Application Analysis of High Voltage Frequency Converter in Synchronous Motor

High voltage timing has a large number of applications in high voltage and high power drives due to its high power factor, stable operating speed and simple design at low speed, such as high power fans, water pumps, oil pumps, etc. For large power low-speed loads, such as crushers, reciprocating compressors, etc., the use of multi-poles can not only improve the power factor of the system, but also save the gear shift mechanism, such as a gearbox, reduce the failure rate of the system and simplify the maintenance of the system.

Due to the complex physical process and difficult control of synchronous motors, the speed/position must be installed in the previous high-voltage synchronous motor speed control system, which increases the failure rate and reduces thesystem reliability.

The multi-level type of the unit series has the characteristics of low cost, high grid-side power factor, small grid-side current harmonics, d-shape sine wave output voltage,Torsion and high reliability. The field of motor frequency conversion speed regulation has reached a very wide range of applications. The application of unit series multi-level inverters to synchronous motors will effectively improve the reliability of synchronous motor frequency conversion speed control systems, reduce the cost of synchronous motor frequency conversion transformation, and improve the benefits of energy-saving transformation. Flat inverters open up a vast new market. After a large number of theoretical analysis, computer simulations and physical system experiments, Lide Huafu technicians solved the problemskeys such as synchronous motor start full stage and successfully applied the multi-level high voltage inverter of the unit series at the end. of April, 2006. It is used in 1000kw/6kv synchronous motor of synthetic ammonia factory of Juhua Co., Ltd. The following will briefly introduce the main technical issues in practical application.

1. Power frequency synchronous motor starting and excitation process

In order to better explain the working characteristics of synchronous motor, first briefly introduce the starting process and industrial frequency excitation of the synchronous motor.

When the grid voltage directly drives the synchronous motor to operate at power frequency, the starting and energizing of the synchronous motor is a relatively complicated process. When the armature winding of the synchronous motor is closed at high voltage, the exciter devicetion of the synchronous motor is warned via the high voltage auxiliary contact to prepare for excitation. At this time, the excitation device automatically connects a degaussing resistor to the synchronous motor excitation winding to prevent high voltage from being induced on the excitation winding, and at the same time supply part of the starting torque when starting. After the armature winding of the synchronous motor is energized, the motor begins to accelerate under the joint action of the start winding and the field winding connected to the demagnetizing resistor. When the speed reaches 95% of synchronous speed, the excitation device selects an appropriate time to start excitation according to the voltage induced on the excitation winding, and the motor is driven into synchronous speed operation . If the salient effect of the synchronous motor is strong and the load ofstart is weak, the synchronous motor has already entered the synchronous working state before the excitation device finds a suitable excitation synchronization. In this case, the excitation device will carry out excitation according to the principle of delayed excitation, that is to say, it will force excitation 15s after the high voltage has been turned off.

2. The whole process of starting when the frequency converter drives the synchronous motor

When using the frequency converter to drive the synchronous motor, use a method dDifferent from the above method: start with excitation.

Before the inverter sends 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, then the inverter supplies the armature winding of the synchronous motor. To go outthe correct voltage to start the motor.

The main difference between synchronous motor and ordinary asynchronous motor is that when the synchronous motor is running, the angle between the armature voltage vector and the position of the magnetic pole of the rotor should be within a certain range, otherwise it will cause system lag. At the start of the motor start, the angle between the two is arbitrary, and the angle should be controlled within a certain range through an appropriate comprehensive process, then the motor enters a stable synchronous working state. Therefore, the problem of starting the whole stage is the key problem of the operation of the synchronous motor driven by the frequency converter.

The whole process of starting a synchronous motor driven by a frequency converter is mainly divided into the following steps:

(1) Excitation of the excitation device. The excitation system transmits acertain excitation current to the excitation winding of the synchronous motor and establishes a certain magnetic field on the rotor of the synchronous motor.

(2) The inverter applies a certain DC voltage to the armature winding of the synchronous motor to generate a certain stator current. At this time, a certain stator current is generated on the synchronous motor and a strong magnetic field is established on the stator. The rotor starts rotating under the action of the electromagnetic force between the stator and the rotor, so that the rotor poles gradually approach the opposite end of the stator poles. At this time, the direction of rotation of the rotor may be the same as that of the motor in normal operation, or it may be opposite.

