1 Introduction
AC asynchronous motors are widely used in various sectors of the national economy due to their simple structure, extremely high operational reliability, strong environmental adaptability, and excellent driving performance. However, their fatal flaw is the large starting shock, which can adversely affect the power grid and equipment. To solve the starting problem, various methods have been used for a long time, such as star/delta starting, autotransformer reduced-voltage starting, saturated reactor and switching transformer reduced-voltage starting, and water resistance reduced-voltage starting. However, these methods all rely on reducing the starting voltage to start the motor. Since the starting torque is proportional to the square of the voltage, the starting torque decreases significantly when the stator voltage is reduced. Therefore, reduced-voltage starting is only suitable for no-load or light-load starting and is not applicable to motors requiring heavy-load starting. Furthermore, because ordinary soft starters reduce voltage without reducing frequency, a large starting current inevitably occurs during starting due to the large slip, thus greatly limiting the application range of soft starters.
This article introduces a new soft-start method that increases the starting torque of an electric motor by reducing the starting frequency through control signals.
2 thyristor soft starter
2.1 System Overall Structure
The thyristor soft-start system is shown in Figure 1. In Figure 1, QS is a high-voltage disconnector, QF1 and QF2 are vacuum circuit breakers, SCR is a (ordinary) thyristor, and M is a medium-voltage motor. QF1 is responsible for switching the main circuit on and off, and QF2 is responsible for bypassing the power devices.
In the SCR soft starter, there are a total of 6 SCRs, each containing m SCRs in series (the value of m is determined by the voltage level and the withstand voltage of the thyristor).
2.2 Basic Working Principle of the System
By utilizing the switching characteristics of thyristors, the conduction time of the thyristors is changed by adjusting their firing angle, thereby controlling the output voltage of the motor and thus controlling its starting characteristics. After the motor starting process is completed, the AC contactor is energized (as shown in Figure 1, i.e., QF2 is energized), short-circuiting all the thyristors and directly connecting the motor to the power grid.
2.3 Advantages and disadvantages
The advantage of soft start is its good starting characteristic curve, which allows the thyristor conduction angle to gradually move forward from zero degrees, the motor terminal voltage to gradually rise from zero until it reaches the rated voltage, and the starting current to rise linearly from zero to the set value, thereby meeting the starting torque requirements and ensuring successful start-up.
Although soft starters offer advantages unmatched by traditional starting methods, such as smooth starting and adjustable starting time, making them an ideal replacement for traditional step-down starters, soft starters using thyristor voltage regulation control of induction motors, while reducing voltage, still operate at the mains frequency. This results in a low power factor and increased reactive power, limiting their application to light-load conditions and making them unsuitable for heavy-load starting.
3. Staged frequency conversion soft start
3.1 Theoretical Overview
Traditional soft starters typically employ reduced-voltage starting or current-limiting starting at the 50Hz power frequency to suppress the inrush current during motor startup, resulting in low starting torque. Since starting torque is directly proportional to the square of the voltage and inversely proportional to the frequency, reducing the voltage during startup and correspondingly reducing the frequency can increase the starting torque. This paper employs a graded frequency conversion method, using different frequency points within the 0-50Hz range for graded frequency conversion. This causes the soft starter's output voltage frequency to start from a lower value and gradually increase until it reaches the 50Hz power frequency. While graded frequency conversion achieves frequency conversion, it cannot provide continuous frequency changes like a frequency converter; it only allows for graded frequency changes, with each stage frequency being 1/P of the 50Hz power frequency, i.e., a 50Hz division.
3.2 The concept of triggering
(4) After the newly turned-on SCR is triggered, the direction of the current flowing through the newly turned-on SCR branch (before the anti-parallel SCR is turned on) is irreversible, regardless of whether it is subjected to the grid line voltage or the phase voltage. That is to say, if the current is zero, it will remain zero (until it is triggered again).
3.4 Conditions for SCR to be turned on
Zero-crossing triggering makes a triggered SCR potentially turn on, but it does not necessarily cause the SCR to turn on. Zero-crossing triggering is a necessary condition for an SCR to turn on, while a sufficient condition for an SCR to turn on is that not only has zero-crossing triggering occurred, but the SCR connected in antiparallel has also stopped turning on. If the SCR connected in antiparallel is still turning on, then because the trigger pulse is a wide pulse with a width greater than 150, the turn-on of the zero-crossing triggered SCR will be delayed until the SCR connected in antiparallel turns off.
3.5 Definition of 'Do not perform zero-crossing triggering'
'Not performing zero-crossing triggering' is relative to 'performing zero-crossing triggering'. They are mutually exclusive. The inevitable consequence of all SCRs performing wide-pulse triggering at zero-crossing moments is that all thyristors are essentially short-circuited wires. The trigger pulse generator always unconditionally generates all zero-crossing trigger pulses. The only difference is that controllable gate circuits exist in the transmission path, and turning off these gate circuits corresponds to 'not performing zero-crossing triggering'.
3.6 Numerical representation of the trigger mechanism (hereinafter referred to as the trigger mechanism)
The first trigger is defined as the triggering of SCR #1 (at the leading edge of the #1 trigger pulse, the #6 trigger pulse still exists). Since (without affecting generality) the first trigger is always guaranteed, its corresponding trigger sequence number is '1'. The second trigger is sent to SCR #2. If a second trigger does not occur, the trigger sequence number is '10'; if a second trigger occurs, the trigger sequence number is '11'. The third trigger is sent to SCR #3. If a third trigger does not occur, the trigger sequence number is '1*0', where * is 0 or 1 depending on whether the second trigger occurs. If a third trigger occurs, the trigger sequence number is '1*1'. The fourth trigger is sent to SCR #4. If a fourth trigger does not occur, the trigger sequence number is '1**0', where ** is 00, 01, 10, or 11 depending on whether the second and third triggers occur. If a fourth trigger occurs, the trigger sequence number is '1**1', and so on. (Further details omitted.)
3.7 Periodicity of Trigger Mechanisms
The fundamental frequency after frequency division triggering is 50p Hz, and its period is 20p ms. The total number of triggering beats in a 20p ms period is 6p. The repetition period represented by the digital representation of the triggering system is also 6p. For example, if the digital representation of the second beat of the triggering system is '10', then the digital representation of the 6p+2 beat is still '10'.
4. Conclusion
AC-AC graded frequency converter soft starters achieve low current and high torque characteristics in the initial starting stage of the motor by changing the triggering sequence and control algorithm of the thyristor trigger pulses without altering the main circuit structure of traditional thyristor soft starters. The emergence of AC-AC graded frequency converter soft starters has effectively solved the problem of heavy-load motor starting, ushering in a new era for soft starter development. Since the main circuit structure of traditional thyristor soft starters remains unchanged, costs are not increased, providing favorable competitive conditions for graded frequency converter soft starters to expand their application scope and enter the market.
