1544
Vol 05, Article 06368; June 2014
http://ijesr.in/
International Journal of Engineering Sciences Research-IJESR
ISSN: 2230-8504; e-ISSN-2230-8512
CLOSED LOOP SPEED REGULATION OF DC MOTOR
USING PHASE CONTROLLED THYRISTOR CONVERTER
SOWMYA.M*, PRADEEPPRABHU.K, K SURYA SEN
*
Electrical and Electronics, KVGCE, Sullia, D.K., Karnataka, India
Sowmya_729@rediffmail.com
ABSTRACT
This paper describes the speed regulation of DC Drive by closed loop method; Single quadrant, armature
voltage control. Armature voltage control can regulates speed below base speed. DC drive is composed of fully
controlled thyristor bridge converter, a control system, to control and minimize the speed errors. To achieve a
good speed regulation cascade control structure is used. Inner fast current loop, which also limits the current to
safer limit operation of motor and converter, and slower outer speed loop. PI controllers are used in current
controller and speed controller to achieve a good dynamic response of the system with fast transient response,
minimum overshoot and zero steady state error. The system is designed with respective subsystem models and
transfer functions. Finally integrated prototype model is developed using MATLAB SIMULINK and simulated to
achieve the desired results.
Keywords: – Speed control, DC Drive, PI controller, thyristor converter, armature control, cascade control
Introduction
In manufacturing industries the quality of a product is of high concern. A flexible, reliable, robust system is
necessary to carry out the process. An electrical drive can be defined as a power conversion means characterized
by its capability to efficiently convert electrical power from an electrical power source (voltage and current) into
mechanical power (torque and speed) to control a mechanical load or process. Electrical drives form the link
between the energy supply and the mechanical processes. Variable speed DC Drive consists of the power
electronic converters (phase controlled thyristor), control unit for control and operation purposes. . Although
AC motors are mainly used in industry for high speed operation (over 2500 rpm) because they are smaller,
lighter, less expensive, require virtually no maintenance comparing to their DC counterparts, the latter are still
used. The reasons for this are that they exhibit wide speed range, good speed regulation, starting and
accelerating torques in excess of 400% of rated, less complex control and usually less expensive drive
Inherently straight forward operating characteristics, flexible performance and efficiency encouraged the use of
D.C motors in many types of industrial drive application. Most multi-purpose production machines benefit from
adjustable speed control, since frequently their speeds must change to optimize the machine process or adapt it
to various tasks for improved product quality, production speed. Developments in the design of controlled
rectifiers and DC–DC converters, has made the realization of control of DC motors easier. Today, DC motors
are still used in several applications as in industrial production and processing of paper pulp, textile industries.
Speed Regulation of DC Motor
The speed regulation is achieved by varying the applied armature voltage. To supply a required DC voltage to
the armature a variable speed DC drive is designed. Since it is voltage controlled, the field supply is kept
constant. Closed loop method with cascade control structure is used. Cascade control is used to achieve fast
rejection of disturbance before it propagates to the other parts of the loop.The overall closed-loop system is as
shown below. The design of control loop starts from the innermost (fastest) current loop and proceeds to the
slowest outer speed loop. The performance of the outer loop is dependent on the inner loop; therefore the tuning
of the inner loop has to precede the design and tuning of the outer loop. By this the dynamics of the inner loop
can be simplified and the impact of the outer loop on its performance could be reduced.
Figure 1- block diagram of closed loop speed control DC Drive
System design
The overall closed-loop system is as shown below.. Accuracy and the dynamic responses are better in a closed
loop system. In the closed loop system, acceleration and the deceleration of the motor can be controlled
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Vol 05, Article 06368; June 2014
International Journal of Engineering Sciences Research-IJESR
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ISSN: 2230-8504; e-ISSN-2230-8512
according to the requirement. The output of the tachogenerator is compared with a preset reference voltage. The
differences between these two signals are fed as an actuating signal to control the elements of the system. There
are two closed paths in the dc control scheme. One is the outer speed control loop and the other is the inner
current control loop. Both the loops are controlled with the help of P-I controllers having their own gains and
the time constants according to the requirement of the system. The output of the speed P-I controller which is
the function of the speed error is used to command for current P-I controller and compared with armature
current. The current P-I controller output control the triggering circuit of the converter. The armature current
tends to increase to a large value when the motor is accelerating. A current limit can be obtained in this system
by limiting the maximum current command voltage. If the motor speed drops below the set point, the resulting
error signal causes the gate pulse generator to advance the firing pulses, and increase the rectifier output voltage.
Consequently the speed rises to the desired level.
Figure-2-block diagram of DC drive
Model of DC Motor
Figure-3-Equivalent circuit of separately excited DC motor
Where:-Ea(t) =Input voltage, ia(t) =Armature current, Ra = Armature resistance, La = Armature inductance, Eb(t)
=Back e.m.f , Tm= Developed Torque, ωm=Motor angular velocity, J=Motor moment of inertia, B=Viscous
friction coefficient, Kb=Back e.m.f constant , KT=Torque constant.
