GE SPM Series, Instruction Manual

Page: 24 / 126

3-
2
SPM Synchronous Motor Protection & Control GE Power Management
3.1 OVERVIEW 3 SYNCHRONOUS MOTOR APPLICATIONS
3
If I
1
is an equivalent current in the stator causing the transformer action, then I
1
will be about 180° from I
2
(or
I
FD
), and the flux will be 90° behind I
FD
. Very significantly, then, the point of maximum-induced flux (Ø) occurs
as the induced field current I
FD
passes through zero from negative to positive; maximum rate of change of cur-
rent. See the figure below.
Figure 3–2: TYPICAL TRANSFORMER ROTOR FLUX AND CURENT (CONSTANT SLIP)
The rotor angle at which I
1
and I
2
go through zero depends upon the reactance-to-resistance ratio in the field
circuit. A very high value of reactance-to-resistance shifts the angle toward –90°. Reactance is high at low
speed (high frequency). At high speed (low slip, low frequency), reactance decreases and the angle shifts
toward 0° if the circuit includes a high value of resistance. As the stator goes beyond –45°, the torque
increases (due to increased stator flux). At this point,
I
FD
yields a convenient indicator of maximum flux and
increasing torque from which excitation is applied for maximum effectiveness.
If the field discharge loop is opened at the point of maximum flux, this flux is "trapped." Applying external exci-
tation in correct polarity to increase this trapped flux at this instant makes maximum use of its existence. At this
point the stator pole has just moved by and is in position to pull the rotor forward into synchronous alignment.
See the figure below.
Figure 3–3: TYPICAL ROTOR FLUX AND CURRENT AT PULL-IN