Setting a Star-Delta Timer: Transition and Changeover Time
How do you set a star-delta timer's transition time? The transition time (dead-time) is set as a separate rotary or DIP-selected interval, typically tens of milliseconds up to a few seconds, on the dedicated star-delta timing relay or timer module per IEC 60947-4-1. It exists to open the star contactor fully before the delta contactor closes, because closing both together shorts two line conductors through the motor windings. This article covers what the timer is actually timing, how to set run-up time versus transition time, the arithmetic behind total cycle time, typical rotary/DIP procedures on ABB CT-D and Schneider Zelio Time hardware, and the field symptoms of a transition set too short or too long.
What the Star-Delta Timer Is Actually Timing
A star-delta starter uses three contactors: line, star, and delta. The star-delta timing relay controls two intervals in sequence, not one. First it holds the star contactor closed while the motor accelerates on reduced (line/√3) voltage. Second, once it drops out star, it holds both contactors open for a short dead-time before it drives delta closed. Confusing these two settings is the single most common wiring-desk error on a star-delta panel.
Cheap timer modules sold as "star-delta timer" often only expose one dial and use a fixed internal transition time, commonly 50 ms. Multifunction relays from the Schneider Zelio Time RE22R range or the timing and control relays collection expose both intervals independently, which matters once motor size or contactor mechanical release time pushes outside that fixed default.
Run-Up Time: Sizing the Star Interval
Run-up time is not a timer spec — it is a motor and load spec that the timer must be set to match. It covers how long the motor needs on reduced voltage to reach roughly 80-85% of rated speed, where the current transient at the star-to-delta switch stays manageable. Light loads (fans, pumps at low inertia) run up in a few seconds. High-inertia loads (large flywheels, centrifuges) can take 15-30 s or longer.
Set the star interval too short and the motor switches to delta while still well below rated speed, producing a current and torque spike close to a direct-on-line start — defeating the purpose of star-delta starting. Set it too long and the motor sits at reduced torque longer than needed, which matters on loads that need to overcome static friction or a check valve on start.
Transition (Dead) Time: Why It Cannot Be Zero
Between star and delta, the timer must hold both contactors open. A contactor does not open the instant its coil de-energizes — the moving contacts and arc take a finite mechanical and electrical release time, commonly in the 10-40 ms band depending on contactor size and whether it is AC or DC coil operated. If the delta contactor closes before the star contactor's contacts have fully parted, line L1 and L2 (or whichever pair overlaps) are connected through the motor winding with close to zero impedance between them — a line-to-line short, not a motor fault.
What we see in the field: undersized dead-time settings pass a bench test with a new contactor and fail six months later once contact wear slows the release slightly — the fault looks intermittent, but the timer setting was marginal from day one.
Calculating Total Star-Delta Cycle Time
The timer's two settings combine into one number worth writing on the panel schedule: total time from start command to full delta run.
Formula: Star-Delta Total Cycle Time — Source: IEC 60947-4-1, motor starter coordination practice
ttotal = tstar + ttransition
| Symbol | Description | Unit |
|---|---|---|
| ttotal | Total elapsed time from line contactor pickup to delta contactor pickup | s |
| tstar | Run-up interval with star contactor closed, set on the timer's first range | s |
| ttransition | Dead-time between star contactor drop-out and delta contactor pickup, set on the timer's second range or fixed internally | s (typically 0.02-2 s) |
ttransition is small next to tstar — often under one second against a run-up of 5-20 s — which is exactly why it gets overlooked on the setting sheet. It is the term that determines whether the starter survives or arcs, not the term that determines starting performance.
How to Set the Timer: Rotary Range, DIP and Potentiometer
Multifunction timing relays such as the ABB CT-D star-delta variants and Schneider Zelio Time RE22R combine a function selector, a time-range selector, and a fine-adjust potentiometer (or a digital rotary/push-button set on newer multifunction units). The sequence for a rotary/DIP unit:
1. Confirm the module is set to the star-delta function (not on-delay or off-delay — many multifunction units default to function A). 2. Select the run-up time range on the first range switch (for example the 1-10 s or 10-100 s band) so the potentiometer resolution covers the target run-up time, then fine-set with the dial. 3. Select the transition time range separately — some units fix this internally at a single value (commonly ~50 ms) with no user adjustment; check the datasheet function table before assuming it is adjustable. 4. Power-cycle and time a dry run with contactors only (motor disconnected or on a bench) to confirm both intervals against a stopwatch before connecting the motor.
