With the rapid development of the national economy, the number of high-voltage motors is increasing. Because the direct current of the large motor is 5~7 times of the rated current, the starting torque is only 0.4 to 1.6 times of the rated torque. It can be started directly under the condition of power grid (less than 10% of the grid voltage when the motor starts up) and the process conditions (starting torque satisfy). But the excessive starting current, too small starting torque and too long start-up time have caused great harm to the motor and the power network. It often causes the increase of the power grid voltage and the harmonic voltage fluctuation, even the front step, greatly increasing the burden of the power grid and the pollution of the power grid, which seriously affects the safe operation of the power grid. At the same time, it has also caused great harm to itself. Therefore, a soft starter should be connected between the power supply and the motor to solve these problems.
The appearance of the soft starter of the thyristor motor is a good solution to the above problems. It makes up the shortcomings of the traditional soft starter, reduces the starting current of the motor, reduces the power distribution capacity, and prolongs the service life of the motor and the related equipment. Starting parameters can be adjusted and easy to maintain.
1, the principle of thyristor motor soft starter regulating voltage.
The thyristor phase-shifting soft starter is a waveform that changes the sinusoidal AC voltage and turns it into a non sinusoidal alternating current, by adjusting its duty ratio, as shown in Figure 2.
Notes:
(1) alpha: control angle. Refers to the time to join the trigger pulse.
(2) Q: conduction angle. Each half cycle thyristor leads the angle. The larger the control angle is, the smaller the conduction angle is, and the sum is +q=p. It changes the average voltage of alternating current, and its average voltage is controllable and smooth.
2. Type selection of thyristor
Thyristor is the most important power device in the soft starter of the motor, and the reliability of the whole machine is closely related to the correct selection of the rated current and rated voltage of the thyristor. The principle of selection should first consider the reliability of work, that is, current and voltage must have enough allowance. Next, we should consider the economy, that is, the cost performance. Finally, we should consider the beauty of installation and the minimized volume.
For the high voltage motors of 6kV and 10kV, the thyristors need to be connected in series after the voltage is high. 6kV requires 6 thyristors for each phase (2 anti parallel connection, 3 series connection), 10kV 10 thyristors for each phase (2 anti parallel connection, 5 series connection). So the voltage for each thyristor is about 2000V, so the positive and reverse voltage of the selected thyristor is VDSM and VRSM above 6500v.
For the selection of thyristor rated current, the rated working current of the motor must be considered. In general, the current of the thyristor should be 3~4 times the rated current of the motor.
In the soft starter of the thyristor high voltage motor, the AC phase controlled voltage regulation composed of 2 independent thyristor devices is used to counter the 1 thyristors in the positive and negative half cycle. Therefore, the conformance of the parameters of the 2 anti parallel devices is higher. Including thyristor trigger parameters, maintaining current parameters and so on. As far as possible, positive and negative half wave symmetry, otherwise there will be DC component current flow through the motor. Because the motor is electrical inductance for the load of the winding, the high DC amount will make the motor stator heat seriously, even burn out the motor winding, so that the motor will be scrapped.