In short. The generator controller is the electronic decision layer that starts and stops the engine, watches voltage, frequency, current, oil pressure, coolant temperature and battery condition, raises alarms, shuts the set down to protect it, and in most Indian installations also senses mains failure and commands transfer. It is not by itself a synchronising or plant management system. Its value lies in how its thresholds and timers are configured and tested, not in its feature list.
What the controller does, in order
Every generator above a few kVA now carries a microprocessor controller in place of the relay logic and analogue gauges of older panels. Its job is a chain: measure, compare, decide, act, record. It measures engine parameters through sensors and electrical parameters through voltage and current inputs, compares each with a configured threshold, decides whether the condition is a warning, a shutdown or a transfer event, operates relays to crank, fuel, trip or close breakers, and logs the event. On a standby set in auto mode the same controller runs the whole AMF sequence described on the AMF control page: it qualifies the mains failure, starts and stabilises the engine, commands the transfer, waits for the mains to return, retransfers, cools down and stops. The AMF panel that houses it adds the contactors or breakers, the battery charger and the field wiring that turn its decisions into action.
Measurements, alarms and shutdowns
The distinction between a warning and a shutdown is the most important configuration decision on the controller. A warning tells the operator something needs attention; a shutdown stops the set to prevent damage, which during a power cut means the site goes dark. Each protection should be classed deliberately for the site, within the limits the engine manufacturer sets.
| Parameter | Typical warning | Typical shutdown | Note |
|---|---|---|---|
| Lube oil pressure | Low pressure pre-alarm | Pressure below engine limit after start delay | Never bypassed; seconds of low pressure destroy bearings |
| Coolant temperature | High temperature pre-alarm | Above engine limit | Usually the first sign of an airflow or radiator problem |
| Speed and frequency | Under or over frequency outside window | Overspeed, typically 110 to 115 percent | Overspeed is a hard trip on every engine |
| Voltage | Outside window for a set time | Sustained over or under voltage | Delays prevent trips on motor starting dips |
| Current and kW | Overload pre-alarm | Overcurrent or over-kW after delay | Backs up the breaker, does not replace it |
| Battery | Low or high voltage, charger fail | Not normally a shutdown | The most common reason a standby set fails to start |
| Starting | Fail to start after configured attempts | Locks out further cranking | Usually three attempts of about 10 seconds each |
| Emergency stop | Not applicable | Immediate | Hard-wired, never routed through software alone |
Modern controllers also display phase voltages and currents, kW, kVA, kVAr, power factor, kWh, run hours, phase sequence and earth fault where the wiring allows. What appears depends on the installed model, the sensors fitted and the configuration; a retrofitted controller gains only the measurements its sensors actually provide.
What a standalone controller is not
A controller brochure can suggest that one device does everything. In practice there is a hierarchy, and knowing where a controller sits in it prevents expensive assumptions. A basic auto-start controller runs one engine and its protections. An AMF controller adds mains sensing and transfer control for one set and one mains supply. A synchronising controller adds the ability to match voltage, frequency and phase angle and close onto a live bus, plus load sharing with other sets, and it needs a governor and AVR that accept external bias signals. Above that, a master controller or plant management system coordinates several sets, decides how many to run, sheds and restores load, and manages the utility connection. Each level requires its own hardware, wiring and commissioning. The master controller page describes the top of the hierarchy.
Communication and remote monitoring
Most current controllers offer a serial or Ethernet port speaking Modbus, and many offer a cloud gateway with a mobile application that shows status, alarms and run hours. Three cautions apply. The capability depends on the exact model and firmware, so it must be confirmed before anyone relies on it. Remote viewing and remote control are different risk categories; a system that can start or stop the set from a phone needs access control and a written operating procedure, and many sites deliberately allow viewing only. And a gateway needs power during an outage, a SIM with a live data plan and a signal at the generator location, which is often a basement or a corner of the compound. Integration with a building management system through Modbus is routine, but the register map and alarm priorities should be agreed at design stage and tested at commissioning. The communications and remote monitoring page covers the options.
Control power and the readiness chain
The controller runs on the engine starting battery, so everything it does depends on 12 V or 24 V DC being present. A mechanically perfect set becomes unavailable if the battery is flat, the charger has failed, a control fuse has blown or the controller was left in manual or off after a service visit. A charger fed from a circuit that is dead during an outage keeps the battery up between outages but not through a long one with repeated cooldown stops. The controller should be configured to alarm on charger failure and low battery voltage, and those alarms should reach someone. The battery and starting system page explains what to check and how often.
Configuration, testing and the event log
A controller leaves the factory with default settings that suit no site exactly. Commissioning should set the mains failure thresholds and delays, the crank sequence, the warm-up and cooldown timers, the protection classes and delays, and the display units, then prove each function by simulation: pull the oil pressure sensor plug, lift the temperature sensor wire, drop a mains phase, and confirm the controller does what the design says. The settings should be recorded and a copy kept off the set. Afterwards, the event log is the most useful diagnostic tool on the machine; an unexplained nuisance shutdown is usually explained in two minutes by reading the last twenty events. Controller faults themselves, from corrupted configuration to failed outputs, are covered on the controller diagnostics page, and replacement modules and sensors on the controllers and sensors page.
Related photographs and videos
Common mistakes
- Leaving the controller in manual or off after a service visit; the next outage passes with the set silent.
- Accepting factory default thresholds; the set trips on under-voltage every time a large motor starts.
- Classing low fuel or high temperature pre-alarms as shutdowns; a nuisance condition blacks out the site in the middle of an outage.
- Assuming the brochure’s cloud monitoring exists on the delivered unit; the installed model has no gateway and nobody is told of a charger failure.
What to send us
Send a photograph of the controller face showing the make and model, a photograph of the panel interior if it can be opened safely, the alarm or shutdown text exactly as displayed, and a description of what the set was doing when the event occurred. For a new installation, tell us whether AMF, synchronising or remote monitoring is required. Use the service and AMC enquiry and we will advise on diagnosis, reconfiguration or replacement.
Questions engineers ask
What is the difference between a generator controller and an AMF panel?
The controller is the electronic module that makes decisions; the AMF panel is the enclosure that contains the controller together with the mains and generator contactors or breakers, the battery charger, protection devices and terminals. Most modern controllers include AMF logic, but the panel is still needed to switch the load.
Can an old generator be fitted with a new controller?
Usually yes, provided the engine has or can be fitted with the sensors the controller expects and the alternator sensing and current transformers are wired in. A retrofit is a common way to add auto-start, protections and remote monitoring to a sound older set, and it costs a small fraction of a new machine.
Why does my generator show “fail to start” when the engine is fine?
The controller has cranked the configured number of times without seeing speed or voltage. The usual causes are a weak battery, a fuel solenoid or stop lever not releasing, air in the fuel after a filter change, or a speed sensing fault, so the controller never knows the engine fired. Check battery voltage under crank before anything else.
Can I monitor my generator from my phone?
If the installed controller has a communication port and a gateway with a working data connection, yes, usually for status and alarms. Whether remote start and stop should be enabled is a separate decision that depends on who has access and whether someone on site could be endangered by an unexpected start.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.