To study the variation of speed of a dc shunt motor by) armature control and (b) field control

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OBJECT: To study the variation of speed of a dc shunt motor  I. With armature voltage under constant field excitation , and II. With field excitation under constant armature voltage


INDIVIDUALLY note the complete name-plate data/information of the motor/transformer that is/are required for the experiment and obtain the signature of the teacher before the start of the experiment. It must also be confirmed that the fuse/meter ratings being used are suitable.
Fig. 1 Speed Control of D.C. Shunt Motor

PROCEDURE: 
I. Variation of Speed with Armature Voltage under constant field excitation:
a) Connect the circuit as shown in Fig. 1.
b) Start the motor with maximum external resistance, 40 in the armature circuit and minimum resistance 185 in the field circuit;
c) Decrease the external armature resistance to about zero and then increase the in the field resistance to bring the motor to the rated speed;
d) Keeping the field resistance undisturbed, increase the armature resistance and for each case tabulate Armature Voltage, Va, Speed, N and the Field Current, If;

II. Variation of Speed with Field excitation (current) under constant armature voltage:
a) Return to the status of I.
c) (Same armature and field resistances and rated speed);
b) Keeping the armature resistance undisturbed at about zero, vary the field resistance and tabulate Armature Voltage, Va, Speed, N and Field Current, If;
c) Set and fix armature resistance to a higher value and repeat II.b);       

RESULTS : 
 i) Plot,1: Speed vs Armature Voltage for different sets of constant field current;
ii) Plot,2: Speed vs Field Current for different sets of constant armature voltage. (Plot on graph papers);

DISCUSSION : 
 i) Discuss and analytically explain (with equations)  the nature of the plots;
ii) Draw an equivalent circuit diagram of the DC shunt motor and analytically express the essential relations;
iii) What are the limitations and merits of the two methods of speed control?
iv) Why is the armature resistance set at a maximum, and the field resistance set at minimum at start?
v) What will happen when the field circuit gets open-circuited, while the machine is running?