in the firing circuit of thyristor drive. This operation suffers from difficulties due to the nonlinear relation between thyristor output and DC input. In the present paper, an opto-isolator based linearization technique of a typical thyristor driven pump has been proposed. The design, fabrication and the necessary circuit diagram along with theoretical explanations of the resultant output has been described. The operation of the linearized thyristor driven pump has been studied experimentally and the experimental data before and after linearization are reported. The characteristic graphs are found to have very good linearity.

1. ISA Transactions 51 (2012) 220–228
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ISA Transactions
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An opto-isolator based linearization technique of a typical thyristor driven pump
S.C. Bera, R. Sarkar∗
, N. Mandal
Instrumentation Engineering, Department of Applied Physics, University of Calcutta, 92 A.P.C Road, Kolkata-700009, India
a r t i c l e i n f o
Article history:
Received 15 February 2011
Received in revised form
30 August 2011
Accepted 12 September 2011
Available online 6 October 2011
Keywords:
Motor load current
Thyristor driven pump
Opto-isolator
PID controller
Inverse derivative control action
Linearization technique
a b s t r a c t
A thyristor driven pump is operated by varying the DC input signal in the firing circuit of thyristor
drive. This operation suffers from difficulties due to the nonlinear relation between thyristor output
and DC input. In the present paper, an opto-isolator based linearization technique of a typical thyristor
driven pump has been proposed. The design, fabrication and the necessary circuit diagram along with
theoretical explanations of the resultant output has been described. The operation of the linearized
thyristor driven pump has been studied experimentally and the experimental data before and after
linearization are reported. The characteristic graphs are found to have very good linearity.
© 2011, ISA. Published by Elsevier Ltd. All rights reserved.
1. Introduction
A thyristor driven pump has many applications in the
processing industry. In this pump, the speed or output flow rate
is adjusted by changing the dc input voltage in the firing circuit
of the thyristor unit. An adjustable speed drive can often provide
smoother operation compared to an alternative fixed speed
mode of operation. Moreover, energy conservation, acceleration
control, different operating speed for different process recipes,
easy adjustment of rate of production, easy maintenance of process
variables at desired values, accurate positioning of moving object,
accurate control of torque or tension etc. are the other advantages
of thyristor or other solid state operated pumps or motors. The
other solid state devices used in variable speed drives may be BJT,
MOSFET, SIT, IGBT, COOLMOS etc. Thyristor or SCR devices also may
be of various types such as BCT, LASCR, TRIAC, RCT, GTO, FER-CTH,
MTO, ETO, IGCT etc. [1–5]. Power MOSFET and IGBT are voltage
controlled devices whereas a thyristor is a current controlled
device. MOSFET and IGBT are easier to drive than thyristors and
are being used in power applications. However IGBT produces
large amount of distortion in the input voltage to the induction
motors which may result in instability of the drive system. So
IGBT should not be used as a power switch without harmonic
∗ Correspondence to: Asansol Engineering College, Department of Electrical
Engineering, Vivekananda Sarani, Kanyapur, Asansol-713305, West-Bengal, India.
Tel.: +91 9433249918; fax: +91 341 2256334.
E-mail addresses: scb152@indiatimes.com (S.C. Bera),
rajan_maa@rediffmail.com (R. Sarkar), nirupama_cal@rediffmail.com (N. Mandal).
filters [6]. A thyristor drive unit does not require such filters and
can produce an output voltage without distortion in the firing
angle within π/3 [1–5]. The current limits of commercial IGBT
are now available as high as 6.5 kV, 2.4 kA and the switching
frequency may be 25 kHz and even up to 100 kHz with few
soft switching techniques [1–5]. A Thyristor is a low cost high
voltage high current device with a switching frequency as low
as 50–60 Hz. The maximum voltage rating may be as high as
6.5 kV and the current rating may be as high as 3 kA [1–4,7,
5]. IGBT has a high input impedance like MOSFET and a low
on-state conduction loss. It has no second breakdown problem
like BJT or MOSFET. It is used in high frequency applications
like, SMPS, induction heating, etc. and also in medium power
application such as dc and ac motor drives, UPS systems, relays and
contactors. There are different applications of thyristor devices in
HVDC transmission, HVAC transmission, Static Var Compensation
(SVC), converters for medium voltage drives, and also to certain
pulse power applications. The thyristor operated electric motor
driven control valves are also used in the processing industry
[1–4,7,5]. Various researchers have taken am interest in studying
the operation of thyristor or solid state operated pumps or
motors. Daniel-Berhe and Unbeaten [8] have studied the nonlinear
continuous time dynamics of a thyristor driven dc motor and have
estimated the physical parameters of the motors. The integral
cycle switching technique has been utilized by Shephard and
Gallagher [9] for thyristor control of dc motor loads. A phase
locked loop technique has been proposed by Gupta et al. [10]
for thyristor control. Stanton [11] has explained the importance
of instrumentation regarding the measurement of parameters
of thyristor control which produces non-sinusoidal waveforms
0019-0578/$ – see front matter © 2011, ISA. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.isatra.2011.09.005

