Transcript Interpreting the Circle Diagram ECE 441 1

Interpreting the Circle Diagram
ECE 441
1
Graphical Interpretation
• The blocked rotor
voltage, EBR, is
assumed constant.
• Pgap=EBRIrcosθr is
proportional to Ircosθr.
• Ircosθr is the projection
of the current phasor
on the horizontal axis.
ECE 441
2
What happens when the load
on the motor is increased?
• Start with the motor
partially loaded and
operating with a slip
of 0.05.
• Increasing the shaft
load will slow the
motor down,
increasing the slip.
• The magnitude and
phase of the rotor
current increase.
ECE 441
3
Graphical Interpretation continued
• The current phasor
elongates and rotates
clockwise to a
position that results in
an Ircosθr sufficient to
carry the load.
• If the load continues
to increase, a
“breakdown” point will
be reached where
Ircosθr decreases!
ECE 441
4