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Design and Construction of a New SRM Sensorless Driver
M.Rafiee 1, E.Afjei2 and A.Siadatan3 1-Young Researchers Club, West Tehran Branch, Islamic Azad University, Tehran, IRAN
[email protected] 2- Department of Electrical Eng., Faculty of Electrical & Computer Eng., Shahid Beheshti University G.C., Tehran, IRAN
[email protected] 3- Department of Electrical Eng., Faculty of Electrical & Computer Eng., Shahid Beheshti University G.C., Tehran, IRAN
Abstract- A new sensorless circuit for Switched Reluctance
Motor (SRM) is designed, constructed and tested in this paper. The circuit uses three steps to control the SRM. One of them is used in motor starting. The other one controls the motor in low speed and the third one is used in high speed. These steps are implemented by an inexpensive AVR microcontroller chip. An asymmetric bridge converter is used to excite the winding of the motor phases. The presented sensorless board is then constructed and tested on two SRMs which are three-phase 12 by 8 and three-phase 6 by 4 SRMs. The parameters of these motors which are required in sensorless techniques are obtained by Finite Element Method (FEM) analysis. The operational waveforms and results that are obtained in laboratory are illustrated in different figures.
Keywords: Sensorless, SRM, Asymmetric Bridge Converter.
I. INTRODUCTION
SRM is a special machine with salient rotor and stator poles which has many advantages like simplicity, low cost, high efficiency, ability to work in harsh environment, etc. The no winding rotor poles make the motor to be able to achieve higher speed. On the other hand, the motor cooling is much easier. The simplicity of SRM causes it requires less maintenance.
The SRM driver uses some power electronic devices and transistors; as a result it wasn’t a favorable motor for many of years till improvement in power electronic in last decades. SRM controlling needs the rotor position information. The stator phase should be turned on in a position of the rotor at which positive torque is generated and the rotor rotates. In general, two methods are used in SRM control: direct and indirect (sensorless) strategies. [1], [2]
In direct strategies, position sensors are used to detect the rotor position. Using position sensors in motor construction causes some disadvantages like less efficiency, less reliability, more wiring, increasing the cost, etc. so sensorless strategies are preferred often.
Most of the sensorless strategies are listed in Fig.1. As it's illustrated, the techniques are classified based on the operation speed of the motor.
This paper presents a novel sensorless circuit to control three-phase SRMs. In the presented board an inexpensive controller component is used so the board is low cost and very simple.
Fig.1. Sensorless methods
An asymmetric bridge converter with two switches in
each phase is used to transfer the command pulses of the microcontroller to the transistors in order to excite the phase windings. The sensorless circuit is then constructed and tested on two three-phase SRMs and the results obtained in laboratory are compared to each other. This paper uses FEM analysis to achieve the parameters of the SRMs
The paper is organized as follows: Section II presents the one phase model of SRM. Section III explains the specifications of the used SRMs. In section IV, the selected three-phase SRMs are simulated. In Section V the controller is constructed and tested in laboratory on both of the selected SRMs and operational results are obtained. The paper is then ended in section VI by conclusions.
II. ONE PHASE MODEL OF SRM
One phase of a SRM has RL circuit modeled so by applying the voltage to the winding of one phase, the equation (1) can be written:
V λ Ri (1) Where V is the supplied voltage, R is the winding
resistance, i is the winding current and λ is flux linkage and calculated by: λ θ, i L θ i 2
Where θ is the rotor position angle respect to the stator pole and L is the inductance of the phase winding and varies with the rotor position variation in the form of Fig.2. So λ depends on both i and θ.
Using equations (1) and (2), equation (3) is obtained for an unsaturated motor: V L didt i dLdt Ri 3
The angle speed of the motor is ω θ , so equation (3) is also able to be written as: V L didt iω dLdθ Ri 4
The expression e iω Lθ is the back EMF voltage of the
motor. [17] In order to use the SRM as a motor, the phase should be
turned on in positive torque region at which torque can be generated and rotates the rotor. So rotor position is the most important parameter which is required in SRM driving. By detecting the rotor position, the adequate fire pulses can be generated and sent to each phase of the motor.
