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CHAPTER 5
DESIGN AND DEVELOPMENT OF DOUBLE WINDING
SYNCHRONOUS RELUCTANCE MOTOR
5.1
INTRODUCTION
Synchronous reluctance motors are generally employed in synthetic
fiber industry, glass making machinery and small textile industry. The power
factor and efficiency of this motor ranges between 0.35 to 0.5 and 55 % to
75% respectively. The product of power factor and efficiency is between 0.25
and 0.32. Construction modification and efficient operation is suggested in
this report to improve efficiency and power factor of synchronous reluctance
motor.
DWIM overcomes certain limitations like power factor, efficiency
and energy conservation compared to conventional induction motor. Second
set of winding of DWIM can be connected to a small three phase load or
single phase loads which do not depend on a separate supply. There exists
speed reduction in DWIM, which causes slight reduction in terminal voltage.
The voltage regulation will be better if the speed of the machine is maintained
constant irrespective of shaft load. Double Winding Synchronous Reluctance
Motor (DWSyRM) model presented in this thesis work aimed to have better
voltage regulation in the second set of winding and performance
improvement.The detailed literature survey of synchronous reluctance motor
based on the performance improvement and design improvement is discussed
below:
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The torque exerted in synchronous reluctance motor depends on
pole arc to pole pitch ratio and saliency of the machine. Reluctance offered by
direct and quadtrature axes vary with respect to saliency ratio.
Matsuo and Lipo (1994) explained that d, q inductance difference
and saliency ratio of a segmented type synchronous reluctance motor are
proportional to torque and power factor. Efficiency of a synchronous machine
over its wide range of operation can be improved by compensating the core
loss currents. Control scheme using a vector controller is used to compensate
core loss currents of the machine. Inductance ripple in the machine is
estimated and incorporated in a machine model using a direct torque ripple
measurements (Fletcher et al 1995).
Matsuo et al (1996) presented an optimum efficiency control
scheme of synchronous reluctance motors in which controller is to seek a
combination of d and q-axis current components, which provides minimum
input power at a certain operating point in steady state. Obe and Senjyu
(2006) investigated performance of reluctance machine using d–q rotor
reference frame equations derived in space-vector model obtained by applying
the concept of winding functions. Core loss and saturation components are
also considered for the dynamic model. Out of two windings, one winding is
connected to the supply and the other fed with a balanced capacitor. In this
machine, the developed torque is superior to a brushless doubly-fed
reluctance machine.
Hiroshi (2000) presented a stator-flux-oriented control scheme to
be used in a synchronous machine without a position sensor at medium and
high electrical frequencies. For a given speed and torque, power losses in the
machine are given as a function of stator flux. Saliency of a synchronous
reluctance motor can be improved with a slit rotor. The stator teeth are
divided and are made of powder magnetic core. In the rotor, stainless sheets
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are inserted along with soft magnetic metal sheets with adhesive to strengthen
rotors and thereby saliency ratio is improved (Masayuki 2006).
Ogunjuyigbe et al (2007) analyzed the performance improvement
of synchronous reluctance motor with balanced capacitor in auxiliary
winding and the operation of the machine is explained using electromagnetic
and circuit concepts.
From the above discussions, the availability of several of types of
reluctance motors and their speed control and improvement of performance
are studied. Double winding induction motors are popular for energy
conservation and speed control applications. In this chapter, the main
motivation is to improve efficiency and power factor of Double Winding
Synchronous Reluctance Motor (DWSyRM).
Design modification and
efficient operation are presented.
5. 1.1
Construction and Principle of Operation
The structure of synchronous reluctance motor is the same as that
of the salient pole synchronous machine, except that the rotor does not have
any field winding. Stator has a three phase symmetrical winding which
creates rotating magnetic field in the air gap rotor pulled into the speed of
magnetic field.
Vagati (1994) presented a modern synchronous reluctance motor
drive in which the performance in terms of torque-per-volume is compared
with brushless and induction motors Due to inherent simplicity, robustness of
construction and low cost, synchronous reluctance machines have been
popularly used in many low power applications such as fiber spinning mills,
where large number of motors operate synchronously with a common power
supply. Synchronous reluctance motor is a self starting machine. At the time
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of starting, the machine runs as an induction motor but as speed increases,
reluctance torque locks the rotor into synchronous speed.
