Must be same

Must be different

Must be low

Must be high

Frequency must be same

Voltage must be different

Sequence must be different

Polarity must be different

To get more voltage

To share more loads

To improve efficiency

To maintain constant frequency

Dark and bright lamp method

Ward - leonald method

Over excitation method

Under excitation method

All 3 lamps in dark

All 3 lamps in bright

Two lamps bright and one lamp dark

One lamp bright and two lamps dark