ETCEE - 2014 NATIONAL CONFERENCE ON EMERGING TRENDS IN COMPUTER & ELECTRICAL ENGINEERING

Analysis of Open Loop Speed Control of

Brushless DC Motor
Kajal D. Parsana1, Hitesh M. Karkar2
1
M.E. Student, AITS Rajkot
2
Electrical Department, AITS, Rajkot, India
1
kajal.parsana@gmail.com
2
hmkarkar@aits.edu.in

Abstract- The main goal of today’s applications and conventional DC motor and induction motor, BLDC
future applications are high efficiency and low motor offers many advantages like higher speed, no
manufacturing cost. Now these goals are achieved by maintenance, lower noise because of brushes and
the use of brushless DC motors (BLDC) in a more commutator are absent also offers linear speed-torque
number of applications. The BLDC motor has characteristics, high starting torque and better heat
various applications used in industries like in removal [3].
drilling, lathes, spinning, electric vehicles, elevators, In the residential and commercial applications BLDC
home appliances where BLDC replace the motor is used, so the speed control of BLDC motor is
conventional brushed DC motor. They can be used very essential. The simple control technique of BLDC
where space is less because of its compact size. motor is open loop technique [4]. As there no brushes
Further, the BLDC motors offer a good control are present so commutation of BLDC motor is done by
which is very much required for the vehicles and electronic circuit. Inverter can be used as an electronic
they are suitable for variable speed applications. So circuit. Open loop with PWM technique is simulated in
in this paper overview of BLDC motor and MATLAB/SIMULINK.
MATLAB simulation of open loop speed control A. BLDC motor
technique with PWM duty cycle are represented. Operating principle and model of BLDC motor are
This PWM duty cycle control technique enable described as follows:
greater efficiency and versatility of the brushless DC
1) Operating principle:
motor to provide flexible control and novel cyclic
BLDC motor having mainly three parts for its
operation. Speed-torque waveform is obtained from
construction: stator, rotor, hall sensor. Stator is made up
this control technique. Different speed is obtained at
coil placed in a slot to form a winding. Rotor is made up
different load condition.
of permanent magnet and can alter from 2-8 pole pairs
with alternate N and S poles. As compared to
Keywords- Brushless DC motor, PWM, Inverter,
conventional DC motor, in BLDC motor there is no
Gate driver circuit, Hall sensor
brushes and commutator. So commutation is done by
I. INTRODUCTION electronically. Thus, in the BLDC motor, the rotor
Since 1980’s the new prototype concept of permanent position must be known to energize the phase pair and
magnet brushless motor has been developed. There are control the phase voltage. So BLDC motor uses three
two types of permanent magnet brushless motor depends Hall sensors embedded in the stator for sensing the rotor
upon the back EMF waveform, Brushless AC (BLAC) position. BLDC motor is as shown fig. 1.
and Brushless DC (BLDC) motors.[1] BLDC motors
have trapezoidal back EMF and quasi-rectangular
current waveform due to concentrated winding [2]. Now
a day’s BLDC motor is mostly used in home appliances.
BLDC motor has various applications in industries like
drilling, lathes, elevators, electric vehicle, aircrafts,
automotive, military component, hard disk drive, HVAC
system and instrumentation. BLDC motors have
compact size and high efficiency. As compared to Fig. 1 BLDC motor

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NATIONAL CONFERENCE ON EMERGING TRENDS IN COMPUTER & ELECTRICAL ENGINEERING ETCEE - 2014

