RIEFING What two things determined the polarity of the generated voltage? y is i
ID: 2249876 • Letter: R
Question
RIEFING What two things determined the polarity of the generated voltage? y is it important that the current in the generator produce a magnetic field having the same direction as the residual magnetism? If you reversed the motor's direction of rotation, why would you have to reverse the field connections of the generator? Why doesn't a self excited generator start producing full rated voltage as soon as the armature starts to turn? What effect does "flashing the field have on the direction of the residual magnetic field? 3-6Explanation / Answer
1. The two things which determine the polarity of induced voltage are:
a. Direction of rotation of conductor
b. Direction of the magnetic lines of force.
2. If the field current produces a magnetic field which is in the same direction of the residual magnetic field in the poles, there will be an increase in the strength of the field. This helps in an easy build up of voltage. However if the field current produces a current which produces a magnetic field such that it opposes the residual magnetic field, the net field strength will reduce and the generator will not be able to build up the voltage.
3. If the direction of rotation of prime mover is reversed, the direction of rotation of generator reverses and hence the polarity of voltage developed due to residual magnetic field reverses. This leads to a current in the field winding which opposes the flow of field current since the field terminals are not reversed. Hence the generator fails to build up voltage. When both the direction of rotation and field terminals are reversed, the current due to residual magnetic field and the field current are in the same direction hence both of them aid together and generator is able to build up the voltage. Hence if the prime mover rotation is reversed, the field connections also needs to be reversed.
4. As the field winding is connevted in parallel to the armature of self excited generator, the current in the field winding is also due to the emf produced by the generator. There is no seperate excitation given to the field winding. Hence as the armature of generator starts to rotate, there is a small voltage developed due to residual magnetic field which develops a small current through the field winding. If the flux produced by the field winding aids the flux due to residual magnetic field, the net mmf increases and hence the voltage across the generator also increases. This increase in the voltage furthee increases the current through the field winding and this process continues until rated voltage is attained. Hence there is a incremental increase in the terminal voltage of a self excited generator.
5. In case the genrator fails to build up voltage, flashing the field helps to reverse the direction of magnetic field produced by the residual flux so that the polarity of terminal of the generator remains the same and the generator will be able to build up voltage again due to reversal of magnetic field due to residual flux. In case of field flashing, the direction of rotation of armature and the output polarity remains unchanged. Only the polarity of residual magnetic field reverses.
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