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GEH-5201, Synchronous-motor Control
SECTION 2 - Starting Synchronous Motors
very-light loads in synchronism without external excitation.
Reluctance torque can also pull the motor into step if it is
lightly loaded and coupled to low inertia.
It is convenient to make an analogy of a synchronous motor
to a current transformer for the purpose of demonstrating
angular relationship of field current and flux with rotor position.
If 11
*
is an equivalent current in the stator causing the
transformer action, then 11 will be about 180. degrees fn;>m 12 (or
I FD ), and the flux will be 90. degrees behind I FD . Very significantly,
then, the pOint of maximum-induced flux (0) occurs as the
induced field current IFD passes through zero going from
negative to positive; maximum rate of change of current. See
Figure 2.
The rotor angle at which 11 and 12 go through zero will
depend upon the ratio of reactance to resistance in the field
circuit. A very high value of reactance to resistance will shift the
angle toward minus 90. degrees (-90.°). Reactance is high at low
speed (high frequency). At high speed (lOW slip, low frequency),
reactance decreases and the angle will shift toward 0. if the
circuit includes a high value of resistance. As the stator goes
beyond minus 45 degrees (-45°), the torque increases (essen-
Electrical
Degrees
Rotor Angle
135
45
90.
0.
(-45)
225
135
180.
Figure 2. Transformer action of rotor flux and current
(constant slip) for
a
typical motor
tially due to increased stator flux). At this point IFD yields a very
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 amperes to the
current path in correct polarity to increase this trapped flux at
this instant makes maximum use of its existence. At this point
*11 is not an actual current. The transformer action is due to
stator flux (not shown) cutting the rotor winding.
4
Electrical
Degrees
Rotor Angle
Trapped Flux
135
45
90.
(-45) Pull-in
315 .-"'-..
0.
270. 0.
360.
Applied IFD
Field Discharge
Loop Opened
IFD
(Induced)
Figure 3. Rotor flux and current at pull-in (typical design)
the stator pole has just moved by and is in position to pull the
rotor forward into synchronous alignment. See Figure 3.
Approximate Sta-
tor Position For
-1
Rotation Maximum Field
i-
-45?..
R
Pole Flux (Depend-
N
\
~
ing Upon Design &
~+-+---'r_s
-y
-90.
0
RDIS)
RDIS
N4-
Approximate Sta-
tor Position For
Maximum Torque
When Maximum
Rotor Flux Occurs
At -45
0
Note: Angular displacement of rotor with
respect to stator is given a negative
sign to indicate the motoring con-
dition.
Figure 4. Angular displacement of rotor
It has been established that salient-pole torque near
synchronous speed is a function of both slip and field-discharge
resistance. Figure 6 shows the combined effects of cage torque
and salient pole torque for a typical motor. Figure 5 shows the
effect of a higher value of discharge resistance on a medium-
torque motor. Obviously, without salient-pole torque the motor
would cease to accelerate certain loads at some point on the
speed axis.
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