Synchronous motors are started using several reduced voltage methods. The most common is starting across the line with full ac voltage to the armature windings. The squirrel cage windings begin the task of accelerating the motor from zero speed. As the motor speed increases, the discharge resistor provides the torque required for the motor to reach synchronous speed. When the synchronous speed is reached, the starting resistor is switched out of the field circuit and excitation can be applied to lock the stator and field poles into synchronism.
It is important to properly time the application of excitation to the main field. The purpose of the dc excitation system is to apply current to the field winding, creating a rotating electromagnet field that couples the rotor field to the rotating ac field in the armature winding when the motor is operating at synchronous speed. When dc excitation is applied to the motor field, the position of the rotor with respect to the stator magnetic field determines the reaction of the rotor.
If the N and S rotor and stator poles are aligned, such that the magnetic flux lines flow easily from the rotor through theairgap, the rotor flux will lock in step with the stator flux and the motor will become synchronous.
If the rotor poles are 180 electrical degrees out of phase with the stator poles, but motor acceleration is decreasing the angle of displacement, it is likely that accelerating torque plus magnetic attraction will combine to draw the rotor rapidly into pole alignment with the stator. Synchronizing additionally depends on the slip frequency between rotor and stator. Synchronizing torque from the magnetic linkage of rotor and stator must be sufficient to accelerate the rotor to keep it locked in step.
Another approach to motor startup is switching out the starting resistor and applying excitation based upon time after the motor ac supply power is applied. Here, a dc contactor closes applying excitation to the field after a fixed time. This approach can be used if the acceleration time of the motor is known and the motor is able to reach nearly synchronous speed without excitation.
In some applications, a speed signal is used to apply excitation when the motor has accelerated
to 92 - 95% of rated speed. The precise timing for switching out the starting resistor and applying dc to the main field is monitored by electronics on the rotating field.
The most straightforward approach to synchronous motor starting is to monitor the frequency of the voltage across the field starting resistor, as the motor nears synchronous speed, the slip frequency approaches zero across the resistor. At a specific slip frequency and rotor angle, dc is applied to the main field and the starting resistor is switched out to provide a very smooth transition from “starting” to synchronous operation. Application of the field can be most reliably performed using solid state devices instead of mechanical breakers or contactors. Most brushless excitation systems include such a scheme; similar devices can be used with brush-type systems to perform the same function.
It is important to properly time the application of excitation to the main field. The purpose of the dc excitation system is to apply current to the field winding, creating a rotating electromagnet field that couples the rotor field to the rotating ac field in the armature winding when the motor is operating at synchronous speed. When dc excitation is applied to the motor field, the position of the rotor with respect to the stator magnetic field determines the reaction of the rotor.
If the N and S rotor and stator poles are aligned, such that the magnetic flux lines flow easily from the rotor through theairgap, the rotor flux will lock in step with the stator flux and the motor will become synchronous.
If the rotor poles are 180 electrical degrees out of phase with the stator poles, but motor acceleration is decreasing the angle of displacement, it is likely that accelerating torque plus magnetic attraction will combine to draw the rotor rapidly into pole alignment with the stator. Synchronizing additionally depends on the slip frequency between rotor and stator. Synchronizing torque from the magnetic linkage of rotor and stator must be sufficient to accelerate the rotor to keep it locked in step.
Another approach to motor startup is switching out the starting resistor and applying excitation based upon time after the motor ac supply power is applied. Here, a dc contactor closes applying excitation to the field after a fixed time. This approach can be used if the acceleration time of the motor is known and the motor is able to reach nearly synchronous speed without excitation.
In some applications, a speed signal is used to apply excitation when the motor has accelerated
to 92 - 95% of rated speed. The precise timing for switching out the starting resistor and applying dc to the main field is monitored by electronics on the rotating field.
The most straightforward approach to synchronous motor starting is to monitor the frequency of the voltage across the field starting resistor, as the motor nears synchronous speed, the slip frequency approaches zero across the resistor. At a specific slip frequency and rotor angle, dc is applied to the main field and the starting resistor is switched out to provide a very smooth transition from “starting” to synchronous operation. Application of the field can be most reliably performed using solid state devices instead of mechanical breakers or contactors. Most brushless excitation systems include such a scheme; similar devices can be used with brush-type systems to perform the same function.
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