Table of Contents
Advertisement
3 SYNCHRONOUS MOTOR APPLICATIONS
3.2 COLLECTOR-RING MOTORS
Control functions for starting the synchronous motor include the following:
•
Applying power to the stator; at full voltage or reduced voltage.
•
Shunting the field with a discharge resistor (FDRS).
•
Sensing rotor speed.
•
Sensing rotor angle.
•
Applying excitation at optimum speed and angle.
•
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 with a field contactor. Optimum appli-
cation of excitation (that is, closing the field contactor) requires accurate sensing of motor speed and rotor
angle. This SPM provides this function. Optimum speed for pull-in varies with motor design and with the field
discharge resistor value. Adjustment of the control to apply field at various values of motor speed is important.
The 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 rotor flux (see Figure 3–3:: TYPI-
CAL ROTOR FLUX AND CURRENT AT PULL-IN on page 3–2). Maximum utilization of motor pull-in capability
depends 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 the frequency of the voltage across FDRS. Voltage across FDRS is not actually "induced field volt-
age," 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.
The SPM detects the proper rotor speed (PRS) and rotor angle (PRA) signal, implemented in the Field Pro-
grammable Gate Array (FPGA). Outputs from the PRS and the circuits are used to determine the proper time
to close the Field Application Relay (FAR), based on the percent synchronous slip setpoint. When the proper
rotor speed and the proper rotor angle conditions are met as determined by the FPGA, the CPU delivers a sig-
nal 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 2–4: PANEL AND TERMINAL LAY-
OUT on page 2–5 for details. The speed at which the motor is to synchronize (PRS) can be programmed from
90 to 99.5% of synchronous speed.
GE Multilin
Courtesy of store.ips.us
SPM Synchronous Motor Protection and Control
3.2 COLLECTOR-RING MOTORS
3.2.1 STARTING AND SYNCHRONIZING
3
3-
5
Advertisement
Table of Contents
