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EXPERIMENT NO.11

Understanding the No-Load characteristics (magnetic saturation curve, B-H curve) of

separately excited DC generator on Machines Trainer ED-5300

CCOOMMPPOONNEENNTTIIAALL EEQQUUIIPPMMEENNTTSS

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CCHHAARRTT OOFF MMOODDUULLEE CCOONNFFIIGGUURRAATTIIOONN AANNDD CCOONNNNEECCTTIIOONN

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CCIIRRCCUUIITT DDIIAAGGRRAAMM

TTHHEEOORRYY

The magnetic characteristics, also known as “No load characteristics” or “Open Circuit”

Characteristics are the relation between generated emf (Eo) and field current (If) at a given

speed (N). It illustrates the behavior of the magnetic circuit of the field system. The magnetic

characteristics of a generator are actually the B-H curve of its magnetic circuit. The magnetic

characteristics of all types of generators will be similar if same magnetic material is used.

Field current is obtained from an external d.c. source and it can be varied by a field rheostat.

E.m.f. Equation of a DC generator is:

Eg =

(

)

Where;

Eg = e.m.f of the Generator

N= Armature speed in rpm

Ø= Flux/pole in Weber

Z= Total number of armature conductor

P= Number of poles

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A= Number of parallel paths

For a given generator Z, P and A are constant So,

Eg ∞

If the speed of generator N is held constant then,

Eg ∞

It is obvious that when If is increased from its initial small value, the flux and hence

generated e.m.f. Eg increase directly as current so long as the poles are unsaturated. But as the

flux density increases, the poles become saturated, so a greater increase in If is required to

produce a given increase in voltage.

PPRROOCCEEDDUURREE

Install each module on the practicing board rack and connect the wires according to

circuit diagram.

Check the follows before turning the power “ON”

o VR of field Rheostat is at position of maximum resistance 100(Ω).

o The velocity regulation VR of driving motor is at MIN.

Switch the power supply “ON”.

Switch the power of driving motor “ON”.

Adjust the velocity regulation of driving motor so that the revolution of motor should be

700 (rpm) using digital tachometer.

Turn the field rheostat to clockwise slowly to increase field current If in sequence of

0.1[A], 0.2[A],. . . . . . . . . 1.0[A] and record the generated e.m.f. against each value of If.

Turn the field rheostat back to max value of 100(Ω).

Adjust the velocity regulation of driving motor so that the revolution of motor should be

1000 [rpm] using digital tachometer.

Gradually increase the field current by decreasing Field Rheostat and record the generated

e.m.f. against each value of If.

Repeat the same process for N=1500[rpm]

Tabulate the readings and draw the curve between If and Eo.

Switch “OFF” power supplies and remove all the connections.

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OOBBSSEERRVVAATTIIOONNSS AANNDD CCAALLCCUULLAATTIIOONNSS

RREEVVIIEEWW QQUUEESSTTIIOONNSS::

Q.1 State the difference between separately excited and self-excited dc generators.

Answer:

Field Current (If) A

If=0

If=0.2

If=0.4

If=0.6

If=0.8

If=1.0

If=1.2

If=1.4

If=1.6

If=1.8

N1= 700

[rpm]

Generated

e.m.f.

(Eo) V

N2= 1000

[rpm]

Generated

e.m.f.

(Eo) V

N3=1500

[rpm]

Generated

e.m.f.

(Eo) V

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Q.2 What is magnetic saturation of poles?

Answer:

Q.3 Draw the Graph between generated emf (Eo) and field current (If) for N1,N2 &

N3