1. Figure 1 shows a cross-section of a wind turbine salient pole three-phase synchronous generator. It has 8 poles and 48 slots. Figure 1: Cross-Section of a synchronous generator. Answer the following questions: (i) Discuss the advantages and disadvantages of using 8 poles instead of 2 poles. [ 8 ] (ii) The generator is connected to a 50 Hz grid. Calculate its synchronous speed. [ 2 ] Copyright 2021 v01 © University of Southampton Page 2 of 6

1. Figure 1 shows a cross-section of a wind turbine salient pole three-phase synchronous generator. It has 8 poles and 48 slots.

Figure 1: Cross-Section of a synchronous generator.

Answer the following questions:

(i) Discuss the advantages and disadvantages of using 8 poles instead of 2 poles.

[ 8 ]

(ii) The generator is connected to a 50 Hz grid. Calculate its synchronous speed.

[ 2 ]

Copyright 2021 v01 © University of Southampton Page 2 of 6

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(iii) Explain why steel laminations are used to construct the stator and rotor cores.

[ 6 ]

(iv) Explain the reasons for having semi-closed stator slots rather than fully open ones.

[ 2 ]

(v) The stator has a 3-phase fully pitched double layer lap winding. Label the windings with letters indicating the winding pattern over at least two poles of the machine. You can crop Figure 1 to have an enlarged 2 pole part showing large slots that you can label, but a neat sketch will be acceptable.

[ 5 ]

(vi) On a copy of Figure 1 label the rotor field current pattern by drawing dots and crosses inside the polygons representing the field winding or by colouring them. Label the rotor poles with N or S to indicate the polarity of the pole faces as seen on the surface of the rotor. Sketch the flux distribution in the machine (you may use FEA to produce the flux plots but a neat hand sketch is also acceptable).

[ 7 ]

(vii) Explain the reasons for stranding and twisting the stator coil wires. Does this need to be done in the rotor field winding?

[ 4 ]

[Total marks 34]

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2. A synchronous generator as shown Figure 1 is used in a grid-connected 100 kVA wind turbine. The grid frequency is 50 Hz. A 1:3 gear is used to increase the speed of the generator. The stator of the generator has a bore diameter of 500 mm and its active length is 500 mm. The airgap, the clearance gap between the rotor and the stator is 1.5 mm. The field winding of each pole has 250 turns. The stator coils have 1 turn each.

(i) Explain why a speed step-up gear is commonly used in wind turbines rather than coupling the turbine directly to the generator.

[ 4 ]

(ii) Estimate the rotor pole coil field current to produce a peak no load flux density of 0.85 T – state the assumptions made. Suggest a method for exciting the rotor field coils.

[6]

(iii) Estimate the width of the stator teeth and back of core such that the flux density in those parts doesn’t exceed 1.65 T when the peak airgap field flux density in the gap is 0.85 T.

[ 8 ]

(iv) Estimate the peak no-load emf per phase when the gap flux density is 0.85 T.

[ 6 ]

(v) The mass of the stator core laminations is approximately 465 kg. The specific loss in the laminations at 1.5 T and 50 Hz is 3.5 W/kg. The hot stator winding resistance is 5 m .

(a) Estimate the stator core loss. Assume the maximum lamination flux density is 1.65 T.

[ 3 ]

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(b) Estimate the stator copper loss when the machine supplies 90 kW at 0.9 power factor lagging into the grid which has a line to line voltage of 400 V rms.
[ 4 ]

(c) The friction, windage, field winding and exciter losses are about 2 kW. Estimate the efficiency of the machine when it produces 90 kW at 0.9 power factor.

[ 2 ]

[Total marks 33]

The post 1. Figure 1 shows a cross-section of a wind turbine salient pole three-phase synchronous generator. It has 8 poles and 48 slots. Figure 1: Cross-Section of a synchronous generator. Answer the following questions: (i) Discuss the advantages and disadvantages of using 8 poles instead of 2 poles. [ 8 ] (ii) The generator is connected to a 50 Hz grid. Calculate its synchronous speed. [ 2 ] Copyright 2021 v01 © University of Southampton Page 2 of 6 appeared first on My Academic Papers.

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