a synchronous machine with amorphous core

Dr. Man Mohan et al. / International Journal of Engineering Science and Technology (IJEST)

A SYNCHRONOUS MACHINE WITH AMORPHOUS CORE Dr. MAN MOHAN Electrical Engineering Department, Faculty of Engineering DEI, Dayalbagh, Agra-282005, India Email: [email protected] Er. A. K. GOYAL Director, Construction and Environment Management Group (CEMG), Khandari, Agra-282002, India Abstract: Amorphous alloy is being seen as a good substitute of grain oriented steel; it has low core loss, less magnetising current and less noise, compared to grain oriented steel. It is being used in transformers as a core material nowadays. Amorphous alloy is available in form of C-core structure. In synchronous machines open slots are preferred on armature side; so, there is a possibility of developing a structure with open slots, by placing C-cores side by side, on armature side. Here, a synchronous machine with amorphous C-core structure is proposed; the performance of proposed synchronous machine is evaluated and compared with conventional synchronous machine. It is found that the efficiency of proposed synchronous machine is higher than that of conventional synchronous machine. Keywords: Synchronous machine design, slots, amorphous-core. 1.Introduction Synchronous machines are important electromechanical energy converters; they are generally used as alternating current (AC) generators. They supply electrical power to all sectors- agriculture, industrial, commercial and residential. Synchronous machines are also used as constant speed motors, or as reactive power compensators. Synchronous generators usually operate together or in parallel, forming a large power system supplying electrical energy to loads. For these applications synchronous machines are built in large units. There are two types of synchronous machine- salient pole type and round rotor type. The selection of type of a synchronous machine depends on speed of prime mover (Sawhney 2009). For high speed prime movers, round rotor type synchronous machines are preferred, and for low speed prime movers, salient pole type synchronous machines are preferred (Say 1970). Construction of a salient pole type synchronous machine is shown in Figure-1; here, rotor has saliency. The rotor pole shoe is made of laminated steel, and rotor pole body is made of forged steel. Field winding is fixed on rotor pole body, which is excited by dc supply. The armature winding is housed in slots of stator. The stator teeth and yoke, both are made of grain oriented steel. There are two types of slots (semi-closed slots and open slots) are generally used in rotating machines. In induction machines, semi-closed slots are preferred to reduce the noise level, as the air gap between stator and rotor is very less (Alger 1965, Boldea,Nasar 2010). In synchronous machine air gap between stator and rotor is comparatively much larger, so noise is not a problem; therefore open slots are preferred in synchronous machines. Advantages with open slots are ease in winding, better cooling and less machining cost. Nowadays, amorphous alloy (Fe78B13Si9) is being seen as a good substitute of grain oriented steel; it has low core loss, less magnetising current and less noise, compared to grain oriented steel (Boyd E.L. and Borst J.D. 1984). It is being used in transformers as a core material. Amorphous alloy is available in form of C-core structure (Manmohan 2012). If two C-cores are placed side by side, then two open slots with a tooth in between them are formed. So, there is a possibility of developing a structure with open slots, by placing C-cores side by side, on armature side. The construction of proposed synchronous machine with amorphous core is shown in Figure-2. Comparing between Figure-1 and Figure-2, there is no change in the material used in stator yoke and field poles; however in Figure-2, stator teeth are made of amorphous alloy. In a synchronous machine, maximum part of iron loss occurs in stator teeth. The stator teeth made of amorphous alloy have reduced iron loss in the machine; the iron loss in amorphous alloy (Fe78B13Si9) is about 1/10 of iron loss in conventional grain oriented steel. Air gap between two C-cores helps in cooling.

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Dr. Man Mohan et al. / International Journal of Engineering Science and Technology (IJEST)

2.Design considerations (Juha Pyrhonen et al 2008, Sawhney 2009) 2.1.Main dimensions: Main dimensions of stator of a synchronous are shown in Figure-3.

For synchronous machine having KVA input Q, output coefficient C0, synchronous revolutions per second (rps) ns, dia of stator bore D, length of stator L and number of stator poles P, the main dimensions are obtained by following equations (1) and (2)D2.L = Q/C0. ns ------(1). L= 1.5(π. D/ P) ------------(2). Output coefficient is given byC0 = 11. Bav. ac. kws./1000. -----(3). Here, Bav is average flux density in air gap, ac is ampere conductor per meter of stator periphery, and kws is stator winding factor. Selection of Bav, ac, and kws is done according to machine specifications. The selected values are- Bav = 0.6 T, ac= 34000 ampere conductor per meter and Kws = 0.955. 2.2. Stator conductors and slots: Flux per pole Φm = Bav.( π. D/ P). L; Stator turns per phase, Ts = Es/(4.44. f. Φm. kws); Here Es is per phase voltage of stator side. Numbers of stator slots Ss = 3.P.qs; qs is slots/pole/phase for stator; it is taken equal to 3.25 for small and medium rating synchronous machines.

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Dr. Man Mohan et al. / International Journal of Engineering Science and Technology (IJEST)

Stator conductors per slot Zss = 6.Ts / Ss. Current per phase Is = Q / (3. Es ) = Ifl . Cross sectional area of stator conductor, as = Is / δ. Current density δ is taken equal to 4 A/mm2. as = (a x b). From standard wire table suitable values of conductor dimensions (a and b) are selected. 2.3. Stator slot dimensions: An open slot of stator is shown in Figure-4. Wss = (b+7) mm, dss = (4a +14) mm.