2. S.C. Bera et al. / ISA Transactions 51 (2012) 220–228 221
with high harmonic content. Riyaz et al. [6] have studied the
performance of a thyristor driven variable voltage induction motor
and have developed simple and flexible models for simulation
and validation of the system. Bennett and Jones [12] have studied
the operation of thyristor driven dc and ac motors in various
applications. Pauwels [13] have shown that a good amount of
energy can be saved by using a thyristor driven variable speed
pump. Huang et al. [14] have presented an on-state model for
MCST and the results are compared with MCT and IGBT. It has been
observed that during the on-state, MCST operates like thyristor
at a low anode voltage and enters IGBT with increasing anode
voltage. Zehringer et al. [15] have described two types of current
IGBT modules which are evolved from opposing requirements
of tractions and power applications. Liu and Daley [16] have
proposed an analytic physical dynamic model for the tuning of
nonlinear PID control of hydraulic systems. In this technique, a
nonlinear PID control with an inverse of dead zone is implemented
to overcome the dead zone in the hydraulic systems. Montanari
et al. [17] have developed a nonlinear control technique of an
induction motor without using any actual speed sensor. Here,
stator current has been taken as a measure of motor speed. Ben
Azza et al. [18] also have proposed a single phase induction motor
drive with sensorless indirect stator field control using tuning of
stator resistance and the estimated speed of the induction motor
is measured by measurements of stator currents.
The nonlinear relation between motor speed and dc input
voltage of a thyristor may cause operational problems in speed
control of the motor or output flow rate control of the pump. In the
present paper, an opto-isolator based linearization technique of a
thyristor driven pump has been studied. In an ordinary thyristor
driven motor it has been observed that the output voltage of
the thyristor, motor speed, motor load current and outlet flow
rate of water produced by a thyristor driven motor and pump
etc. are all nonlinearly related with the dc input signal of the
thyristor firing circuit. Moreover, the response time of the motor
driven pump to any change of load is found to be very small.
So when an ordinary PID controller is used to adjust the speed
of the motor to obtain a desired flow rate, it may suffer from
wide fluctuations from the desired value. In order to overcome
these difficulties of a thyristor driven motor coupled to a pump,
a modified design of the control system with PIDD−1
controller has
been described in this paper. After proper tuning of this control
loop, its performance has been experimentally studied. A quite
linear characteristic of the pump with much more stable operation
has been observed. The experimental results are presented in this
paper. Moreover, the measured output speed and flow rate of the
pump are automatically maintained at the values as dictated by the
set point value. Hence pump speed and flow rate follow a linear
relationship with the set point value taken as the input signal of
the thyristor firing circuit.
2. Method of approach
Let us consider an ordinary thyristor operated ac/dc universal
motor driven pump lifting water from a reservoir tank through a
pipeline as shown in Fig. 1. The thyristor firing circuit has the input
dc supply in the range 1–5 V so that the pump can be operated from
a minimum to maximum range to obtain a water flow rate through
the pipeline in a given range. Under this condition the thyristor
output voltage, load current, the pump output flow rate and motor
speed vary nonlinearly with input dc voltage signal of the firing
circuit.
Now in order to design an electronic linearization network with
the motor load current linearly varying with the dc input voltage
of the thyristor driven pump, it is required to measure the load
current derived from thyristor output and the measuring system
Fig. 1. Circuit diagram of a thyristor operated electric motor driven pump before
linearization.
should be isolated from the supply voltage. So an opto-isolator
based measuring circuit of the load current is designed as shown
in Fig. 2.
In Fig. 2, an A.C. ammeter (A) is used to measure the load
current of the thyristor, passing through the pump motor. A light
emitting diode (LED) D1 shunts the ammeter and produces light
of an intensity according to the load current of the pump motor.
The light produced by the LED is allowed to be incident on a
light dependent resistor (LDR) R1. The resistance R1 of the LDR unit
changes in accordance with the intensity of the light emitted by
LED. The LED and LDR combination is placed in a black enclosure,
so that no external light can interfere with their operation. The LDR
is used as the input resistance of an inverting amplifier with op-
amp A1. The input signal of the amplifier is a stabilized negative dc
voltage signal (−V) so that the output of the amplifier is a positive
dc voltage signal and is given by,
V1 =
R2V
R1
(1)
where R1 is the resistance of LDR for a particular load current IL.
Let RD be the forward resistance of the LED (D1) and RA be the
resistance of the A.C. ammeter (A), both connected in parallel and
placed in series with the motor circuit. Let Im be the current passing
through the ammeter. So the current ID passing through the LED
will be given by,
ID =
ImRA
RD
. (2)
Now the light power PL emitted by LED is directly propor-
tional [19,20] to the current (ID) flowing through it i.e.,
PL ∝ ID
or,
PL = K1ID (3)
where K1 is the constant of proportionality.
From (2) and (3),
PL =
K1ImRA
RD
. (4)

