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Design of a segment-stator induction motor with optimum efficiency.

Since their invention, electrical machines have been undergoing redesign and improvement to meet the growing requirements of modern industry. Over the past few decades, special motors have been gaining popularity and their application has been expanding [1]. Engineers and researchers have spared no efforts in fully developing special motors, such as brushless dc machines, servo motors, stepper motors, variable reluctance motors, pole-shaded motors, and disc motors. Thanks to those efforts theses kinds of machines are now well known and understood. This made them attract attention and find a wide range of applications in both industry and consumer products. Another important class of special machines, known as electrical machines with opened-magnetic conductor, includes [2]: linear motors, motors with arc-segment stator, and multi-rotor machines. Linear motors have reached state-of-the-art development and found applications in many areas, such as electric transport and textile industry [3]. However, motors with segment stator were completely overlooked and received much less attention than other types of motors. The low efficiency of this kind of motors was the major factor behind this lack of interest [4]. Previous attempts to overcome this drawback concentrated mainly on the improvement of the stator's design of the motor. An important contribution, in this respect, was the introduction of special stator windings with compensating elements [5]. Another work attempted the increase of the air-gap at the edges of the stator [6]. In the present work, a different approach was adopted.

Our approach focused on the redesign and construction of the secondary element (rotor) with novel windings configurations. A prototype stator was built and its performance was theoretically and experimentally evaluated. A set of new windings configurations adequate for the proposed motor is presented. A general comparison between the conventional induction motor and the proposed induction motor with segment stator is provided. Methods for improving the efficiency of the proposed motor are also given. In the present work, the developed induction motor with segment stator was used to drive a cotton module builder machine. Results of this practical application are provided and discussed in the paper. At the present the efficiency of the proposed motor is increased to more than 50%.

Design of Induction Motor with Segment Stator

Stator Construction

Induction motor with segment stator can be designed and manufactured by cutting a predefined arc-section of the stator of a conventional induction motor. The rotor will be kept intact and will represent the secondary element of the motor. The air-gap region where the rotor meets the stator is referred to as 'the active region' [5]. The rotor side that is not exposed to the stator will be known as 'the passive region'.

Having cut the stator, the magnetic field will no longer be rotating but traveling [7]. This creates an important phenomenon at the edges of the active region. This phenomenon is called 'edge effect'.

The synchronous speed of the motor with segment stator is now given by [5]:

[n.sub.s] = 60f/p [alpha] (1)

Where f is the supply frequency, p is the number of pole pairs and a is the central angle made by the stator segment, taken as a ratio of the complete angle 360[degrees]. In our prototype a was taken equal to 1/3. This means that our stator segment represents about third of a conventional stator, as shown in Fig. 1.

[FIGURE 1 OMITTED]

Secondary Element's (Rotor) Construction

Instead of using a solid rotor, windings were fitted onto the secondary element (rotor) in order to reduce the non-magnetic gap and increase the magnetic permeability of the motor with a segment stator [8]. This modification has a very important advantage represented in the reduction of the rotor radial edge effect, induced as a result of continuous changing of the rotor blocks (parts) in the active region, which in turn causes a reduction in torque and efficiency.

Windings of conventional induction motor can be fitted onto rotors of motors with segment stator. A set of special rotor windings is suggested. These are: insulated short-circuited lap winding and insulated short-circuited wave winding. These windings are basically used to produce the useful mechanical torque, but they can also be used for other purposes, such as speed control.

Block, as a part of wave winding, is defined here as a closed electrical circuit [9], which is formed from two or more elementary phases by connecting their starts and ends together, as shown in Fig.2. Therefore, the rotor winding will consist of a few blocks electrically not connected together. In this investigation, it has been found that the block wave winding is the most effective winding, which can be used for motors with a segment stator. The short-circuited loop of block wave winding can be formed by connecting two elementary phases together, the first one forms the forward (phase 1) and the second one forms the inverse (phase 2) winding waves, as illustrated in Fig. 2 (a).

[FIGURE 2 OMITTED]

Other windings configurations and topologies, such as conventional wave winding, single layer and double layer wave windings, can be derived from block wave winding. One of the advantages of block wave winding is that its block might have any length multiple of stator's pole pitch.

When the active region is short, the secondary winding can be constructed from a group of blocks, distributed over the entire length of the rotor. This will decrease the resistance and reduce the length of the passive region of rotor winding in which rotor current is flowing.