2.4. Stator core: Depth of stator core (dcs) is shown in Figure-5. Cross sectional area of stator core, Acs = (Φm/2)/ Bcs. Here, Bcs is flux density in stator core, which is taken about 1.2 Tesla. dcs = Acs/L; Outer dia of stator, Do = (D+2.dss+2.dcs). 2.5.Air gap Length of air gap, lg = [0.8.(ATfo)./{800000. (Bav/0.74). Kg}]; Here, No load field mmf, ATfo=(2.7 Ts. Is. kws/P); Gap contraction factor Kg=1.15, for open slots. 2.6.Poles Flux in pole body, Φp = 1.2 Φm; Taking flux density in pole body,Bp=1.6 T. Cross-sectional area of pole body Ap = Φp/Bp. Width of pole body, bp = Ap/L. Depth of pole yoke, dy= Φm/(2.4. L). 2.7.Damper bars: Cross-sectional area of damper bars, Ad = 0.2. ac. ( π. D/ P)/ δb. Current density in damper bars, δb=4 A/mm2.

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2.8. No-load loss: Iron loss in stator teeth= specific iron loss x mass of stator teeth. Iron loss in stator core = specific iron loss x mass of stator core. Total iron loss = Iron loss in stator teeth + Iron loss in stator core; Total No load loss = Total iron loss + Friction and windage loss; 2.9. Copper loss: Load loss = stator copper loss + Field winding copper loss; On basis of physical dimensions, resistances of stator and field winding conductors are calculated, and then copper losses are calculated. 2.10 Efficiency(η): η = output/(output+Noload loss+copper loss); 2.11.StatorTempererature rise: Loss dissipation from back of core, L1= (π.Do.L)/C1; Loss dissipation from inner surface of stator, L2=(π.D.L)/C2; Loss dissipation from from sides, L3= (π/4).(Do2 – D2)/C3; C1,C2,C3 are cooling coefficients. Stator Temperature rise, θm = Total stator losses/(L1+L2+L3). 3.Resuts and Discussion Machine Specifications: 3000KVA, 6.6KV, 3-phase, 50 Hz, star connected, 187.5 rpm, salient pole synchronous generator. On basis of specifications, main dimensions of stator are calculated. After obtaining main dimensions, turns per phase, number of stator slots and conductors per slot are calculated. On basis of per phase stator current, conductor size is calculated. After obtaining conductor size and number of conductors per slot, physical dimensions of a stator slot is determined. Stator winding resistance is calculated on basis of physical dimensions of stator conductors and then stator copper loss is calculated. Masses of stator teeth and yoke are calculated on basis of their physical dimensions; further iron losses are determined by multiplying masses with specific iron loss. After obtaining dimensions of field poles and pole body, dimensions of field winding are calculated; further losses in field system are calculated. Finally, no load loss, copper loss, efficiency and temperature rise are calculated. Above simulation is done in MATLAB-7.5; obtained results are shown in Table-1. 4.Conclusion The proposed synchronous machine with amorphous core, has higher efficiency and less temperature rise, compared to conventional synchronous machine. Acknoledgements Authors are thankful to leader in the area of electrical machines- Prof. R.C. Goyal, Prof. D.R. Kohli, Prof. V.K. Varma, Prof. Bhim Singh, Prof. S.P. Srivastava, Prof. D.A. Rao, Prof.D.K. Chaturvedi, friends and family members. References [1] [2] [3] [4] [5] [6] [7]

Alger P.L., (1965) The nature of poly phase induction machines, Gorden and Breach Science publishers, New York. Boldea I. and Nasar S. A., (2010), The induction machine Design handbook, second edition, CRC press. Boyd E.L. and Borst J.D. (1984) Design concepts for an amorphous metal distribution transformer, IEEE Trans. Power Apparatus and Systems,vol.103,no.11,pp.3365-3372. Juha Pyrhonen, Tapani Jokinen and Valeria Hrabovcova (2008) Design of Rotating Electrical Machines, John Wiley & Sons. Manmohan (2012) An overview on amorphous core transformers, Journal of Emerging trends in Engineering and applied sciences (JETEAS), Issue-April,vol.2,No.3, pp. 217-220 Sawhney A.K. (2009), A Course in Electrical Machine Design, Dhanpat Rai Publication, India. Say M.G. (1970), Performance and design of AC machines: Pitman, London.

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Table-1 Description

Synchronous

Machine

Synchronous Machine

with grain oriented steel

with amorphous core

(conventional)

(proposed)

Dia of stator bore (D); meters

3.2

3.2

Length of stator (L); meters

0.44

0.44

Outer dia (Do); meters

3.49

3.49

Numers of stator slots

312

312

4

4

Area cross-section of stator conductor (as); mm

65

65

Size of stator conductor (a,b);mm

5, 13

5, 13

Depth of stator slot(dss); mm

34

34

Width of stator slot (Wss); mm

20

20

Minimum tooth width of stator; mm

12.2

6.1+6.1

Depth of stator core (dcs); mm

112

6.1+105.9

No load field mmf; A

5761

5248

Full load field mmf; A

9200

8552

Numbers of field poles

32

32

Field winding turns per pole

42

40

Total iron loss; kw

46

24

Stator copper loss; kw

31.2

31.2

Friction and windage loss; kw

21

21

Field copper loss; kw

26.3

25

Total losses; kw

124.5

101.2

95%

96%

49

39

Stator conductors per slot 2

Efficiency at power factor 0.8 lagging o

Stator temperature rise; C

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