How does across the line starting work?

Answer 1

There are mainly two types of starting methods. Reduced voltage starting and across the line starting. Reduced voltage starting comes in the configuration of the wye-delta starter. Across the line starter is the combination of voltage source, motor contactor and motor load.

The utility company's policy is set, to allow only a certain horsepower load to utilize across the line starting. This is in the neighbourhood of 25 HP up to 50 HP. High horsepower loads draw high current, up to 600%, of the motors full load rated amps. This high current inrush tends to draws the line voltage down and to prevent this condition wye-delta starters must be used.

In the electrical market today solid state soft starts have taken the place of wye delta starters.

When using a three phase contactor across the line for motor starting, a three phase overload block must be used. The wiring of the contactor consists of taking L1, L2 and L3 of the supply voltage and connecting it to the top of the contactor on terminals L1, L2 and L3 of the contactor. The motor load is connected to T1, T2 and T3 on the bottom of the overload block.

If you are using a three wire control for operating the contactor, the contactor must have a N.O. contact on it to seal the contactor in once the contactor coil has been energized.

The overload block has three N.C. contacts, one for each leg of L1, L2 and L3. These three contacts are wires in series to the stop - start circuit.

Years ago on old schematic diagrams these contacts used to be placed on the down stream side of the magnetic starter's coil (between the coil and the neutral). When the overloads tripped the voltage was left on the control circuit which was found to be dangerous for trouble shooting. Now for safety reasons, the overload contacts are positioned in the beginning of the start- stop circuit so that when a overload trips, the whole stop - start circuit becomes de-energized.