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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
REDUCTION OF SWITCHING LOSSES IN BLDC
MOTOR USING SOFT SWITCHING TECHNIQUES
1 MR.
D. K. PATEL, 2 MR. J. B. SARVAIYA
1 M.E.
2 Assist.
Ivth Sem, Electrical S.S.Engineering College Bhavnagar, ,
Professor, Electrical Department S.S.Engineering College.Bhavnagar, India,
[email protected],[email protected]
ABSTRACT : A BLDCM can be described as an inverted brush dc motor with its magnet being the rotor and its
stationary windings forming the stator. BLDC Motor is fast gaining popularity because of their superior
performance in terms of high efficiency, fast response, light weight, precise and accurate control, maintenance
free operation, brushless construction, the operating characteristic of a motor is important for its control,
modeling and optimum performance. It is usually supplied by a hard switching PWM inverter, which normally
has number of detrimental effect is produce. In order to reduce the effects, many soft switching techniques are
designed or develop. The purpose of this paper is to build a modeling and simulation of hard switching and soft
switching inverter for estimate or discriminate the switching losses in BLDCM using soft switching techniques,
for see reduction in switching losses by using MATLAB SIMULINK.
Index Terms :BLDCM (Brushless DC Motor), Hard And Soft Switching, PWM Inverter, MATLAB.
I. INTRODUCTION
In this paper we propose a simulation model for an
entire BLDCM drive and its actual implementation.
In this model the trapezoidal back EMF waveforms
are modeled as a function of rotor position, so that
position can be actively calculated according to the
operating speeds. Moreover, the switching function
concept is adopted to model the PWM inverter. This
in turn results in obtaining the detailed voltage and
current waveforms [1]. BDCM has been widely used
in industrial applications because of its high power
density, As compared to a conventional DC brush
motor, BLDC motors are DC brush motors turned
inside out, so that the field is on the rotor and the
armature is on the stator. In BLDC motor, field
excitation is provided by a permanent magnet and
commutation is achieved electronically instead of
using mechanical commutators and brushes. In BLDC
motor, the mechanical ‘rotating switch’ or
commutator/brush gear assembly is replaced by an
external electronic switch synchronised to the rotor's
position.There are two main types of BLDC motors;
trapezoidal type and sinusoidal type.In the trapezoidal
motor the back-emf induced in the stator windings
has a trapezoidal shape and its phases must be
supplied with quasi-square currents for ripple-free
torque operation.
The sinusoidal motor on the other hand has a
sinusoidal shaped back-emf and requires sinusoidal
phase currents for ripple-free torque operation.The
shape of the back-emf is determined by the shape of
the rotor magnets and the stator winding distribution.
The sinusoidal motor needs a high resolution position
sensor because the rotor position must be known at
every time instant for optimal operation. It also
requires more complex software and hardware. The
trapezoidal motor is a more attractive alternative for
most applications due to its simplicity, lower price
and higher efficiency. [3]
Figure 1 Basic block diagram of BLDC
II. CONSTRUCTION OF BLDC MOTOR
Figure 2 Construction of BLDC Motor
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ELECTRICAL ENGINEERING
· LA
· RA
s1
s3
a
· Vdc
s5
· LB
· RB
· ia
ea
·
·
eb
·
BLDC works on basic principal of Motor, when
current carrying conductor placed in magnetic field it
experiences force and its direction is define by
Fleming left hand rule and force is define by F=BIL.
In BLDC, stator has winding but rotor has no winding
so it does not require brushes. Rotor has permanent
magnet in cylindrical form which continuously
magnetic field generating.
BLDC Motor requires controlled alternating cycle
voltages which are generated by SPWM Inverter. The
main function of inverter is converts fixed direct
cycle voltage in to variable alternating cycle voltage.
SPWM means sinusoidal pulse width modulation
technique is used to operate inverter and reduces
harmonic contents. In SPWM, pulse width is
changing but amplitude of pulse is not changing.
SPWM is one of type of PWM techniques among
them SPWM is widely used in industries for
controlling purpose.
BLDC classified in to two categories, trapezoidal
type and sinusoidal type. BLDC is classified by
generating back electro motive force (EMF) shape if
it is trapezoidal then BLDC is trapezoidal type.
The trapezoidal Motor is a more attractive for most
applications due to its simplicity, lower price and
higher efficiency.
Hall sensor uses to sense the position of rotor of
BLDC Motor and that signals are given to inverter
circuit for controlling purpose.
