Speed control of sensorless brushless DC motor by computing back EMF from line voltage difference

International Journal of Advances in Applied Sciences (IJAAS)
Vol. 9, No. 2, June 2020, pp. 125~131
ISSN: 2252-8814, DOI: 10.11591/ijaas.v9.i2.pp125-131  125
Journal homepage: http://ijaas.iaescore.com
Speed control of sensorless brushless DC motor by computing
back EMF from line voltage difference
R. Sivakami1
, Thangaraj. K2
1
Department of Electrical and Electronics Engineering, Christ Institute of Technology, India
2
Department of Electrical and Electronics Engineering, Sri Manakula Vinayagar Engineering College, India
Article Info ABSTRACT
Article history:
Received Jun 26, 2019
Revised Feb 11, 2020
Accepted Mar 23, 2020
Sensorless operation of permanent magnet brushless direct current (BLDC)
motor drive controls the rotating speed with different applied voltage.
No phase lagging is produced which leads to increase the efficiency and
minimize the torque pulsation of the BLDC motor. Initially, motor can be
started by following the v/f method then allows the sensorless mode after
reaching the minimum speed of 500-1000rpm. The Sensorless BLDC motors
are highly used due to higher efficiency, reliability power, acoustic noise,
smaller, lighter, greater dynamic response, better speed versus torque
characteristics, higher speed range and longer life. Thus, the source voltage
spikes and switching losses are reduced. This method can be demonstrated
through MATLAB simulation and DSP TMS 320LF2407A is used in
the experimental setup to get the output.
Keywords:
Back electromotive force
(EMF) detection
Brushless DC (BLDC) motor
Sensorless control
Sensorless brushless DC motor
Digital signal processor (DSP)
This is an open access article under the CC BY-SA license.
Corresponding Author:
R. Sivakami,
Department of Electrical and Electronics Engineering,
Christ Institute of Technology,
Ramanathapurram Revenue Village, Commune, Villianur, Puducherry 605502, India.
Email: sivakamieee@gmail.com
1. INTRODUCTION
The Sensorless BLDC motor is widely used in applications including domestic device, automotive,
aerospace, consumer, medical, automated industrial equipment and instrumentation. These motors are used in
a great number of industrial sectors because; their architecture is suitable for any safety critical applications.
The Sensorless BLDC motors are highly used due to higher efficiency, reliability power, acoustic noise and
smaller, lighter, greater dynamic response, better speed versus torque characteristics, higher speed range and
longer life. It has some advantages such as elimination of motor neutral voltage, fixed phase shift circuit, low
starting speed and cost effective.
The BLDC motor is electronically commutated by power switches instead of brushes. The voltage
spikes are created by the residual current that is blocked when the armature current is produced by power
switches. Reduction of commutation torque ripple is already achieved in the position of sensorless BLDC
motor drive [1]. Analysis, design and implementation of a high performance are achieved in cost effective
sensorless scheme for BLDC Motors [2]. BLDC motors, also known as Permanent Magnet Direct Current
Synchronous motors, are one of motor types that have more rapidly gained popularity, mainly because of
their better characteristics and performance [3]. The BLDC motor without position and speed sensors has
attracted wide attention. The two typical control methods for BLDC motor are square wave control and
sinusoidal current control methods [4]. In BLDC motors, only two out of three phases are excited at any time
leaving the third winding floating [5]. The back emf in the floating winding can be measured to determine
the switching sequence for the commutation of power switching devices in the three-phase inverter [6].

