In short. Automatic mains failure logic watches the utility on all three phases, waits a confirmation delay so that momentary dips do not start the engine, cranks the generator, proves voltage and frequency, transfers the load, and reverses the process once the utility has been stable for a longer return delay before cooling and stopping the engine. Each step depends on a confirmed state rather than a command sent, and each abnormal branch needs a tested response.
The normal sequence step by step
An AMF controller is a state machine, and it is the first layer described in the generator controls centre. It moves from one state to the next only when a condition is confirmed, and every transition has a timer. The table below shows the normal sequence for a single diesel generator with a contactor or breaker changeover on a 415 V supply, with timer ranges typical in Indian installations. Actual values are set for the site.
| Step | Condition confirmed | Typical timer | Purpose |
|---|---|---|---|
| Mains failure detection | Voltage or frequency outside limits on any phase | 2 to 10 s confirmation | Ignore momentary dips and DISCOM auto-reclose attempts |
| Start | Crank command issued, engine fires, crank disconnect on speed or oil pressure | Up to 3 attempts of 5 to 10 s, rest 5 to 10 s | Protect starter motor, detect failed start |
| Warm-up and proving | Voltage and frequency within window, usually 0.9 to 1.1 pu and 47.5 to 52 Hz | 3 to 10 s | Do not connect an unstable source to the load |
| Transfer to generator | Mains contactor open confirmed, then generator contactor closed confirmed | Interlock delay 0.5 to 2 s | Break before make, avoid paralleling two sources |
| Mains return detection | All phases healthy and stable | 30 s to several minutes | Avoid repeated transfers on a flickering supply |
| Retransfer | Generator contactor open confirmed, then mains contactor closed | Interlock delay 0.5 to 2 s | Return load to utility |
| Cooldown and stop | Engine runs unloaded | 2 to 5 min | Let turbocharger and coolant temperatures settle before shutdown |
Why the delays exist
The confirmation delay on failure is not hesitancy. Telangana feeders see frequent short interruptions as the DISCOM’s auto-reclosers operate, and a controller that cranked for every one would wear the starter, exhaust the battery and transfer the load needlessly. Two to five seconds covers most reclose cycles. The return delay is longer again, because a returning supply is often unstable for a minute or more.
The voltage and frequency proving window matters more than most installers appreciate. If the generator contactor is allowed to close while the engine is still recovering from cold cranking, the load sees low frequency, the AVR under-excites to protect the alternator, and motors may stall. The deeper discussion of how a set responds to being loaded is in generator transient response.
Signals versus confirmed states
The commonest design weakness in AMF schemes is treating a command as proof of an outcome. The controller energises the generator contactor coil and assumes the load has transferred. If the contactor is mechanically jammed or the coil is burnt, the load sits dead while the controller displays a healthy picture. Good practice wires auxiliary contacts from both switching devices back to the controller, confirms each step before moving on, and raises a transfer-failed alarm if confirmation does not arrive. The same applies to engine running. Crank disconnect should use alternator frequency or a speed signal, with oil pressure as a backup, rather than a fixed time.
Abnormal conditions and the response to each
A functional description that only covers the happy path is incomplete. Each case below should have a written response and a commissioning test.
- Engine fails to start after the permitted attempts. The controller locks out, raises a fail-to-start alarm locally and to any remote system, and does not keep cranking. The load remains on the dead mains until an operator intervenes or a second set is started.
- Engine starts but voltage or frequency does not come into the window. The controller holds the transfer, alarms for under-voltage or under-frequency, and after a timeout shuts the set down rather than connecting a bad supply.
- Set trips on load for an electrical or engine fault. The generator contactor opens, the alarm latches, and automatic restart is blocked until the fault is reset by a person.
- Control power fails. The changeover should fail to a safe state, normally mains connected, and the loss of DC should itself be alarmed through a battery-fail or charger-fail output.
- Operator leaves the controller in manual or off. The set will not respond to a mains failure, so the not-in-auto condition must be alarmed and preferably reported remotely.
The changeover hardware
AMF logic drives a changeover, and the changeover’s type shapes the logic. Contactor-based changeovers below about 400 A are simple and fast but rely on electrical and mechanical interlocks to prevent both sources closing together. Breaker-based changeovers with motorised ACBs give fault withstand and proper isolation but have closing times of a few hundred milliseconds that the interlock delays must respect. Dedicated transfer switches, described under automatic transfer switches, bring their own controller and need only a start signal and a running feedback from the set. The panel options themselves are described under AMF panels.
Mains sensing done properly
Sense all three phases, and sense them at the point that represents the supply the load depends on, normally the incoming terminals of the mains breaker. Set under-voltage pickup around 80 to 85 percent and over-voltage around 110 to 115 percent of nominal, with phase-loss and phase-sequence detection enabled. In our experience single-phase sensing, inherited from an old relay panel, is the most frequent reason a factory’s motors run single-phased for an hour while the generator sits idle.
Commissioning the sequence
An AMF installation is commissioned by causing a mains failure, not by pressing the test button. With the site prepared and the load either disconnected or willing, open the incoming breaker and record the time of each event from the controller log, confirm the transfer and retransfer, and check voltage on each phase at the load during the generator period. Then repeat with a simulated failed start by isolating the fuel solenoid, and again with the controller deliberately left in manual to prove the alarm. Save the controller configuration file, note the timer settings in the handover record. The service side of this work is under ATS service.
Related photographs and videos
Common mistakes
- Setting the mains failure delay to zero or near zero. The set cranks on every DISCOM recloser operation, the battery and starter wear out early and the load sees unnecessary transfers.
- Using a fixed crank time instead of crank disconnect on speed or frequency. The starter stays engaged on a running engine or disengages before a cold engine has fired.
- Trusting the contactor command without feedback. A jammed or burnt contactor leaves the load dead while the controller shows a successful transfer.
- Omitting the not-in-auto alarm. A technician leaves the selector in manual after a service and the facility discovers it during the next outage.
What to send us
Send a photograph of the controller and the changeover panel with the door open, the generator nameplate, and a description of the fault or the behaviour you want, for example the delay before start, the behaviour during short outages or the retransfer timing. If a UPS is in the circuit, note its autonomy. For a new scheme, a modernisation of an old relay panel or a scheme that has stopped behaving, use the application engineering enquiry.
Questions engineers ask
How long should the generator take to pick up the load after a power cut?
For a healthy set with a typical configuration, about 10 to 20 seconds from the start of the outage. That is the sum of the mains failure confirmation delay, cranking, the voltage and frequency proving period and the transfer interlock. Loads that cannot accept that gap need a UPS, since no diesel set transfers instantly.
Why does my generator start but the load does not transfer?
Usually the controller has not received confirmation that the generator output is within its voltage and frequency window, or the generator contactor has failed to close and its feedback is missing. Check the proving limits, the alternator sensing wiring, the contactor coil and the auxiliary contacts before suspecting the controller itself.
Should the AMF transfer the neutral?
It depends on the earthing arrangement. Where the generator neutral is separately earthed at the set, a four-pole changeover avoids two parallel neutral-earth paths. Where the generator neutral is bonded only at the main earth bar, a three-pole changeover is common. The choice must follow the earthing design and the protection scheme, not habit.
Can one AMF controller handle two generators?
Not in the single-set sense. Two sets serving one load need either a generator-to-generator transfer scheme, where one is duty and one is standby, or a synchronising and load-sharing scheme where both can run together.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.