This depends on which timer family is on the panel — some star-delta modules are single-function with one adjustable dial and a fixed transition, others are full multifunction units off the same 22.5 mm platform used for on-delay and off-delay functions, so the same base relay covers several starter types once configured. See how to select a timing relay for choosing between single-function and multifunction parts, and how to wire a timing relay for the socket and terminal layout common to both.
Common Setting Mistakes and Field Symptoms
Transition set to zero or near-zero on a unit that allows manual adjustment: audible bang or visible arc flash at the contactor pair on every start, breaker or fuse nuisance trips, contactor contact pitting concentrated on the delta contactor. Run-up set too short: high current draw persists past the switch point, thermal overload relay trips on start, motor hums audibly at the transition. Run-up set too long: no fault, just wasted acceleration time and unnecessary heat in the motor windings on repeated starts. Function selector on the wrong mode: timer never times out, or times out and drops everything instead of sequencing star then delta — check this first when a "timer" fault is reported, since it is a setup error, not a component failure.
Some panel builders still lock the transition dial with thread-locker or a sealed cover after commissioning, precisely because it is the one setting where a stray bump during maintenance has consequences beyond a mistimed cycle. For related failure patterns unrelated to star-delta specifically, see general timing relay troubleshooting for contact and range-selection faults.
Where This Fits in a Star-Delta Panel
The timer is one of three coordinated devices — line contactor, star contactor, delta contactor, all sized to the motor's rated and star-mode current — and none of the settings above substitute for correct contactor and overload sizing. For the wiring sequence and terminal-by-terminal connections between the timer and the contactors, see wiring a contactor for star-delta motor starting. For starters above roughly 30-40 kW where mechanical switching itself becomes the constraint, compare against a soft starter, which removes the star/delta transition arc question entirely by ramping voltage electronically instead of switching windings.
Frequently Asked Questions
What is a typical transition (dead) time on a star-delta timer?
Commonly in the tens of milliseconds up to roughly 1-2 s, sized above the star contactor's rated mechanical release time. Many low-cost star-delta timer modules fix this internally around 50 ms with no user adjustment; multifunction units expose it as a separate range.
Can the transition time be set to zero?
No. Zero transition time means the delta contactor can close before the star contactor's contacts have fully parted, connecting two line conductors through the motor winding with near-zero impedance between them. Even units that allow a very low setting still need it above the contactor's release time.
How long should the star (run-up) interval be set?
Long enough for the motor to reach roughly 80-85% of rated speed on reduced voltage — typically a few seconds for light loads and up to 30 s for high-inertia loads. It is a motor/load parameter, not a fixed timer default, and should be confirmed against observed acceleration behavior.
Does every timing relay support star-delta function?
No. Single-function on-delay or off-delay relays do not. Star-delta requires either a dedicated star-delta timer module or a multifunction relay (such as ABB CT-D or Schneider Zelio Time RE22R families) with the star-delta function selected on its function switch.
What happens if the run-up time is set too short?
The motor switches to delta before it has reached sufficient speed, producing a current and torque transient close to a direct-on-line start. Symptoms include thermal overload tripping on start and an audible motor hum or growl at the switch point.
Why does the transition setting matter more than it looks like on paper?
It is small in absolute terms — often well under a second against a 5-20 s run-up — but it is the setting that prevents a line-to-line short through the motor winding at every single start. An undersized transition can pass initial testing on a new contactor and only fail once contact wear extends the release time.
Conclusion
Two numbers, two purposes: run-up time matches the motor and load, transition time protects the contactor pair from overlap. Set the function selector to star-delta first, confirm whether the transition interval is fixed or adjustable on the specific module, size run-up to the load's acceleration, and leave margin on transition above the contactor's rated release time. Time a dry run with a stopwatch before the motor is connected — it catches a wrong function-selector setting before it becomes a bang at commissioning.