5.2
DESIGN PROCEDURE
A DWSyRM consists of two sets of three phase windings in the
same stator core. A squirrel cage salient pole rotor is constructed from the die
cast rotor.
5.2.1
Representation of the Proposed Scheme
As a proof for the discussion, a 3kW, 415V, 4-pole, 3-phase
Squirrel Cage DWSyRM has been designed, fabricated and tested.
Representation of DWSyRM is shown in Figure 5.1. Initially the machine
starts as induction motor and pulls into synchronous speed due to reluctance
torque.
Figure 5.1 Representation of DWSyRM
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The stator windings of a double winding induction motor can be
arranged with different shift angles between them. In double winding
induction motor, shift angle of 60 degrees or zero degrees are the best choice.
In the proposed model, to obtain optimum utilization, both the windings are
placed with zero degree phase angle displacement between them.
5.2.2
Design Considerations
The design of DWSyRM is affected by various constraints such as
thermal limit, overload capacity and utility of stator slots. The energy
conserving DWSyRM is ideal to be used for low power operations due to the
limitation in thermal insulation value.
5.2.3
Design of stator
Design procedure for the stator of DWSyRm is similar to stator
design of DWIM. In the proposed model, the same stator of Double Winding
Induction Motor is used. Design of stator winding is presented in chapter 2 is
used here. Highlights of the design details presented below:
Number of poles
=4
Synchronous speed
= 1500 rpm
Diameter of the core
= 0.139 m
Length of the core
= 0.11m
Number of turns per phase
= 228
Current density
= 5.2 1A/mm2
Electrical loading
= 18000A/ m2
Magnetic loading
= 0.44 Wb/m2
89
5.2.4
Design of rotor
In order to obtain reluctance torque, saliency is made in the rotor by
removing a set of rotor slots in uniform intervals around the rotor periphery of
die-cast rotor. Construction of salient rotor is shown in Figure 5.2.
Figure 5.2 Rotor of Double Winding Synchronous Reluctance motor
Higher Ld/Lq ratios yield higher power factors, which corresponds
to reduced copper losses and reduced volt ampere ratings of the inverter
driving machine. L/ ratio of the designed machine is unity. Due to air gap
variations, reluctance torque is exerted in the rotor. Nearer to synchronous
speed, rotor is magnetically lacked into revolving magnetic field.
90
5.2.5
Performance of DWSyRM
Xu (1991) presented a vector control to synchronous reluctance
motor considering saturation and iron losses of the core. Various direct and
indirect methods of testing of induction motor are available. Leonardi (1996)
presented a numerical method based analysis using current and flux density
waveforms to calculate iron losses of an electric machine which is based on
finite element analysis.
Performance improvement of two-stator winding induction motor
to develop reluctance torque with good power factor is obtained when one of
the stator winding is fed with a balanced capacitor (Obe and Senjyu 2006). In
order to predict the performance of designed DWSyRM, a series of testing
have been carried out by loading both windings.
Conventional load test has been carried out considering each
winding separately to determine machine performance. Testing has been
carried out with various combinations of electrical and mechanical loads. The
following tables and figures shows the details of reading observed and
performance characteristics for various load combinations.
Table 5.1 shows the reading observed considering one set of
winding, while second set of winding is kept unloaded. Maximum efficiency
obtained is 64.2% and the corresponding power is 0.56 and the corresponding
product of efficiency and power factor is 0.56. Figure 5.3 shows power factor
and efficiency characteristics.
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Line
current (A)
Input
power (W)
Torque
in (Nm)
Output
power (W)
415
2.0
680
0
0
0.0
0.47
415
2.5
980
3.4
534
54.5
0.55
415
3.0
1280
5.0
785
51.4
0.59
415
3.5
1520
6.0
943
62.0
0.60
415
4.0
1740
7.0
1100
63.2
0.61
415
4.5
1900
7.6
1194
62.8
0.58
415
5.0
2080
8.4
1320
63.4
0.58
415
5.5
2200
9.0
1414
64.2
0.56
Power
factor
Input
voltage
% Efficiency
Table.5.1 Brake Test on DWSyRM
Figure 5.3 Efficiency and power factor characteristics (DWSyRM)
92
Table 5.2 shows reading observed considering both windings.
