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The starting method is chosen by answering three questions: Can the supply and transformer handle the starting current? How much torque does the load need at starting? Is speed adjustment needed during operation? A comparison of the four methods listed in the HEM catalogue:
| Method | Starting current | Starting torque | Suitable for |
|---|---|---|---|
| Direct on line (DOL) | Highest, full Is | Full starting torque | Small and medium power, strong supply, heavy starting load |
| Star-delta | About one third of DOL | About one third | Motors wound 400/690 V that start unloaded or lightly loaded |
| Soft starter | Limited to an adjustable degree | Falls with voltage | Pumps, fans, conveyors; reducing mechanical shock |
| Inverter (VFD) | Can be held near rated current | Can deliver high torque at low speed | Processes needing speed control, smooth starting under heavy load |
Practical approach: at small ratings and on a strong supply, direct-on-line starting is the simplest solution with the highest torque. As power increases, starting current must be limited; if the load starts unloaded or lightly loaded, star-delta is enough, and if a shock-free start is wanted, a soft starter is sufficient. When both low current and high starting torque are needed under heavy load, or when speed is to be adjusted, the inverter comes to the fore.
Always state the starting method in your order; star-delta requires a motor wound for 400/690 V. For a worked assessment, see our article on voltage and starting method for an 18.5 kW motor.