Motor Starting Current

When an induction motor starts, the rotor is stationary and the slip is 1.0, presenting very low impedance to the supply. The resulting inrush current — the locked-rotor current — is typically 5 to 8 times the motor's rated full-load current for direct-on-line starting. This high current persists for several seconds during acceleration and causes significant voltage dips that can affect other equipment on the same supply. Motor starting methods reduce the impact: star-delta starting reduces starting current to about one-third, soft starters provide adjustable current limiting, and variable frequency drives eliminate inrush entirely by ramping voltage and frequency gradually. Protection devices must be selected to withstand the starting current without nuisance tripping while still protecting the motor against stalled-rotor conditions. Cable sizing for motor circuits must account for the starting duty, and the voltage drop during starting must not exceed limits that would prevent the motor from accelerating to full speed. IEC 60034-12 Code letters classify motors by their locked-rotor current ratio relative to rated current.