Here we have used back electromagnetic force wave
instead of Hall sensors for sensing the position of
rotor of BLDC Motor. Back electromagnetic force is
depending on radius of rotor, angular velocity of
rotor, length of rotor, magnetic flux density of rotor,
and no. of turns per phases.
III. OPERATION OF BLDC MOTOR
The three phase BLDC Motor is operated in a twophase-on fashion, i.e. here third phase is off, so two
phases produce the highest torque, which two phases
are energized depends on the rotor position. The
signal generated from the position sensors in the form
of three digit number that change over every 60
degree (electrical deg.) as shown in Fig 3. The figure
shows also ideal current and BACK EMF waveforms
·
b
· ib
c
· ic
· LC
· RC
s4
s2
s6
n
ec
·
·
Figure 4 BLDC driver circuit
Table 1
Switching sequence
Switchin Se Pos. sensor
g
q.
H1 H2 H3
No
Interval(
s.
0-60
0 1 0 0
deg.)
60-120 1 1 1 0
Switch Phase current
120-180 2
0
1
0
Q3 Q6 Off +
-
180-240 3
0
1
1
Q3 Q2 -
+
off
240-300 4
0
0
1
Q5 Q2 -
Off +
300-360 5
1
0
1
Q5 Q4 off -
A
Closed
Q1 Q4 +
B
C
-
Off
Q1 Q6 +
Off -
+
IV. MATHEMATICAL MODEL OF BLDC MOTOR
The 3-phase star connected BLDC Motor can
described by following four equation from the Figure
3,
............. (1)
.............. (2)
.............. (3)
.................. (4)
Where
denote the phase-phase
voltages, phase currents and BACK EMF
respectively, in three phase a, b and c.
= Resistance of winding per phase in ohm
= Inductance of winding per phase in m H
= Electrical Torque in mNm
Load Torque in mNm
Figure 3 Ideal back-EMFs, phase currents and
position sensor signals
Rotor Inertia in g
Friction Constant in Nm s (assumed value)
................................................. (5)
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ELECTRICAL ENGINEERING
................................................ (6)
............................................. (7)
.......... (8)
Where
are the BACK EMF Constant
and the Torque Constant,
.................................................... (9)
Where
Electrical Angle
Rotor Angle
Gives the Trapezoidal waveform of the BACK
EMF.
One period of this function can be written as,
...
....... (10)
For simplification of implementation in MATLAB,
equation 1 - 4 must be written in state space form.
Each voltage equation is linear combination of the
other two voltage equations only two equations are
required. By reducing one equation and eliminating
one variable using the current relationship,
The voltage equations become,
........... (11)
........... (12)
And the complete model is,
…….... (13)
........... (14)
independent. By moving switching frequency far off
the resonant frequency and using a simple buck type
voltage converter, we have been able to make a
switching converter with very low output impedance.
By accepting inherent switching losses, the buck
converter’s output voltage is accurately controlled by
‘pulse width modulation ‘(PWM) of the switching
transistors. One of the biggest challenge connected to
hard switching is certainly connected to
electromagnetic noise generated in the switching
moment, especially in hard switching circuits the
problem is exaggerated by a desire to shorten the
switching moment to minimum .The shorter the
switching moment, higher the frequency of the noise
As the frequency goes up, the more apparent the
noise problem and the challenge of controlling the
noise increases.
Why soft switching inverter is required?
· Due to switching loss,
· Device under Stress,
· EMI Problems,
· Effect on Machine insulation,
· Machine Bearing Current,
· Machine Terminal Overvoltage.
This is the main reason why many design engineers
choose a ‘soft switching’ design.
VI. SOFT SWITCHING
In order to minimize the size of necessary reactive
power components, we have used relatively high
switching frequency: 20 kHz in on module and 35
KHz in another module .By using latest technology
within IGBT’s we have been able to reduce the
switching loss. Hard switching is opposed to soft
switching. When we make soft switching circuits we
start out with hard switching circuit and than add
circuitry (power components) to make it soft. Softmeans to achieve smooth current /voltage transitions
in the switching moment. By ‘hard Switching we
simply means that no special circuitry is added to
make the circuit soft. In order to get smooth
transitions, the fundamental principle for all ‘Soft
Switching’ techniques is to switch in a moment at
zero voltage and zero current, in the main switching
devices. At high switching frequency soft
switching techniques (ZVS or ZCS) are used to
achieve good efficiency and reduced switching
stress. In Zero-Voltage Switching (ZVS), the voltage
across device is zero just before turn–on. On the other
hand in Zero-Current Switching (ZCS), the current
through device is zero just before turn-off. Fig 5 (a)
and Fig 5 (b) illustrate the ZVS and ZCS switching
trajectory and also shown in Fig 6.Typical switching
trajectories of power switches.
