In short. A generator feeds the plant’s power control centre (PCC), which feeds the motor control centres (MCCs), which feed the motors. Integration means deciding where the generator incomer lands, how it is interlocked with the transformer incomer, which MCC feeders return first after a transfer, what the capacitor bank does on generator supply, and how running motors change the fault duty. Get those five decisions right and a healthy set stops tripping on the first restart.
What a PCC and an MCC each do
In an Indian industrial plant the PCC is the main 415 V switchboard. It receives the transformer incomers and the generator incomers, holds the bus coupler, carries the automatic power factor correction (APFC) bank and sends high-current feeders onward. The MCC is organised around motors: each cubicle holds a breaker or fuse, a contactor, an overload relay and a starter, whether direct-on-line, star-delta, soft starter or variable frequency drive, with control wiring back to a PLC. A process plant may have one PCC and six or more MCCs spread across the shop floor. The wider design of these boards is covered under LV switchboards, PCC and MCC architecture; here the question is how the generator fits into them.
Where the generator incomer should land
There are three common arrangements, and the right one depends on how much of the plant the generator is meant to carry.
| Arrangement | How it works | Suits | Watch for |
|---|---|---|---|
| Generator incomer on the main PCC bus | DG and transformer incomers interlocked; whole bus transfers | Generator sized for full plant load | Entire motor population restarts at once |
| Essential bus section on the PCC | Coupler opens on mains failure; DG feeds one section only | Generator sized for part of the plant | Loads on the wrong section, coupler interlock logic |
| Separate essential PCC fed by ATS | Dedicated board downstream of an automatic transfer switch | Retrofits, buildings, utilities loads | Neutral switching, feeders left on the main PCC |
Whichever is chosen, the generator breaker and the transformer breaker on the same bus need an electrical and preferably a mechanical interlock so that the two sources cannot be paralleled unless the plant has been engineered for utility paralleling. Bus loading must be checked for the configuration where the generator carries the essential section through the coupler, because the coupler and the section bus then carry current they never see in normal operation.
Motors decide the transient behaviour
A generator is a soft source compared with the grid, and the MCC is where that matters. A motor started direct-on-line draws six to eight times its full-load current at a lagging power factor near 0.2 for the first second or two. The generator sees a sudden reactive load, its terminal voltage dips, the AVR forces excitation and the engine governor recovers the speed. A voltage dip beyond 15 to 20 percent can drop out contactors on other MCC feeders and reset drives.
Starting kVA = Motor kW × (kVA per kW at start)
The kVA per kW at start is taken from the motor’s code letter or manufacturer data, typically 6 to 7 for a standard squirrel-cage machine. Worked example (illustrative): a 75 kW DOL pump draws about 75 × 6.5, around 490 kVA, at the instant of starting. On a 500 kVA generator already carrying 200 kW this produces a dip well beyond 20 percent; on a 1,000 kVA set it is tolerable. The starter type changes the picture: star-delta roughly thirds the starting kVA, a soft starter limits current to around three times full load, and a VFD starts the motor at little more than full-load current but adds harmonic current the alternator must absorb. The motor starting on generators guide gives the full method and the starting method selector compares options.
Restoration: which motors come back, and in what order
After a transfer to generator the worst thing the MCC can do is re-energise every motor feeder at once. Contactors that dropped out on mains failure are usually arranged to stay out until a restart command, and the restart order should follow process need and generator capacity: cooling-water pumps and compressors first, then conveyors, then auxiliaries, with a few seconds between groups so each starting transient settles before the next begins. A plant with a PLC-controlled MCC can implement this in logic; a plant without one uses timed restart relays on the starter control circuits. The approach is detailed under restoration and restart sequencing. The same hierarchy supports load shedding when generation falls short, which belongs to load management.
Capacitor banks on generator supply
The APFC bank on the PCC is tuned for the grid-connected plant. On generator supply it can cause trouble in two ways. At light load, switched-in capacitors push the power factor leading, and a synchronous alternator supplying a leading load can lose voltage control and over-excite, with the AVR unable to hold the setpoint. During motor restarts the APFC controller may hunt, switching steps in and out as the load swings. The usual practice is to inhibit the APFC bank, or limit it to a small fixed step, whenever the generator breaker is closed, using an auxiliary contact from the generator incomer into the APFC controller. Where the generator runs for long periods and the DISCOM power factor penalty is not in play, this costs nothing.
Running motors contribute to faults
Induction motors act as generators for the first few cycles of a fault, feeding current back into the bus from their spinning rotors. On a utility-fed PCC this is a modest addition; on a generator-fed PCC, where the source contribution is smaller and decaying, the motor contribution can be a larger share of the total. For an MCC bus fault, current arrives from the PCC and from the motors on that MCC together. For a PCC bus fault, every running motor on every MCC feeds back through its feeder. The short-circuit study must include this, and the feeder breakers must be rated for the current they actually carry in each case, as explained in motor contribution to fault current.
Related photographs and videos
Common mistakes
- Placing the generator incomer on the main bus of a plant the generator cannot fully carry. The set overloads and trips within seconds of every transfer.
- Allowing all motor contactors to re-close together when generator voltage returns. The combined starting kVA collapses the voltage and the transfer fails.
- Leaving the APFC bank live on generator supply. The alternator runs at a leading power factor, voltage climbs and the AVR alarms.
- Rating MCC feeder breakers on the source contribution only. The motor contribution adds to the fault through the breaker and the rating is exceeded.
What to send us
Send the single-line diagram showing the PCC, MCCs and the proposed generator incomer, the motor list with kW and starting method for every motor above 15 kW, the APFC bank rating and controller type, and photographs of the PCC incomer and coupler cubicles. Tell us which process loads must return first after an outage. Use the application engineering enquiry page.
Questions engineers ask
Can the generator feed the whole PCC or only an essential section?
Either, provided the generator is sized for whatever bus it feeds. Most plants in our experience size the set for an essential section containing process-critical motors, lighting and utilities, and let the rest of the PCC stay dead until mains returns. Feeding the whole bus needs a set sized for the full maximum demand plus the largest restart transient.
Why do my motors trip when the plant transfers to generator?
Usually because several motors restart together and the combined starting current pulls the generator voltage down far enough for contactors to drop out or undervoltage relays to operate. Staggering the restarts, switching large motors to soft starters, or inhibiting the APFC bank on generator supply normally resolves it.
Should the APFC panel be switched off when running on generator?
In most plants, yes. Inhibit the automatic controller through an auxiliary contact from the generator breaker so that no capacitor steps switch in. If the plant runs on generator for long hours and needs some correction, leave a single small fixed step that keeps the power factor lagging under the lightest expected load.
Does adding VFDs to the MCC make generator operation easier?
Starting becomes easier because a drive ramps the motor up at near full-load current. Steady-state operation becomes harder because six-pulse drives inject fifth and seventh harmonic currents that heat the alternator and distort its voltage. The usual guidance is to keep drive load below about a third of generator capacity or to specify an oversized alternator and line reactors.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.