Starting a large induction motor directly across the line draws a very high inrush current — often 6-8 times the rated current — which can cause voltage dips, mechanical stress, and nuisance tripping on smaller supplies. Choosing the right starting method balances cost, simplicity, and how much that inrush current actually matters for your installation.
Direct-On-Line (DOL) Starting
The simplest method — the motor is connected directly to full supply voltage via a contactor and overload relay. Starting current can be 6-8x rated current, and starting torque is correspondingly high (100% of rated torque or more).
Best for: Small motors (typically up to 5-7.5kW, though this varies by supply capacity and local regulations) where the inrush current doesn't significantly disturb the supply, and where high starting torque is actually needed (e.g. loaded conveyors, crushers).
Star-Delta (Wye-Delta) Starting
The motor starts in "star" configuration (reduced voltage across each winding, roughly 1/√3 of line voltage) then switches to "delta" (full voltage) once it reaches near-running speed. This reduces starting current to roughly 1/3 of DOL current, at the cost of reduced starting torque (also roughly 1/3).
Best for: Medium-to-large motors (roughly 7.5kW to 90kW+) driving loads that don't need high starting torque — pumps, fans, and other loads with a easy/low-torque starting characteristic. Requires a motor wound for delta-connected running (6 terminals brought out) — not usable on all motors.
Variable Frequency Drive (VFD) Starting
A VFD (also called a Variable Speed Drive) controls both voltage and frequency supplied to the motor, allowing a smooth, controlled ramp-up from zero speed with starting current typically limited to rated current or just above — the gentlest start of the three methods, with full control over acceleration profile.
Best for: Applications needing speed control during normal operation (not just starting) — pumps and fans with variable flow/pressure needs, conveyors requiring soft start/stop, or any process benefiting from energy savings at partial speed. Higher upfront cost than DOL or star-delta, but often pays back through energy savings and reduced mechanical wear, on top of the starting benefit.
Quick Comparison
| Method | Starting Current | Starting Torque | Relative Cost | Speed Control |
|---|---|---|---|---|
| DOL | 6-8x rated | ~100% | Lowest | No |
| Star-Delta | ~2-3x rated | ~33% | Low-Medium | No |
| VFD | ~1-1.2x rated | Adjustable, up to 150%+ | Highest | Yes, full range |
Making the Choice
Start with your motor size and supply capacity — if the supply can comfortably handle DOL inrush and the application doesn't need speed control, DOL is the simplest, most reliable, and cheapest option. If inrush current is a concern but speed control isn't needed, star-delta is a proven, cost-effective middle ground. If the process benefits from speed control or energy savings at partial load, a VFD is usually worth the extra investment.
TEKTOW supplies motor starters, contactors, and control equipment. Request a quote with your motor rating and application details.