Fig.2. Inductance versus rotor position
III. SELECTED SRMS
Both of the selected SRMs are built in a same motor case with a same size. The constructed three-phase 6 by 4 and 12 by 8 SRMs are illustrated in Fig.3.a and Fig.3.b respectively.
(a)
(b)
Fig.4. Three-phase SRM: a: 6 by 4, b: 12 by 8 The specifications of the presented 6 by 4 three-phase
SRM are presented in table I. The rotor pole arc is 32° while the stator pole arc is 30°. The difference between the arc of the rotor and stator poles helps the motor to have lower torque ripple. The stator has the diameter of 60 mm and the rotor has the diameter of about 35 mm. There is 0.25 mm air gap between the rotor and stator poles. Each coil in this motor includes 200 turns winding. The motor has about 50 watts power in 12 volts supply voltage. The nominal speed of the motor is 5000 rpm.
TABLE I:
The specifications of the 6 by 4 three-phase SRM
Nominal Power 50 Watts Nominal Voltage 12 volts Nominal speed 5000 rpm
Stator core outer diameter 60mm Stator core inner diameter 52mm
Stator pole arc 30° Air gap 0.25mm
Rotor core outer diameter 35mm Rotor shaft diameter 8mm
Rotor pole arc 32° Number of turns per pole 200
The specifications of the introduced 12 by 8 three-phase
SRM are presented in table II. The rotor pole arc is 16° while the stator pole arc is 14°. The stator has the diameter of 60 mm and the rotor has the diameter of about 35 mm. There is 0.25 mm air gap between the rotor and stator poles. Each coil in this motor includes 150 turns winding. The motor has about 50 watts power in 12 volts supply voltage. The nominal speed is about 3000 rpm in this SRM.
Both of the presented 6 by 4 and 12 by 8 SRMs are three-phase but the 12 by 8 SRM has lower speed than the 6 by 4 SRM because it has more poles. It is able to generate higher torque level with lower torque ripple than the 6 by 4 SRM.
Inductance (Henry)
TABLE II: The specifications of the 12 by 8 three-phase SRM
Nominal Power 50 Watts Nominal Voltage 12 volts Nominal speed 3000 rpm
Stator core outer diameter 60mm Stator core inner diameter 52mm
Stator pole arc 14° Air gap 0.25mm
Rotor core outer diameter 35mm Rotor shaft diameter 8mm
Rotor pole arc 16° Number of turns per pole 150
IV. THE SIMULATION OF THE SELECTED SRMS
The selected SRMs analysis has been done by Magnet CAD package.[18] The software uses FEM which is a numerical technique in order to simulate electrical machines.
In the software, the rotor of the SRM is rotated from Fully Unaligned (FU) position to Fully Aligned (FA) position then to FU position and different parameters of the motor are obtained.
Rated current of 3A is applied to the stator phase winding. The flux density throughout the SRM is shown in Fig.5.a for FU position, Fig.5.b for SOA position, Fig.5.c for Half Aligned (HA) position and Fig.5.d for FA position.
As it's illustrated the maximum value of flux density is about 1.4 Tesla in FA position.
The same work is performed again for different current values and also for the selected 6 by 4 SRM
Fig.6 shows the magnetization curve of the selected 12 by 8 three-phase SRM for the phase current of 0.1A to 3A and different rotor positions. In the beginning of alignment position, the flux has its minimum value and in fully aligned position, the flux has its maximum value. In a specified position of the rotor, if the current of the phase winding increases, the flux will increase as well.
(a)
(b)
(c)
(d)
Fig.5. flux density throughout three phases 12 by 8 SRM in (a): FU, (b): SOA, (c): HA, and (d): FA position
Fig.6. Magnetization curve of the selected 12 by 8 SRM
Fig.7 illustrates the magnetization curvby 4 three-phase SRM for the phase rated3A and different positions of the rotor.
Fig.7. Magnetization curve of the selected
V. CONSTRUCTION AND OPERATIONAL PRESENTED SENSORLESS CIR
In this section the circuit is designlaboratory. The controller is an Amicrocontroller board. It operates withfrequency. It generates desired pulses to cusing Pulse Width Modulation (PWM) tecphase excitation is done by an asymmetriwith two IRF540 N-channel Mosfets. Adriver is used in the gate driver circuit for ethe Mosfets. The board has a 12v DC powLm7805 voltage regulator to generate 5v microcontroller and other digital chips.
