In short. AMF and ATS logic turns source measurements and breaker states into start and transfer decisions. Good logic is a clear state machine with qualified inputs, timeouts, interlocks and alarms. The most common real failure is not the logic but the system left in the wrong mode.
Inputs
- Utility voltage, frequency and phase sequence.
- Generator voltage, frequency and running signals.
- Breaker or contactor positions.
- Mode selector: AUTO, MANUAL, TEST, OFF.
- Emergency stop, fault and lockout signals.
- Remote start or inhibit signals.
States
| State | Entry | Exit |
|---|---|---|
| Normal on utility | Utility OK | Utility fails for set time |
| Starting | Start command | Generator running or fault |
| Warm-up and stabilise | Running | Source acceptable |
| On generator | Transfer complete | Utility returns for set time |
| Retransfer | Return confirmed | Transfer complete |
| Cool-down | Off load | Timer ends |
| Fault or lockout | Protection or failed sequence | Manual reset |
Interlocks
The two source breakers must be mechanically or electrically interlocked so both cannot close together. Use feedback from auxiliary contacts to confirm positions, not only command outputs. See interlocking.
Test modes
- Test off load: start and run without transfer.
- Test on load: transfer to generator, then return.
- Simulated mains failure: safest realistic test for the sequence.
Define who may run each test and ensure the system returns to AUTO afterwards.
Alarms for readiness
- Not in AUTO.
- Low fuel, low coolant, low battery voltage, charger fault.
- Breaker not in service position.
- Failure to start or transfer.
- Controller fault or communication loss.
Neutral and earthing
Where the generator neutral is earthed, switch the neutral during transfer. See earthing architecture and ATS service.
Illustrative example
After a service visit, a generator is left in MANUAL. On the next outage, nothing starts. The fix is not only better training but also an alarm and a remote indication that shows “Standby not ready” whenever the selector is not in AUTO, plus a post-service checklist step.
A state machine for AMF
| State | Entry condition | Exit |
|---|---|---|
| Mains healthy | Voltage and frequency within window | Mains out of window for failure delay |
| Start sequence | Failure confirmed | Engine running or failure to start |
| Warm-up | Engine running | Voltage and frequency stable for set time |
| On generator | Transfer completed | Mains healthy for return delay |
| Retransfer | Mains stable | Load back on mains |
| Cool-down | Generator off load | Timer complete, engine stops |
| Fail-to-start | Crank attempts exhausted | Alarm, lockout, reset |
Start attempts
Typical cranking: three attempts of 5 to 10 seconds with 5 to 10 second rests, with the starter protected against overheating. After the last failure, alarm and lock out.
Permissives for transfer
- Generator voltage and frequency inside the window.
- Mains breaker confirmed open (or interlock released).
- No generator protection trip active.
- Mode in automatic.
Abnormal sequences to design for
- Mains returns while generator is warming up.
- Mains fails again during cool-down: return to load without a full restart.
- Generator fails while on load: retransfer to mains if available.
See interlocking and state machine engineering.
The master engineering question
At this moment, if the utility failed, would everything in the chain be ready, and how would anyone know without a test?
What to send us
Send controller model and the single-line diagram. Send your application details.
Questions engineers ask
What is the difference between AMF and ATS?
AMF starts the generator; ATS switches the load. Many panels combine both.
Why feedback from the breaker?
A command does not prove the breaker moved.
How often to test?
Regularly, including on-load tests, per site policy.
Can logic be customised?
Yes, within the controller’s limits and with documented testing.
This guide gives general principles. Follow the manufacturer’s instructions.