The principle of BLDC motor is based on

synchronising the magnetic field produced by the stator
winding and magnetic field by rotor winding. According
to rotor position stator winding is switched on and off,
so that the magnetic field produced by the stator and
rotor will be synchronized. This interaction of magnetic
field produces electromagnetic torque [5]. This Torque
increases with increase in stator current. These three
stator windings are fed from a three phase inverter
bridge. Inverter topology is as shown in fig. 2. The three
phase inverter operation can be divided into six modes
(1-6) according to current conduction states and
conduction sequence. The switches in are operated such
that each phase carries current only during the
120° period when the back EMF is constant. [6]
Commutation sequence decides according to the Hall
sensor signal. Hall sensor produces high or low signal
according to N or S pole passes near the sensor. The
three currents are controlled to take a form of quasi Fig. 3 Back EMF and Current waveform
square waveform in order to synchronize with the So the electromagnetic torque can be expressed as:
trapezoidal back EMF to produce constant torque. The
waveforms of Back EMF and current are shown in fig. ܶ௘ = (݁௔ ݅௔ + ݁௕ ݅௕ + ݁௖ ݅௖ )Τ߱௥ (2)
3.
where ܶ௘ is the electromagnetic torque and ߱௥ is the
rotor speed. The speed equation is given below:
ௗఠೝ
= (ܶ௘ െ ܶ௅ െ ‫߱ܤ‬௥ )Τ‫ܬ‬ (3)
ௗ௧

where B is the damping constant, J is the moment of

inertia, and ܶ௅ is the load torque. The electrical
frequency is:

௉
߱௘ = ቀ ቁ ߱௥ (4)
ଶ

where P is the number of pole .

Fig. 2 BLDC motor drive system II. CONTROL TECHNIQUE OF BLDC MOTOR
1) Modelling: In resent BLDC motor is widely used in home
BLDC motor having a trapezoidal flux distribution, appliances and commercial application. So speed control
but given non sinusoidal flux distribution , model of of BLDC motor is very essential. Speed control in a
BLDC motor in terms of phase variable is derived.[7] BLDC motor involves changing the applied voltage the
The analysis of a BLDC motor is represented the motor phases. This can be done using a sensored method
following circuit equations: based on the concept of pulse width modulation.
1) Open loop control technique:
ܸ௔ ܴ 0 0 ݅௔ ‫ܮ‬ ‫ܯ‬ ‫݅ ܯ‬௔ ݁௔ An interesting property of brushless DC motors is
ௗ
൥ܸ௕ ൩ = ൥ 0 ܴ 0 ൩ ൥݅௕ ൩ + ௗ௧ ൥‫ܯ‬ ‫ܮ‬ ‫ܯ‬൩ ൥݅௕ ൩ + ൥݁௕ ൩ that they will operate synchronously to a certain extent.
ܸ௖ 0 0 ܴ ݅௖ ‫ܯ‬ ‫ܯ‬ ‫݅ ܮ‬௖ ݁௖ This means that for a given load, applied voltage, and
(1) commutation rate the motor will maintain open loop
lock with the commutation rate provided that these three
where V a , V b , V c , are the stator phase voltages, R is the variables do not deviate from the ideal by a significant
stator phase resistance, i a , i b , i c are the stator phase amount. The ideal is determined by the motor voltage
currents, e a , e b , e c are the phase back emf, L is the self and torque constants. How does this work? Consider
inductance of each phase, that when the commutation rate is too slow for an
M is the mutual inductance. applied voltage, the BEMF will be too low resulting in

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 ETCEE - 2014 NATIONAL CONFERENCE ON EMERGING TRENDS IN COMPUTER & ELECTRICAL ENGINEERING

