3 Phase Dol Digital Auto Switch Preventer

A Digital Auto Switch Preventer (also known as an Automatic Phase Preventer, or Digital Phase Preventer) is an electronic control device used mainly in three-phase electrical systems to automatically manage power supply and protect equipment from phase-related faults.
⚙️ 1. Purpose / Function

A Digital Auto Switch Preventer automatically:

• Detects phase failure, phase reversal, or low/high voltage conditions.

• Ensures that connected 3-phase motors, pumps, or equipment receive correct phase sequence and healthy voltage.

• Prevents operation under unsafe conditions (which can burn motors or cause malfunction).

• In some models, automatically switches between main and backup power sources (Mains/Generator)

�� 2. Working Principle

Here’s how it works step-by-step:

a) Phase Sensing

• The preventer continuously monitors all three phases: R (Red), Y (Yellow), and B (Blue).

• It uses voltage sensing circuits (usually with microcontroller or comparator ICs) to check:

  • Presence or absence of each phase.

  • Correct phase sequence (R-Y-B).

  • Voltage level within limits.

b) Fault Detection

• If any of the following occurs:

  • Phase failure (one phase missing)

  • Phase reversal (wrong sequence)

  • Under-voltage or over-voltage

• The circuit immediately disconnects the output relay, preventing power from reaching the connected motor/control panel.

c) Relay Control

• When all conditions are normal:

  • The relay energizes, allowing supply to pass through to the load (motor, contactor, etc.).

• When fault is detected:

  • The relay de-energizes, cutting off the load supply.

d) Auto Reconnection

• Once the fault is cleared (all phases OK, voltage stable), the preventer automatically:

  • Waits for a short delay time (e.g., 60 seconds)

  • Then reconnects the supply to the load.