3. 222 S.C. Bera et al. / ISA Transactions 51 (2012) 220–228
Fig. 2. Circuit diagram of proposed thyristor operated electric motor driven pump after linearization.
Now one terminal of LDR is connected to a stabilized negative
dc voltage source (−V) and the other end is connected to a virtual
ground inverting terminal of op-Amp A1. So if the current passing
through the LDR in dark condition is Id and in the lit condition is Iph
then the resistance of the LDR is given by
R1 =
V
Id + Iph
. (5)
Again, in the dark region Iph is zero and Id is very small. So the
value of R1 is very high in the dark region. In the lighted region the
incident light power Pinc on the LDR surface is proportional to the
developed power PL by LDR i.e.
Pinc ∝ PL
or,
Pinc = K2PL (6)
where K2 is a constant of proportionality.
Combining (4) and (6) we have,
Pinc =
K1K2ImRA
RD
. (7)
Now the current Iph produced in the LDR material by the
incident photons is given by,
Iph =
ηqPinc
hν
(8)
where,
q = Electronic charge
η = Quantum efficiency of generation of electron–hole pairs
by the incident photons
h = Plank’s constant
ν = Frequency of light, emitted by LED
From (5) and (8) we have,
R1 =
V

Id + ηqPinc
hν
. (9)
Hence from (1) and (8) the output voltage V1 of op-Amp A1 is
given by,
V1 = R2

Id +
η q Pinc
hν

. (10)
Combining (6) and (10),
V1 = R2

Id +
ηqK2PL
hν

. (11)
From (4) and (11) we get,
V1 = R2

Id +
ηqK1K2ImRA
hνRD

. (12)
Now from the shunt principle,
Im =
RDIL
RD + RA
. (13)

4. S.C. Bera et al. / ISA Transactions 51 (2012) 220–228 223
Combining (12) and (13), we get,
V1 = R2
[
Id +
ηqK1K2RAIL
hν (RA + RD)
]
(14)
or,
V1 = K3 + K4IL (15)
where,
K3 = R2Id and K4 =
ηqK1K2R2RA
hν(RA + RD)
. (16)
Now assuming no losses of a motor driven pump in the ideal
case, the power consumed in a pump motor may be given by the
simple relation, power = mass flow rate of the liquid × the height
to which the liquid is lifted × acceleration due to gravity.
Hence under ideal conditions the motor load current is directly
proportional to the mass flow rate of the liquid if the motor supply
voltage, power factor and height of fluid lift are kept constant.
However, in actual cases there is always loss of energy in a motor
and pump. So the relation between motor load current (IL) and
mass flow rate (Q ) of the liquid may be nonlinear as shown by the
following equation.
IL = ILO +

∂IL
∂Q

QO
Q +
1
2!

