DC generator - principle - parts - types - function - e.m.f. equation
- General concept of rotating Power machine In rotating machines, there are two parts, the stator and rotor. Rotating Power machines are also of two types - DC and AC machines. Power machines are widely used. In DC machines the stator is used as a field and the rotor is used as an armature, while reverse is the case for AC machines. That is synchronous generators and synchronous motors. The induction motor is another kind of AC machine, which is singly excited; that is AC supply voltage is only given to the stator and no supply is given to the rotor. In DC machines and synchronous machines, the field is always excited.
- Generator:An Power generator is a machine which converts mechanical energy into Power energy
- Principle of the generator:To facilitate this energy conversion, the generator works on the principle of Faraday's Laws of Electromagnetic Induction.
- Faraday's Laws of Electromagnetic Induction: There are two laws. The first law states
- First law: Whenever the flux linking to a conductor or circuit changes, an emf will be induced. The
- second law states: The magnitude of such induced emf depends upon the rate of change of the flux linkage. :
- Types of emf: According to Faraday's Laws, an emf can be induced, either by the relative movement of the conductor and the magnetic field or by the change of flux linking on a stationary conductor.
- Dynamically induced emf: In case, the induced emf is
due to the movement of the conductor in a stationary
magnetic field as shown in Fig 1a or by the movement of
the magnetic field on a stationary conductor as shown in
Fig 1b, the induced emf is called dynamically induced emf
As shown in Figs 1a & 1b, the conductor cuts the lines of force in both cases to induce an emf, and the presence of the emf could be found by the deflection of the needle of the galvanometer `G'. This principle is used in DC and AC generators to produce electricity.
- Statically induced emf: In case, the induced emf is due to change of flux linkage over a stationary conductor as shown in Fig 2, the emf thus induced is termed as statically induced emf. The coils 1 and 2 shown in Fig 2 are not touching each other, and there is no Power connection between them.
- According to Fig 2, when the battery (DC) supply is used
in coil 1, an emf will be induced in coil 2 only at the time
of closing or opening of the switch S. If the switch is
permanently closed or opened, the flux produced by coil
1 becomes static or zero respectively and no emf will be
induced in coil 2. EMF will be induced only when there is
a change in flux which happens during the closing or
opening of the circuit of coil 1 by the switch in a DC circuit.
Alternatively the battery and switch could be removed and coil 1 can be connected to an AC supply as shown in Fig 2. Then an emf will be induced in coil 2 continuously as long as coil 1 is connected to an AC source which produces alternating magnetic flux in coil 1 and links with coil 2. This principle is used in transformers.
- Production of dynamically induced emf: Whenever a conductor cuts the magnetic flux, a dynamically induced emf is produced in it. This emf causes a current to flow if the circuit of the conductor is closed. For producing dynamically induced emf, the requirements are:
- magnetic field
- conductor
- relative motion between the conductor and the magnetic field. If the conductor moves with a relative velocity 'v' with respect to the field, then the induced emf `E' will be E = BLV Sinθ Volts where ,
Symbol Quantity Unit Description B Magnetic Flux Density Tesla (T) The strength of the magnetic field through which the conductor moves. L Effective Length of the Conductor in the Field Metres (m) The length of the conductor that is within and interacting with the magnetic field. V Relative Velocity Metres/second (m/s) The speed at which the conductor moves relative to the magnetic field. θ Angle Between Conductor and Magnetic Field Degrees (°) or Radians (rad) The angle at which the conductor cuts across the magnetic field lines.
Let us consider Fig 3a in which conductors A to I are placed on the periphery of the armature under magnetic poles. Assume for this particular generator shown in Fig 3a, the value of BLV = 100V.
Accordingly the conductor A induces an emf = BLV Sin θ where θ = zero and Sin zero is equal to zero = 100 x 0 = zero. emf induced in
Conductor B = BLV Sin 30° = 100 x 0.50 = 50 volts.
emf induced in Conductor C = BLV Sin 90° = 100 x 1 = 100 V.
emf induced in Conductor D = BLV Sin 135° = BLV Sin 45° = 100 x 0.707 = 70.7 volts.
emf induced in Conductor E = BLV Sin 180° = Sin 180°= 0 = 100 x 0 = zero.
Likewise for every position of the remaining conductors in the periphery, the emf induced could be calculated. If these values are plotted on a graph, it will represent the sine wave pattern of induced emf in a conductor when it rotates under N and S poles of uniform magnetic field.
As in Fig 3b the emf induced by this process is basically alternating in nature, and this alternating current is converted into direct current in a DC generator by the commutator.
Fleming's right hand rule: The direction of dynamically induced emf can be identified by this rule. Hold the thumb, forefinger and middle finger of the right hand at right angles to each other as shown in Fig 4 such that the forefinger is in the direction of flux and the thumb is in the direction of the motion of the conductor, then the middle finger indicates the direction of emf induced, i.e. towards the observer or away from the observer.
Imagine a conductor moving in between north and south poles in an anticlockwise direction as shown in Fig 5a.
Applying Fleming's right hand rule, we find that the conductor 1 which is moving upwards under the north pole will induce an emf in the direction towards the observer indicated by the dot sign and the conductor 2 which is moving down under the south pole will induce an emf in the direction away from the observer indicated by the plus sign.
Fig 5b indicates the current direction in the form of an arrow. The dot sign indicates the pointed head of the arrow showing the current direction towards the observer and the plus sign indicates the cross-feather of the arrow showing the current direction away from the observer