ISSN 2277-2685
IJESR/June 2014/ Vol-4/Issue-6/324-332
J.E. Muralidhar et al./ International Journal of Engineering & Science Research
ANALYSIS OF FOUR SWITCH AND SIX SWITCH CONVERTER FED BLDC
MOTOR WITH FIXED AND VARIABLE SPEEDS
J.E.Muralidhar*1, Dr. P. Varanasi2
1
Assoc. Prof, EED, MuffakhamJah College of Engineering and Technology, Hyderabad (A.P), India.
2
Prof, Department of Electrical & Electronics Engineering, BRECW, Hyderabad (A.P), India.
ABSTRACT
Brush less dc motors has only decades of history. They have been gaining attention from various industrial and
household appliance manufacturers because of its high efficiency, high power density and low maintenance
cost, silent operation, compact form, and reliability. Brushless DC electric motor are asynchronous motors that
are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC
electric signal to drive the motor. Moreover, reducing of the drive components is more attractive for low cost
applications. This paper presents a comparative study on control of four-switch and six-switch Inverter fed
BLDC motor drive along with fixed and variable speed with PI control system has been presented using
MATLAB/SIMULINK.
Keywords: BLDC motor; four switch inverter; six switch inverter, proportional integrator.
1. INTRODUCTION
Permanent magnet motors with trapezoidal back EMF and sinusoidal back EMF have several advantages over
other motor types. Most notably, (compared to dc motors) they are lower maintenance due to the elimination of
the mechanical commutator and they have a high-power density which makes them ideal for high-torque-to
weight ratio applications [1]. The permanent magnet brushless dc (BLDC) motor is gaining popularity being
used in computer, aerospace, military, automotive, industrial and household products because of its high torque,
compactness, and high efficiency [3]. A conventional BLDC motor drive is generally implemented via a six
switch, three-phase inverter and three Hall-effect position sensors that provide six commutation points for each
electrical cycle. Cost minimization is the key factor in an especially fractional horse-power BLDC motor drive
for home applications. It is usually achieved by elimination of the drive components such as power Switches
and sensors. Therefore, effective algorithms should be designed for the desired performance. Recently, a four
switch, three-phase inverter topology has been developed and used for a three-phase BLDC motor drive.
Reduction in the number of power switches, switching driver circuits, losses and total cost are the main features
of this topology. A four-switch three-phase BLDC motor drive is proposed to simplify the topological structure
of the conventional six-switch inverter. The uncontrollable phase current causes unsymmetrical voltage vector
and its waveform is much of distortion from rectangular. The direct current control based on hysteresis avoids
this problem and it senses currents of phases A and B individually by two current sensors and then switches
them separately [2,4].
2. PRINCIPLE OF OPERATION
A bldc motor is a permanent magnet synchronous that uses position detectors and an inverter to control the
armature currents. The bldc motor is sometimes referred to as an inside out dc motor because its armature is in
the stator and the magnets are on the rotor and its operating characteristics resemble those of a dc motor. Instead
of using a mechanical commutator as in the conventional dc motor, the bldc motor employs electronic
commutation which makes it a virtually maintenance free motor. 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 operation. The
sinusoidal motor on the other hand has a sinusoidally shaped back – emf and requires sinusoidal phase currents
*Corresponding Author
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
for ripple free torque operation. The shape of the back – emf is determined by the shape of rotor magnets and
the stator winding distribution.
The sinusoidal motor needs high resolution position sensors 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 simplicity, lower price and higher efficiency. Bldc
motors exist in many different configurations but the three phase motor is most common type due to efficiency
and low torque ripple.
(a)
(b)
Fig 1: BLDC (a) Motor Cross Section (b) Phase Energizing Sequence
This type of motor also offers a good compromise between precise control and number of power electronic
devices needed to control stator currents. Position detection is usually implemented using three Hall - an effect
sensor that detects the presence of small magnets that are attached to the motor shaft.
3. PRINCIPLE OPERATION OF SIX-SWITCH BLDC MOTOR
Typically, a Brushless dc motor is driven by a three-phase inverter with, what is called, six-step
o
commutation. The conducting interval for each phase is 120 by electrical angle. Fig.1 shows a cross section of
a three phase star connected motor along with its phase energizing sequence. Each interval starts with the rotor
and stator field lines 1200 apart and ends when they are 600 apart. Maximum torque is reached when the field
lines are perpendicular. The commutation phase sequence is like AB-AC-BC-BA-CA-CB. Each
conducting stage is called one step. Therefore, only two phases conduct current at any time, leaving the third
phase floating. In order to produce maximum torque, the inverter should be commutated every 600 so that
current is in phase with the back EMF. The commutation timing is determined by the rotor position, which
can be detected by Hall sensors as shown in the Fig.2 (H1, H2, and H3).
Fig 2: Ideal Back-Emf’s, Phase Currents, and Position Sensor Signals
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Current commutation is done by inverter as shown in a simplified from in Fig.3. The switches are shown as
bipolar junction transistors but MOSFET switches are more common. Table I shows the switching sequence, the
current direction and the position sensor signals.