∂2
IL
∂Q 2

QO
Q 2
+ · · · (17)
where ILO is the load current for a given mass flow rate QO and IL
is the load current when the mass flow rate increases from QO by
Q .
Or,
IL = ILO + a Q + b Q 2
+ · · · (18)
where,
a =

∂IL
∂Q

QO
and b =
1
2!

∂2
IL
∂Q 2

QO
. (19)
For small values of Q , the higher order terms are negligible
and (18) may be reduced to
IL = ILO + a Q + b Q 2
. (20)
From (14) and (20) we get,
V1 = K3 + K4(ILO + a Q + b Q 2
) (21)
or,
V1 = K5 + K4(a Q + b Q 2
) (22)
where
K5 = K3 + K4ILO. (23)
If the mass flow rate increases from an initial value Qo to a value
Q by a very small amount Q then Q = Q − Qo and (22) is
reduced to
V1 = K5 + K4[a(Q − Qo) + b(Q − Qo)2
]. (24)
If we start from no flow condition then Qo = 0 and (24) reduce
to
V1 = K5 + K4[aQ + bQ 2
]
or,
V1 = K5 + K6Q + K7Q 2
(25)
where,
K6 = K4a and K7 = K4b. (26)
Fig. 3. Circuit diagram of a linearizing PID controller with inverse derivative control
action.
For smaller pumps there is an almost linear relation between
flow rate Q and load current IL. Hence b = 1
2′

∂2IL
∂Q 2

QO
may be
assumed to be zero or negligible. So K7 = K4b may be assumed to
be zero or negligible and (25) may be reduced to
V1 = K5 + K6Q . (27)
Let VZ be the output voltage of the zero adjustment potentiome-
ter RZ and R3 = R5 = R6 = R. So the output (Vm) of the differential
amplifier circuit consisting of op-Amp A2 is given by,
Vm =
R4
R3
(V1 − VZ ) (28)
or,
Vm =
R4
R3
[K3 + K4IL − VZ ]
or,
Vm = K8 + K9Q + K10Q 2
(29)
where,
K8 =
R4
R3
(K5 − VZ ), K9 =
K6R4
R3
and
K10 =
K7R4
R3
. (30)
For smaller pumps K10 is very small since K7 is negligible and
(29) is reduced to
Vm = K8 + K9Q . (31)
By adjusting the gain K9 or gain adjustment potentiometer R2 and
zero adjustment voltage VZ or zero adjustment potentiometer RZ , the
output voltage Vm in the range 1–5 V may be obtained. In order to
obtain a linear characteristic between Vm and Q as well as to obtain
a linear characteristic of the thyristor driven pump, the measuring
circuit shown in Fig. 2 is combined with a modified PID controller
circuit with inverse derivative control action to form a linearization
network of a thyristor operated pump as shown in Fig. 3.
In Fig. 3, the measured signal (Vm) is subtracted from the input
or set point signal (VS ) to obtain the error or deviation signal (Ve)
by the differential amplifier consisting of op-amp A3.
Assuming, R7 = R8 = R9 = R10.
So,
Ve = VS − Vm. (32)
Combining (29) and (32) we have,
Ve = VS − K8 − K9Q − K10Q 2
. (33)
The linearizing controller circuit consists of only two op- Amps
A4 and A5 as shown in Fig. 2. This controller circuit is separately
shown in Fig. 3.
The deviation signal Ve is now applied at the input terminal A of
the amplifier A4. The output signal VB at the output terminal B of

5. 224 S.C. Bera et al. / ISA Transactions 51 (2012) 220–228
Fig. 4. Equivalent circuit of Fig. 3.
the amplifier A4 is obtained from the equivalent circuit shown in
Fig. 4.
Since the input impedance of the operational amplifier is high,
the input signals e1 and e2 at the inverting and non-inverting
terminals respectively of the amplifier A4 may be assumed to be
identical. Again, due to the high input impedance of the op-Amp,
negligible current passes through the input resistance R11 from the
input deviation signal source (Ve). Hence, the input signal e2 at
the non-inverting terminal of A4 will be almost equal to the input
signal Ve.
i.e. e1 ≈ e2 and Ve ≈ e2. (34)
Now assuming, R11 = R12
e1 =
R12
(R12 + R13)
VB. (35)
Hence,
Ve ≈ e1 =
R12
(R12 + R13)
VB
or,
VB = αVe (36)
where,
α =
R12 + R13
R12
. (37)
Hence, the equivalent circuit of the network of amplifier A4
between the terminals ‘A’ and ‘D’ may be as shown in Fig. 4 and
the Laplace transform of the circuit is given by,
I(s) =
[αVe(s) − Ve(s)]

