dc_MACHINES

dc_MACHINES - 1 EE 4420 Electric Machine Analysis Fall 2005...

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1 EE – 4420 Electric Machine Analysis Fall 2005 DIRECT CURRENT MACHINES 1. CONSTRUCTION AND PRINCIPLE OF OPERATION A scheme of a primitive, single coil winding dc machine with two magnetic poles is shown in Fig. 1. The stator has salient poles that are excited by a field winding supplied from the dc source. The field current I f produces the flux Φ f . The rotor consists of a single-turn coil, which is connected to two semicircle copper segments (moveable with the coil), which together with two (stationary) “brushes” constitute the commutator . If the coil rotates the emf e is induced in both sides of the coil with the direction that can be determined by the right-hand rule. After one half of turn the direction of the induced voltage in the coil’s sides changes, but due to the commutator the polarity of the voltage across the brushes remains unchanged. V Φ commutator armature winding field winding + - ω e,i e,i R L + - V f I f I a d q θ Fig. 1 A scheme of the single-turn dc machine The flux distribution in the air gap is symmetrical with respect to the pole axis called direct axis or d-axis (Fig. 2). The brushes are placed along quadrature (or q ) axis , perpendicular to the d-axis . The voltage induced in the coil changes with respect to θ angle as shown in Fig. 3.b. This voltage is rectified be the commutator, so its shape, when measured across the brushes is as shown in Fig. 3.c. When more multi-turn coils are wound on the rotor (Fig. 2), each coil is connected to the copper segment of the commutator (Fig. 2.b) and the rectified voltage across the brushes is changing more smoothly (Fig. 3.d).
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2 (a) I n field winding (stator) armature winding (rotor) S N F + - (direct axis) (quadrature axis) d q (b) commutator segment made of copper brush Fig. 2.a A scheme of the multi-coil dc machine; b - commutator (a) B θ N S brushes
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3 (b) e a t (c) e a t (d) E a t Fig.3.(a) Distribution of the magnetic flux density on the rotor circumference; waveforms of: (b) the voltage induced in the coil, (c) voltage across the brushes – for single-coil armature winding; (d) voltage waveform across the armature terminals for multi-turn armature winding + - + - Fig. 4 Lap-type winding of the armature Fig. 5 Wave-type winding of the armature
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4 The coils are connected in series and there are no end terminals. There are two types of armature winding: Lap winding (Fig. 4), and Wave winding (Fig. 5). If the iron core is removed from the rotor and the winding is stretched out it looks like as shown in Fig. 6. Fig. 7 shows the circuit diagram of the dc machine with both windings: - field winding and - armature winding. + - E Fig. 6 Diagram of the armature winding E V V f I f Φ f Fig. 7 Circuit diagram of the dc machine The induced emf in one side of the coil turn is: e Blv = (1) where: B is the flux density in the air gap,
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5 l is the active length of the coil side, v is the speed of the coil related to the flux density B .
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This note was uploaded on 02/26/2012 for the course EE 4150 taught by Professor Wu during the Fall '10 term at LSU.

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dc_MACHINES - 1 EE 4420 Electric Machine Analysis Fall 2005...

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