Fig 3: Simplified BLDC Drive Scheme
Table 1: Switching Sequence
Implementation of a BLDC motor can be developed in the similar manner as a three phase synchronous
machine. Since its rotor is mounted with a permanent magnet, some dynamic characteristics are
different. Flux linkage from the rotor is dependent upon the magnet. Therefore, saturation of magnetic flux
linkage is typical for this kind of motors. As any typical three phase motors, one structure of the BLDC motor is
fed by a three phase voltage source as shown in Fig.3. The source is not necessary to be sinusoidal. Square
wave or other wave- shape can be applied as long as the peak voltage is not exceeded the maximum
voltage limit of the motor.
4. PRINCIPLE OPERATION OF FOUR-SWITCH BLDC MOTOR
According to the operating modes, one can derive the following current equations: Table II implies that due to
the characteristics of the BLDC motor, such as two-phase, only two phases (four switches) needed to be
controlled, not three phases. Therefore, based on Table II, one can develop a switching sequence using four
switches as follows:
Table 2: Rotor position signal Vs reference current
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Fig 4: Proposed four-switch converter topology for three-phase BLDC motor
As shown in Table 3, the two-phase currents need to be directly controlled using the hysteresis current control
method by four switches. Hence, it is called the direct current controlled PWM scheme
Table 3: Switching Sequence of Four switch BLDC motor
Closed Loop Operation
The closed loop operation carried out by the voltage controller (PI controller) processes the error signal and
produces appropriate current signal (IS). The current signal (IS) is multiplied with unit sinusoidal template
which is produced by using phase locked loop (PLL), to produce IS sinωt. The load current iL subtracted from
the IS sin ωt to produce the reference current signal iS*. As the boost inductor current can’t be alternating, the
absolute circuit gives the absolute value of the reference current signal iS* that is iC*. The actual signal (iC) and
the required reference signal (iC*) are given to the current controller to produce the proper gating signal. The
current controller adopted is a hysteresis current controller. Upper and lower hysteresis band is created by
adding and subtracting a band ‘h’ with the reference signal iC* respectively shown in the Fig. 8. The inductor
current is forced to fall within the hysteresis band. When the current goes above the upper hysteresis band, i.e.
iC *+h, the pulse is removed resulting the current forced to fall as the current will flow through the load. When
the current goes below the lower hysteresis band i.e. iC *-h, the pulse is given to the switch, so the current
increases linearly.
Fig 5: Adopted control scheme for the Closed Loop operation
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
5. MATLAB MODELING AND SIMULATION RESULTS
Here the simulation is carried out by four cases, in that applied to constant speed reference and variable speed
reference.
1.
2.
BLDC Motor Drive with six switches.
BLDC Motor Drive with four switches.
Case i: Six switch Inverter fed BLDC drive with Constant Speed
Fig 6: Matlab/Simulink model of six switch inverter fed BLDC with fixed speed of 2000 r.p.m
Fig 7: Simulated output wave forms of Stator currents and EMF for Six Switch inverter fed BLDC Motor
under constant speed
Fig 8: Simulated output wave form of Speed for Six Switch inverter fed BLDC Motor under constant
speed
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Fig 9: Simulated output wave form of Torque for Six Switch inverter fed BLDC Motor under constant
speed
Case ii: Six switch Inverter fed BLDC drive with Variable Speed
Fig 10: Matlab/Simulink model of six switch inverter fed BLDC with variable speed
Fig 11: Simulated output wave forms of Stator currents and EMF for Six Switch inverter fed BLDC
Motor under variable speed
Fig 12: Simulated output wave form of Speed for Six Switch inverter fed BLDC Motor under variable
speed
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Fig 13: Simulated output wave form of Torque for Six Switch inverter fed BLDC Motor under variable
speed
Case iii: Four switch Inverter fed BLDC drive with Constant Speed
Fig 14: Matlab/Simulink model of four switch inverter fed BLDC with fixed speed of 2000 r.p.m
Fig 15: Simulated output wave forms of Stator currents and EMF for four Switch inverter fed BLDC
Motor under constant speed
Fig 16: Simulated output wave form of Speed for four Switch inverter fed BLDC Motor under constant
speed
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Fig 17: Simulated output wave form of Torque for four Switch inverter fed BLDC Motor under constant
speed
Case iii: Four switch Inverter fed BLDC drive with Variable Speed
Fig 18: Matlab/Simulink model of four switch inverter fed BLDC with variable speed
Fig 19: Simulated output wave forms of Stator currents and EMF for four Switch inverter fed BLDC
Motor under variable speed
Fig 20: Simulated output wave form of Speed for four Switch inverter fed BLDC Motor under variable
speed
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J.E. Muralidhar et al./ International Journal of Engineering & Science Research
Fig 21: Simulated output wave form of Torque for four Switch inverter fed BLDC Motor under variable
speed
6. CONCLUSION
The simulation model of the BLDC motors drive system with PI control based four switch and six switch three
phase inverter on MATLAB/Simulink platform is presented under fixed and variable speeds. The performance
of the developed algorithm based speed controller of the drive has revealed that the algorithm devises the
behavior of the PMBLDC motor drive system work satisfactorily. And also in this paper, the four-switch
inverter topology is studied to provide a possibility for the realization of low cost and high performance threephase BLDC motor drive system over the conventional inverter topologies.
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