RD + 1
CDs
 . (38)
Hence in the frequency plane, the voltage signal at ‘D’ will be
given by,
VD(s) = Ve(s) + RDI(s)
or,
VD(s) =
(1 + TDs)

1 + TDs
α
Ve(s) (39)
where,
TD = αRDCD =
[
(R12 + R13)RDCD
R12
]
. (40)
Now this voltage signal is again input to the non-inverting
terminal of the second amplifier A5 through the resistance R − RD
of the derivative potentiometer R. Since the current drawn by A5
is negligibly small, the voltage signal e4(s) at the non-inverting
terminal at the amplifier A5 will be approximately equal to VD(s),
i.e. e4(s) = VD(s). (41)
Let the voltage signal at the output terminal E of the amplifier
A5 be ‘m’ and that at its inverting input terminal be e3. Hence in
the frequency plane, the voltage signal VF (s) at the point F of the
circuit diagram as shown in Fig. 3 is given by,
VF (s) =
RP
(R14 + R′)
m(s) = Km(s) (42)
where
K =
RP
(R14 + R′)
. (43)
This voltage is being discharged through RI and CI . Hence the
voltage across the capacitor CI will be given by,
VF (s) =
[
1 +
1
RI CI s
]
e3(s). (44)
From (41) and (43) we have,
Km(s) =
[
1 +
1
TI s
]
e3(s) (45)
where,
TI = RI CI . (46)
Now, from the op-Amp A5 we have,
e3(s) = e4(s). (47)
From (39), (41), (45) and (47) we have,
Km(s) =
[
1 +
1
TI s
]
VD(s)
Km(s) =
[
1 +
1
TI s
]
(1 + TDs)

1 + TDs
α
Ve(s).
Thus the transfer function of the controller circuit as shown in
Fig. 3 is given by
m(s)
Ve(s)
= KC
[
1 +
1
TI s
]
(1 + TDs)

1 + TDs
α
 (48)
where, KC = 1
K
.
The above transfer function is the transfer function of a
PID controller with inverse derivative control action where the
proportional constant KC , integral action time TI , derivative action
time TD and dynamic gain α are given by,
KC =
1
K
=
(R3 + R′
)
RP
, TI = RI CI , TD = αRDCD
and α =
(R12 + R13)
R12



. (49)
From (29), it is observed that Vm is directly related with the
speed of the pump or mass flow rate (Q ). Hence if the speed of
the pump or the mass flow rate of the liquid deviates from an ideal
linear value as dictated by set-point, then a control action will be
generated which will change the firing angle of the thyristor until
the desired flow rate is restored. Hence Vm is forced to follow VS
i.e. the mass flow rate of the fluid produced by the pump. Thus Vm
must be linearly related with speed of the pump and mass flow rate
under steady condition and after proper tuning of the control loop.