 ISSN: 2252-8814
Int J Adv Appl Sci, Vol. 9, No. 2, June 2020: 125 – 131
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The terminal voltage of the floating winding with respect to the neutral point of motor is needed to get
the zero-crossing time of the back emf [7]. Integration starts when zero crossing of the back emf occurs and
the integration stops when the threshold set value is reached, which gives the commutation instantly [8].
Frequency independent phase shifter for sensorless control of BLDC motor, which can shift the zero-crossing
point of input signal with a specified phase delay [9]. An extended Kalman filter estimator for a brushless dc
motor has been developed by Jung and Ha for speed and rotor position estimation. Rotor position information
is determined indirectly by sensing the back emf from one of the three motor terminal voltages of three phase
motor [10]. An obstacle to applying the extended Kalman filter algorithm to rotor position estimation is
the need to set the appropriate values for the covariance matrix parameters, which reflect the uncertainties in
modeling and measurements [7]. The sensorless control technique based on Zero Cross Point (ZCP) of
the back emf has been widely used for the low-cost application [8]. Zero crossing detector (ZCD) is used to
detect the ZCP of back emf and thus the pulse is generated by shifting 300
from ZCP. In the ZCP method,
the back emf cannot be obtained when the BLDC motor is at standstill or operating nearly zero speed [9].
Therefore, a special control is needed for smooth and reliable sensorless control operation of BLDC
motors [10] further [11] didn’t discuss the details of sensorless starting of the motor. However, direct
commutation instant detection technique proposed in [10] and [11] lacks this flexibility to advance
the commutation instant, which is possible to implement using the back emf zero-crossing detection
techniques. In [12] designed and implemented an integrated circuit for the sensorless operation of BLDC
motor by sensing the motor terminal voltages. Frequency independent phase shifter [13] for sensorless
control of BLDC motor, which can shift the zero-crossing point of input signal with a specified phase delay.
An extended Kalman filter estimator for a brushless dc motor has been developed [14] for speed and rotor
position estimation. The commutation instant increases the torque production particularly at high speed
operation of BLDC motor [15]. The approach to zero crossing detection were used to reliably start the BLDC
machine in sensorless operation. In this work, the approach is extended to propose a simple running
mode algorithm.
In this work, a simple and reliable method for the detection of back emf zero-crossings for
the sensorless operation using MATLAB/SIMULINK and also hardware implementation through DSP.
In hardware circuit DSP is needed to produce correct instant for pulse generation. The zero crossings of
the back emf are estimated indirectly from the terminal voltages that are measured with respect to dc negative
terminal and this method does not involve any integration. Further, since the line voltages are used,
the requirement of neutral potential and the common mode noise has been eliminated.
Along with this model of BLDC motor, the dynamic behavior such that no-load, load and load
transients of the motor have been studied. Simulation has been carried out to study the effectiveness of
the sensorless control based on detection of zero crossing from the terminal voltages’ differences. Section I
describes the proposed back emf zero-crossing estimation method and presents the simulation results of
the proposed method. Section IIpresents hardware implementation through DSP. Section III presents
the conclusion.
2. PROPOSED ZERO-CROSSING BACK EMF ESTIMATION METHOD
Consider a BLDC motor having three stator phase windings connected in star [8]. The rotor consists
of Permanent magnets. The Brushless dc motor is driven by a three-phase frequency inverter in which
the devices are triggered with respect to the rotor position as shown in Figure 1.
Figure 1. Equivalent circuit BLDC motor

Int J Adv Appl Sci ISSN: 2252-8814 
Speed control of sensorless brushless DC motor by computing back EMF … (R. Sivakami)
127
The phase A terminal voltage with respect to the stator, is given (1).
dia
v R I L ea a a andtan
  
(1)
Where, Ra is the stator resistance, La is the phase inductance ean is the back emf, and ia is the phase
current of the “A “phase. From above equations, the line voltage 𝑉𝑎𝑏 may be determined (2).
( )
( )
d i ia b
v v v R i i L e ea b an bndtab an bn

      
(2)
Consider the interval, when phases A and C are conducting and phase B is open as indicated by
the shaded region. In this interval, phase A winding is connected to the positive terminal of dc supply,
phase C is connected to the negative terminal of dc supply and phase B is open. Therefore, 𝑖𝑎 = −𝑖𝑐 and
𝑖𝑏 = 0. It can be seen from Figure 1 (shaded region) that the back emf in phases A and C are equal and
opposite. Therefore, in that interval 𝑉𝑎𝑏𝑏𝑐 may be simplified (3)
2 2 bn
v v v e e e eabbc ab bc an bn cn      
(3)
The difference in line voltages waveform, is an inverted representation of Back EMF waveform.
The Back EMF value in resistance, inductance, Back EMF (RLE) representation of the phase but not referred
to ground.
3. SIMULATION AND RESULTS
The sensorless method is simulated in MATLAB/SIMULINK. The motor and zero crossing detector
parameters are given in Table 1 & Table 2.
Table.1. Motor details
Parameter Value
Motor Rating 1.1hp
Rated current 4.52A
No. of Poles 4
Rated Speed 3000rpm
Supply Voltage 320V
Rated Torque 2.25N-m
Table 2. Zero-crossing detector parameter
Parameter Value
Step down transformer 230/9V
Filter Inductance 40 mH
Filter capacitance 15, 0.1µF
Resistance 15K, 10K, 1.5 KΩ
Operational amplifier TL082
The circuit diagram of simulation of BLDC MOTOR, in sensorless mode is shown in Figure 2.
From the sensed terminal voltages with respect to negative dc bus (𝑉𝑎𝑏, 𝑉𝑏𝑐, 𝑉𝑐𝑎), line voltages, and
subsequently their differences (𝑉𝑐𝑎𝑎𝑏, 𝑉𝑎𝑏𝑏𝑐, 𝑉𝑏𝑐𝑐𝑎) are determined.

