Can't soft starter and inverter be clearly separated? Let's learn together today.

The inverter changes the output frequency and voltage at the same time, that is, it changes the n0 on the motor running curve, so that the motor running curve moves down in parallel. Therefore, the frequency converter can make the motor with a smaller starting current, and at the same time make the motor starting torque reach its maximum torque, that is, the frequency converter can start the heavy load.

Soft start only changes the output voltage, does not change the frequency, that is, does not change the motor running curve of n0, but to increase the steepness of the curve, so that the motor characteristics become soft. When n0 is unchanged, the torque (rated torque, maximum torque, locked-rotor torque) of the motor is proportional to the square of its terminal voltage, so the soft start greatly reduces the starting torque of the motor, so the soft start is not suitable for heavy-duty starting of the motor.

The inverter is functionally irreplaceable by soft start. The inverter can realize constant torque start, that is to say, it can have the same torque as high speed at low speed, but soft start cannot be realized.

The working principle of 1. soft start equipment

A three-phase anti-parallel thyristor (silicon controlled rectifier) is connected in series between the three-phase power supply and the motor. Using the thyristor phase shift control principle, the motor terminal voltage gradually increases from zero with the conduction angle of the thyristor and the motor speed gradually increases until the start-up process is completed when the start-up torque is met. At this time, the bypass contactor is turned on (to avoid harmonic pollution to the power grid during operation and prolong the life of the thyristor the operation) and the motor, when parking, the bypass contactor is cut off first, and then the conduction angle of the thyristor in the soft starter is gradually reduced from large to large, so that the three-phase power supply voltage is gradually reduced, the motor speed is gradually reduced from large to zero, and the parking process is completed.

When starting, the conduction angle of the thyristor starts from 0 and gradually moves forward. The terminal voltage of the motor starts from zero and gradually rises according to the preset function relationship until the starting torque is met but the motor starts smoothly, and then the motor runs at full voltage. From the working principle, the soft driver is actually a voltage regulator, the output only changes the voltage, and does not change the frequency. This is different from the frequency converter.

　　Advantages of 2. soft-start devices

The advantages of soft start are:

1) There is no impulse current when starting, and the starting current rises linearly from zero to the set value by gradually increasing the thyristor conduction angle;

2), is a constant current starting, soft starter can be introduced into the current closed-loop control, so that the motor in the starting process to maintain constant current, to ensure that the motor starts smoothly;

3), according to the load situation and power grid relay protection characteristics, can freely stepless adjustment to the best starting current.

4), can frequently start the motor, soft start allows 10 times/hour, without overheating the motor.

(The traditional decompression starting methods of squirrel-cage motors include Y-△ starting, self-coupling decompression starting, reactor starting, etc., all of which belong to stepped decompression starting. Its disadvantage is that secondary impulse current will appear during the starting process.)

When the motor runs at full speed, the bypass contactor is used to replace the soft starter that has completed the task to reduce the heat consumption of the thyristor and improve the system efficiency. In this mode, a soft starter is used to start multiple motors.

1. inverter concept

From the speed formula of asynchronous motor n = 60f(1-s)/p (where: n-motor speed, f-power frequency, p-pole number of motor, s-slip rate), it can be seen that when p is constant and the frequency of power supply is uniformly changed, the synchronous speed of the motor can be continuously changed. This speed regulation is called frequency conversion speed regulation.

The torque formula of three-phase asynchronous motor is:

　　S R2

M = C U12 formula [2]

　　R22 （S X20）2

C: is a constant related to the characteristics of the motor itself; U1: input voltage;

R2: rotor resistance; X20: rotor leakage reactance; S: slip rate

It can be known that the torque of M∝ U12 is proportional to the square of the power supply voltage. When the normal input voltage is set to be M2, the voltage drop causes the electromagnetic torque M to drop a lot. Since M2 remains unchanged, the balance relationship between M and M2 is destroyed, resulting in the decrease of motor speed and the increase of slip S. It also causes the change of rotor voltage balance equation, causing the rotor current I2 to rise. That is, the stator current I1 increases (as can be known from the transformer relationship); At the same time, the increase of I2 is also the torque M sent on the motor shaft and rises again until it is equal to M2. At this time, the motor speed tends to a new stable value.

The torque formula of three-phase asynchronous motor is:

　　S R2

M = C U12 formula [2]

　　R22 （S X20）2

C: is a constant related to the characteristics of the motor itself; U1: input voltage;

R2: rotor resistance; X20: rotor leakage reactance; S: slip rate

　　T=9550*P/n

　　P(KW)n(r/min)T(N*m)

Classification and principle of 2. frequency converter

The key equipment of the variable frequency speed regulation device is the motor variable frequency governor, also called frequency converter.

The frequency converter has AC-AC frequency converter, which is composed of three-phase anti-parallel gate crystal tube reversible bridge type and adopts the principle of natural commutation of power grid.

