International Journal of Industrial Electronics and Electrical Engineering, ISSN: 2347-6982
Volume- 2, Issue- 1, Jan.-2014
SENSORLESS CONTROL OF BLDC MOTOR USING THIRD HARMONIC
BACK EMF
1
ASHISH P.R, 2VINCENT G
1
2
M-Tech scholar Industrial Drives and Control Govt. RIT, Kottayam
Assistant professor Electrical Engineering Department Govt. RIT, Kottayam
Email: ashishpalakkadan@gmail.com, Vincent@rit.ac.in
Abstract- This paper presents particular methods for deployment of the sensorless Permanent Magnet Brushless DC (PM
BLDC) motor drive system. The typical trapezoidal waveform of the motor internal voltages (or back emf) contains a
fundamental and higher order frequency harmonics. In particular, the third harmonic component is extracted from the stator
phase voltages while the fundamental and other polyphase components are eliminated via a simple summation of the three
phase voltages. The resulting third harmonic signal keeps a constant phase relationship with the rotor flux for any motor
speed and load condition, and is practically free of noise that can be introduced by the inverter switching, making this a
robust sensing method. In contrast with indirect sensing methods based on detection of the back-emf signal that require
heavy filtering, the third harmonic signal needs only a small amount of filtering to eliminate the switching frequency and its
side bands.
I.
EMF zero crossing is sensed with respect to the
negative dc bus potential. In [3], Kim and Ehsani
define a function depending on the measured
voltages, currents, and the derivative of the currents,
which indicates the switching instants. After
prepositioning, Kim and Ehsani [3] advance the
switching pattern by 60 electrical degrees and let their
sensor less algorithm take over. Since their functions
are dependent on the computation of derivatives of
currents, the method requires digital implementation
and could be affected by sensor noise. Detecting the
free-wheeling diode conduction in the open phase
gives the zero-crossing point of the back EMF
waveform [4]. This approach of rotor-position
sensing works over a wide speed range, especially at
lower speed. The main drawback of this scheme is the
requirement of six additional power supplies for the
comparator circuits to detect current flowing through
the free-wheeling diode. Integrating the back EMF
waveform of the unexcited phase is another method
to extract the position information for the phase
commutation [5]. 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 instant. Frequency
independent phase shifter is proposed by Jung and Ha
[8] for sensorless control of BLDC motor, which can
shift the zero-crossing point of input signal with a
specified phase delay. However, direct commutation
instant detection technique proposed in [6] and [7]
lacks this flexibility to advance the commutation
instant, which is possible to implement using the back
EMF zero-crossing detection Techniques.
An
extended Kalman filter estimator for a brushless dc
motor has been developed by Jung and Ha [9] for
speed and rotor position estimation. An obstacle to
applying the extended Kalman filter algorithm to
rotor position estimation is the need to set appropriate
values for the covariance matrix parameters, which
reflect the uncertainties in modeling and
INTRODUCTION
PERMANENT MAGNET (PM) motors have been
widely used in a variety of applications in industrial
automation and consumer appliances because of their
higher efficiency and power density. Brushless dc
(BLDC) motors, with their trapezoidal electromotive
force (EMF) profile, which requires six discrete rotor
position information for the inverter operation. These
are typically generated by Hall-effect switch sensors
placed within the motor. However, it is a well-known
fact that these sensors have a number of drawbacks.
They increase the cost of the motor and need special
mechanical arrangements to be mounted. Further,
Hall sensors are temperature sensitive, and hence
limit the operation of the motor. They could reduce
system reliability because of the extra components
and wiring. Furthermore, sensorless control is the
only reliable way to operate the motor for
applications in harsh environments. The BLDC motor
without position and speed sensors has attracted wide
attention and many papers have reported work on
this. These methods are based on, using back EMF of
the motor [1]–[3], detection of the conducting state of
freewheeling diode in the unexcited phase [4], back
EMF integration method [5].Back EMF estimation
methods typically rely on the zero crossing detection
of the EMF waveform. The technique of estimating
back EMF by sensing the terminal voltages with
respect to a virtual neutral point is proposed in [1].
The neutral point will not be stable during pulse
width modulation (PWM) switching. Low pass filters
have been used to eliminate the higher harmonics and
to convert the terminal voltages into triangular
waveform signals. Delay is introduced in the sensed
signal due to heavy filtering, which also varies with
the operating speed. Therefore, this method is well
suited only for a narrow speed range. Indirect back
EMF sensing technique is proposed in [2] without the
need of neutral or virtual neutral potential. The back
Sensorless Control of BLDC Motor Using Third Harmonic Back EMF
35
International Journal of Industrial Electronics and Electrical Engineering, ISSN: 2347-6982
Volume- 2, Issue- 1, Jan.-2014
measurements. The parameter values are often chosen
by trial and error. Advancing the commutation instant
increases the torque production particularly at high
speed operation of BLDC motor as proposed and
analyzed by authors in [10] and [11].
