JohdotaStar-delta starting reduces the starting current of a large motor. In star the voltage across each winding is lower, so the current is about a third of the direct-on-line starting current. The starting torque is also about a third or less.
Requirements
The delta voltage of the motor must be the same as the mains voltage. On 400 V mains the rating plate must therefore read 400/690 V. The motor must have six winding ends in its terminal box.
Power circuit
Main contactor K1 supplies the winding starts U1, V1 and W1. Star contactor K2 connects the winding ends U2, V2 and W2 together. Delta contactor K3 connects the end of each winding to the start of the next winding: U1–W2, V1–U2 and W1–V2.
Changeover time
The motor typically stays in star for 6–10 seconds. Switch to delta only when the motor has reached about 85% of its rated speed. Otherwise the changeover causes a large current peak.
Remember
- K2 and K3 must never be energised at the same time. Interlock them with each other's NC contacts.
In your region
Showing notes for
United States, Canada
This is called wye-delta starting. The motor must have all its winding ends brought out: six or twelve leads. A six-lead motor starts in wye with L1, L2 and L3 on T1, T2 and T3 and with T4, T5 and T6 joined. It runs in delta with L1 on T1 and T6, L2 on T2 and T4, and L3 on T3 and T5. Nine-lead dual-voltage motors cannot be started wye-delta.
Source: Siemens: Basics of Control Components, part 3 (STEP 2000 course) · WEG: motor installation manual, NEMA and IEC lead markings
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
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