Engineering · Motors & Drives
Induction Motor Starting Methods
Why not just switch every motor straight onto the supply?
An induction motor that is switched straight onto the supply draws six to eight times its rated current while it accelerates. That can dip the supply voltage, trip breakers and shock the machine. Four ways of starting trade this inrush against starting torque, smoothness, speed control and cost: direct on line (DOL), star-delta, soft starter and variable frequency drive (VFD). Pick a load, set each starter, and watch all four start the same motor on the same plots. Figures are indicative and depend on the motor and starter data sheets.
| Starter | Peak current | Peak torque | Time to speed | Supply voltage dip |
|---|
Current plot: line current as a multiple of rated current. Torque plot: motor torque as a multiple of rated torque; the motor can only speed up while its torque is above the load torque. Voltage plot: voltage at the motor terminals as a percentage of line voltage. Speed plot: percentage of rated speed. The motor model is a simplified equivalent circuit with a starting current of about 6.5 times rated and a starting torque of about 1.4 times rated, so the shapes are right but the numbers are illustrative. The star-delta changeover transient and the voltage dip in the supply are not modelled in the curves; the dip column uses starting kVA divided by the supply fault level, a common approximation.
| Parameter | DOL | Star-delta | Soft starter | VFD |
|---|
Torque and current are shown against the motor’s rated values. Star-delta only works on a motor with six terminals whose windings are rated for the full line voltage in delta. Reduced-voltage starting lowers current and torque together, so the load must still be able to accelerate on the reduced torque.
Starting multiples are typical planning ranges, not guarantees. Always confirm with the motor and starter manufacturer’s data and your supply authority’s rules on inrush and voltage dip.