Soft starters, motor starters, and variable frequency drive are all used to control motors, but their prices are different.
What is the difference between their functions?
How can we choose them for motor better?
This article is an introduction and comparison of these three products.
What is a soft starter?
A soft starter is an over-control device. It is a three-phase parallel thyristor connected in series between the power supply and the controlled motor, and the internal digital circuit controls the moment when the thyristor is turned on in the alternating current. If the silicon is turned on, the output voltage of the soft starter will be high. If the thyristor turns on again within one cycle of the alternating current, the output voltage of the soft starter will become very low. Therefore, the voltage at the motor terminal will gradually increase during the start-up process. Thereby, the starting current and torque of the motor are controlled, so that the motor can start smoothly. The soft starter can only change the level of the power supply voltage, but cannot change the frequency of the power supply.
What are the functions of the soft starter?
First of all, the soft starter is equipped with a current control loop, so it can monitor the current in the motor circuit in real time, and capture the current change in time. And the thyristor will be automatically disconnected when overloaded, so that it can play the role of overload protection and the alarm signal for the motor.
Secondly, when the soft starter is in a working state, it can track and monitor the current change trend in the three-phase line anytime and anywhere. If there is a current interruption, the soft starter will act as a fault for the motor for the first time—phase protection.
Finally, the thermal relay element in the internal structure of the soft starter can be used to monitor the degree of heat dissipation of the thyristor. If it exceeds the preset value, the thyristor will be automatically disconnected and an alarm signal will be issued, so that the staff can make timely emergency maintenance.
●Frequently start and stop.
●Lower motor starting current, lower power capacity demand, and avoid extra cost for adding power supply facility.
●Lower starting stress and extend the motor lifespan.
●Soft start and soft stop, avoid pump oscillation and other unnecessary harm caused by sudden start and stop.
●Various methods of start and large-scale current-voltage setting, are applicable for different kinds of load and can improve the process control.
●More reliable and complete protection functions can better protect the motor and related equipment.
●Overload protection function: When the motor is overloaded, the thyristor is turned off and an alarm signal is issued.
●Short circuit protection: if there is a short circuit in the line, it will automatically stop.
●Voltage mismatch and phase loss protection: Undervoltage: When the voltage drops a lot, or a current interruption occurs, the phase loss protection translation can be made.
●Remove the harm to the power grid
● Reduce damage to electrical equipment
What is a motor starter?
Motor starter is an electrical device used to start and stop a motor safely. Similar to relays, motor starters turn power on/off, and unlike relays, they also provide low voltage and overcurrent protection.
The main functions of the motor starter：
Start the motor safely
Stop the motor safely
Reverse motor direction
Protect the motor from low voltage and overcurrent
What is a VFD?
Variable frequency drive (VFD) converts the frequency power into alternating current of various frequencies and controls the management, control equipment of the AC motor by changing the frequency of the working power of the motor.
Choosing Between a VFD and a Soft Starter
The main problem solved by the soft starter is to reduce the starting current of the large load and reduce the impact on the power grid. The start-up of large equipment will produce a very large start-up current, which will cause a large voltage drop. In this case, a soft starter is often used to make the starting process without impact, therefore the motor starts smoothly and less power supply is needed.
Variable frequency drive is mainly used in places with speed regulation. It can control the speed of the three-phase motor, such as speed regulation of CNC machine tool spindle motor, mechanical conveyor belt, large fan, and transmission control of heavy machinery. Inverters can be used for applications, etc. In general, its functionality is much more practical than soft starters.
| SOFT STARTER | VFD | |
| Principle | A semiconductor-based motor starter that safely starts and stops motors. | VFD is a semiconductor-based motor starter that safely starts, stops, and changes the speed of a motor. |
| Working process | At full speed, use a contactor to bypass the soft starter. | VFD operates during the entire operation of the motor. |
| Current conversion | Soft starter uses only thyristor to regulate the AC voltage. | VFD converts AC to DC and then back to AC of the desired value. |
| Transfer motor parameters | Soft starter can only change the supply voltage. | VFD can change voltage and frequency. |
| Torque | Soft starter produces low to moderate cranking torque. | VFD produce high starting torque. |
| Harmonic | No harmonics are generated in the system. | VFD might generates harmonics when working in the system. |
| Filters | Soft starter does not require any additional filters. | VFD require filters due to harmonic injection in the line. |
| Motor application | Soft starter is used in constant speed motor applications. | VFD is for variable motor speed applications |
| Size | Compared to a VFD, soft starter is smaller in size. | VFD is larger in size. |
Compared with soft starter and motor starter, the frequency converter has more functions and better protection for the motor. In most projects, we recommend to use variable frequency drive working together with the motor.