All windings construction in terms of the number of their series-connected conductors or half-sections (n), can be referred to block winding as follows:

a) If the block consists of one half-section (bar), i.e. n=1, this is a squirrel cage winding.

b) When two half-sections, which located from each other approximately by one pole pitch, are connected together (n=2), we get a short-circuited lap winding (Fig. 2 (b)).

c) If the number of series-connected half-sections, which located from each other approximately by one pole pitch, are more than two (n=3,4,5,....), we get a block wave winding (Fig. 2 (c)).

If n is too large we get wave winding (Fig. 2 (d)).

e) If the start and the end of each phases of block wave winding are connected in short or through an additional resistance, we can get a phase winding similar to a rotor winding of conventional IM.

Taking into account the above mentioned remarks, the block wave winding can serve as a basis for analysis of multiphase secondary windings not only for motors with a segment stator, but also for linear induction motors.

Mathematical Modelling of Motors with a Segment Stator

Using method of equivalent circuits (MEC), a mathematical model was developed for a motor with a segment stator with arbitrary winding onto the secondary element [10]. Using this mathematical model, the performance of this motor is investigated. Some assumptions were made:

1. Magnetic permeability of stator's and rotor's iron core is infinity.

2. The increased air gap magnetic reluctance, as a result of two-side slotting, is taken into account using Carter's coefficient.

3. Shunt fluxes are considered by including shunt resistances in the equivalent circuit.

4. A system of symmetrical primary linear voltages is assumed.

5. Leakage fluxes of primary and secondary circuits are considered by including leakage inductances.

6. Electro-conductive material of the secondary element is assumed as a homogeneous distributed layer on the surface of the core.

7. Secondary element moves by a constant speed with respect to the stator. Fig.3 illustrates a model of a 2-pole motor with a segment stator with a secondary element containing slots in which arbitrary winding can be placed. The number of slots per pole per phase (q) for this machine is equal to one. Fig. 4 reveals the simplified magnetic equivalent circuit for this machine. Where [F.sub.s(A,B,C)]- mmf of stator (armature) phase, [F.sub.i]- mmf of [in.sup.th] slot of the secondary element, [R.sub.sh]- Magnetic reluctance of the shunt parts, [R.sub.[sigma]i]- Magnetic reluctance of the air gap, [[PHI].sub.[sigma]i]- Magnetic flux in the air gap, and [[PHI].sub.i]- Magnetic flux in the stator core, i=1,2,3,....- slot (tooth) number, or number of the certain part in the equivalent circuit..

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Electromagnetic Analysis of the Motor with a Segment Stator

Using the model described in the previous section, and taking into account the diagram connection of the stator's winding,, we might write the Electrical State (voltage) Equation (ESE) for all stator phases :

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [R.sub.1], [L.sub.1]- Resistance and leakage inductance of stator's winding, and [PSI].sub.A(B,C)]- Flux linkage of stator's phase with the motor's magnetic field. When stator's phases are in Y connection, usually the linear voltages are given, therefore the voltage equation should be written in the following form:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Similar equations can be written for [V.sub.BC] and [V.sub.CA].

For Squirrel-Cage Winding

Using MEC, the ESE of the secondary element can be written for each particular bar. For example, for squirrel-cage winding bars:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

where [i.sub.b]- Current in the nth bar of the secondary element, [R.sub.b]-Resistance of the bar, [L.sub.b]- Leakage inductance of the bar, and v- Speed of the secondary element.

If the bars of the secondary element are connected in more complicated diagram than the squirrel-cage, then the equations system should include an additional equation which takes in consideration the connection of these bars and their relative positions with respect to the stator, which will increase the total number of equations. This is undesired, because their solution becomes more difficult and the time required increases.

For Wave Winding

Consideration of the diagram connection of the secondary element winding can be done another way. Instead of writing a set of ESE for particular bars included in a certain short-circuited winding's loop, it is enough to put one equation for the whole loop, taking into account the successive connection of those bars, in the predefined loop. Hence, the total number of ESE will be decreased.

For example; for a wave winding, the ESE for the first loop along the axes linked with the secondary element, may be written as:

[R.sub.1][i.sub.1] + [L.sub.1] [partial derivative][i.sub.1]/[partial derivative]t = [partial derivative][[psi].sub.1]/[partial derivative]t [[psi].sub.1] = ([[empty set].sub.1] - 2[[empty set].sub.4] + [[empty set].sub.1] -[[empty set].sub.10])[N.sub.k2] (5)

where [R.sub.1], [L.sub.1]- Resistance and inductance of the loop, [[PSI].sub.1]-Total flux linkage of the loop with stator's magnetic flux, and [[PHI].sub.i]-Magnetic fluxes linked with loop bars.

