Comparison of inverter and soft starter: Features and applications
Engines often have great energy needs to start and quickly reach top speed. Soft starter and inverter can both be used to reduce inrush current and limit torque to protect mechanical equipment, avoid factory voltage drop and prolong motor life by reducing starting current operation of high-power motors or engines that require continuous starting and stopping. So when to use inverter, when to use soft start?
What is soft start? How does the soft starter work?
A soft starter is a device used to assist the starting process of an AC electric motor, which protects the motor from damage caused by sudden large currents when starting and avoids voltage drop in the power grid by gradually increase the voltage supplied to the motor from a predetermined voltage up to the rated voltage (for soft stop, vice versa).
Working principle of soft starter:
The structure of the soft starter consists of 3 pairs of thyristors (SCRs) connected in reverse parallel. In the disconnected state, the thyristor prevents current from flowing, when in the open state, the thyristor gradually opens the trigger angle (opening angle of the semiconductor valves) allowing the current to flow slowly, the motor starts. and gradually accelerate. The voltage is controlled by controlling the valve opening angle. When the valve is fully open, the voltage reaches the rated voltage value and at that time the motor will reach the maximum allowable speed. Since the motor torque is proportional to the square of the voltage, the current is proportional to the voltage, the acceleration torque and the starting current are limited through adjusting the r.m.s. value of the voltage.
When the motor reaches the rated speed, the bypass contactor in the soft start is closed, and the system automatically bypasses the mains without going through the thyristor.
Soft start is commonly used in applications such as:
- Ask the engine to accelerate and increase torque slowly when starting or stopping the engine
- High starting current limiting for large motors to avoid problems with mains voltage drop or motor damage
- Start-up speed control to avoid sudden torque or tension damage to mechanical systems such as conveyors, belt drive systems, gears, couplings
- Apply pump motor to avoid sudden increase in pressure at the start of pump, causing water hammer to break the pipeline.
Benefits of a soft starter:
- Support start-up process, reduce grid voltage drop
- Motor protection in case of overcurrent, overvoltage, phase loss, input and output phase fault, underload, load short circuit protection, overload protection
- Increase engine life and mechanical components
What is an inverter? What are the benefits of starting with an inverter?
Inverter is a device that changes the frequency of the current applied to the coil inside the motor, thereby controlling the motor speed steplessly not only when starting the motor but also during the operation of the motor. device.
Working principle of inverter:
The operating principle of the inverter is quite simple. A 1-phase or 3-phase AC power supply is first rectified and filtered into a flat DC power by a diode bridge rectifier and a capacitor. As a result, the inverter’s cosphi power factor is independent of the load and has a value of at least 0.96. Then, this DC voltage is converted (inverted) into a symmetrical 3-phase AC voltage through the IGBT (isolated gate bipolar transistor) system by pulse width modulation (PWM) method.
The 3-phase AC output system can change the amplitude and frequency values steplessly depending on the controller, thereby changing the motor speed as required during operation.
Benefits of using an inverter:
- Helps engine start smoothly, reducing mechanical wear
- Easily change the motor speed to meet the technological requirements of different equipment and systems
- Full range of motor protection functions such as overcurrent, overvoltage, phase loss, phase reversal, etc. to help increase motor life
- Efficiently save energy, protect equipment in the system
- Integrating various control functions to improve technology and increase production productivity
So when to use inverter, when to use soft start?
To choose the right startup method for your application, it is necessary to clearly define your needs, use purposes and balance the investment to achieve optimal efficiency. Please see the comparison table between the inverter and the soft starter below for a more objective assessment before making a choice of whether to use a soft starter or use an inverter:
| Inverter | Soft start |
|---|---|
| It can change the motor speed flexibly, reverse the motor not only when starting and stopping the engine but also during the operation of the device. | Only accelerates or decelerates during starting and stopping the engine for “soft” starting or stopping, not reversing the motor. |
| Adjusting acceleration and deceleration time more flexible, wide adjustment range makes starting the engine extremely smooth | The adjustment range of the acceleration and deceleration time is narrow, starting is heavier, especially with large engines, the starting is quite difficult. |
| More protection functions for engines and mechanical systems | Few motor protection functions, only basic protection functions |
| It can start as many times a day and during the life of the equipment without affecting the engine and mechanical system. | Depending on engine capacity and factory, the number of starts may be limited |
| Changing the motor speed by frequency changes should not affect the starting torque | Changing the motor speed by changing the voltage should make the starting torque weak, need to pay attention to the load factor when choosing a soft starter |
| One inverter can be used for speed regulation and “soft stop” for multiple motors | A soft starter is used for a single engine only |
| Larger in size and takes up more space | More compact when compared to inverters of the same capacity |
| Higher price | Cheaper |
Thus, the common feature of soft starter and inverter is that they are both used to control the motor (mainly speed) in a soft and energy-saving way, but while soft starter is only used to When “softening” the process of starting or stopping the machine, the inverter allows to control the motor speed during operation, reverse the motor rotation and many other flexible control features. This not only helps to meet the technology in many cases, but also helps to save energy in many different applications.
Inverters are larger in size and more expensive to invest than soft starters, especially in high power applications. Therefore, if your application only requires current limiting during starting, the grid is stable, no motor speed changes are required, and the operating conditions have no other demanding requirements, A soft starter can be an effective solution. However, although the initial investment of the inverter can be more expensive, it can in many cases result in energy savings of up to 50%, which in turn can save operating costs over the life of the device. .
In summary, the choice between a soft starter or an inverter often depends on the specific application, system requirements and cost (both initial investment and life cycle costs of the system). Selecting the wrong equipment can cause waste and damage to the engine and other equipment in the plant, so the user should consider all factors before making a decision, or call Immediately consult our experts for the most effective solutions.
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17/08/2021
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