The concept of soft starter and the difference between frequency converter

Soft starter is a set of motor soft start, soft stop, light load energy saving and a variety of protection functions in one of the novel motor control device, foreign known as Soft Starter. Its main components are connected in series between the power supply and the controlled motor three-phase anti-parallel gate and its electronic control circuit. By using different methods to control the conduction angle of the three-phase anti-parallel gate tube, the input voltage of the controlled motor can be changed according to different requirements, and different functions can be realized. Soft starters and frequency converters are two completely different products. Inverter is used in the need for speed control, its output not only changes the voltage and at the same time change the frequency; soft starter is actually a voltage regulator, for motor starting, the output only changes the voltage and does not change the frequency. The inverter has all the soft starter functions and rich protection functions, but its price is much more expensive than the soft starter and its structure is much more complicated. Performance and Application of Soft Starter

The moment the motor starts, the current is 5-7 times its own rated current. When a 30KW motor starts, 30KW becomes 150KW-210KW. At this time, it has a great influence on the whole power load and the bearing capacity of components. How to start the motor, relative to reduce some of the starting current? There is no soft starter era, by the star delta buck actuator, self-coupling buck starter to achieve the purpose. They can reduce the starting current to 3 to 5 times, but this facility uses more components, the structure is more complex, the space occupied is also very large, a more than 150KW motor step-down starting cabinet, I am afraid to occupy a cabinet, if the number of motor working conditions, the cabinet will need a considerable large workshop.

Star-delta and auto-coupled buck start-up devices are switched between star and delta to achieve buck-down current. Every time you start and stop, you have to switch once. The switching sound is loud. If you encounter frequent start and stop places, the components are vulnerable and the maintenance volume will also increase. With the soft starter of the motor, the step-down start of the motor becomes easy. Motor soft starter, also known as "solid-state soft starter", which uses SCR and integrated components. Light weight, small size, easy to install, and reliably complete the smooth start and stop of the motor. There are domestic and imported soft starters, imported soft starters are: ABB, AB, Schneider. The price of domestic soft start is only equivalent to the price of self-coupling buck cabinet. Compared with the motor inverter, they have something in common: smooth start and stop. The difference: the inverter can make the motor speed, and soft start can not. Soft starters are mainly used in motors with high power and no need for speed regulation. Its main purpose is to reduce the starting current of the current. If the inverter is used, although the same purpose can be achieved, the cost will increase a lot. The use of star-triangle and self-coupling is reduced, and there are many shortcomings. The selection of soft start should be economical and simple. method. Large motors in various working conditions can be soft-started.

Several ways of soft start

The soft start of the motor mainly adopts the following methods:

1. Reduced supply voltage start-up

2. Decrease power frequency start

3. Starting with reduced excitation current

The soft starter mainly uses thyristor phase shift to reduce the motor voltage and realize soft start.

The so-called soft start of the motor is essentially that the motor starts slowly with a lower current, which has a small impact on the power grid, and at the same time can reduce the load margin of the transformer and control circuit, and at the same time increase the service life of the equipment. Generally, when the AC motor is started directly, the starting current is 6 to 10 times of the trial operation current, and after the soft start technology is adopted, the starting current is reduced to 1 to 3 times.

Frequency converter:

Definition: the voltage, frequency fixed alternating current into voltage, frequency variable alternating current converter called inverter.

Function: Reduce the impact load caused by the motor when starting, control the motor speed, lengthen the starting time, smooth the current, achieve the purpose of soft start, and improve the efficiency of the power grid and the motor. In fact, the inverter is mainly used in energy saving, through the adjustment, change the output voltage, current, frequency. General speed control motor using frequency converter.

Disadvantages:

The cost is high, and the price is much more expensive than microcomputer protection.

2. As the current frequency converter almost all adopt PWM control mode, such pulse modulation form makes the frequency converter generate high harmonic current on the power supply side during operation, and cause voltage waveform distortion. After the power system is polluted by harmonics, it will affect the operation efficiency of the system, and damage the equipment and even endanger the safe operation of the power system.

3. Overload makes the frequency converter jump more frequently. When overload occurs at ordinary times, generally speaking, the motor has strong overload capacity. As long as the motor parameters in the frequency converter parameter table are properly set, the motor overload is generally not likely to occur. The inverter itself is prone to overload alarm due to poor overload capacity.

Soft Starter:

Definition and function: it is connected in series between the power supply and the controlled motor, and the internal thyristor trigger conduction angle is controlled by microcomputer to realize AC voltage regulation, so that the input voltage of the motor gradually rises from zero to a preset function relationship until the start is completed, and the full voltage of the motor is given, I .e. soft start. during the soft start process, the starting torque of the motor gradually increases and the rotation speed gradually increases until the thyristor is fully turned on, the motor works on the mechanical characteristics of the rated voltage to achieve smooth starting and reduce the starting current. Avoid starting overcurrent trips. When the motor reaches the rated number of revolutions, the starting process ends, providing the rated voltage for the normal operation of the motor.

Disadvantages:

1. The power frequency cannot be adjusted, so the motor cannot be started from zero voltage and zero frequency, and zero impact start cannot be realized.

2. Can not speed.

3. The soft starter exits the system after starting the motor and loses the protection function.

1. Inverter is used for the need for speed control, the output of the inverter not only changes the voltage but also can change the frequency at the same time.

2. The soft starter is actually a voltage regulator, which protects the motor. When the motor is started, the output of the soft starter only changes the voltage without changing the frequency.

The inverter has all the functions of the soft starter, but its structure is complex and the price is much more expensive than the soft starter.

The frequency converter controls the electrical equipment by adjusting the frequency, and can adjust the speed and start, such as the variable frequency pump used in industry. The soft start is basically far away from changing the resistance connected in series in the circuit to make the current from small to large. This method is used to start the high-power motor, which is used to reduce the fluctuation caused by the direct start of the high-power motor to the power grid, or cause some unimportant The trouble of load unloading.