EFFECT OF AIR GAP IN MAGNETIC CIRCUIT
OBJECT:-
To study the effect of air gap on magnetizing current at constant flux density.
APPARATUS:-
Sr No. Name of components Range No. Reqd. Remark
1 A choke with variable air gap
2 Single phase variac
3 Voltmeter
4 Ammeter
THEORY:-
The magnetic circuit has exact analogy with the electrical circuit. The electromotive force (e.m.f.) corresponds to (m.m.f.) magneto motive force. He current corresponds to the flux (?) and the resistance corresponds to the reluctance (s) and it is the product of current and number of turns in coil.
? = m.m.f flux = m.m.f i.e. m.m.f = NI amp turns or {AT}
S reluctance
DEFINATION:-
M.M.F = The magnet pressure which sets up or tends to set up magnetic flux in a magnetic circuit is called magneto motive force
OR
The work done in moving a unit magnetic pole (1 webber) once round the magnetic circuit is called magneto motive force.
i.e. m.m.f = NI or AT
The flux is measured in Webber or mili Webber
The reluctance is given by the formula
S = L
�a
Where,
L= Length of magnetic path
A = Area of cross section
� = Permeability of the material
S = l/a�0.�r
� = �0.�r
WHERE:- �0 = Permeability of free space of air
�r = Relative permeability of the material
S = The opposition offered to magnetic flux by magnetic circuit is called S.
Magnetic flux (?) = the amount of magnetic line of force set up in magnetic circuit is called magnetic flux. Unit (webber) it is analogous to electric current, ampere in electric circuit.
Permeability of iron is very high compared to air. Hence its reluctance is low compared to air. For producing same flux density air gap, m.m.f required is more than that for iron.
In our experiment a � 3 limb variable air gap choke is taken. On each limb of oil is found, the coil on the central limb is connected to a variable supply other two coils are connected in series and a voltmeter is connected across it iron core consists of two parts. Lower parts are fixed while the upper parts can be moved and hence an air gap can be created and change in the magnetic circuit of the choke.
When current is passed through the central coil, flux is established in magnetic circuit and it links with other two coils as a result emf is induced in them.
When the air gap increases the reluctance increases. So flux decreases and emf also decreases, the magnetizing current should be increased. Then it can be seen that to maintain constant flux density more magnetizing current is required with the increase in air gap.
PROCEDURE:-
1. Set up the connections as per the circuit diagram
2. At zero air gap gradually. Increase the magnetizing current with the help of variac till the emf = 40 V
3. Increase the air gap step by step and maintain induced emf constantly by variac by increasing magnetizing current
4. Note down length of air gap, ammeter and voltmeter readings
5. Draw graph of magnetizing current V/S air gap in amp
OBSERVATION:-
Voltage V = ----------------------- volts constant
SR.NO AIR GAP IN mm MAGNETISING CURRENT IN AMPS [1 M]
1
2
3
4
5
CONCLUSION:-