Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
DC MOTOR CONTROL USING CHOPPER
MOHAMMADREZA HAIRI YAZDI
School of Mechanical Engineering
University of Tehran
College of Engineering, Karegar St.North,
Tehran
IRAN
myazdi@ut.ac.ir
NAZANIN AFRASIABI
Department of Mechatronics Engineering, Kish
International Campus, University of Tehran
Kish Island
IRAN
n.afrasiabi@ut.ac.ir
Abstract: - This paper deals with DC motor control in a high performance manner. The DC motor control
system is designed and performed using a chopper circuit. The four different parts of control of DC
motors will be considered here. Using chopper as a converter the speed of DC motor is controllable. The
chopper firing circuit gets signal from controller and then by supplying variable voltage to the armature of
the motor the desired speed chopper is achieved. There are two different types of control loops, current
controller and speed controller. The controller used is Proportional-Integral type. Using this controller the
delay is removed and fast control is achieved. Separately excited DC motor is designed and the complete
layout of DC drive mechanism is achieved. The current and speed controller is designed and in order to
get stable and high speed control of DC motor, the speed controller is optimized using modulus hugging
method. After obtaining the entire model of DC drive system, the model is utilized with MATLAB
(SIMULINK). DC motor drive is computer simulated and analyzed under various speeds and load
torques.
Key-Words: - chopper circuit, four quadrant control, DC motors, control loops, MATLAB (SIMULINK).
1. Introduction
more complex and expensive. D.C motor is considered
as a SISO (Single Input and Single Output) system
which has torque/speed characteristics and is compatible
with most mechanical loads. By proper adjustment of the
terminal voltage [3] the mentioned characteristic makes
a D.C motor controllable over a wide range of speeds. In
this article controlling DC motor speed using Chopper as
power converter and PI as speed and current controller is
investigated. A chopper is a static power electronic
device that converts fixed dc input voltage to a variable
dc output voltage. Chopper systems have smooth control
capability and are highly efficient and fast in response. A
chopper can be used to step down or step up the fixed dc
input voltage [2] like a transformer.
High performance motor derives are necessary parts of
industrial applications. A motor drive system with high
performance has special characteristics such as good
dynamic speed command tracking and load regulating
response. DC motors are well known for their excellent
control of speed for acceleration and deceleration. In a
DC motor the power supply directly connects to the field
of the motor and causes a precise voltage control which
is essential for applications which need control of speed
and torque.
Because of various advantages such as simplicity, ease
of application, reliability and favorable cost, DC drives
have long been a backbone of industrial applications. In
comparison with AC drive systems DC drives are less
and are normally cheaper for low horsepower ratings.
DC motors are identified as adjustable speed machines
for many years and a wide range of options have evolved
for this purpose. Adjustable speed AC drives would be
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
2. Principle of chopper operation
current are the two set values. When the load current
gets its maximum value, chopper is switched off and
the chopper is switched on when the load current goes
below lower limit. By setting maximum and minimum
level of current it is possible to control the frequency
of chopper.
A chopper is a "on" or "off" semiconductor switch
which is so high in speed. It connects source to load and
disconnect the load from source at a fast speed. As
shown in Fig1. During the period Ton, chopper is on and
load voltage is equal to source voltage Vs. During the
period Toff, load voltage is zero and chopper is off. In
this manner, a chopped dc voltage is produced at the
load terminals [2].
Current limit control involves feedback loop, the
trigger circuit for the chopper is therefore more
complex. For the power control in chopper circuit
PWM technique is the commonly chosen control
strategy [2].
2.2 Basics of Separately Excited DC
Motor
In Fig2 a separately excited dc motor is shown. When
a separately excited dc motor is excited by a field current
of if and an armature current of ia flows in the circuit, the
motor develops a back EMF and a torque to balance the
load torque at a particular speed. The field current if
is independent of the armature current i a. Each
winding is supplied separately. Changing the armature
current does not have any effect on the field current. The
if is generally much less than the ia. Instantaneous field
current:
Figure1.Chopper Circuit and Voltage and Current Waveform.
Average Voltage,
Vo= (Ton/ (Ton+Toff))*Vs
(1)
= (Ton/T)*Vs =aVs
Ton=on-time.
Toff=off-time.
T=Ton+Toff = Chopping period.
Fig2. Separately excited dc motor
a=Ton/Toff.
2.1 CURRENT- LIMIT CONTROL
In this different control method, the on and off of
chopper circuit is set by the previous set value of load
current. Maximum load current and minimum load
(2)
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
Where
and
If Ra is a small value (which is usual) or when the
motor is loaded lightly, i.e. Ia is small,
are the field resistor and inductor,
respectively.
Instantaneous armature current:
(8)
(3)
That is if the field current is kept constant, the motor
speed depends only on the supply voltage.
Where Ra and La are the armature resistor and
inductance respectively.
The developed torque is:
The motor back emf is expressed as:
(9)
(4)
The required power is:
Kv is the motor voltage constant (in V/A – rad/Sec and
is the motor speed (in rad/sec).
(10)
By changing the armature current (Ia) for a fixed field
current, or flux (If) it is possible to satisfy the torque
demand.
The torque developed by the motor is:
(5)
The motor speed can be controlled by controlling Va
(voltage control) and controlling Vf (field control).
