Automatic 3-Phase Induction Motor Starter

The starter circuit presented here offers two main advantages: single-phase prevention and automatic star-to-delta conversion. -- R. Lakshmanan

58204
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Relay ratings

RL1 and RL2 are normal control relays, which are used to energise relay RL3. PCB-mounted OEN Type 57 relays rated for 12 volts (or equivalent) may be used for the purpose. RL3 and RL4 are required to support the complete line current during star as well as delta configurations. Hence the contacts must be rated to withstand full line-to-line voltage and expected full current of the motor in delta configuration. Thus heavy-duty power relays of appropriate voltage and current rating for 12V coils must be used. The relays are to be mounted outside the PCB on the chassis of a suitable metal cabinet, which must be earthed properly to avoid any risk of shock.

Before connecting the motor to the circuit, proper operation of the relays must be checked. The windings must be connected as shown in the circuit diagram.

Construction

A single-side PCB for the automatic 3 phase induction motor starter circuit is shown in Fig. 2 and its component layout in Fig. 3.

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