*Description of the circuit

The control system for a star delta contactor is sequence circuit. The star contactor and delta contactor are interlocked relative to each other with a ‘either-or’ condition. Switching from ‘star’ to ‘delta’ is done automatically by means of a time relay.

In the circuit above, the star contactor is energised fist, then the mains contactor. Circuits in which the mains contactor is energised fist, followed by the star contactor, are also common.

*Switching on

Pressing pushbuttom S1Q energises star contactor K1M and time relay K1T. The NC contact of contact K1M in current path 4 interlocks delta contactor K3M, and the NO contact of K1M in current path 3 energises mains contactor K1M. Contactor K2M holds itself in via NO contact K2M in current path 4

Motor M1M runs in star

When the set time has elapsed , contact K1T in current path 1 opens the circuit and star contacctor k is de-energised. NC contact K1M in current path 4, which has interlocked the delta contactor to prevent it closing, closes, and delta contactor K3M is energised. Mains contactor K2M remains energised.

Motor M1M runs in delta.

*Switching off

Pressing pushbuttom switch S0Q opens the control circuit, de-energised contactors K2M, K3M and motor M1M is switched off.