These observations lead to the application of variable
DC voltage for controlling the speed and torque of DC
motor. Generally, the speed correspond to the rated V a,
rated Ia and rated If. Ia and If are kept constant to satisfy
torque demand. Va is maintained at the rated value and If
is reduced to increase speed. However, the power
developed by the motor (= torque x speed) remains
constant. This phenomenon is known as Field weakening
[4].
where (Kt - Kv) is the torque constant. (in V/Arad/s)
Sometimes it is written as:
(6)
For normal operation, the developed torque must be
equal to the load torque plus the friction and inertia,
i.e.:
2.3
The motor speed can be easily derived:
MODELIMG OF DC
MOTOR FOR DRIVE
SYSTEM
The basic principle behind DC motor speed control is
that the output speed of DC motor can be varied by
controlling armature voltage for speed below and up to
rated speed keeping field voltage constant. The output
speed is compared with the reference speed and error
(7)
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
signal is fed to speed controller. Controller output will
vary whenever there is a difference in the reference
speed and the speed feedback. The output of the speed
controller is the control voltage Ec that controls the
operation duty cycle of (here the converter used is a
Chopper) converter. The converter output give the
required Va required to bring motor back to the desired
speed. The Reference speed is provided through a
potential divider because the voltage from potential
divider is linearly related to the speed of the DC motor.
The output speed of motor is measured by Tachogenerator and since Tacho voltage will not be perfectly
dc and will have some ripple. So, we require a filter with
a gain to bring Tacho output back to controller level [1].
The basic block diagram for DC motor speed control is
show below:
From motor’s basic armature equation, after taking
Laplace Transform on both sides, we will get [1]:
(14)
Suppose the motor with the following specifications:
Model of this DC Motor is shown in Fig3.
Fig3. Closed loop system model for speed control of dc motor [1].
Fig3. Block Model of Separately Excited DC Motor
The torque equation will be given by:
(11)
3. Controller design
For the correction to the output which will keep on
adding or subtracting a small amount to the output until
the motor reaches the set-point, an improvement is
required. This can be done by keeping a running total of
the error speed observed for instant at regular interval
(say 25ms) and multiplying this by another gain before
adding the result to the proportional correction found
earlier. This approach is based on what is effectively the
integration of the error in speed.
So we use a PI (proportional-integral) controller. The
job of fast-acting correction is done by the proportional
term does the job of fast-acting correction which will
Assuming absence (negligible) of friction in rotor of
motor, it will yield: B=0
Therefore, new torque equation will be given by:
(12)
Equation for back emf of motor will be:
Also,
(13)
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
produce a change in the output as quickly as the error
arises. The integral action takes a finite time to act but
has the capability to make the steady-state speed error
zero [5].
We use saturation block as limiting speed error and
armature current error [1]. Speed error limiting is
between zero and triangular signal amplitude and
armature current error limiting is between zero and rated
current.
We use two loop for armature current and motor speed.
Since chopper takes a fixed DC input voltage and gives
variable DC output voltage. It works on the principle
Pulse Width Modulation technique [2].
Fig7. Speed close loop circuit
Reference speed and motor speed is shown in Fig8.
3.1 MATLAB SIMULATION,
RESULTS AND ANALYSIS
Since chopper takes a fixed DC input voltage and gives
variable DC output voltage, it works on the principle
Pulse Width Modulation technique [2]. It is represented
in Fig 4. We use a triangular wave with amplitude
Vtri=5v and frequency fs=33kHz that compare with Vref
(output of PI controller). This signal is used for turn off
and turn on IGBT switches.
Fig8. Refrence speed and motor speed
Speed error is shown in Fig9.
Simulink Model for Speed Control of Separately
Excited DC motor using Chopper Converter is shown in
Fig 5. Kpwm is shown in Fig4.
Current close loop circuit is shown in Fig6.
Fig9. Speed error
Reference current and motor current is shown in Fig10.
Fig6. Current close loop circuit
Speed close loop circuit is shown in Fig7.
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
In this paper based on closed loop system model and
using chopper as a converter and Proportional-Integral
type Speed and Current controller the speed of a dc
motor has been controlled. At first for controlling speed
of DC motor a simplified closed loop is utilized and
requirement of current controller is studied. After that
DC motor is modeled more completely and a full layout
of DC drive system is achieved. A current and speed
controller is designed. The speed control loop is
optimized through Modulus Hugging approach. A DC
motor specification is taken and corresponding
parameters are driven from derived design approach.
Finally simulation is performed for model with and
without filter used after reference speed the response of
both methods are compared with each other. An analysis
is also performed on the simulation results obtained
under different reference speed and different loads. The
model shows good results under all conditions employed
during simulation.
Fig10. Reference current and motor current
Current error is shown in Fig11.
Fig11. Current error
4. Conclusion
Fig 4. Block Model for Chopper Design
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Global Journal of Science, Engineering and Technology (ISSN : 2322-2441)
Issue 8, 2013 , pp. 67-73
© GJSET Publishing, 2013.
http://www.gjset.org
Fig5. Simulink Model for Speed Control of Separately Excited DC motor using Chopper Converter
References:
[3] Dubey, G.K., Fundamentals of Electrical Drives.
New Delhi, Narosa Publishing House, 2009.
[1] Gopakumar, K., Power Electronics and Electrical
Drives, Video Lectures 1-25, Centre for Electronics and
Technology, Indian Institute of Science, Bangalore.
[4] Salam Dr. Zainal, UTJMB, Power Electronics and
Drives (Version 3-2003).
[5] Infineon Technologies, Basic DC motor speed PID
control
with
the
Infineon
Technologies
[2] Bimbhra, P.S., Power Electronics. New Delhi,
Khanna Publishers, 2006.
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