Primary Reactance Starter (or) Auto Transformer Starters

PRIMARY REACTANCE STARTER (or) AUTO TRANSFORMER STARTERS

The working principle of the primary reactance starter is same as that of primary resistance starter except that voltage drop occurs across the reactor, so that i/p voltage applied to the stator of induction motor reduces.

·         An auto transformer consists of a n auto transformer and a switch as shown in fig.

·         When the switch S is put on start position, a reduced voltage is applied across the motor terminals.

·         When the motor picks up speed, say to 80 percent of its normal speed, the switch is put to RUN position.

·         Then the quato-transformer is cut out of the circuit and full rated voltage gets applied across the motor terminals.

·         The switch making these changes from „start‟ to „run‟ may be air break (for small motors) or may be oil-increased (for large motors) to reduce sparking.

Derivation for autotransformer starter:

When full voltage is applied without suing autotransformers say starter, then current taken by the motor is 5 times the full load current. If V is pre line voltage then voltage/phase across the motor isV / Rt(3) .

Relation between starting and full-load torque:

From Fig. 1(a) it is seen than for star connection of windings, phase current.

when ILY is the line current when windings are connected and Zp is the windings impedance per phase. For delta connection of windings

The ratio of line currents drawn in star and delta –connection is therefore,