6.2 Wye-Delta and Reduced-Voltage Starting
Key Takeaways
- Wye-delta starting starts the motor in wye so each winding sees line voltage over √3; line current and starting torque fall to about one-third of across-the-line values, then the starter transitions to delta to run.
- Large ammonia screws use reduced-voltage starting so inrush does not collapse plant voltage or exceed utility and transformer limits.
- Start the compressor unloaded; one-third torque may not accelerate a loaded screw, and an early transition still slams the bus with near full inrush.
- Star and delta contactors must be interlocked; both closed at once is a winding short.
- Soft starters and VFDs are common alternatives that also limit starting current without a wye-delta lead package.
A NEMA Design B motor started across the line (full voltage on a simple starter) draws locked-rotor current of about six times FLA. On a 300 HP screw that is a four-digit ampere spike for a few seconds. Lights dip, other motors stall or drop out, and the utility or the plant transformer may not be allowed to serve that start. Reduced-voltage starting exists to cut that inrush. Wye-delta is the method CIRO candidates are expected to understand in an ammonia machinery room.
What wye-delta starting actually does
The motor is built as a six-lead machine that can be connected in wye or in delta. The run connection is delta: each winding sees full line-to-line voltage (480 V on a 480 V bus). The start connection is wye: each winding sees line voltage / √3, about 58% of run voltage (about 277 V on a 480 V bus).
Torque of an induction motor varies with the square of winding voltage. (1/√3)² = 1/3, so starting torque is about one-third of across-the-line starting torque. Line current during a wye start is also about one-third of delta locked-rotor current. If across-the-line locked-rotor current is 6 × FLA, wye-start line current is on the order of 2 × FLA. That is still above FLA, but it is a start the transformer can often live with.
Those one-third factors are why operators must start large screws unloaded. Slide valve at minimum, capacity unloaders open, pump discharge not dead-headed. One-third torque against a fully loaded compressor may not accelerate the rotor. The motor then sits near locked rotor in wye, overheats, and the overload or the wye timer expires into a bad transition.
The starter, the timer, and the interlock
A typical wye-delta starter has three power contactors plus a transition timer:
- A line (main) contactor that connects the motor to the 480 V bus.
- A wye (star) contactor that ties the winding ends together for start.
- A delta (run) contactor that reconnects the windings in delta.
The wye and delta contactors are mechanically and electrically interlocked. If both close at once, the windings are paralleled in a way that puts a bolted short across the motor. A failed interlock is not a nuisance trip; it is a flash and a ruined stator. Never defeat those interlocks to “get the compressor up.”
Open-transition wye-delta briefly disconnects the motor when it leaves wye and before delta closes. During that gap the collapsing field can produce a voltage that is out of phase with the line, so the delta close can create a second current spike almost as ugly as a full-voltage start. Closed-transition designs insert resistors or an intermediate path so the motor is never fully open. Closed-transition costs more and is kinder to the bus.
The transition timer should hand the motor to delta only when speed is near synchronous (small slip). Transition too early and you still impose near locked-rotor current in delta. Transition too late and you overheat the wye-connected windings. On a screw package, watch amps, listen for the machine coming up to pitch, and confirm the manufacturer’s start sequence (often including oil-pump proof and unloaded slide valve) before you blame the timer.
Why ammonia plants still use it on large compressors
A 300 HP class screw on 480 V is a serious kVA load even at FLA. Six times that current on an across-the-line start can:
- Exceed the utility’s starting-current limit or the plant’s transformer inrush budget.
- Dip voltage so other motors on the same bus stall, drop contactors, or single-phase on undervoltage.
- Nuisance-trip instantaneous breaker settings that were cut too close to inrush.
- Annoy a neighboring process that cannot tolerate a 480 V sag.
Wye-delta is a relatively inexpensive electromechanical answer when the motor is specified with six leads. It is common on large condenser fans and pumps as well as compressors. It is not a speed control and it is not a substitute for unloading. Once the motor is in delta, it is a full-voltage machine; capacity control is the slide valve, vanes, or cycling — not the starter.
Part-winding and autotransformer starters are other reduced-voltage families. Autotransformer starting generally gives more torque per ampere than wye-delta and is used when the load needs a harder start than one-third torque. Part-winding requires a motor wound for it. CIRO’s working picture is still: cut inrush, then run at full voltage, and know which method your MCC actually uses before you change a timer.
Soft starters and VFDs as alternatives
A soft starter uses SCRs (or similar) to raise voltage smoothly. The usual current limit is in the 250% to 400% of FLA band — more current (and more torque) than a wye start, still far below 6 × FLA. Soft starters do not require six motor leads, so they retrofit three-lead motors. They still need a bypass contactor or equivalent thermal plan for long running, and they still need the compressor unloaded if breakaway torque is high.
A VFD limits starting current even more tightly (often near FLA or a modest multiple) because it ramps frequency and voltage together. The drive also becomes the capacity and rotation tool. VFDs bring harmonics, motor-insulation stress, and shaft-current issues that belong in the next electricity chapter; for starting current they are the most flexible reduced-inrush option in a modern room.
None of these devices changes the meaning of FLA. FLA remains the thermal baseline for overload protection while the motor is running. Starting means (wye-delta, soft start, VFD, autotransformer) change only the path to FLA. Overload relays must still ride through the remaining start current and still trip on a stalled or overloaded run. If someone “turns up” overloads because wye-delta starts nuisance-trip, the real questions are transition timing, loaded starts, low voltage, or a mechanical bind — not a new FLA.
Inrush, FLA, and what the operator watches
| Method | Typical line current during start | Typical starting torque vs across-the-line | Notes in an NH3 plant |
|---|---|---|---|
| Across-the-line | about 6 × FLA | 100% | Simple; brutal on large screws |
| Wye-delta (in wye) | about 2 × FLA (one-third of LRA) | about 33% | Needs six-lead motor; start unloaded |
| Soft starter | often 2.5–4 × FLA, adjustable | depends on voltage ramp | Three-lead motors OK |
| VFD | often about 1–1.5 × FLA | controlled by torque limit | Also provides speed control |
On a start attempt, a supervisor should see: control power healthy, all interlocks made (oil pressure or oil-pump proof as the package requires, discharge-valve state, high-level cutout, emergency stop reset), unloaded compressor, then a current spike that falls as speed rises, then — on wye-delta — a transition bump that should be smaller than the original spike if timing is right. A current that stays at locked-rotor value is a fail-to-accelerate condition: loaded start, seized compressor, single-phasing, or a starter that never left a bad connection.
If the plant recently swapped a motor, confirm it is six-lead before you expect the existing wye-delta starter to work. Landing only three leads on a wye-delta starter is a misapplication. If the plant recently swapped a starter to a soft start, the old six-lead motor can usually be reconnected in permanent delta in the box; leaving it in a half-made wye-delta scheme is how you get mysterious low torque.
Reduced-voltage starting is an electrical courtesy to the bus. It is not permission to ignore mechanical starting rules. Unload, prove oil, bump for rotation (next section), then start. Those steps matter more on a 300 HP screw than on a 5 HP condenser fan, but the physics is the same.
A six-lead compressor motor is started in wye on a 480 V bus, then run in delta. What should a CIRO operator expect during the wye-start interval?
Why do industrial ammonia plants put wye-delta or other reduced-voltage starting on large screw compressors instead of a simple across-the-line starter?
Locked-rotor current on a NEMA Design B motor started across the line is typically about six times FLA. What is the correct relationship between that inrush and the overload relay?
A three-lead 480 V pump motor needs lower starting current, but it cannot be reconnected for wye-delta. Which substitution is the usual plant alternative?