Magnetic fluxes coefficients depend on the number of bars in the particular slot, and the signs of these coefficients depend on the selected positive direction of the current in the bars.

Computation of motors with multiphase winding onto the secondary element using method of MEC, should be performed for the instantaneous values of all quantities, since the position of particular loops is continuously changing

Theoritical Computation of Motors with a Segment Stator

Calculations with Lap and Wave Windings

Calculations were performed for the proposed motor with a block lap and wave windings on the secondary element. Results were compared with the experimental data obtained from a series of tests on a motor with a segment stator with different types and configurations of the rotor's windings.

In Fig. 5, calculations results are presented for instantaneous values of currents in the blocks of lap and wave windings, when secondary element moves with respect to the stator with a slip s=0.2, (x=0 - coordinate of the stator's entrance edge). It is clear that the current in the block wave winding is more sinusoidal (curve 2), especially when the block has completely entered the active region (x>2). The current surges, when the block enters or leaves the active region, are less than the corresponding for lap winding (curve 1). This emphasizes the fact that using a block wave winding enables elimination of radial edge effects, which are related to the entering -leaving action of the secondary loops into the motor active region.

[FIGURE 5 OMITTED]

The total power losses in the block wave winding is less than the one resulting from the use of lap winding, since the number of wave winding blocks located in the active region is 3 times less than the number of lap winding blocks.

Characteristics of Segment Stator Motor

The above mentioned fact is revealed in Fig. 6, which shows the mechanical characteristics and efficiency, [eta], of the motor with a segment stator for lap and wave windings. The curves shown emphasize the importance of using block wave windings to eliminate edge effects, especially at small values of slip,

Fig. 7 shows the impact of electromagnetic permeability on the characteristics of motor with a segment stator with a lap and wave windings on the secondary elements. By increasing the electromagnetic permeability, the mechanical characteristic becomes stiffer and the motor maximum efficiency increases. But, when the resistance of the lap windings is reduced, the amplitudes of additional rotor current components increase. This leads to an increase in the negative forces at the edge intervals and consequently reduction of resultant forces, especially for small slips.

[FIGURE 6 OMITTED]

Figure 8, shows the calculated rotor currents in the blocks of a wave winding for a 2-pole stator at a slip s=0.5. The solid curve is based on a symmetrical system of voltages, whereas the dashed curve is based on a symmetrical system of stator currents. As shown in Fig. 9, the distribution of magnetic field in the active region exhibits more distortion in the case of symmetrical stator voltages (curve 1) than in the case of symmetrical stator currents (curve 2).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Laboratory Testing

Experimental Test Rig

In order to validate the theoretical investigation and calculations about the efficiency of using secondary windings, an experimental prototype of the motor with a segment stator, shown in Fig. 10, was built and tested using different rotor constructions.

[FIGURE 10 OMITTED]

The segment stator was made on the basis of the stator of a conventional induction motor, in which 2/3 of its magnetic conductor (iron core) was removed. Refer to Fig. 1. The construction of experimental pinch shown in Fig. 10, enables testing of different rotor constructions keeping the same value of the air gap 1.2 mm. Motor with a segment stator was tested and analyzed with the following rotor modifications:

Squirrel-cage rotor ([[epsilon].sub.0]=7)

Rotors with lap and wave windings ([[epsilon].sub.0]=7)

Squirrel-cage rotor, with cuttings (Fig. 11) and without cuttings in the end-rings ([[epsilon].sub.0]=15)

Rotor made from ferromagnetic segments.

Rotor made from ferromagnetic segments, displaced from each other by a certain distance.

The basic data for the experimental model are presented in table1. Magnetic field density, in the air gap and in the shunt zones, is determined using measuring coils, which are distributed on the stator active surface and outside of it by a distance equal to pole pitch. The load and speed regulation of the motor with a segment stator is performed by a DC machine, which is connected to a regulated voltage. The mechanical forces produced by the motor with a segment stator were determined by the armature reaction of the DC machine by means of dynamometer. Voltage regulation for experimental motor with a segment stator was a provided by means of induction regulator.

