Before the frequency converter outputs voltage to the stator of the synchronous motor, the excitation device first supplies a certain excitation current to the excitation winding of the synchronous motor, and then the frequency converter outputs an appropriate voltage to the armature winding of the synchronous motor to start the motor.
The angle between the armature voltage vector and the rotor pole position of a synchronous motor must be within a certain range. If it is outside this range, the synchronous motor may lose synchronization. Before the motor starts, the angle is arbitrary, so it must be controlled within this range for the motor to gradually enter a stable synchronous operating state. Based on this phenomenon, we can determine that the biggest obstacle when using a frequency converter with a synchronous motor is the starting and synchronizing phase.
The starting and synchronizing process of a frequency converter driving a synchronous motor mainly consists of the following steps.
(1) Excitation device is activated.
(2) The armature winding of the synchronous motor will be subjected to the DC voltage applied by the frequency converter, which will generate a certain stator current.
(3) The frequency converter slowly rotates the voltage vector applied to the armature winding in the direction of motor rotation. Finally, the synchronous motor enters the synchronous operation state, and the process is completed.
(4) The frequency converter adjusts the output voltage according to the preset acceleration and v/f curve (i.e. magnetic flux setting) to gradually accelerate to the given frequency.
Steady-state operation and excitation regulation during operation of frequency converter-driven synchronous motor
Because frequency converters use a control method that eliminates the need for speed/position sensors when driving synchronous motors, and because the output waveform of a frequency converter is a multi-level PWM waveform, which is the same as the waveform when controlling an asynchronous motor, the frequency converter is similar to a sinusoidal voltage source, with no torque pulsation and high reliability.
Under normal operating conditions, manually adjust the excitation current to minimize the inverter's output current and keep it constant. When adjusting the excitation current in real time, the inverter can measure the reactive power it outputs to the synchronous motor, send excitation command signals to the excitation device, and adjust the excitation current.
