SELECTION COMPARISON
Overview
This is one of the most basic and confusing choices in motor control.
In one sentence, the soft starter only "starts the motor smoothly". After the start is completed, the motor runs at full speed; the variable-frequency drive can continuously change the speed during operation.
If the motor has been running at full speed after starting and just wants to reduce the starting shock, then a soft starter is enough, with lower cost and smaller size.
If speed adjustment is required during operation, or if you want to save energy through speed adjustment, you must use a variable-frequency drive.
function comparison
| Comparison point | soft starter | variable-frequency drive |
|---|---|---|
| core function | Control the starting process and reduce starting impact | Continuously adjust motor speed |
| operating status | Run at full speed after startup | Continuous speed change during operation |
| energy-saving potential | It only reduces the startup impact and does not save energy during operation. | Significant energy saving when load is variable |
| typical applications | Fan, water pump (constant speed), crusher, compressor | Pumps, fans, and transmission equipment that require speed regulation |
| size and cost | smaller and more economical | larger and more expensive |
| common combinations | Bypass contactor, switching after startup | Configure control method according to load type |
How to Decide
- Whether it is necessary to change the speed during operation - if necessary, choose a variable-frequency drive; if not, consider a soft starter.
- Do you want to save energy through speed regulation? If the speed regulation of pumps and fans has obvious energy-saving effects, you should choose a variable-frequency drive.
- Nature of the starting problem - If it is just excessive starting current that causes grid fluctuations or mechanical shock, a soft starter can solve it.
- Budget and cabinet space – soft starters have an advantage when both are limited.
- Whether frequent starts and stops are required - In the case of frequent starts and stops, it is necessary to confirm the startup frequency capability of the selected equipment.
Corresponding series reference
| Brand | soft starter | variable-frequency drive |
|---|---|---|
| Schneider Electric | ATS22 (Altistart 22), ATS480 (including ATS430) | ATV320, ATV630, ATV930 |
| ABB | PSE, PSTX | ACS180, ACS380, ACS580, ACS880 |
Common Pitfalls
Think that the soft starter can adjust the speed.
The soft starter only controls the voltage during the starting phase. After the starting is completed, the motor runs at full speed and does not have the ability to continuously adjust the speed.
To save money, replace the variable-frequency drive with a soft starter.
If the process itself requires speed adjustment, the soft starter cannot meet the requirement, and it will still need to be modified later, which will increase the cost.
Ignore startup frequency.
In situations of frequent starts and stops, it is necessary to confirm the allowable number of starts and heat dissipation conditions of the equipment. You cannot select based on current alone.
Ignore bypass configuration.
Some soft starters require an external bypass contactor, which should be confirmed when purchasing.
Frequently Asked Questions
Can a soft starter be used as a variable-frequency drive?
No. The soft starter only controls the starting process. After starting, the motor runs at full speed and cannot continue to adjust the speed. If the process requires speed regulation, a variable-frequency drive must be selected.
Which water pump is more suitable?
Pumps that run at a constant speed can use soft starters; pumps that need to be adjusted according to flow or pressure can use variable-frequency drives to significantly save energy and make long-term operation more cost-effective.
What should I do if ATS48 is discontinued?
The official successor direction is ATS480, but the power, current and control power supply need to be checked item by item. For details, see our ATS48 → ATS480 replacement page.
What information is required for selection?
The rated power and current of the motor, supply voltage, control power supply voltage, starting frequency and load type are required. You can send us a photo of the motor nameplate to assist in selection.
Need help confirming a part number or application?
Send the complete part number, BOM, nameplate or product photos. Documents and images can be shared through WhatsApp, Telegram or WeChat.