The presented SRMs are controlled inpresented sensorless circuit: from standstilhigh speed. Note that, the SRMs are condition in the presented paper.
In standstill, training pulse method wloop technique is used to start the motor rphases is selected and trains of pulse are apThis is done for other phases frequentfrequency according to the desired starting
In low speed, the magnetization curve the rotor position. One phase is excited acurrent of the phase is sampled by 1MHASC712 current sensor. The output of the low amplitude analogue signal which is cdata by a power full 10 bit Analogue to(ADC).
The flux is calculated by equation (10)the rotor position is obtained by using phase current and the magnetization curvefully aligned position (θdesired), the phase isnext phase is excited; otherwise, the procthe desired rotor position.
In High speed, current waveforimplemented to detect the rotor position. active phase is sampled by the current sento digital data by the ADC. When the slwaveform is zero, the rotor is in the begin
ve of the selected 6 d current of 0.1A to
d 6 by 4 SRM
RESULTS OF THE RCUIT
ned and tested in Atmega128 AVR h 16 MHz clock control the SRM by chnique. The SRM ic bridge converter
A HIP2500 Mosfet each phase to drive
wer supply and uses in order to use for
n three steps by the ll, in low speed and driven in no-load
which is an open-rotation. One of the pplied to the phase. tly with a desired g speed.
is used to estimate t first and then the
Hz frequency by an current sensor is a
converted to digital Digital Converter
) then. The value of the value of flux, e. If the rotor is in s turned off and the cesses continue till
rm technique is The current of the
nsor and converted lope of the current nning of alignment
position which means the phproperty is basically because ophase winding. In fact, in unaligphase inductance is low whicincreases very quickly after phacurrent waveform is positive). Iposition, the phase inductance swhich prevents the phase currenthe value of the phase current islope of the current waveform inductance increases which decreases (the slope of the curre
The constructed circuit is fifor the 6 by 4 three-phase SRM
The operational results are of about 800 rpm of the 6 by 4 S
As it is shown, in this stewhich means we are able to algorithms (Torque or speed ccycle of the pulses.
(a)
(b)
(c)Fig.14. Operating in 800rpm speed in 6of the microcontroller board (b): Drain
Output voltage of ASC
ase should be excited. This of the inductance value of the gned position, the value of the ch causes the phase current ase excitation (the slope of the In the beginning of alignment starts to increase immediately nt to increase. In this position, is constant for a moment (the is zero). After that, the phase causes the phase current
ent waveform is negative). irst adjusted and programmed in laboratory. shown in Fig.8 for the speed
SRM. ep, chopped pulses are used have any under-load control ontrol) by changing the duty
)
)
) 6 by 4 SRM: (a): PWM output signal n voltage of the upper transistor (c): C712 current sensor
The constructed sensorless driver programmed for the presented 12 by 8 thre
The operational results in the speed oare shown in Fig.9.
(a)
(b)
(c) Fig.17. Operating in 1050rpm speed in 12 by 8 SR
signal of the microcontroller board (b): Drain voltage(c): Output voltage of ASC712 curren
VI. CONCLUSION
This paper presented the designing,testing of a sensorless control circuit for on6 by 4 three-phase SRMs. The driver control algorithms for three ranges of spSRMs which were: training pulse techniqumagnetization curve method (in low spwaveform technique (in high speed). Eachtechnique was able to be used in specified
An inexpensive Atmega128 AVR mused as the main control component. Aconverter with N-channel Mosfets and driver was used to switch the phase winand transmit the command pulses of the
circuit is then ee-phase SRM. of about 1050 rpm
RM: (a): PWM output e of the upper transistor nt sensor
construction and ne 12 by 8 and one board used three
peed to control the ue (from standstill), peed) and current h sensorless control range of speed.
microcontroller was Asymmetric bridge
HIP2500 Mosfet nding of the SRMs
microcontroller to
the transistor gates. The boardselected SRM and tested in labowere obtained.
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