In short. An alternator’s output voltage is fixed by how its stator coils are connected. A twelve-lead machine can be wired for several voltages, but the kVA stays the same, so a lower voltage means proportionally higher current through cables, breaker and current transformers. In India the standard connection is series star, 415 V line to line and 240 V line to neutral, neutral earthed. Any reconnection is a design change to be checked against the nameplate and the installation.
How the stator produces three voltages
The stator carries three groups of coils spaced 120 electrical degrees apart. As the rotor field sweeps past, each group produces a sinusoidal voltage displaced by a third of a cycle from its neighbours. The magnitude is set by flux per pole, turns in series and speed, so for a given machine at 1,500 rpm the only variable left open is how the coil groups are connected to each other and to the terminals.
In a star connection the three phase windings share a common point, the neutral, and the line-to-line voltage is root three times the phase voltage. In a delta connection the windings form a closed triangle, the line-to-line voltage equals the phase voltage, and there is no neutral. Indian low-voltage practice uses star almost exclusively, because the 240 V single-phase supply for lighting and small loads is taken between a line and the neutral, and because the neutral provides the reference for earth fault protection.
The balanced relationships
VLL = √3 × VLN, and I = S ÷ (√3 × VLL)
Here VLL is the line-to-line voltage in volts, VLN is the line-to-neutral voltage in volts, S is the apparent power in volt-amperes and I is the line current in amperes for a balanced load. As a worked example (illustrative), a 500 kVA alternator connected in series star at 415 V has a phase voltage of 415 ÷ 1.732 ≈ 240 V and a full-load line current of 500,000 ÷ (1.732 × 415) ≈ 696 A. If the same machine were reconnected for 240 V line to line, its full-load current would be 500,000 ÷ (1.732 × 240) ≈ 1,203 A, and every conductor and device between the alternator and the load would have to carry that. Power factor does not enter this calculation; it decides how much of the kVA is useful kW, not the current. Our current calculator guide covers the single-phase and unbalanced cases.
What a reconnectable machine offers
Most industrial alternators from a few tens of kVA upward have twelve leads, giving two coil groups per phase and four connections: series star, parallel star, series delta and parallel delta. Smaller machines may have six leads, allowing only star or delta, and single-voltage machines have four leads with the connection made internally. The table shows each connection on a machine wound for 415 V in series star.
| Connection | Line-to-line voltage | Line-to-neutral voltage | Full-load current at 500 kVA | Neutral available |
|---|---|---|---|---|
| Series star | 415 V | 240 V | About 696 A | Yes |
| Parallel star | 208 V | 120 V | About 1,390 A | Yes |
| Series delta | 240 V | None | About 1,203 A | No |
| Parallel delta | 120 V | None | About 2,406 A | No |
These figures are illustrative; exact voltages come from the nameplate diagram, and the regulator’s set point and sensing must change with the connection. In Indian service the lower-voltage connections are rare and appear mostly on machines imported from 60 Hz markets. A 12-lead machine also allows a dedicated single-phase zig-zag or double-delta connection, at a reduced rating the manufacturer specifies, typically around two thirds of the three-phase figure.
Why reconnection is a design change
- Current rating of everything downstream. Terminal lugs, the alternator breaker, busbars, cables and the current transformers feeding the controller and protection relays are all sized for one current. The cable sizing and breaker frame must be redone for the new value.
- Regulator sensing. The AVR senses line-to-line voltage, sometimes through a transformer. It must be re-set and, on some machines, re-wired for the new voltage, or it will regulate to the wrong value.
- Protection settings. Overcurrent pickups, CT ratios, under- and over-voltage thresholds and earth fault settings all assume the original connection.
- Neutral and earthing. Moving to a delta connection removes the neutral, which breaks the earthing arrangement required by the Central Electricity Authority regulations for low-voltage systems and removes the 240 V single-phase supply. The implications are set out under earthing and neutral grounding.
- Rating. The kVA does not change in most cases, but single-phase and some parallel connections carry a reduced rating that must be read from the manufacturer’s data.
Winding pitch and why it matters at 415 V
A coil spanning a full pole pitch produces the highest fundamental voltage but also the largest third-harmonic component. A coil shortened to two thirds of the pole pitch loses a little fundamental and cancels the third harmonic almost entirely, which is why generator alternators are wound that way. Third-harmonic voltages in a star-connected machine appear between line and neutral, are in phase on all three phases, and drive current through the neutral and earth path when the neutral is solidly earthed, as it is in Indian practice. When two sets are paralleled, any third-harmonic mismatch drives circulating current through both neutrals. A two-thirds pitch machine keeps the line-to-neutral waveform clean and makes paralleling straightforward; confirm it on any alternator intended for parallel or heavily single-phase service.
Unbalanced loading
An alternator supplying unequal loads on its phases carries negative-sequence current, which heats the rotor surface, and zero-sequence current, which flows in the neutral. Manufacturers limit the unbalance, commonly to about 10 percent difference between phase currents with no phase above rated, and the voltage on lightly loaded phases rises because the regulator senses an average or a single phase. Distribute single-phase loads across the phases at the design stage and check the balance with a clamp meter at commissioning. The heating consequences are explained under temperature rise, insulation and IP.
Common mistakes
- Reconnecting a 12-lead alternator to a lower voltage without resizing the breaker and cables; the conductors carry up to twice their design current and overheat at the terminals.
- Assuming every alternator is reconnectable because it has a terminal box full of leads; six-lead and four-lead machines cannot be reconnected and the nameplate diagram is the only authority.
- Changing the connection and leaving the AVR sensing as it was; the regulator holds the old voltage on the new connection and the output is badly wrong.
- Loading one phase heavily with single-phase welding or lighting; the other phases rise in voltage, the neutral runs hot and the rotor heats from negative-sequence current.
What to send us
Send a photograph of the alternator nameplate and its connection diagram, a photograph of the terminal box with the links visible, the voltage and current you need at the load, and whether a neutral is required. For an unbalance or voltage complaint, send the three phase voltages and currents measured at the alternator terminals under load. Send your application details and we will confirm what the machine can be connected for and what else must change. Replacement terminal hardware and windings are listed under alternator parts.
Questions engineers ask
Can a 415 V alternator be reconnected to give 433 V or 440 V?
Not by reconnection; the available connections are in fixed ratios of root three and two. The regulator’s set point usually allows adjustment of several percent either way, so 415 to 433 V is within normal AVR range on most machines. Check that connected equipment and the protection settings accept the higher value.
Why does the neutral carry current when my three-phase load is balanced?
Either the load is not as balanced as it appears, or non-linear loads are injecting third-harmonic currents that add in the neutral rather than cancelling. On a full-pitch machine the alternator itself contributes third-harmonic current. Measure the neutral current with a true-RMS clamp meter and compare the phases.
Can I draw 240 V single-phase from a three-phase generator?
Yes, between any line and the neutral of a star-connected machine, within the unbalance limits the manufacturer allows. For a wholly single-phase load, a 12-lead machine can be reconnected in a dedicated single-phase arrangement at a reduced rating, which is a better use of the machine than loading one phase of a three-phase connection.
Does changing the connection change the kVA rating?
For the standard three-phase connections, no; the winding carries the same current per coil and the kVA is unchanged. Single-phase connections and some parallel arrangements carry a reduced rating stated by the manufacturer, and the regulator and cooling must be checked at the new current.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.