DC Generator 1. THE DIRECTION OF CURRENT DUE TO INDUCED VOLTAGE: UNDERSTANDING FLEMING S RIGHT HAND RULE - The direction of current flow in the conductor is given by Fleming s right hand rule Figure 1: Fleming s right hand rule - The directions are: Thumb indicates: relative motion of conductor Pointing finger indicates: flux Middle finger indicates: current Figure 2: Change in current direction Key point : If the direction of relative motion of the conductor is reversed keeping the flux direction same or if the flux direction is reversed keeping the direction of relative motion of conductor same, the direction of induced emf and hence the direction of current is reversed. Stamford University Bangladesh Page 1
2. THE AMOUNT OF INDUCED EMF - The induced emf in conductor depends on the position of plane of flux direction and plane of conductor s rotation. - Mathematically it is found that if the plane of rotation is parallel to the plane of flux, there will no cut of flux; hence there will be no induced emf. - The flux cutting, and hence induced emf is maximum when the plane of rotation is perpendicular to the plane of flux. - In between these two extremes, for any angle between the plane of rotation and plane of flux, the induced emf depends on the perpendicular projection of rotation to the plane of flux. - The induced emf is given by E= BL(vsinα) Where, B = magnetic field L = active length of the conductor v = relative velocity of the conductor α = angle between the plane of rotation and plane of flux Plane of rotation α Plane of flux Figure 3: Flux induced in conductor Key point: For an induced emf in the conductor, not only the relative motion between conductor and flux is necessary but also the plane of rotation should not be parallel to the plane of flux. 3. OPERATION OF A SINGLE COIL DC GENERATOR - A single coil dc generator is composed of three elements: o Armature: The moving part of a magnetic device consisting of one or more coils that are electrically connected to create the rotatable section of a generator. Stamford University Bangladesh Page 2
o Slip Rings: A slip ring is a method of making an electrical connection through a rotating assembly, commonly found in electrical generators. o Brushes: conducts current between rotating and stationary parts of a generator. Figure 4: Single coil dc generator Figure 5: Operation of a single coil dc generator. Stamford University Bangladesh Page 3
- Initially the conductor is in the position A. In this position it has no induced emf in it, because the plane of motion is parallel to the plane of flux. - Next the rotor starts to rotate clockwise i.e. Leg 1 [darker portion of the conductor] is moving toward the south pole and Leg 2 [lighter portion of the conductor] is moving toward the north pole [see position B]. - As a result Leg 1 is moving downward and Leg 2 is moving upward. - Emf starts to induce in the conductor, hence it causes to current flow in the direction given by Fleming s right hand rule. - The induced emf and consequently the current is maximum when plane of motion is perpendicular to the plane of flux [see position C]. - In the next quarter, the emf starts to reduce [see position D] until the conductor reaches the position E and at this position the induced emf is zero again. - Moving toward the next quarter [see position F] results in such a way that Leg 1 is now moving upward and Leg 2 is in downward i.e. the exact opposite phenomenon is happening to the position B. - According to Fleming s right hand rule now, the current flow in the external circuit is opposite to that of in the first half cycle. - The current in opposite direction becomes maximum at the position G. In next quarter the current reduces [see position H] and become zero after finishing one cycle [see position A]. 4. MAKING THE CURRENT UNIDIRECTIONAL IN THE EXTERNAL CIRCUIT: - The current produced from a dc generator using slip sing is alternating in every cycle in the external circuit. - To make this current unidirectional in the external circuit, slip ring is replaced by split ring. The split ring has two parts. Each part is connected to the corresponding brushes. - The split ring is allowed to rotate at the same velocity of the rotation of conductor while the brush positions are remain unchanged - As a result, after every half cycle the one part of split ring goes in touch of other brush form the first one to whom it did touch in first half cycle. - Consequently, current becomes unidirectional in the external circuit. Stamford University Bangladesh Page 4
Figure 6 : Making current unidirectional References: 1. U. A. Bakshi, V. U. Bakshi, Electrical Technology, 4 th Edition 2009, Technical Publication Pune. 2. whs.wsd.wednet.edu Stamford University Bangladesh Page 5