Used tor reduced-voltagestarting of larae induction motors As the part of Auto Transformer Starter (ATS) Panel

Function

1.Limits Inrush Current: Reduces theinitial voltage applied to the motor(e.g., 50%, 65%, 80% of line voltage)to minimize the impact on thepower grid. The line current drawnfrom the supply is reduced to 1/K² of the direct-on-line (DOL) startcurrent.

2.Higher Starting Torque: Comparedto star-aeita starters, it provides ahigher starting torque at the samecurrent reduction level (thoughtorque drops with V², it remainsrelatively high).

3.Automatic Transition: After themotor accelerates, it switches tofull-rated voltage for continuousoperation, and the auto-transformeris disconnected.

Principle

It operates on electromagneticinduction but uses a single windingwith multiple taps. The primary(supply) and secondary (motor) sharea common series winding (the "auto"

part), meaning they are electricallyconnected.

1.Let the supply voltage be U1, tapvoltage be U2, and turns ratio K =N1/N2＞1,Then U2=U1/K

2.Power Transfer: It transfers powervia both electromagnetic induction(magnetic flux) and direct electricalconduction (series winding). Thismakes it smaller and more efficientthan a double-winding transformerof the same rating.

3.Current Reduction: If the motordraws current I2, the supply current I1 is only I2/K. Since voltage is alsoreduced by K, the total supplycurrent during start is reduced by K² times compared to afull-voltage start, greatly reducinggrid stress.

Key Caveat

The starting torque is proportional to U². So if K = 2 (voltage reduced to 50%), the starting torque drops to 25%. Therefore, this method is onlysuitable for no-load or light-loadstarts. Also, auto-transformers provide

no electrical isolation between inputand output.