1 A load has been added in the second set of winding in addition to
mechanical load. Figure 5.4 shows efficiency and power factor characteristics.
The maximum efficiency obtained is 79.7% and the corresponding power
factor is 0.77, product of efficiency and power factor is 0.613.
Input
Voltage
Line
Current (A)
Input
Power (W)
Torque
in Nm
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power Factor
Table 5.2 Mechanical load with 1A Electrical Load (DWSyRM)
415
2.1
1040
0.0
0
680
680
65.4
0.70
415
2.5
1360
2.2
346
680
1026
75.4
0.76
415
3.0
1640
4.0
628
680
1308
79.7
0.77
415
3.5
1960
5.6
880
680
1560
79.6
0.79
415
4.0
2200
6.0
942
680
1622
73.7
0.77
415
4.5
2380
6.8
1068
680
1748
73.4
0.74
415
5.0
2540
7.6
1194
680
1874
73.7
0.71
415
5.5
2680
8.0
1257
680
1937
72.2
0.69
Figure 5.4 Efficiency and power factor with 1A Electrical load
(DWSyRM)
93
Table 5.3 shows the reading observed considering both the
windings. Second set of winding is loaded to a load current of 2A in addition
to mechanical load. Figure 5.5 shows efficiency and power factor
characteristics. The maximum efficiency obtained is 81.9% and the
corresponding power factor is 0.87, product of efficiency and power factor is
0.713.
Input
Voltage
Line
Current (A)
Input
Power (W)
Torque
in Nm
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
%
Efficiency
Power
Factor
Table. 5.3. Mechanical load with 2A Electrical Load (DWSyRM)
415
415
415
415
415
415
415
2.8
3.0
3.5
4.0
4.5
5.0
5.5
1720
1840
2300
2480
2720
2920
3140
0.0
0.8
3.0
4.4
5.6
6.6
7.0
0
126
471
691
880
1037
1099
1340
1340
1340
1340
1340
1340
1340
1340
1466
1811
2031
2219
2377
2439
78.0
79.7
78.7
81.9
81.6
81.4
77.7
0.87
0.86
0.93
0.87
0.85
0.82
0.80
Figure 5.5
Efficiency and power factor with 2A Electrical load
(DWSyRM)
94
Table 5.4 shows reading observed considering both windings.
Second set of winding is loaded to a load current of 3 A in addition to
mechanical load. Figure 5.6 shows efficiency and power factor characteristics.
Maximum efficiency obtained is 82.5% and the corresponding power is 0.9,
product of efficiency and power factor is 0.743.
Input
Voltage
Line
Current (A)
Input
Power (W)
Torque
in ( Nm)
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power
Factor
Table 5.4 Mechanical load with 3A Electrical load (DWSyRM)
415
3.4
2260
0
0
1820
1820
80.5
0.94
415
4.0
2640
2.0
314
1820
2134
80.8
0.93
415
4.5
2940
3.8
597
1820
2417
82.2
0.92
415
5.0
3180
5.4
848
1820
2668
82.5
0.90
415
5.5
3420
6.0
942
1820
2762
80.7
0.88
Figure 5.6 Efficiency and power factor with 3A Electrical load (DWSyRM)
95
Table 5.5 shows the reading observed considering both windings.
Second set of winding is loaded with load current of 4A in addition to
mechanical load. Figure 5.7 shows efficiency and power factor characteristics.
Maximum efficiency obtained is 82.7% and the corresponding power is 0.96,
product of efficiency and power factor is 0.794.
Input
Voltage
Line
Current (A)
Input
Power (W)
Torque
in(Nm)
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power
Factor
Table 5.5 Mechanical load with 4A Electrical load (DWSyRM)
415
4.2
2880
0
0
2280
2280
79.1
0.97
415
4.5
3100
1.0
157
2280
2437
78.6
0.97
415
5.0
3400
3.4
534
2280
2814
82.7
0.96
415
5.5
3680
4.2
660
2280
2940
79.8
0.94
Figure 5.7 Efficiency and power factor with 4A Electrical load
(DWSyRM)
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Maximum efficiency and corresponding power factor for each
combination of electrical and mechanical loads are presented in the previous
section. In order to identify the maximum power factor and efficiency,
another combination of load test has been carried out in which the mechanical
load is kept constant and electrical load has been varied. Table 5.6 shows
reading observed with mechanical load of 346 Watts and variable electrical
load.