This paper proposes a simple and reliable method for
the sensorless operation using the third harmonic
back EMF and back EMF of any one phase . In this
paper, the third harmonic back EMF is extracted
directly from the phase voltages. The method does
use back EMF zero crossing for the commutation.
Switching pulses is generated from the zero crossing
of third harmonic back EMF. Unlike the method
described in [3], this scheme is easy to
implement.The organization of this paper is as
follows. Section II describes operation principle of
BLDC Motor. Section III describes proposed third
harmonic method. Section IV presents the simulation
results of the proposed method and Section V
presents the conclusion.
the motor, the inverter commutates the current every
60° according to the rotor position given by hall
sensors or sensorless method and thus the motor
rotates once through the six commutation steps. The
conducting interval for each phase is 120° and only
two phases are conducted at any time, leaving the
third phase floating. Fig. 2 shows the phase current
and commutation sequence using hall sensors.
II.
III.
Fig. 2. Phase current and commutation sequence
OPERATION PRINCIPLE OF BLDC
MOTOR
PROPOSED THIRD HARMONICS
METHOD
The sensorless control proposed in this paper is based
on the estimation of the air gap flux via the third
harmonic component of the stator phase voltages. The
use of the stator third harmonic voltage for the PM
BLDC motor discussed in this paper is based on the
symmetrical three phases Y-connected motor with air
gap flux distribution. The summation of the threestator phase voltages results in the elimination of all
poly phase components and all the characteristic
harmonics components. The resulting sum is
dominated by the third harmonic component,
maintaining constant phase displacement with the
fundamental air gap voltage for any load and speed.
Fig. 3 presents the idealized air gap flux density
distribution for the PM BLDC motor with surface
mounted magnets. The resultant air gap flux has a
dominant third harmonic component linked to the
stator windings, inducing a third harmonic
component in each one of the phases. The summation
of the three-stator phase voltages only results in the
third harmonic plus other high frequency
components.
A general BLDC motor has three phase stator
windings and is driven by an inverter which
constitutes of six switches. Fig. 1 shows the
equivalent circuit of am Y connection BLDC motor
and the inverter topology.
Fig. 1. BLDC motor and inverter block diagram
Assuming that the stator resistance and inductance of
all the windings are equal, the voltage equation of a
BLDC motor can be expressed as
(1)
where Vx, ix, and ex(x=a,b,c) denote the terminal
voltage, the phase current, and the phase BEMF
respectively. R is a stator resistance and L is a stator
inductance. Vn is the neutral voltage of a Y
connection motor The electromagnetic torque
equation is given by
(2)
Where Te denotes the torque and ωm is the rotor
speed. Unlike a BLAC motor, a BLDC motor is
powered by rectangular current. In order to operate
Fig. 3. Air gap flux distribution and its fundamental and third
harmonic components
Fig. 4 depicts the motor internal voltage
corresponding to back-EMF, third harmonic signal,
Sensorless Control of BLDC Motor Using Third Harmonic Back EMF
36
International Journal of Industrial Electronics and Electrical Engineering, ISSN: 2347-6982
Volume- 2, Issue- 1, Jan.-2014
integral signal, and commutation signal. The zero
crossings of the third harmonic component occur at
60 electrical degrees, exactly at every desired current
commutation instant. The integrator of third harmonic
introduces a phase shift of π2 from the zero crossings
of the third harmonic. Finally the detected zero
crossing of the integrator output generates required
phase commutation timing signals.
Fig. 6. Torque Wave form
Fig. 4. Commutation signal using third harmonic signals
IV.
Fig. 7. Current Wave form
SIMULATION RESULTS
CONCLUSION
The proposed sensorless method is simulated in
MATLAB/SIMULINK
software.
The
motor
parameters used for simulation and hardware
implementation are given in Table I. The informative
results are displayed in
Fig. 5 through to Fig. 7.
A simple technique to detect third harmonic back
EMF zero crossings for a BLDC motor using the
phase voltages is proposed. The control method of the
sensorless PM BLDC Motor is based on the detection
and processing of the stator third harmonic voltage
component. The proposed control method can operate
over a wide speed range, using the third harmonics
voltage of the stator voltage and proposed position
detection circuit. In addition, the reduction in size
achieved by eliminating the rotor position sensor and
the encoder may be critical in many applications.
Simulation results are shown which validate the
suitability of the proposed method
TABLE I
BLDC MOTOR PARAMETERS
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Fig. 5. Speed Wave form
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