Timing Relays in Star-Delta Motor Starters
How does a timing relay work in a star-delta motor starter? It runs the timing sequence between the three contactors: closing the star contactor, holding it through the motor's run-up interval (commonly a few seconds up to roughly 30 s for medium-size motors), then opening star and holding a short transition dead-time, typically tens of milliseconds, before closing delta. Get the transition too short or skip it, and the star and delta contactors overlap, bridging two line phases through the motor windings. This piece covers contactor sequencing, run-up and transition setting, panel wiring, timer selection, and the mistakes we see in installed star-delta starters.
How the Timer Sequences Star, Transition and Delta
A star-delta starter uses three contactors: line (or main), star, and delta. The star-delta timing relay decides when the star and delta contactors get their coil signal — the line contactor stays closed through the whole cycle. On power-up, line and star close together and the motor runs in star, drawing roughly a third of its direct-on-line starting current. The timer counts the run-up time, then de-energizes star. After the transition dead-time it energizes delta, and the motor finishes accelerating and runs at full torque in delta.
Two output configurations exist. A single-function on-delay timer plus an interposing relay can build the sequence from separate parts, but most panels now use a dedicated two-output star-delta/multifunction timer — Schneider Zelio Time RE22R or ABB CT-VWD/CT-YDD/CT-SDD — that gives both the run-up and transition timing from one 17.5–22.5 mm modular unit. Both output contacts are change-over (SPDT), sized to switch a contactor coil, not the motor current itself, so a modest amp rating covers nearly every panel.
Setting the Run-Up (Star) Time
Run-up time is set by a rotary range selector plus a dial, not left at a factory default. It should cover the time the motor needs to reach close to rated speed under its actual load — a lightly loaded pump accelerates faster than a high-inertia fan or a loaded conveyor drive of the same frame size. Set it too short and the delta contactor closes while the motor is still drawing near-starting current in star, which defeats the point of star-delta starting and stresses the delta contactor's contacts.
What we see in the field: timers left at whatever range the previous panel builder used, on a motor with a different load. A five-second star time works for a small unloaded pump; the same setting under a loaded gearbox can leave the motor still accelerating when delta closes.
Why the Transition Dead-Time Is Safety-Critical
Star and delta contactors must never be closed at the same instant. If they are, two line phases bridge through the motor windings and draw a fault current well above the motor's rated current, tripping upstream protection at best and welding contacts at worst. The transition dead-time exists to guarantee separation: the timer holds both contactors open for a short interval after star opens, long enough for the star contactor's main contacts — and any arc — to fully clear before delta is commanded closed.
Formula: Star-Delta Cycle Time — Source: IEC 61812 timing relay definitions, general star-delta starting practice
tcycle = tstar + ttrans
| Symbol | Description | Unit |
|---|---|---|
| t_star | Run-up time the motor spends connected in star before changeover | s |
| t_trans | Transition dead-time between opening star and closing delta | ms |
| t_cycle | Total time from star contactor closing to delta contactor closing | s |
Some panel builders shorten the transition to cut the total starting time. This depends on the contactor's rated break time and the arc-clearing behavior of the specific contactor — shave it too far and the safety margin the setting exists for disappears. See setting a star-delta transition time for how to size it against contactor data.
Wiring the Timer Into the Starter Circuit
The timer's supply is usually the same control voltage as the contactor coils, tapped after the start/stop circuit so it only runs while the starter is commanded on. Its output contacts feed the star and delta contactor coils through electrical interlocking — normally-closed auxiliary contacts on each contactor wired into the other's coil circuit — so a contactor welded closed cannot be bridged by the other closing regardless of what the timer commands. A mechanical interlock between star and delta contactors adds a second layer where panel space allows it.
Full step-by-step terminal wiring, including 2-wire vs 3-wire trigger arrangements, is covered separately: see how to wire a timing relay and, for the contactor side of the same circuit, wiring a star-delta motor starter.
Overload protection sits on the line side or, in some designs, in the delta branch — check the manufacturer's diagram before assuming the overload relay sees the full motor current in both star and delta. A thermal or electronic overload sized for the motor's full-load current, not the reduced star current, protects the winding correctly through both stages of the start.
Selecting a Star-Delta Timer for the Panel
Match the timer to three things: the timing range needed (most multifunction star-delta timers cover roughly 0.1 s to tens of seconds or more on the star side, with a separate fixed or adjustable transition range), the supply voltage — multi-voltage AC/DC models cut inventory across panel types — and the DIN-rail width available, since a two-output multifunction timer at 17.5–22.5 mm replaces two single-function timers plus an interposing relay. Contact rating matters too: the output only has to switch the contactor coil current, not the motor current, so a modest SPDT/DPDT rating is normally enough.
General selection criteria that apply across timing relay types, not just star-delta, are covered in how to select a timing relay. For the contactors themselves, see selecting the right contactor.
Star-Delta Timing vs Soft Starter Ramping
A star-delta timer only gives two fixed current steps — star, then delta — not a ramped start. For motors that start frequently, drive high-inertia loads, or need reduced mechanical shock on start and stop, a soft starter ramps voltage continuously instead of switching between two connections, and is worth comparing against a star-delta setup at the quotation stage. Browse soft starters alongside the timing and control relays and contactors used in a star-delta build.
Common Field Mistakes in Star-Delta Timing Setup
Three mistakes account for most of the star-delta timer complaints we see. First, a run-up time copied from another panel without checking the motor's actual load and inertia. Second, a transition time trimmed toward zero to shave a second off starting, without checking the contactor's break time first. Third, missing or defeated electrical interlocking, so a timer fault or a welded contactor has no independent block against a phase-to-phase closure. None of these are timer defects — they are setup and interlock decisions made once at commissioning and rarely revisited.
Frequently Asked Questions
What does the timer actually control in a star-delta starter?
It sequences the star and delta contactors: closing star, holding for the run-up time, opening star, holding the transition dead-time, then closing delta. It does not switch motor current directly — the contactors handle that; the timer only orders the coil operations.
How long should the run-up (star) time be set?
Set it to cover the time the motor needs to reach close to rated speed under its actual load, not a fixed default. High-inertia or heavily loaded drives need a longer run-up than a lightly loaded pump; too short a setting switches to delta before starting current has dropped, defeating the purpose of star-delta starting.
Why not set the transition time to zero?
A zero or too-short transition risks the star and delta contactors closing at nearly the same instant, bridging two line phases through the motor windings and drawing a high fault current. The transition dead-time gives the star contactor time to fully open and its arc to clear before delta closes.
Can one timer relay handle both the run-up and transition timing?
Yes. Multifunction star-delta timers such as the Schneider Zelio Time RE22R or ABB CT-VWD/CT-YDD provide both timing stages and two output contacts from a single 17.5–22.5 mm modular unit, instead of wiring two separate single-function timers plus an interposing relay.
When is a star-delta timer not the right choice?
For motors that start frequently, or where controlled acceleration and reduced mechanical shock matter, a soft starter is the better fit. Star-delta gives two fixed current steps, not a ramped start, and the transition still needs careful setting regardless of how good the timer is.
Conclusion
A star-delta timer's job is narrow but exacting: hold the motor in star long enough to bring starting current down, then leave enough dead-time before delta closes that the two contactors never overlap. Set the run-up time from the motor's real load, size the transition against the contactor's break time rather than trimming it for speed, and back the timer with proper electrical interlocking. Get those three right and the rest of the timing relay engineering guide applies the same way it would to any other timing application.