6. S.C. Bera et al. / ISA Transactions 51 (2012) 220–228 225
Fig. 5. A photographic view of the experimental setup.
3. Design
The above linearization control circuit has been designed and
fabricated using a commercially available thyristor driven single
phase motor integrated with a centrifugal pump (ac/dc cont. Tullu-
25 Pump, M/s Tullu Motors (p) Ltd., India) sending tap water
through a 25 mm diameter pipeline. The thyristor drive unit
is made by M/s. Microtech Industries, Kolkata, India, with the
provision of 1–5 V dc input in its firing circuits has a 230 V, 50 Hz
single phase, ac supply. The LED and LDR combination used as an
opto-isolator is placed in a black polyvinyl chloride (PVC) tube of
4 mm internal diameter and 6 mm outside diameter and 15 mm
length closed at both ends with black PVC sheets. The remainder
of the circuits is fabricated on a printed circuit board (PCB) and
connections are made as shown in Fig. 2. The op-Amps used in
the circuits were OP-07, the color of the LED was selected to be
red and the LDR was selected to be a small low cost type 4 mm
diameter available in the market. All resistances were selected to
Fig. 6. On-line display of motor load current.
be 1/2 W, with 1% tolerance. A photographic view of experimental
setup is shown in Fig. 5. The on-line display of the motor current
using Storage CRO in the form of voltage drop across a 10 , 1 A
standard resistance connected in series with the motor load circuit
is shown in Fig. 6.
4. Experiment
The experiment is performed in two steps using tap water
at room temperature as the process liquid. In the first step, the
characteristic graphs of the thyristor driven pump before control
are determined. The thyristor drive unit is connected to the pump
as shown in Fig. 1. The output flow rate of water produced by
the pump is measured with a rotameter and the speed of the
pump motor is measured by a digital tachometer. The dc input
voltage of the thyristor firing circuit is now increased in steps
1 2 3 4 5
100
150
200
250
D.C input(volts)
Thyristoroutput(volts)
D.C input(volts)
1 2 3 4 5
1400
1600
1800
2000
2200
2400
Motorspeed(rpm)
D.C input(volts)
1 2 3 4 5
0.1
0.15
0.2
0.25
Loadcurrent(amp)
(a) Thyristor output voltage vs. D.C input
characteristic.
(b) Motor speed vs. thyristor input characteristic
before control.
(c) Motor load current vs. thyristor input
characteristic.
D.C input(volts)
1 2 3 4 5
0
2
4
6
8
10
Flowrate(LPM)
(d) Output flow rate vs. thyristor input characteristic.
Fig. 7. Characteristics of thyristor drive unit and thyristor driven pump.

7. 226 S.C. Bera et al. / ISA Transactions 51 (2012) 220–228
1 2 3 4 5
120
140
160
180
200
D.C input(volts)
Outputvoltage(volts)
1 2 3 4 5
D.C input(volts)
Percentagedeviationfromlinearity
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
(a) Thyristor output voltage vs. D.C input characteristic along with percentage deviation from linearity.
1 2 3 4 5
1600
1800
2000
2200
2400
D.C input(volts)
Motorspeed(rpm)
1 2 3 4 5
D.C. input(volts)
Percentagedeviationfromlinearity
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
(b) Observed speed vs. thyristor input characteristic along with percentage deviation from linearity.
1 2 3 4 5
0.15
0.16
0.17
0.18
0.19
0.2
Loadcurrent(amps)
1 2 3 4 5
D.C input(volts)D.C input(volts)
Percentagedeviationfromlinearity
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
(c) Motor load current vs. thyristor input characteristic along with percentage deviation from linearity.
D.C input(volts) D.C input(volts)
1 2 3 4 5
3
4
5
6
7
Flowrate(LPM)
1 2 3 4 5
Percentagedeviationfromlinearity
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
(d) Output flow rate vs. thyristor input characteristic along with percentage deviation from linearity.
Fig. 8. Characteristic graphs of thyristor driven pump after control.
and in each step, the thyristor output voltage across the pump
motor, motor speed, motor load current and corresponding reading
of the rotameter are monitored. Characteristics graphs are drawn
by plotting thyristor output voltage, pump speed, motor load
current and output flow rate against thyristor input voltage. The
characteristic graphs thus obtained are shown in Fig. 7(a)–(d)
respectively.
Now in the second step, the pump is connected with the
proposed linearization or control circuit and the characteristic
parameters of the controller are determined by using the ultimate
closed loop tuning method of Ziegler and Nichols. In this method,
the controller was kept in proportional mode by shortening the
derivative capacitor CD and integral capacitor CI in close loop. Now
the gain of the controller was increased in steps by varying the
gain adjustment potentiometer RP and at each step the controller
output was observed in storage CRO until a sustained oscillation
with constant amplitude was obtained. The time period of this
sustained oscillation was measured from CRO. This time period
is the ultimate time period TU . The corresponding gain of the
controller was calculated from the value of RP from (49). This
gain gives the ultimate gain KU . From these values of TU and KU ,
the tuned parameters of the PID controller are calculated using

