6 min read
The current drawn when a motor starts can reach six to eight times the rated value. That surge stresses both the supply and mechanical parts such as belts, couplings and bearings. There are three common answers, and none replaces the others.
Star-delta: cheap but blunt
The motor starts in star at reduced voltage, then switches to delta. Starting current falls to about a third, but so does torque, so a loaded machine may not accelerate in star. The current surge at transition is also felt mechanically. The motor must have six terminals and its delta voltage must match the supply.
Soft starter: kind to the mechanics
It ramps the voltage up to smooth the start, with no transition surge. On pump lines it noticeably reduces water hammer, and on conveyors belt slip. It does not change running speed, so it saves no energy. On frequently started duties, thyristor heating must be considered.
Drive: the most expensive and most capable
Beyond starting, it controls speed. On variable-flow pumps and fans the cubic law brings marked energy savings, and it adds process control, protection functions and fault data. In return it needs more panel space, more heat-load allowance and a harmonics assessment.
| Criterion | Star-delta | Soft starter | Drive |
|---|---|---|---|
| Initial cost | Low | Medium | High |
| Starting current | Reduced | Controlled | Lowest |
| Speed control | No | No | Yes |
| Energy saving | No | No | High on variable flow |
| Panel space | Little | Medium | More |
- Simple loads that start unloaded and run rarely — star-delta is still economical.
- Fixed-speed lines where water hammer or belt stress is a problem — soft starter.
- Pumps, fans and conveyors with varying flow or speed — drive.
- Where positioning or synchronised motion is needed — consider a servo drive.
Frequently asked questions
How many contactors does star-delta need?
Three: mains, delta and star contactors. The star and delta contactors must be mechanically and electrically interlocked, and a timer relay or PLC output handles the transition timing.
Should I remove the star-delta gear when fitting a drive?
With a drive the motor is connected directly in delta and the star-delta contactor group becomes unnecessary. On critical lines, if a bypass is wanted against drive failure, the existing contactors can be reused for that purpose.
Does a soft starter protect the motor?
Most models include electronic overload protection, but short-circuit protection must be provided separately. A suitable fuse or circuit breaker must be fitted upstream.
Above what power does a drive start to make sense?
It is decided by running hours and load characteristic rather than power. A small variable-flow pump running long hours pays back quickly, while a large motor running a few hours a month takes far longer.