ATS & AMF Explained

How an automatic standby generator takes over when mains power fails.

An automatic mains-failure system monitors the normal utility supply and controls the sequence required to start the generator and transfer the electrical load. When mains power returns and is stable, the system transfers the load back and allows the generator to cool down before stopping.

ATS and AMF: what is the difference?

ATS generally refers to the automatic transfer switching equipment that transfers the load between mains and generator supplies. AMF refers to the automatic mains-failure control function that monitors the mains supply and manages the generator start, transfer, return and shutdown sequence. In practical standby installations the functions work together as one automatic backup-power system.

1. Mains supply is monitored

The controller continuously monitors the incoming mains supply. If the supply fails or moves outside the configured operating limits, the AMF sequence can begin after the programmed delay.

2. Start delay prevents unnecessary starts

A short configurable delay can prevent the generator from starting for a momentary mains disturbance. If the mains failure remains present after the delay, the controller sends the engine start command.

3. The generator starts

The engine cranks and starts automatically. The controller monitors generator conditions and allows the set to reach acceptable operating voltage and frequency before the load is transferred.

4. The load transfers to generator supply

Once the generator supply is stable, the transfer equipment disconnects the load from mains and connects it to the generator. Interlocking is used so the mains and generator sources are not connected together unintentionally in a standard open-transition arrangement.

5. The generator carries the load

The generator supplies the connected load while the controller continues monitoring operating conditions and the mains supply. Correct generator sizing remains important because motor starting currents and changing site loads can affect performance.

6. Mains power returns

When the mains supply returns, the controller normally waits for it to remain stable for a programmed period. This avoids transferring the load back during an unstable restoration.

7. The load transfers back to mains

After the return delay, the transfer equipment reconnects the load to the mains supply and removes it from the generator supply.

8. Cool-down and shutdown

The generator can continue running without load for a programmed cool-down period before the controller stops the engine and returns the system to standby, ready for the next mains failure.

Controls and system configuration

The exact sequence, delays, alarms and protection settings depend on the controller and the electrical design of the installation. GenTech SA generator builds use automatic control systems configured for the application, with SmartGen controls used across many of our generator packages.