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128
Figure 2. Simulation of BLDC motor in sensorless mode
The simulated pulse for inverter, phase currents, back emf waveforms are shown in Figure 3,
Figure 4, Figure 5 and Figure 6.
Figure 3. PWM pulse for the inverter Figure 4. Phase current waveform of SBLDCM
Figure 5. Back EMF waveform of SBLDM Figure 6. Simulated speed waveform of sensorless
BLDC motor
From the line voltage difference, waveforms zero crossings are detected using the zero-crossing
detection model. These zero crossings are decoded to corresponding signals using Zero Crossing Detector
(ZCD) decoding system and are fed to Hall sensor decoder system to provide gate signal to the inverter by
using two loop control method. The simulated speed waveform is shown in Figure 6.
A simple technique to detect the back emf zero crossings for a BLDC motor using the line voltages
is proposed. It is shown that the method provides an amplify version of the back emf. Only three motor
terminal voltages need to be measured thus eliminating the need for motor neutral voltage. Running

129
the machine in sensorless mode is then proposed, in this paper, making use of the novel zero-crossing
detection algorithm. While starting relies on triggering the devices at the zero crossings detected using
the proposed algorithm, continuous running is achieved by realizing the correct commutation instants
300
delay from the zero crossings. The motor is found to start smoothly and run sensorless even with load
and load transients. Simulation and experimental results are shown which validate the suitability of
the proposed method.
4. HARDWARE IMPLEMENTATION AND TEST RESULTS
Terminal voltage used as the input of the zero-crossing detector is sensed from the motor terminal.
Sensed terminal voltage is stepped down to 9V by using step-down transformer that is used as input to
the LC filter. Step down voltage contains some spikes that will be given to the voltage divider circuit for
driving the voltage, and that output will be given to the voltage follower circuit for equalizing the input
impedance, from that output will be given to the input of the zero-crossing detector.The Block diagram of
the proposed Zero Crossing Detector is shown in Figure 7.
ZCD will detect the positive zero crossing and negative zero crossing gives the low and high pulse.
From that output will get the square waveform. Zener diode is used to maintain the output amplitude at 5.1V.
Output of the ZCD will be gives to the DSP processor. The experimental setup of the proposed Zero Crossing
Detector is shown in Figure 8.
Figure 7. Block diagram of the zero-crossing detector
Figure 8. The experimental setup of zero-crossing detector
The experimental setup of the proposed sensorless BLDC motor drive is illustrated in the Figure 9.
A motor with 820 W, 4600 rpm, six pole BLDC motor drive coupled to a DC generator connected to
a variable resistor for loading are the parameters of the motor used in the simulation study and listed in
Table 1. The proposed sensorless method is implemented using TI TMS320LF2407A digital signal processor
(DSP). The BLDC motor is driven by a three-phase power IGBT module. The three terminal voltages of

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the motor is sensed and fed to the analog-to-digital converter (ADC) of the DSP through a resistive divider.
In order to reduces the magnitude. The voltage spikes that appear in the sensed terminal voltage waveform
due to the free-wheeling diode conduction during phase commutation are filtered out by the sample and hold
method as discussed in Section III. The high-frequency switching noise due to PWM (Pulse width
modulation) is filtered out by synchronizing the ADC sampling during the PWM on time.
Figure 9. The experimental setup of sensorless BLDC motor
5. HARDWARE RESULTS
The output of zero crossing detectors is given to DSP (TMS 320 LF 2407) and then detects the zero-
crossing point. From that ZCP the gate pulses are produced with the phase shift of 30o
and then the pulses are
given to inverter. The sensorless BLDC motor current waveform for 120o
conduction mode at various speeds
under no load, full load and transient condition, Inverter pulse and Phase current waveform.
The experimental speed waveform of BLDC motor in sensorless operation at no load and on load condition is
shown in Figure 10 and Figure 11.
Figure 10. Speed waveform from serial monitor port
2407 for BLDC
Figure 11. Speed waveform from serial monitor port
2407 for BLDC motor in sensorless operation at no
load. Motor in sensorless operation at on load.
6. CONCLUSION
In this work, a simple technique to detect the back emf zero crossings from the line voltage
difference by running the machine in sensorless mode is studied. It is shown that the method provides an
amplified version of the back emf. Only three motor terminal voltages need to be measured and eliminate
the need of motor neutral voltage. While starting the triggering devices at the zero-crossing detected point by
using the proposed algorithm, continuous running is achieved by realizing the correct commutation instants
300
delay from the zero crossings point. The BLDC motor starts smoothly and run sensorless even with load
and load transients occur due to changes in load.

131
REFERENCES
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