Starlit Electricals Product Information

Listing ID #3867239

A 3 Ph Star connected Auto transformer with a changeover switch forms an Auto Transformer starter. Initially the stator of the induction motor is supplied with reduced voltage through the Auto Transformer using a suitable tap. This limits the starting current to a safe value. Usually 3 tapings per phase (50%, 65%, 80% or 60%, 70%, 80%) or as per customer’s requirement are provided to give that % of line voltage across motor terminals. When the motor attains about70 to 80% of normal speed, then the motor is connected in delta connection which cuts off the Auto Transformer from the circuit and full voltage is applied across motor terminals. The loading the system and power loss involved are less in this type and it can be used in delta as well as star connected motors. The provisions of several tappings make the adjustments possible to suit local conditions.

Relation between starting torque and full load torque of an Auto Transformer Let Transformation ratio = N2/N1 = K Starting Current 1st (motor) = K Isc lsc= Starting current at normal voltage So current drawn from the mains = 1st (Mains) =K(Klsc) For motor, Tst/Tf= (Ist/lf) * (Ist/lf) * Sf = K*K*(lsc/lf)*(lsc/lf)*Sf. Sf=Slip at full load Thus with Auto Transformer starting for starting torque of K*K of that obtainable by direct switching Only K x K of short ckt current is taken from the supply. Also the torque obtainable can be varied be selecting suitable tap. The provision of taps helps in adjusting to local conditions. Auto-transformers for motor starting duty, both LT upto 1200 HP/ H.T upto 2500 HP, oil and air cooled constructions are manufactured. The typical duty cycle is 6 starts per hour and less than 20 seconds per start. Duty cycles can be offered to suit various motor characteristics.