V. HARD SWITCHING
This is the simplest method of switching. It requires
less no. of power inductors/capacitors in the circuit.
This means we reduce the cost, complexity and power
loss of the circuit. The output voltage is load
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ELECTRICAL ENGINEERING
(a) ZVS trajectory
Heat–sinking
requirement
Hardware count
Overall power
density
EMI problem
dv/dt problem
Modulation scheme
Maturity
Cost
(b) ZCS trajectory
Norm
Possibly lower
Norm
Norm
More
Possibly higher
Severe
Severe
Versatile
Mature
Norm
Low
Low
Limited
Developing
Higher
Figure 5 Soft switching trajectory
I
Safe Operating Area
On
Hard-switching
snubbered
Soft-switching
Off
V
Figure 6 Typical switching trajectories of power
switches.
VII. SOFT COMMUTATION TECHNIQUES
As a consequence, switching loses now tend to
predominate Conventional PWM power converters
when operated in a switched mode operation, the
Power Switches have to cut off the load current
within the turn-on and turn-off times under the hard
switching conditions. Hard switching refers to the
stressful switching behavior of the power electronic
devices the switching trajectory of hard-switched and
soft switched power devices. Dissipative passive
snubbers are usually added to the power circuits so
that the dv/dt and di/dt of the power devices could be
reduced, and the switching loss and stress be diverted
to the passive snubber circuits. However, the
switching loss is proportional to the switching
frequency, thus limiting the maximum switching
frequency of the power converters.During the turn-on
and turn-off processes, the power device has to
withstand high voltage and current simultaneously,
resulting in high switching losses and stress.
IX. ESTIMATION OF BLDC MOTOR AND
INVERTER LOSSES [9]
The BLDC motor losses are mainly the winding
copper losses and iron losses. Of these, the core or
iron loss is the most difficult to compute accurately.
The permanent magnet and rotor back iron
experience little variation in flux and therefore do not
generate significant core loss. On the other hand, the
stator teeth and stator back iron experience flux
reversals on the order of Bmax (maximum steel flux
density) at the fundamental frequency. With the
knowledge of Bmax and frequency f, the core loss of
the stator can be roughly approximated.
· BLDC Motor Losses are below:-
VIII. DIFFERENCE
Hard
Switching
Soft Switching
A. Copper losses:In a 3-phase BLDC motor, the permanent magnets
rotate and the armature remain static..The copper
losses are defined as the resistive heating losses that
occur in the stator windings. These losses are
expressed for a three-phase winding as in (15).
Severe
Norm
Almost zero
Possibly
Higher
................... (15)
B. Stator core losses:There are three components in iron losses which are
the eddy-current losses, the hysteresis losses, and the
BETWEEN
SOFT AND HARD
SWITCHING: [1]
Table 2
Comparison between hard and soft switching
Switching loss
Overall efficiency
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excess or residual losses. Basically, there are several
methods to determine the iron losses. The benefit of
having different methods is to compare the results
obtained to ensure the accuracy. The iron losses are
expressed as in (16).
........ (16)
The determination of iron losses requires the
knowledge of the flux density variations, the
frequency, and the magnetic material characteristics
for all the motor magnetic parts. In BLDC motor, iron
loss not only appears in the stator but also in the
rotor.
C. Eddy current losses in the magnetic core:Analytical approach to calculate the eddy current
density distribution in permanent magnets is based on
known distribution of magnetic flux density in the airgap of BLDC motor. The eddy losses are expressed
as in (17).
.................. (17)
Where, Ke = the eddy-current constant, f = frequency,
t = thickness, B = flux density
The parameters used can be found from the
datasheets provided by the manufacturer of the
electric motor.
D. Hysteresis losses:The hysteresis is an effect within the ferromagnetic
materials. The area between the B-H curve and the B
axis represents the work done (per unit volume of
material). Theoretically, the total power lost over one
complete cycle is proportional to the area within the
hysteresis loop. Because this effect is related to an
area, hysteresis loss is roughly proportional to the
square of the working flux density. In fact, the nonlinearities will, for iron material, reduce this to about
B1.6. For ferrite grade 3C8 it is 2.5. The data sheets
sometimes have graphs of loss versus flux density on
a log scale. These can be used to
estimate the index n. The hysteresis losses are
expressed as in (18).