more motor current. The motor will react by is used to supply the 3 phase stator winding of the
accelerating to the next phase position then slow down BLDC motor. In decoder Hall sensor detects rotor
waiting for the next commutation. In the extreme case position and according to this rotor position back emf is
the motor will snap to each position like a stepper motor generated. Gate circuit produces PWM signal which is
until the next commutation occurs. Since the motor is given to Inverter Bridge. These PWM techniques are
able to accelerate faster than the commutation rate, rates used to switch ON and OFF the switches. In order to
much slower than the ideal can be tolerated without vary the speed, these signals should be PWM at a much
losing lock but at the expense of excessive current. higher frequency than the motor frequency [9]. The
Now consider what happens when commutation is PWM frequency should be at least 10 times that of the
too fast. When commutation occurs early the BEMF has maximum frequency of the motor. When the duty cycle
not reached peak resulting in more motor current and a of PWM is differed within the sequences, the average
greater rate of acceleration to the next phase but it will voltage supplied to the stator reduces, thus lowering the
arrive there too late. The motor tries to keep up with the speed. If the DC voltage is much greater than the motor
commutation but at the expense of excessive current. If rated voltage, the motor can be controlled by limiting
the commutation arrives so early that the motor cannot the percentage of PWM duty cycle corresponding to that
accelerate fast enough to catch the next commutation, of the motor rated voltage [5]. So, the commutation
lock is lost and the motor spins down. This happens sequence of the inverter bridge depends on rotor
abruptly not very far from the ideal rate. The abrupt loss position. According to this, electromagnetic torque is
of lock looks like a discontinuity in the motor response produced and also rotor speed will be analyzed.
which makes closed loop control difficult. An
alternative to closed loop control is to adjust the
commutation rate until self locking open loop control is
achieved. This is the method we will use in our
application. [8]
When the load on a motor is constant over its
operating range then the response curve of motor speed
relative to applied voltage is linear. If the supply voltage
is well regulated, in addition to a constant torque load,
then the motor can be operated open loop over its entire
speed range. Consider that with pulse width modulation
the effective voltage is linearly proportional to the PWM
duty cycle. An open loop controller can be made by
linking the PWM duty cycle to a table of motor speed
values stored as the time of commutation for each drive
phase. The block diagram of open loop speed control of Fig. 5 Open loop speed control of BLDC motor
BLDC motor is as shown in fig. 4 simulation circuit

IV. SIMULATION RESULT

Speed and torque waveform when load torque=3 Nm
is as shown in fig. 6

Fig. 4 Block diagram of open loop technique

III. MATLAB SIMULATION CIRCUIT

Simulation circuit is as shown in fig. 5 here 24 V DC Fig. 6 waveform of rotor speed and electromagnetic
voltage is given to Inverter Bridge. This inverter bridge torque

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NATIONAL CONFERENCE ON EMERGING TRENDS IN COMPUTER & ELECTRICAL ENGINEERING ETCEE - 2014

TABLE 2 REFERENCES
[1] M.V.Ramesh, S.Karmakshaiah and J.Amarnath, “Speed Torque
characteristics of the BLDC motor in either direction on load
Simulation Result of BLDC Drive using ARM controller” ISSN vol.2, pp.37-48, 2011.
[2] G.R.Arab Markdesh, S.I. Mousavi and A. Kargar, “Position
Sr. No. Load Rotor Sensorless Direct Torque Control of BLDC Motor” IEEE vol.3,
Torque Speed 2008.
[3] Ming-Fa Tsai, Tran Phu Quy and Chun-Shi Tseng, “Model
(N.m) (RPM) Construction and Verification of a BLDC Motor Using matlab
simulink and FPGA Control” IEEE vol.1, pp.1797-18002,
1 0 678.5 2011.
[4] G.R.P Lakshmi and S. Paramasivam, “Dcpic based power
assisted steering usin brushless direct current motor” American
2 3 570 Journal of Applied Science, pp.1419-1426, 2013.
[5] H.R. Hiziroglu, “On the Brushless DC Motors” ELECO, pp.35-
3 5 507.4 39, 2009.
[6] Alphonsa Roslin Paul, “Brushless DC Motor Control Using
Digital PWM Techniques” ICSCCN vol.79, pp. 733-738
(2011).
[7] M.R. Feyzi and M. Ebadpour, “A New Single Current Strategy
V. CONCLUSION for High-Performance Brushlees DC Motor Drives” IEEE
power electronic tranjection vol.1, pp.000419, 2011.
In this paper open loop speed control with PWM duty [8] Padmraja Yedamale, “BLDC motor fundamentals” Microchip
cycle is simulated. Simulation is done at different load technology.
torque and according to that different speed is obtained. [9] Vijay Bolloju, “PWM Control Methods Increases Efficiency,
At no load condition full speed is obtained but when Reliability and Extend Battery Lifetime” Bodo;s power systems,
August-2007.
load torque is applied the motor speed falls below the
rated speed.

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