Synchronous motors, like synchronous generators, consist of a fixed stator and a field that rotates concentric with the stator. The stator contains armature windings that are electrically connected to the ac supply system while the rotor contains a field winding that is electrically connected to a source of excitation (dc). Since the primary purpose of the field winding is to create a rotating magnetic field, the field winding is wound around “poles” attached to the rotor in a configuration that produces magnetic north and south poles that are 180 electrical degrees apart. During starting of the motor, the field winding is not effectively coupled with the armature windings in the stator, and no net torque is produced in the field when ac power is connected to the stator winding. To produce starting torque, a supplementary winding is provided on the rotor that effectively couples electromagnetically with the armature windings. This winding is a “squirrel cage” arrangement of bars placed across each poleface that are electrically shorted at each end.
The squirrel cage winding on the rotor is formally known as the damper or ammortiseur winding. When ac power is connected to the stator, current is induced in the squirrel cage winding. This results in a net torque that is applied to the rotor. This squirrel cage winding is also used in the induction motor to produce motor torque during starting and running operation. Torque is produced by the electromagnetic interaction of stator and rotor only when a slip speed exists. Thus, the induction motor speed increases from zero until just below synchronous speed as torque decreases with increasing rotor speed.
The synchronous motor then can be started like an induction motor, with the torque on the rotor dependent on the difference between the rotor speed and the frequency of the power being applied to the stator winding. The torque supplied by the squirrel cage is at a maximum when ac power is first supplied to the stator winding, decreases as the rotor accelerates, and approaches zero as the rotor approaches synchronous speed. The absolute value of the accelerating torque is a function of the resistance of the bars in the squirrel cage:
• Higher resistance bars produce higher starting torque (and hotter squirrel cage
windings)
• Lower resistance bars produce lower starting torque with less heat generation.
If the starting torque produced by the squirrel cage winding is not adequate to roll the rotor, the rotor is said to be “locked” and ac power must be quickly removed from the stator windings to avoid overheating both the armature and the squirrel cage windings.
The stator winding is connected to the ac supply system at startup, and the bars of the squirrel cage winding on the rotor produces an accelerating torque on the rotor. The field winding is connected to a field discharge resistor during startup, and no external excitation is applied to the field.
As the rotor accelerates, the field winding is coupled to the stator field via the armature winding. AC current is induced into the field based upon the difference between the frequency of the applied ac voltage to the motor and the frequency associated with the instantaneous speed of the rotor. If the field winding were open circuited during startup, dangerously high voltages could be induced in the field winding, hence a starting resistor is used to limit the voltage seen by the field winding during startup while dissipating the energy induced in the field. The resistance of the starting resistor also affects the synchronizing torque available as the rotor approaches rated speed:
• Synchronizing torque increases with the resistance of the field discharge resistor.
A low resistance produces lower synchronizing torque.
• Insulation limits of the field winding limit the resistor, because field voltage increases
with increasing values of discharge resistor.
Sizing the starting resistor is an “art” that balances starting torque and allowable field voltage during startup. In some applications, the load is removed from the synchronous motor during startup and applied only after the motor has reached stable operation at synchronous speed, in order to reduce torque required to start.
The squirrel cage winding on the rotor is formally known as the damper or ammortiseur winding. When ac power is connected to the stator, current is induced in the squirrel cage winding. This results in a net torque that is applied to the rotor. This squirrel cage winding is also used in the induction motor to produce motor torque during starting and running operation. Torque is produced by the electromagnetic interaction of stator and rotor only when a slip speed exists. Thus, the induction motor speed increases from zero until just below synchronous speed as torque decreases with increasing rotor speed.
The synchronous motor then can be started like an induction motor, with the torque on the rotor dependent on the difference between the rotor speed and the frequency of the power being applied to the stator winding. The torque supplied by the squirrel cage is at a maximum when ac power is first supplied to the stator winding, decreases as the rotor accelerates, and approaches zero as the rotor approaches synchronous speed. The absolute value of the accelerating torque is a function of the resistance of the bars in the squirrel cage:
• Higher resistance bars produce higher starting torque (and hotter squirrel cage
windings)
• Lower resistance bars produce lower starting torque with less heat generation.
If the starting torque produced by the squirrel cage winding is not adequate to roll the rotor, the rotor is said to be “locked” and ac power must be quickly removed from the stator windings to avoid overheating both the armature and the squirrel cage windings.
The stator winding is connected to the ac supply system at startup, and the bars of the squirrel cage winding on the rotor produces an accelerating torque on the rotor. The field winding is connected to a field discharge resistor during startup, and no external excitation is applied to the field.
As the rotor accelerates, the field winding is coupled to the stator field via the armature winding. AC current is induced into the field based upon the difference between the frequency of the applied ac voltage to the motor and the frequency associated with the instantaneous speed of the rotor. If the field winding were open circuited during startup, dangerously high voltages could be induced in the field winding, hence a starting resistor is used to limit the voltage seen by the field winding during startup while dissipating the energy induced in the field. The resistance of the starting resistor also affects the synchronizing torque available as the rotor approaches rated speed:
• Synchronizing torque increases with the resistance of the field discharge resistor.
A low resistance produces lower synchronizing torque.
• Insulation limits of the field winding limit the resistor, because field voltage increases
with increasing values of discharge resistor.
Sizing the starting resistor is an “art” that balances starting torque and allowable field voltage during startup. In some applications, the load is removed from the synchronous motor during startup and applied only after the motor has reached stable operation at synchronous speed, in order to reduce torque required to start.
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