Experimentally, the phase currents and voltages, active phase power, rotor speed, mechanical force, efficiency and power factor were recorded.

[FIGURE 11 OMITTED]

Effects of Rotor Windings

In order to study the impact of the rotor windings types on the performance, experimental tests were carried out on the motor with a segment stator in which rotor is made from separate block sections and bracelet type windings. Also rotors with double-layer winding were used. Windings with different block lengths (from 1 stator pole pitch till 6) can be obtained using different connection diagrams (Fig. 12).

[FIGURE 12 OMITTED]

Fig. 13 shows the mechanical characteristics of prototype motor with a segment stator, for different rotor constructions. All windings fitted onto the rotor lead to an increase in the mechanical forces produced by the motor, as a result of increasing the permeability. For squirrel-cage rotor and for lap winding, it is clear that the edge effect for small slip values is strong, which results in high break forces.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

Implementing block wave winding with block length equal to 3o and 6o enables eliminates the radial edge effect. However, this will result in an increase in the mechanical developed forces for small slip values. Therefore the efficiency for block wave winding is the maximum among all, as shown in Fig. 14. Data points in Figs. 13 and 14 indicate the calculated values. Comparison between experimental and theoretical shows that the adopted analytical method is correct and the mathematical model is precise.

Fig. 15 shows the power factor as a function of the slip. It is clear that the minimum power factor is for wave winding (n=12).

[FIGURE 15 OMITTED]

This is expected because this winding has a very high leakage inductance. This is considered as a disadvantage for wave winding, but this may be corrected by connecting capacitors in the secondary winding. Table 2, presents the values of mechanical force, efficiency, power factor, in the nominal operation of the motor. Nominal operation is considered as an operation with maximum efficiency.

On-Site Testing

The basic technical data of the experimental gearless drive, using the motor with a segment stator inside a module-builder of raw cotton material (shown in Fig. 16), are presented in Table 3.

[FIGURE 16 OMITTED]

A comparison of the commercial and proposed cotton module-builders In order to support the use of motors with a segment stator in real applications, a comparison of the technical spesifications for commercial and experimental cotton module-builders is provided in Table 4.

These data indicate the efficiency of using motor with a segment stator for such applications. Commercial cotton module-builders, including their drive, have a weight of around 400 kg, whereas our modified module-builder based on gearless drive using motor with a segment stator has a weight of 183 kg. This means that weight is reduced by 55%, which is very important, taking into account that this module-builder has to be mobile. From the previous table, it is shown that the input power for the experimental model has been reduced by 20%, while the other parameters (rotational speed and developed torque) are kept constant.

Conclusions

An induction motor with a segment stator was designed and analyzed. The design of such a motor is so flexible that it can be configured to meet the requirements of the load is going to drive. For example, one can join two arc-sections back-to-back in order to drive two rotors, simultaneously. A prototype based on the design was made and experimentally tested. It has been demonstrated that block wave winding is the most general and universal multiphase winding for rotor of the motor with a segment stator.

Performance characteristics were experimentally determined. The prototype was tested in real application achieving very good results. The results obtained confirm that the designed motor will be suitable for applications requiring compact and inexpensive low-speed drives, such as cement mixers, steel rolling machines, belts and conveyers...etc. In other applications, the driving motor can be replaced by a stator segment (arc-shaped) and the rotating body of the mechanism will be used as a secondary element (rotor). In general, this type of motor will be mostly suitable for systems where reduced weight and free cooling are of a paramount importance. In most applications forced cooling will not be needed. Further improvement of efficiency is foreseen for the future.

References

[1] E.R. Laiwaite, Induction Machines for Special Purposes, London (1966)

[2] M.G. Resin, "Induction motor with disconnected magnetic conductor", B.I Publications Report AC No. 574825, USSR (1977)

[3] B.A. Vinocorov, E. V. Kozachenco, V. A. Vlacov, "Characteristics and ways of improving linear induction motors", Higher Education Journal of Electromechanics, 11, pp. 1014-1017, USSR (1979)

[4] M.G. Murdjikyan, "Induction motor with arc-shaped stator", B.I Publications Report AC No.710095, USSR (1980)

[5] P.A. Fridkin, Gearless Arc-Shaped Electro-Drive, Energy, USSR (1970)

[6] S. Yamamura, H. Ito, J. Ishikawa, "Theories of Linear Induction Motor and Compensated Linear Induction Motor", IEEE Trans., Power Apparatus and Systems, Vol. 91, No. 4, pp. 1700-1708 (1972)