8. S.C. Bera et al. / ISA Transactions 51 (2012) 220–228 227
Ziegler–Nichols’ formula: KC = 0.6KU , TI = 0.5TU , TD = 0.125TU .
The tuned parameters thus obtained were KC = 5.217, TI =
300 µs, Td = 75 µs. The values of CI and CD were selected to be
each of 0.1 µF, 100 V, R12 and R13 were also selected to be equal, so
that the value of α was equal to 2. The values of RP , RI and RD were
than calculated by using the relation (30). Thus the controller was
tuned by adjusting the values of RP , RI and RD to these tuned values
and by removing the short circuiting wires of capacitors CI and CD.
The controller parameters are accordingly adjusted. Now the
dc input voltage of the thyristor drive is increased in steps and
in each step the thyristor output voltage across the pump motor,
motor speed, motor load current and corresponding reading of the
rotameter are monitored. Now the similar characteristic graphs as
in the first steps are drawn and are shown in Fig. 8(a)–(d) along
with their respective percentage deviation curves from linearity.
5. Discussions
From the experimental graphs as shown in Fig. 7(a)–(d), it is
found that the output voltage of the thyristor drive unit, motor
speed, motor load current and the flow rate produced by the pump
all vary nonlinearly with the input dc voltage when no linearization
technique is used.
In order to minimize the nonlinearity of the pump, the proposed
PID controller circuit with inverse derivative control action has
been used. The load current is measured by the opto-isolator based
technique. When the output voltages of the load current measuring
circuit deviates from input set point dc voltages, the thyristor
conduction changes until the deviation reduces to zero. Thus the
load current of the thyristor operated pump is found to follow
the input dc signal and vary linearly with dc input voltage. After
tuning of the control loop the controller parameters are selected.
With these controller parameters the characteristics graphs of the
whole thyristor driven pump are drawn experimentally. These
graphs are shown in Fig. 8(a)–(d) and we observe that all the
characteristics graphs of the thyristor driven pump are quite linear
and the percentage deviation of the characteristic graphs from
linearity lies within ±0.5%, which is within a tolerable limit in
industry applications.
During the design and experimental work, a few obstacles like
obstructions in the pump, malfunction of electronic components,
loose connections of connecting wires etc. were observed. The
future scope of the present work is to extend the technique in a
three phase IGBT based and thyristor based system and to develop
a marketable commercial unit.
The basic aim of the present work is to remove the nonlinearity
of a thyristor driven pump and to operate it linearly with the help of
a 1–5 V dc signal. From the experimental graphs shown in Fig. 7 for
a low capacity pump, it may be concluded that a very good linearity
can be achieved in other high capacity pumps by utilizing the
proposed technique in both single and three phase applications.
The technique can be used in various applications such as the final
control element of a control loop, flow control of a water lift pump
etc.
Due to external disturbance the ordinary electronic PID
controller suffers from derivative over run effect. But due to inverse
derivative control action [5] this derivative over run problem is
easily eliminated in a PIDD−1
controller. The other disturbances
such as malfunctioning of electronic components, malfunctioning
of thyristor drive etc. are easily eliminated due to the negative
feedback technique used in the control system. So a very stable
and linear response of the proposed system has been observed as
shown in Fig. 8.
6. Conclusions
From the study of the present work, the following conclusion
may be drawn regarding the major contributions of the work.
(a) The motor has been successfully coupled with an analog
electronic circuit. Through a suitable DAS card the motor can
also be coupled with a PC based system.
(b) The proposed opto-coupler technique successfully measures
the motor load current and flow rate of the liquid produced by
the pump without using an conventional flow meter.
(c) The nonlinear characteristic of the pump as shown in Fig. 5 in
the revised paper has been successfully linearized by using the
proposed technique.
(d) The linearization technique is very simple and requires
no complicated piecewise linearization technique and no
microprocessor or microcontroller for linearization.
(e) The linear control of pump speed and output flow rate
according to a desired value or set point value has been
successfully implemented.
(f) Only two op-Amps have been used to design the PIDD−1
controller. Thus the controller may be considered to be very
cheap compared to the conventional PID controller where more
IC’s are used.
(g) The inverse derivative control action helps in reducing the
noise due to incidental change of motor speed caused by
various disturbances like sudden change of supply voltage,
sudden malfunction of thyristor drive etc.
Acknowledgments
The authors are thankful to the All India Council of Technical
Education (AICTE), MHRD, Govt. of India for their financial
assistance in the present investigation and the Department of
Applied Physics, University of Calcutta, for providing the facilities
to carry out this work.
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