Like all motors, the induction motor starting directly at full line voltage draws high inrush current several times greater than the normal rated current.
Ship electrical motors high start current problems
- Unnecessary motor-fuse melting or circuit-breaker tripping (a nuisance
since motor starting is not an abnormal fault condition)
- Momentary voltage drop in the cable that may disturb other sensitive components around the motor and may cause light flickers
- Excessive kVA drawn during starting causes heavy stress on the power
lines and the source
The source, cable, and circuit protection all must be sized for such inrush current, which is usually estimated from the code letter included in the motor nameplate.
The code letter is expressed as the starting kVA per horsepower of the motor rating and is known as the locked-rotor or blocked-rotor kVA of the motor. It indicates the severity of the starting stress in terms of the kVA/hp it draws from the source on direct line starting.
The range of codes found on motor nameplates is varies from low 3.14 to high 22.4 kVA/hp.
A motor with higher code letter produces higher stress on the power system design.