[7] F.H. Sarakolov, "Analysis of induction motor with disconnected magnetic conductor", J. Electricity, 5, pp. 30-34, USSR (1982)

[8] M.G. Murdjikyan, "Parameters of rotor's winding of an induction motor with an arc-shaped stator", Proc. of Scientific-technical conference, Sverdlovsk, pp. 31-32, USSR (1976)

[9] J.M. Bani-Younis, The Design and Study of Electrical Machine with Disconnected Magnetic Conductor, PhD Thesis, pp. 45-47, Uzbekistan (1996)

[10] J.M. Bani-Younis, "Mathematical modeling of electrical machines with disconnected magnetic conductor", J. Prob. of Energy and Eng. Computation, 2, pp 30-34, Uzbekistan (1995)

Jihad Al-Khalaf Bani-Younis, Azzeddine Ferrah,

Abdelkader Tami and Munir Bouzguenda

Sohar University, P.O BOX: 44, PC: 311,

Sohar, Sultanate of Oman

E-mail: j.alkhalaf@soharuni.edu.om
Table 1 : Basic data for the laboratory prototype

Quantity                                                Value

Number of stator poles, 2p                              4
Internal stator diameter [m]                            0.230
Angle of the stator active zone                         120 [degrees]
Pole pitch, [tau] [m]                                  0.04
Number of stator slots, z                               12
Stator winding                                          Double-layer
Winding's conductor diameter, d [m]                     0.001
Number of turns per phase, [N.sub.1]                    260
Number of slots per phase per pole, q                   1
Synchronous speed, n [rpm]                              500
Air gap, [delta] [m]                                    0.0012

                                        Rotor 1         Rotor 2

Permeability coefficient                7               15
[[epsilon].sub.0]
Number of slots/ rotor segment          6               6
[z.sub.2]
Rotor winding's type                    Squirrel-cage   Squirrel-cage
                                        Lap winding
                                        Wave winding

Table 2 : Nominal performance of the motor with a segment stator

Quantity              Rotor winding

                      Squirrel cage   Lap     Wave

                      n=1             n=2     n=6     n=12

Slip                  0.30            0.30    0.25    0.30

Mechanical force [N]  8.0             10.5    13.5    17.5

Efficiency [%]        34.5            42.0    47.0    52.5

Cos [phi]             0.64            0.69    0.60    0.57

Pout/Pin              0.15            0.193   0.195   0.21

Table 3 : Technical specification of the experimental gearless drive

Quantity                            Value

Number of stator poles 2p           4
Internal stator diameter [m]        0.23
Angle of the stator active zone     120
Pole pitch [tau] [m]                0.035
Number of stator slots, z           16
Stator winding                      double-layer
Number of turns per phase           210
[N.sub.1]
Number of slots per phase per       1
pole q
Synchronous speed n [rpm]           500
Air gap [delta] [m]                 0.0025
Rotor type                          Solid ferromagnetic material
Power [kW]                          4
Line voltage [V]                    380
Stator current [A]                  11.1
Efficiency [%]                      54
Power factor                        0.62

Table 4 : Technical specifications of the commercial and experimental
cotton module-builders.

General Specifications                  Commercial     Proposed module
                                      module builder       builder

Productivity (by the cotton               35-40             35-40
material), T/h

Maximum throwing distance, m              16-18             18-20

Diameter of the rotating cylinder,         400               400
mm

Length of the rotating cylinder, mm        600               600

Diameter of the vertical cleaning           24               24
rods, mm

The height of vertical cleaning            194               194
rods, mm

Pitch between vertical cleaning             70               70
rods, mm

Number of groups(rows) of vertical          4                 4
cleaning rods, mm

Speed of the rotating speed,               6.6               7.5
rev/sec

Diameter of the receiving end, mm          900               900

IM power, kW                                5                 4

Motor speed, rev/min                       1500              500

System efficiency                          0.61             0.54

Power factor                               0.78             0.62

General Sizes

Length, mm                                 1200             1200

Width, mm                                  800               800

Height, mm                                 1200             1200

Mass, Kg                                   400               183
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Author:Al-Khalaf Bani-Younis, Jihad; Ferrah, Azzeddine; Tami, Abdelkader; Bouzguenda, Munir
Publication:International Journal of Applied Engineering Research
Article Type:Report
Date:Jun 1, 2008
Words:4024
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