Construction of Power Circle Diagram

Construction of Circle Diagram
Conduct No load test and blocked rotor test
on the induction motor and find out the per phase
values of no load current I0, short circuit current ISC
and the corresponding phase angles Φ0 and ΦSC.
Also find short circuit current ISN corresponding to
normal supply voltage. With this data, the circle
diagram can be drawn as follows.
1. With suitable scale, raw vector OA with
length corresponding to I0 at an angle Φ0 from
the vertical axis. Draw a horizontal line AB.
2. Draw OS equal to ISN at an angle ΦSC and join
AS.
3. Draw the perpendicular bisector to AS to meet

the horizontal line AB at C.
4. With C as centre, draw a portion of circle
passing through A and S. This forms the circle
diagram which is the locus of the input
current.
5. From point S, draw a vertical line SL to meet
the line AB.
6. Divide SL at point K so that SK : KL = rotor
resistance : stator resistance.
7. For a given operating point P, draw a vertical
line PEFGD as shown.
then PE = output power, EF = rotor copper
loss, FG = stator copper loss, GD = constant
loss (iron loss + mechanical loss)
8. To find the operating points corresponding to
maximum power and maximum torque, draw
tangents to the circle diagram parallel to the
output line and torque line respectively. The
points at which these tangents touch the circle
are respectively the maximum power point
and maximum torque point.