Input
Voltage
Line
Current (A)
Input
Power (W)
Torque
in (Nm)
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power
Factor
Table 5.6 Constant mechanical load with variable Electrical load
415
2.0
700
2.2
346
0
346
49.4
0.49
415
2.3
1140
2.2
346
380
726
63.6
0.96
415
1.6
1480
2.2
346
720
1066
72.0
0.79
415
3.0
1820
2.2
346
1080
1426
78.3
0.84
415
3.3
2140
2.2
346
1380
1726
80.6
0.90
415
3.7
2480
2.2
346
1680
2026
81.6
0.93
415
4.3
3040
2.2
346
2160
2506
82.4
0.98
415
4.7
3360
2.2
346
2400
2746
81.7
0.99
415
5.2
3680
2.2
346
2640
2986
81.1
0.98
415
5.6
3980
2.2
346
2860
3206
80.5
0.98
415
5.8
4160
2.2
346
3040
3386
81.3
0.99
Table 5.7 shows reading observed with variable electrical load and
constant mechanical load current of 4.5A. When a electrical load of 4A is
applied to second set of stator winding, efficiency is improved to 90.9% and
power factor to 0.91. It is also observed that machine is loaded to total current
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of 8.5A without affecting thermal limit. When compared with conventional
synchronous reluctance motor, efficiency is improved from 64.2% to 90.9%
and power factor from 0.56 to 0.91.
Table 5.7 Efficiency and power factor comparison
Mechanical
load (A)
4.5
4.5
4.5
4.5
4.5
4.5
Electrical
Load( A)
0.0
1.0
1.5
2.0
3.0
4.0
% Efficiency
81.2
84.0
86.5
87.0
87.5
90.9
Power
factor
0.63
0.65
0.65
0.82
0.85
0.91
Performance of DWIM and DWSyRM has been compared. Table
5.8 shows efficiency and power factor comparison between DWIM and
DWSyRM. General observation with DWSyRM is that, when the machine is
loaded with mechanical load above its rated capacity, rotor vibrates and
looses it synchronism. Hence for the safer operation, synchronous reluctance
motor should be operated with in its rated capacity. Efficiency of DWIM is
comparatively higher than DWSyRM for the given load current.
Power factor
of DWSyRM
Power factor
of DWIM
Electrical
load (A)
% Efficiency
of DWSyRM
Mechanical
load (A)
% Efficiency
of DWIM
Table 5.8 Efficiency and power factor of DWIM and DWSyRM
4.5
4.0
90.9
78.6
0.91
0.97
5.0
4.0
94.7
82.7
0.97
0.96
98
5.3
SUMMARY
The DWSyRM presented in this thesis consists of two sets of three
phase identical windings in same stator core. In the rotor, saliency is made by
removing a set of teeth alternatively around the rotor periphery with uniform
distances.
When operated as a DWIM, when the machine is loaded, there is a
voltage drop in the second set of stator winding. When operated as a
synchronous reluctance motor, the speed is constant from no load to full load.
However, near the full load current, rotor loses its synchronism and
mechanical vibration takes place.
A 3 kW, 3- phase, 4- pole, 1500 rpm, 415 V DWSyRM has been
designed and tested. When the machine is operated as conventional
Synchronous reluctance motor, the maximum efficiency and corresponding
power factor is 64.2% and 0.56. The main focus of the thesis is to improve
machine performance at reduced mechanical loads.
For a load current of 3A, when one winding is considered for
mechanical load, efficiency of the machine is 51.4% and power factor is 0.59.
For the same mechanical load, if electrical load of 680Watts tapped in from
the second set of winding, efficiency is improved to 79.7% and power factor
to 0.77.
By utilising the electrical output from the second set of winding,
dependency on separate supply to the connected load to this winding is
reduced. Hence energy conservation becomes possible in this machine. The
main objective of improvement of power factor and efficiency is obtained to
an appreciable extent. However, this machine has to be operated close to the
full load to avoid mechanical vibrations.
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The output power from the second winding can be used for
charging the batteries of UPS system and supplying lighting loads thereby
reliability of supply can be improved. DWSyRMs would be the better option
for rewound and poor performance motors for the same application.