© 2014 IJIRT | Volume 1 Issue 6 | ISSN: 2349-6002
SPEED CONTROL OF SEPARATELY EXCITED DC
MOTOR USING TRIAC
Rishabh Verma, Satya Prakash, Sneha Nivedita
Dronacharya College of Enginering, Gurgaon, India
Abstract: This paper proposes speed control of
separately excited DC Motor using semiconductor
devices like TRIAC and DIAC Devices. It also
explains how we can control speed of separately
excited dc motor for various purposes. There are
various types of speed control of separately excited
dc motor like field control and armature control
method. The speed control of dc motor can be used
for various applications like lathes, fans, pumps
disc and band saw drive requiring moderate
torques, Electric traction, high speed tools, etc.
Keywords: - TRAIC, DIAC, DC Motor, Torque,
Electric Traction.
of feed forward in artificial neural networks is
important for several reasons the dynamical properties
of the system, and in practice it may improve the
performance. They are generally present in most nonlinear dynamical system and can be used to implement
specific structures.
1. Introduction
The development of high performance motor drives is
very important in industrial applications. Generally, a
high performance motor drive system must have good
dynamic speed command tracking and load regulating
response. D.C motors have long been the primary
means of electric traction. D.C motor is considered a
SISO system having torque/speed characteristics
compatible with most mechanical loads. This makes a
D.C motor controllable over a wide range of speeds by
proper adjustment of its terminal voltage. Recently,
brushless D.C motors, induction motors, and
synchronous motors have gained widespread use in
electric traction. However, there is a persistent effort
towards making them behave like dc motors through
innovative design and control strategies. Hence dc
motors are always a good proving ground for
advanced control algorithm because the theory is
extendable to other types of motors. Many practical
control issues (motor control problems):
1) Variable and unpredictable inputs
2) Noise propagation along a series of unit processes
3) Unknown parameters
4) Changes in load dynamics
Under these conditions, the conventional constant gain
feedback controller fails to maintain the performance
of the system at acceptable levels. The incorporation
Speed Control of DC Motor Using TRIAC. DC motors
are widely used in industry because of its low cost, less
complex control structure and wide range of speed and
torque. With the increasing use of power
semiconductor units, the speed control of DC motor is
increasingly getting sophisticated and precise. A
TRIAC is a bidirectional thyristor with three
Terminals. Its three terminals are designated as MT1
(main terminal 1),MT2 & gate by G. Triac can
be turned on in each half cycle of the applied voltage
by applying a positive or negative voltage to the gate
with respect to the terminal 1. To successfully trigger
a TRIAC, Gate current must come from the main
terminal 2 side of the circuit
IJIRT 100923
2. Voltage, current and power Equations
In a separately excited motor, armature and field
windings are excited form two different dc supply
voltages. In this motor,
INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY
1549
© 2014 IJIRT | Volume 1 Issue 6 | ISSN: 2349-6002


Armature current Ia = Line current = IL = I
Back emf developed, Eb = V – I Ra where V
is the supply voltage and Ra is the armature
resistance.
 Power drawn from main supply , P = VI
 Mechanical power developed , Pm = Power
input to armature – power loss in armature =
VI – I2 Ra = I ( V – IRa) = Eb I
3. Speed Control
Generally, the rotational speed of a DC motor is
proportional to the EMF in its coil (= the voltage
applied to it minus voltage lost on its resistance), and
the torque is proportional to the current. Speed control
can be achieved by variable battery tapping’s, variable
supply voltage, resistors or electronic controls. The
direction of a wound field DC motor can be changed
by reversing either the field or armature connections
but not both. This is commonly done with a special set
of contactors (direction contactors). The effective
voltage can be varied by inserting a series resistor or
by an electronically controlled switching device made
of thyristors, transistors, or, formerly, mercury arc
rectifiers.
3.1 Field Control
The speed of dc motor can be controlled by varying
the flux in any one of the following manners.
3.1.1 Series-parallel
Series-parallel control was the standard method of
controlling railway traction motors before the advent
of power electronics. An electric locomotive or train
would typically have four motors which could be
grouped in three different ways:
 All four in series (each motor receives one
quarter of the line voltage)
 Two parallel groups of two in series (each
motor receives half the line voltage)
 All four in parallel (each motor receives the
full line voltage)
This provided three running speeds with minimal
resistance losses. For starting and acceleration,
additional control was provided by resistances. This
system has been superseded by electronic control
systems.
3.1.2 Field divertor method
The speed of a dc motor can be increased by field
weakening. This is done by inserting shunt or diverter
resistances in parallel with the field winding. When the
field is weakened, the back-emf reduces, so a larger
current flows through the armature winding and this
increases the speed. Field weakening is not used on its
IJIRT 100923
own but in combination with other methods, such as
series-parallel control.
3.1.3 Tapped field control
This is another method of increasing the speed by
reducing the flux and it is done by lowering number of
turns of field winding through which current flows. In
this method a number of tapping from field winding
are brought outside. This method is employed in
electric traction.
3.2 Armature Control
3.2.1 Armature resistance control method
This is the most common method employed. Here the
controlling resistance is connected directly in series
with the supply to the motor as shown in the fig. Below
The power loss in the control resistance of dc series
motor can be neglected because this control method is
INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY
1550
© 2014 IJIRT | Volume 1 Issue 6 | ISSN: 2349-6002
utilized for a large portion of time for reducing the
speed under light load condition. This method of speed
control is most economical for constant torque. This
method of speed control is employed for dc series
motor driving cranes, hoists, trains etc.
3.2.2 Shunted armature control
The combination of a rheostat shunting the armature
and a rheostat in series with the armature is involved
in this method of speed control. The voltage applied to
the armature is varies by varying series rheostat R 1.
The exciting current can be varied by varying the
armature shunting resistance R2. This method of speed
control is not economical due to considerable power
losses in speed controlling resistances. Here speed
control is obtained over wide range but below normal
speed.
In steel rolling mills, etc.
5. REFERENCES
[1] J.B.Gupta , publishers of engg. And computer
books. 2010-11 edition.
[2] http://en.wikipedia.org/wiki/DC_motor
[3] Herman, Stephen. Industrial Motor Control. 6th
ed. Delmar, Cengage Learning, 2010.
[4] William H. Yeadon, Alan W. Yeadon. Handbook
of small electric motors. McGraw-Hill
Professional, 2001.
3.2.3 Armature terminal voltage control:
The speed control of dc series motor can be
accomplished by supplying the power to the motor
from a separate variable voltage supply. This method
involves high cost so it rarely used.
4. Conclusion
This paper explains a brief theory about how to control
the speed of a separately excited dc motor with the
help of TRIAC and DIAC. It also explains various
types of methods and the use of speed control of dc
motor in the machinery used in the industrial factories.
5. Applications
The separately excited dc motors are generally used
for paper machines, diesel electric propulsion of ships,
IJIRT 100923
INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY
1551