Electrical Machines02
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eind
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How Magnetic Field can affect its surroundings
FARADAY’S LAW – Induced Voltage from a Time-Changing Magnetic Field
Before, we looked at the production of a magnetic field and on its properties. Now, we will look at the
various ways in which an existing magnetic field can affect its surroundings.
1.
Faraday’s Law:
‘If a flux passes through a turn of a coil of wire, voltage will be induced in the turn of the wire that is
directly proportional to the rate of change in the flux with respect of time’
If there is N number of turns in the coil with the same amount of flux flowing through it, hence:
where: N – number of turns of wire in coil.
Note the negative sign at the equation above which is in accordance to Lenz’ Law which states:
‘The direction of the build-up voltage in the coil is as such that if the coils were short circuited, it would
produce current that would cause a flux opposing the original flux change.’
Examine the figure below:
If the flux shown is increasing in strength, then the voltage built up in the coil will tend to
establish a flux that will oppose the increase.
A current flowing as shown in the figure would produce a flux opposing the increase.
So, the voltage on the coil must be built up with the polarity required to drive the current through
the external circuit. So, -eind
NOTE: In Chapman, the minus sign is often left out because the polarity of the resulting voltage
can be determined from physical considerations.
2.
Equation eind = -dφ /dt assumes that exactly the same flux is present in each turn of the
Lecture 5 & 6
Electrical Machines
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coil. This is not true, since there is leakage flux. This equation will give valid answer if the
windings are tightly coupled, so that the vast ma