(3) The inverter slowly rotates the voltage vector applied to the armature winding in accordance with the direction of rotation of the motor during normal operation. With the rotation of the rotor of the synchronous motorand the rotation of the magnetic field of the stator, the poles of the rotor will pass over the opposite poles of the stator at some time, or the poles of the rotor will accelerate to catch up with the poles of the rotating stator. At this time, the motor rotor magnetic poles are reliably attracted to the stronger stator magnetic poles, and the angle between the two gradually tends to a smaller constant after a small amount of vibration with damping. So far, the synchronous motor enters the synchronous working state, and the whole step process is completed.

(4) The frequency converter adjusts the output voltage according to the preset acceleration and the v/f curve (i.e. the magnetic flux value), and gradually accelerates to at the given frequency. At this time, the rotor angle of the synchronous motor gradually increases to a constant value, and then the poles of the motor rotor are gradually aaccelerated to the desired speed under the attraction of the magnetic field of the stator, and the starting process of the synchronous motor is completed.

In the full-step starting process of a synchronous motor, the selection of the magnetic potential of the stator and rotor and switching between steps are the main control issues. If the magnetic field of the stator is too weak, the stator poles cannot be reliably absorbed during the first pass of the opposite poles of the rotor. After that, the rotor experiences the reverse acceleration of the repulsive force between the same poles. When passing the stator poles next time, the two will be with greater relative speed, the magnetic field of the stator is more unable to effectively pull the rotor poles, which will eventually lead to the failure of the whole step starting. Selecting too large a stator magnetic field can cause the stator core to saturater of the synchronous motor, which will further cause the inverter output overcurrent and the motor will not start.

3. The steady state operation of the synchronous motor driven by the frequency converter and the excitation adjustment during operation

Since the synchronous motor driven by the frequency converter frequency converter uses a control method that does not need to install a speed/position sensor, and the frequency converter output waveform is a multi-level pwm waveform, which is the same than the waveform when controlling an asynchronous motor. Therefore, during operation, the frequency converter can be completely equivalent to a sinusoidal voltage source without torque ripple and has high reliability.

Since the reactive current of the synchronous motor only flows between the motor and the frequency converter and does not enter the mainswater, it is not necessary to control the motor excitation current. Generally, under the typical motor operating conditions, the excitation current can be manually adjusted to reduce the output current of the inverter, and the output power factor is about 1, then the current can be maintained unchanged during the first speed regulation operation. For working conditions that require real-time adjustment of excitation current during operation, the frequency converter can actually measure the output reactive power to the synchronous motor, send a given excitation signal to the excitation device and adjust the excitation current.

4. Synchronous motor fault degaussing

During normal stop, the inverter first drives the synchronous motor to decelerate to the stop speed, and then stops supplying output voltage to the motor armature winding. At this viHowever, the voltage induced by the excitation current on the stator side of the synchronous motor is lower than the long-term withstand voltage on the output side of the inverter. Therefore, during the process of free slip after the motor, maintaining the excitation current will not cause damage to the equipment and will not damage the equipment. Instant degaussing is required.

In the event of a fault, if only the power supply to its armature winding is stopped and its excitation current is maintained, the rotating synchronous motor will constantly send a three-phase alternating voltage to the stator side, this which will endanger the safety of the equipment, and may cause the expansion of the accident. Therefore, when a serious fault needs to be stopped, the frequency converter must request the excitation device to de-energize.

The physical process of synchronous motor decay is as follows:

At the start of decay, the necksynchronous motor excitation rate drops rapidly under the action of device excitation, but due to the main magnetic field of the synchronous motor If there is no sudden change in the pass, a large current is induced on the snubber winding (start winding). At this time, the rotary synchronous motor sends a high three-phase alternating voltage to its stator end (i.e. to the output end of the frequency converter). Subsequently, the current on the snubber winding gradually decreases to zero on the internal resistance of the snubber winding, and the stator voltage of the synchronous motor also gradually decreases accordingly. This attenuation process usually takes several seconds, so the output terminal of the frequency converter must have the ability to withstand a short-time overvoltage in the off state.

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