..................... (18)
Where, Kh = hysteresis constant, f = frequency, B =
Flux- density,
n = Steinmetz exponent.
It is clearly seen that different materials result in
different losses as constants used in calculating the
eddy current and hysteresis losses are different for
different types of material. Therefore the material
selection is also important in achieving the lowest
iron losses. The iron losses can be calculated as in
(19)
is the core loss density of the stator material at the
flux density, B max and frequency, f .
· INVERTER Losses are below:E. Semiconductor Losses:In the IGBT PWM inverter, each switch consists of
an IGBT and connected with a feed-back diode in
anti-parallel. The losses in an IGBT device are
mainly due to conduction, off-state blocking, turn-on
switching and turn-off switching. It is assumed that
the power switches in the inverter are subject to hard
switching and the load is the BLDC motor.
F. IGBT off-state blocking losses:The leakage currents passing through an off-state
power switch, or diode, whose function is to block the
supply voltage from reaching the load, results in offstate power losses and are usually small enough to be
neglected
G. IGBT on-state conduction losses:When a power switch is turned-on, the voltage drop
across the main switching terminals falls to a low
(but, finite) on-state value of Vsat : typically 1-3
volts when fully saturated. The conduction losses are
computed from the average current, I T avg passing
through the on-state switch. The on-state voltage
drop, Vsat is across the switch terminal and the
losses are as in (20).
........ (20)
H. Diode conduction losses:The conduction losses in the feed-back diodes can be
estimated with the same functional equation used
above to calculate Conduction losses in IGBT power
switch. Simply replacing Vsat with diode forward
voltage drop VDfd typically 1 V and I D avg
substituted for ITavg. The diode current has the
same peak value as the IGBT switching current but,
flows only during t off . The average diode current, is
thus proportional to
where is
the peak (flat-topped) approximation of the switching
current and, D is the converter duty cycle. The diode
conduction losses are as below:
..(21)
I. IGBT turn-on and turn-off switching losses:The turn-on behavior of the power switch is
characterized by the rise time of the current and fall
time of the voltage. Similarly the turn-off behavior of
the switch is denoted by the rise time of voltage and
fall time of the current. The switching losses occur in
both the diode and IGBT devices due to the existence
of high current and high voltage. The switching losses
can be computed as in (22).
......... (19)
Where, ρ bi , kg/m3, is the mass density of the back
iron, Vst is the stator volume, and Γ (B max, f ), W/kg,
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……...(22)
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ELECTRICAL ENGINEERING
Where,
tir = the time for current to rise from zero to peak
current, IP at turn-on.
tvf = the time for voltage to fall from Vdc to Von at
turn- on.
tvr = the time for voltage to return to Vdc at turn-off
tif = the time for current to fall from Ip to zero at
turn-off.
X. SIMULATION AND RESULTS:
Figure 10 Inverter output with soft switching
Figure 7 Simulation diagram for BLDC motor
with SPWM inverter
Figure 11 BLDC output without soft switching
Figure 8 SPWM output
Figure 12 BLDC output with soft switching
Figure 9 Inverter output without soft switching
Figure 13 Three phase Iabc, eabc, Theta without
soft switching
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XII. REFERENCES
Figure 14 Three phase Iabc, eabc, Theta with soft
switching
Figure 15 Rotor position without soft switching
Figure 16 Rotor position with soft switching
XI. CONCLUSION
Soft switching using IGBT gives low switching
losses, higher efficiency, reduce torque ripples and
improves speed as compared to hard switching.
The waveforms for stator current, rotor speed,
torque and voltage for the BLDC motor drive with
soft switching are more effective than hard
switching. Soft
switching converter gives
“smoothed waveform with no transient spikes. It
reduces switching losses and stress. Soft -switched
converter can be operated at the very high
frequency.
These converters also provide an
effective solution to suppress EMI.
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Rathi, Aziz Ahmed, Rajiv Kumar: International
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2009
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[5] T.J.E. Miller, “Brushless Permanent-Magnet
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[6] A Kusko And S.M. Peeran, “Definition Of The
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[8] Bldc Motor Modelling And Control – A
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Books
[1st] Jacek F. Gieras, Mitchell Wing: Permanent
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