Starting method of the hottest SCR thyristor frequ

2022-07-29
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SCR thyristor frequency reduction and step-down asynchronous motor starting method

I. disadvantages of SCR thyristor phase-shifting soft start

scr thyristor phase-shifting soft start is to cut off a part of the waveform of sinusoidal AC through the cut-off and controllable conduction of thyristors connected in series between the three-phase AC power supply and the motor, and add the remaining part of waveform pulse to the stator winding of asynchronous motor, as shown in Figure 1, in an attempt to step-down start the asynchronous motor, Limit the starting current of asynchronous motor

however, when the non sinusoidal AC pulse acts on the asynchronous motor, the power factor drops sharply, the electromagnetic torque decreases sharply, and the motor is unable to start. Finally, it starts directly when the conduction angle is 180 degrees and the voltage is 380V, which does not achieve the purpose of soft start. But its electronic digital control mode is very good

II. Advantages and disadvantages of traditional step-down starting mode

according to the principle of step-down current limiting, the traditional step-down starting mode can limit the starting current by reducing the power supply voltage. Years of practice has proved that step-down starting is an effective and good method. This method is only suitable for no-load or light load occasions because it reduces the voltage and reduces the electromagnetic torque of the motor. Since this method only reduces the power supply voltage and does not change the waveform of sinusoidal alternating current, from XL = 2 π f l, it is known that the motor impedance angle and power factor remain unchanged, which is much better than the SCR thyristor phase-shifting soft start, but the starting torque is not enough. The control mode is mainly manual, which is backward

III. principle of variable frequency soft start

the frequency converter starts the asynchronous motor on the frequency ramp, and the slip ratio s between the synchronous speed and the rotor speed is kept within a very small range. The motor basically completes the starting process along the speed ramp under the rated slip ratio, rated current and rated torque. The speed ramp time can be set at will. The perfect performance of the starting process makes people praise it as "soft start"

IV. can the new startup mode of frequency reduction and voltage reduction learn from the principle of soft startup of frequency converter, and make full use of the advantages of SCR, such as high cost performance ratio, low cost and convenient electronic control compared with traditional voltage reduction startup, to innovate and design a simple and easy startup mode and startup equipment that is both voltage reduction and frequency reduction

according to the frequency division discrete control principle, it is envisaged that "the positive half cycle of one cycle of AC and the negative half cycle of the next cycle of AC" can be superimposed into a cycle of "starting AC" through the cut-off and controllable conduction of SCR thyristors connected in series between the three-phase AC power supply and the motor. The frequency of this "starting AC" is just one third of the power frequency, i.e. 17hz. The frequency drops by 2/3 and the voltage drops by 2/3, which changes the starting slip performance of the motor, reduces the starting current and enhances the starting torque

one of the two SCR thyristors in reverse parallel is zero crossing conduction in the positive half cycle of one cycle of power frequency AC; The other is zero crossing conduction in the negative half cycle of one cycle under power frequency AC. Both SCR thyristors adopt zero crossing commutation trigger control mode, which is simple and easy. The synchronous trigger mode can be the automatic synchronous trigger pulse mode generated by the power frequency itself

what do you pay attention to when you buy the machine? Current electricity is added to the winding of the asynchronous motor. The motor electromagnetic torque operates for 10ms, intermittently for 20ms, and then for 10ms in every two power frequency AC cycles. It is pulsating and intermittent, and the electromagnetic torque is discontinuous

intermittently, we talked about a kind of multifunctional ceramic graphene metamaterial. If the voltage and current remain at the motor winding end within 20ms, the starting torque will be continuous. Who will undertake the task? We all know that half wave rectifier capacitor filtering can make the output half wave fluctuating voltage continuous and smooth. Good! We use capacitors to realize the discrete continuous smoothing of "starting AC", that is, capacitor charging and discharging are used to extend and compensate the discrete interval, so as to realize the discrete torque continuous smooth control

however, both SCR thyristor controlled rectifier circuit and SCR thyristor controlled AC circuit belong to phase-shifting control mode, and it is forbidden to connect filter capacitor behind them. The diode rectifier circuit can be followed by the capacitor filter circuit

in the "starting AC" of frequency division discrete control, SCR thyristor is triggered by zero crossing, which is 180 ° half waveguide pass, not a phase shift control mode, and the diode working state is the same as that of diode half wave rectifier circuit; It is feasible for analog diode half wave rectifier circuit to realize the continuous smoothing of "starting AC" with the function of capacitor filtering.

how to connect and connect three-phase capacitors? By analogy, the local compensation capacitor of three-phase asynchronous motor adopts the method of star connection (or angle connection), which is connected in parallel with the motor at the three-phase winding end. It is feasible to simulate the connection of local compensation capacitor

how to select the specifications of capacitors? Are there any principles? From the power frequency power supply voltage of 380V for capacitor operation, the maximum value is 510v, taking 2 times the safety factor, and the withstand voltage value should be AC 1000V; The capacitance C shall not be too large. It discharges through the motor winding, and its time constant shall not exceed 25ms; The capacitance C can be measured from small to large, and empirical data can be obtained from experiments, which should be related to the starting capacity and current of the motor

the motor can be started up to 33% of the rated speed with 17hz reduced frequency and reduced voltage "starting AC"; When the rotor speed exceeds 30% of the rated speed, according to the principle of direct torque control (DTC), the optimal switching period for motor switching is full frequency, full voltage positive, high power factor, small current and large torque

after the reduced frequency and step-down motor is started, the motor shall be switched to full frequency and full voltage operation in time. How to complete full frequency and full voltage switching? Very simply, the switch can be completed as long as the SCR thyristor control angle is zero and enters full conduction or the built-in bypass contactor is connected and short circuited. Smooth discrete capacitor can enter local compensation state or be cut off

how reliable and stable is the system? Due to SCR zero crossing triggering and back pressure blocking, the failure rate of thyristor will be greatly reduced; Due to SCR zero crossing triggering and back voltage blocking, the selection coefficient of rated current of thyristor can be greatly reduced and the cost can be reduced; "Zero delay" switching can be realized from frequency reduction and step-down startup to full frequency and full voltage operation; Since the switching speed of the motor is 33% of the rated speed, the switching will not cause harmful impact on the system; Switching is simple and easy

this starting mode is an ingenious combination of the traditional step-down starting mode and the variable frequency soft starting mode. It is a new starting equipment in Southeast Asia, Indian subcontinent, Latin America and the Middle East markets. It is a completely innovative mode with frequency reduction, step-down and capacitance participation. The starting slip rate s is low. Due to the participation of capacitor, the starting power factor is high and the starting torque is large. The starting is mainly used for the small dynamic current of automobile engine components and is suitable for light and heavy load starting. Due to its superior starting performance, advanced electronic control mode, low cost and high cost performance ratio, it can be asserted that it will

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