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CIRCUIT DIAGRAM FOR MAGNETIZATION CHARACTERISITCS OF D.C.SHUNT GENERATOR

Drop Test for R sh :

DROP TEST R SH :

EM1 LAB ,BVCR ,RAJAHMUNDRY Prepared by K. Nagesh & V.D.Neelima



MAGNETIZATION CHARACTERISITCS OF D.C.SHUNT GENERATOR

AIM: To conduct experiment on given D.C Shunt Generator and to draw its open circuit characteristics

curve and also to calculate critical field resistance and critical speed of the given shunt generator.

APPARATUS:

S.No Name of theEquipment

Type Range Qty.

1 Ammeter M.C 0 – 2 A 1No.

2 Voltmeter M.C 0 – 300V 1No.

3 Rheostat Wire

wound

50 Ω/ 5A 1No.

4 Rheostat Wire

wound

360 Ω/ 1.2A 1No.

5 Rheostat Wire

wound

500 Ω/ 1.2A 1No.

6 Tachometer Digital 0 – 10,000 rpm 1No.

THEORY:

Open Circuit characteristics curve is the relation between field current and the no load

induced E.M.F. It resembles the B-H curve of a magnetic material. Due to residual magnetism in the poles

some E.M.F is generated even when I is zero. Hence the curve starts a little way up. The slight curvature at

the lower ends is due to the magnetic inertia. The first part of the curvature is practically straight line. This

is due to the fact that at low flux densities reluctance of the iron path begin negligible. The generated E.M.F

is directly proportional to the field current. At higher flux densities where I is small iron path reluctance

becomes appreciable and relation between E and I, no longer holds good.Critical field resistance is the another important point to be considered to build up voltage in

the generator. The value of the resistance represented by the tangent to the curve is known as “Critical field

resistance” Rc for a given speed.

Due to residual magnetism in the poles, some e.m.f and hence current, would be generated.

This current while passing through the field coils will strengthen the magnetism of the poles. The mutual

reinforcement of e.m.f and flux proceeds on till equilibrium is reached at some point like P. The point lies

on the resistance line on of the field winding. Let R be the resistance of the field winding. Line OA is

drawn such that its slope equals the field winding resistance. O.C.C is plotted from the given data. Then,

tangent is drawn to its initial portion. The slope of this curve gives the critical resistance for the speed atwhich the data was obtained.

PROCEDURE:

1. Make the connections as per the circuit diagram.

2. Give the supply to the motor by closing DPST switch.

3. Start the motor with the help of 3 – Point starter.

4. Adjust the speed of the motor to its rated value by varying either field ( or ) armature

of the generator.5. Measure the voltage generated in the armature of the generator which gives residual

voltage.

6. Closing the DPST switch and measure armature voltage and field current.




7. Gradually change the field rheostat of generator in steps of 0.1A

8. Note down the corresponding readings.

9. Draw the necessary graph.

TABULATION :

S.No. Fielfd Current

Ish ( I )

Induced e.m.f

E0 (v)1

2

3

4

5

6

7

8

9

Tabulation for Drop Test ( Rsh ):-

S.No. Voltage (v) Current( A )

R sh = V/I (Ω)

1

2

34

Average

PRECAUTIONS:

1. Connections should be tight

2. Take the readings without parallax errors.

3. Fuses should be properly rated.

4. The operation of the 3-Point starter should be slow and uniform.

5. Armature rheostat should be connected in maximum position initially.

6. Field rheostat of motor should be in minimum resistance position.

7. Generator field rheostat should be in the maximum position

Model Graph:-

Rc

A

OCC

EoRsh

P

C


B



Q If

Model Calculations:-

Critical Resistance R C = OP/OQ

Critical Speed NC = N1 x BC/AC

RESULT:-The experiment was done on the given D.C. Shunt Generator and its o.c.c. curve is drawn

and also critical field resistance and critical speed are calculated.

VIVA QUESTIONS

1) What is residual magnetism?

2) What is critical resistance?

3) What will happen if the field circuit is opened?

4) Conditions for building up of voltage?

5) What is critical speed?

6) What does tangent line indicates?

7) Occ is curve is plotted between what parameters?


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