1 Introduction
High-voltage synchronous motors are widely used in high-voltage, high-power electrical drive applications, such as high-power fans, water pumps, and oil pumps, due to their advantages such as high power factor, stable operating speed, and simple low-speed design. For high-power, low-speed loads, such as mills and reciprocating compressors, using multi-pole synchronous motors can not only improve the system power factor but also eliminate the need for speed change mechanisms, such as gearboxes, reducing system failure rates and simplifying system maintenance.
Due to the complexity of the physical processes and the difficulty of controlling synchronous motors, traditional high-voltage synchronous motor speed control systems had to be equipped with speed/position sensors, which increased the failure rate and reduced the reliability of the system.
Unit-series multilevel frequency converters have been widely used in the field of high-voltage, high-capacity asynchronous motor frequency conversion speed regulation due to their advantages such as low cost, high grid-side power factor, low grid-side current harmonics, sinusoidal output voltage waveform with virtually no distortion, and high reliability. Applying unit-series multilevel frequency converters to synchronous motors will effectively improve the reliability of synchronous motor frequency conversion speed regulation systems, reduce the cost of synchronous motor frequency conversion retrofits, and enhance the benefits of energy-saving retrofits. It also opens up a broad new market for unit-series multilevel frequency converters. Through extensive theoretical analysis, computer simulation, and physical system experiments, the technical personnel of Leadway have solved key problems such as synchronous motor starting and synchronizing. At the end of April 2006, they successfully applied unit-series multilevel high-voltage frequency converters to a 1000kW/6kV synchronous motor at the Juhua Group's synthetic ammonia plant. The main technical issues in practical applications will be briefly introduced below.
2. Power Frequency Starting and Excitation Process of Synchronous Motors
To better illustrate the operating characteristics of synchronous motors, a brief introduction to the power frequency starting and excitation process of synchronous motors will be given first.
When a synchronous motor is directly driven by grid voltage at power frequency, the starting and excitation of the synchronous motor is a relatively complex process. When the high-voltage circuit breaker closes the armature winding of the synchronous motor, it notifies the excitation device of the synchronous motor to prepare for excitation. At this time, the excitation device automatically connects a demagnetizing resistor to the excitation winding of the synchronous motor to prevent high voltage from being induced in the excitation winding, while providing some starting torque during startup. After the armature winding of the synchronous motor is energized, the motor begins to accelerate under the combined action of the starting winding and the excitation winding connected to the demagnetizing resistor. When the speed reaches 95% of the synchronous speed, the excitation device selects an appropriate time to engage excitation based on the induced voltage on the excitation winding, and the motor is pulled into synchronous speed operation. If the synchronous motor has a strong salient pole effect and a low starting load, the synchronous motor may have already entered synchronous operation before the excitation device finds a suitable time to engage excitation. In this case, the excitation device will engage excitation according to the delayed excitation principle, that is, forcibly engaging excitation 15 seconds after the high-voltage circuit breaker closes.
3. Starting and synchronizing process when a frequency converter drives a synchronous motor
When a synchronous motor is driven by a frequency converter , a different starting method is used than the one described above: live starting.
Before the frequency converter outputs voltage to the stator of the synchronous motor, i.e. before starting, 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 main difference between synchronous motors and ordinary asynchronous motors in operation is that, during synchronous motor operation, the angle between the armature voltage vector and the rotor magnetic pole position must be within a certain range; otherwise, the system will lose synchronization. At the beginning of motor startup, this angle is arbitrary and must be controlled within a certain range through a proper synchronizing process before the motor enters a stable synchronous operating state. Therefore, the starting synchronizing problem is a key issue in the operation of a frequency converter driving a synchronous motor.
The starting and synchronizing process of a frequency converter driving a synchronous motor mainly consists of the following steps:
(1) Excitation device activation. The excitation system supplies a certain excitation current to the excitation winding of the synchronous motor, thereby establishing a certain magnetic field on the rotor of the synchronous motor.
(2) The frequency converter applies a certain DC voltage to the armature winding of the synchronous motor, generating a certain stator current. At this time, a certain stator current is generated in the synchronous motor, and a strong magnetic field is established on the stator. The rotor begins to rotate under the action of the electromagnetic force between the stator and the rotor, causing the rotor magnetic poles to gradually move closer to the opposite ends of the stator magnetic poles. At this time, the direction of rotor rotation may be the same as or opposite to the direction of rotation when the motor is running normally.