Here, the differential equation of armature circuit isEa(t) = Ra.ia(t) +
+ Eb (t)………………(1)
The Torque equation isTm (t) =
+ B.ωm (t) ………………(2)
The torque developed by motor is proportional to the product of the armature current and field current i.e.
Tm (t) = Kf.if.ia ……………………………………………(3)
Where, Kfis constant.
In armature – controlled D.C. motor the field current (if) is kept constant i.e.
Tm = KT.ia ……………………………………………………(4)
Where, KT= Kf.ifis torque constant.
The back e.m.f. of motor is proportional to the speed i.e.
Eb(t) = Kb. ωm ……………………………………………(5)
Where, Kb is back e.m.f. constant. In order to create the block diagram of system initial conditions are zero and
Laplace transform is implemented to the equations.
Ea(s) = Ra.Ia(s) + sLa.Ia(s) + Eb(s)
Ia(s) =
…………………………(6)
Tm (s) = sJ.ωm(s) + B.ωm(s)
ωm(s) =
……………………………(7)
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Tm(s) = KT.Ia(s) …………………………(8)
Eb(s) = Kb. ωm(s) ……………………….(9)
Block diagram of separately excited DC motor
The DC motor contains an inner loop due to the induced e.m.f. It is not physically seen; it is magnetically
coupled. To decouple the inner current loop from the machine-inherent induced-e.m.f loop, it is necessary to
split the transfer function between the speed and voltage into two cascade transfer functions. First between
speed and armature current and then between armature current and input voltage, represented as
=
.
Where: = J /
3-Phase Thyristor Bridge Converter
The thyristor provides a convenient method of converting AC voltage to a variable DC voltage for use in
controlling the speed of a DC motor. The gating angle of a thyristor with respect to the AC supply voltage,
determines how much rectified DC voltage is available. Entire firing angles are with reference to main circuit.
The gate pulse signals to the thyristor rectifier bridge are fed by six pulse synchronous signal generator.
A simple formula is used to calculate the amount of rectified DC voltage in a 3-phase bridge. Converted DC
voltage (Vdc) is equal to 1.35 times the RMS value of input voltage (Vrms) times the cosine of the phase angle
(cosα).
Vdc = 1.35 x Vrms x cosα
Figure-4-thyristor converter
Model of 3-phase converter
The converter can be considered as a box with some gain and phase delay for modeling and use in control. The
gain
of the linearized controller-based converter for a maximum control voltage
is
= (1.35*V) /
Time delay = 1 / 12*
For 50 Hz supply voltage source, the time delay is equal to 1.66ms.
The converter is then modeled with its gain and time delay as
=
Controller Design
The current controller is of proportional-integral type. It is represented as
Gc(s) = Kc (1+sTc)
sTc
Figure-5- Current control loop
The loop gain function is
G (s) =
This is a fourth order system, and simplification is necessary.
G (s) ≈
; Where , K =
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Vol 05, Article 06368; June 2014
International Journal of Engineering Sciences Research-IJESR
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ISSN: 2230-8504; e-ISSN-2230-8512
The second order model of the current loop is replaced with an approximate first order model.:
Where:
T3 = T1+Tr ; Ti =
; Ki =
;
=
Speed Controller
The speed controller is of proportional-integral type. It is represented as
Figure-6 -speed control loop with reduced inner current loop
The speed loop with the first-order approximation of the current control loop. The loop gain function is
G
This is a fourth order system. The approximate gain function of the speed loop is.
G
Where:
The closed loop transfer function is
=
This transfer function is optimized based on Symmetrical optimum The choice of approximation is based on the
physical constants of the motor, on the converter and transducer gains, and on time delays.After simplification,
the speed controller gain is
; Time constant:
Substituting the above values to the closed-loop transfer function
=
Simulation Model
A prototype model is developed in MATLAB SIMULINK. The model is as shown below
Figure-7- Prototype DC Drive simulink model
Simulation Result
Figure-8-speed and electric torque waveforms
Figure-9-armature voltage and firing angle
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Prototype model is simulated in Simulink for set speed 1200rpm. Figure-8 shows speed and electric Torque
waveforms. Figure-9 shows armature current and firing angle wave form. Figure-10 shows speed
error.waveform.
Figure-10 speed error waveforms
Conclusion
A closed loop speed control DC drive is designed. Armature control is preferred for good speed regulation. But
it provides speed regulation below base speed. To achieve speed above base speed then field weakening
technique has to be used. The model designed is for single quadrant operation, it can be expanded for full 4quadrant operation according to the required application process. The developed prototype model is simulated to
obtain the desired result. Constant speed is achieved with good dynamic response
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