Difference between Soft Start and Variable Frequency Start

Soft starter is a novel motor control device which integrates soft start, soft stop, light load energy saving and multiple protection functions. Its main structure is a three-phase anti parallel thyristor and its electronic control circuit connected in series between the power supply and the controlled motor. Different methods are used to control the conduction angle of the three-phase anti parallel thyristor, so that the input voltage of the controlled motor changes according to different requirements, and different functions can be realized. Soft starter and frequency converter are two different products. The frequency converter is used in the place of speed regulation, its output not only changes the voltage but also changes the frequency; the soft starter is actually a voltage regulator, when it is used to start the motor, the output only changes the voltage and does not change the frequency. Frequency converter has all the functions of soft starter, but its price is much more expensive than soft starter, and its structure is much more complex. What is the difference between soft start and variable frequency start? The following article will give you a detailed answer.

• The difference between the principle of soft start and variable frequency start

1. The principle of soft start is to use the solid state relay (or bidirectional thyristor) to adjust the voltage and speed of the motor through phase shifting trigger (or zero crossing trigger).Soft start is a step-down start, but the step-down value can be adjusted continuously and smoothly. Just like step-down start, it is at the expense of starting torque. The frequency converter changes the voltage and frequency at the same time, and continuously adjusts the speed without reducing the torque
2. The process of starting the motor by variable frequency  is variable frequency speed regulation. During the operation, the motor can be adjusted by forward rotation speed regulation, braking, reverse rotation speed regulation, variable frequency operation, etc. Variable frequency is to start by changing the frequency, it can start with load, there is no impact current, soft start is to start by reducing the voltage, starting torque will be affected, and there is a certain impact current.

• Why do many manufacturers produce soft starter now? What is its development trend?

1. Even though frequency converter can adjust speed and save energy, why do many manufacturers produce soft starter now? What is its development trend? The problem is that the frequency converter needs speed regulation to save energy, and the frequency converter is much more expensive than soft start. In the case of no speed regulation, frequency converter plays the same role as soft start. Why spend more money to do the same thing? Soft starter technology content is relatively low, easy localization, stable performance, price is much lower than frequency converter! The price of frequency converter is higher than that of soft starter, at the same time, the frequency converter with feedback is higher. For example: in special occasions, such as when the load rate is less than 1 / 3 and there is feedback energy, the cost of on-line soft start is very low. High power variable frequency, changing power frequency or industrial variable frequency, the first cut in the off mode is high cost for the frequency converter. There are problems of asynchrony and phase difference. It is much easier to realize the two-way thyristor module of soft start. The reason is that the cost should be reduced. Only when IGBT is manufactured on a large scale in China is the time when frequency converter replaces soft power.                             
2. Soft starter is used to reduce the impact of large power on the motor when starting the power grid. Its working principle is in its specific starting time from OV to full voltage to complete the motor starting process; frequency converter is to change the frequency of the output power supply to achieve motor speed adjustment, it is generally used in the requirements of variable speed equipment, of course, it can also achieve the normal starting process of the motor.
3. Why soft starter? Because there is no need to change the speed of high-power motor equipment, it is used in order to better reduce the damage to motor equipment and power grid in the starting process (such as high-power fan, water pump, etc.).At the same time, the price of soft starter with the same power is much less than that of frequency converter.

• What occasions is soft starter suitable for?

1. In principle, cage asynchronous motor can be used in all kinds of applications where speed regulation is not required. At present, the application range is 380V AC (or 660V), and the motor power is from several kilowatts to 800KW.
2. Soft starter is especially suitable for all kinds of pump load or fan load, where soft start and soft stop are needed. Similarly, the soft starter (without bypass contactor) has the effect of light load and energy saving when the motor is in light load operation for a long time under variable load condition and only in heavy load for a short time or instantaneously.

ATO will provide you with more cheap soft starter and variable frequency drives. For example:20 hp (15 kW) Soft starter, 30 A, 3ph 220v/380v/480v, 15 hp (11 kW) Soft starter, 22A, 3ph 240v/400v/480v, 25 hp (18 kW) Soft starter, 37 A, 3ph 220v/400v/460v, 1 hp (0.75 kW) VFD, 3 Phase 230V, 400V, 480, 1 hp (0.75 kW) VFD, Single Phase Input & Output. ATO will meet all your needs for soft starter and variable frequency drives, please come to consult.