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SECTION 4 - Description of the CR192 Micro-starting
and Protection Module
4.1 Description of operation
4.1.1 Starting and synchronizing
Control functions for starting the synchronous motor
include the following.
1. Applying power to the stator; at full voltage or at
reduced voltage.
2. Shunting the field with a discharge resistor. (FDRS)
3. Sensing rotor speed.
4. Sensing rotor angle.
5. Applying excitation at optimum speed and angle.
6. Reluctance torque synchronizing.
The first step in starting a synchronous motor is to apply
power to the stator by means of a magnetic contactor or circuit
breaker.
Shunting a resistor around the motor field during starting is
accomplished by a field contactor. Optimum application of
excitation (that is, closing the field contactor) requires accurate
sensing of motor speed and rotor angle. This function is pro-
vided by the CR192 {(SPM. Optimum speed for pull-in will vary
somewhat from one motor design to the next, and with the value
of the field discharge resistor. Adjustment of the control to apply
field at various values of motor speed is important. Correct rotor
angle for field application does not vary, and is always the point
where induced field current passes through zero going from
negative to positive; the point of maximum flux in the rotor. See
Figure 3. Maximum utilization of motor pull-in capability will
depend upon the degree to which the control can accurately
sense speed and rotor angle.
Rotor frequency is the most positive electrical parameter
available for indicating speed, and can be sensed by detecting
frequency of the voltage across FDRS. Voltage across FDRS is
not actually "induced field voltage," but is the voltage which is
essentially in time phase relation to the current through the
resistor. That is, the current goes through zero at the same time
the voltage goes through zero.
Figure 11 is included to illustrate the electronic circuits of
the CR 192 pSPM which detect the proper rotor speed (PRS) and
rotor angle (PRA) signal.
Outputs from the PRS and the circuits are fed into a Central
Processor Unit (CPU) to determine the proper time to close the
Field Application Relay (FAR). When
th~
proper rotor speed and
the proper rotor angle conditions are met as determined by the
microprocessor (CPU), the CPU delivers a signal to the FAR
Relay so it can close its contact FAR1-FAR2. FAR picks up field
contactor FC to apply excitation to the motor field and to open
the field discharge resistor loop. See Figure 10.
Speed at which the motor is to synchronize (PRS) can be
programmed over the range of 90 to 99.5 percent speed. (See
Programming Instruction, Section 8.)
4.1.1.1 Reluctance torque synchronizing
A synchronous motor that is lightly loaded and connected to
low inertia load may pull in to synchronism before the rotor poles
are externally magnetized. This is commonly known as reluc-
tance torque synchronizing. This magnetization can result in
sufficient torque to hold the salient poles in direct alignment with
corresponding stator poles and run the motor at synchronous
speed. When load is applied, however, the rotor will begin to slip
since the torque developed is only a fraction of rated torque
under separate excitation. Furthermore, the rotor is polarized by
the stator flux under this condition and can therefore be polar-
ized in any direct axis alignment; occuring each 180 degrees.
External excitation forces pole-to-pole alignment in only one
orientation of the direct axis.
Should the rotor pull in to synchronism 180 degrees away
from the normal running alignment, external excitation will build
up flux in the rotor in opposition to the stator flux. As the external
excitation builds up, correct alignment of rotor to stator will occur
by slipping one pole and the motor will then run in normal
synchronism.
The Field Application Control must respond in such a way
as to proceed with proper application of excitation in the event
the motor does synchronize on reluctance torque. Figures 12
and 13 show how the pSPM automatically responds to reluc-
tance torque synchronizing.
4.1.2 Starting protection
The amortisseur, or cage winding of a synchronous
motor, is probably the element most susceptible to thermal
damage. Its function is essentially operative only during star-
ting, and there are limitations on space available for its con-
struction onto the rotor. Hence, it is usually made of lighter
material than the cage winding of an induction motor. The
cage is also vulnerable to overheating should the motor be
allowed to run out of synchronism with no excitation. In this
case, it runs as an induction motor at some value of slip
which will produce cage current that develops running tor-
que. However, the cage of a synchronous motor is not
designed for continuous operation. Therefore, an important
protective function of the controller is to prevent overheating
of the cage winding both during starting and running out of
synchronism.
Monitoring the starting condition of a synchronous motor
can be accomplished by looking at the frequency of induced
field current, the same procedure used to accomplish synchro-
nizing. Metor designers always place a limit on the time a
particular motor can be allowed to remain stalled ("allowable
stall time"*). An accelerated schedule can then be established.
• "Allowable stall time" is important for the induction motor also,
but the time is usually shorter for the synchronous motor and
varies from one design to another in terms of greater spread.
9
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