What Is a Star-Delta Timer Relay? Types and Selection Criteria
A star-delta timer relay is a control relay that times the changeover from the star contactor to the delta contactor during star-delta starting of a three-phase motor.
A star-delta timer relay is a control relay that times the changeover from the star contactor to the delta contactor during star-delta starting of a three-phase asynchronous motor. The motor accelerates in star connection for the set time; when the time is up, the relay releases the star contactor and, after a short transition delay, energises the delta contactor. In star connection the motor's starting current and torque fall to about one third of their direct-on-line values.
How does it work?
The starter has three contactors, main, star and delta; the relay manages two times:
- Star time: The time set for the motor to run up to speed in star connection.
- Transition time: The short pause after the star contactor opens and before the delta contactor closes. It lets the arc in the star contactor clear and prevents both contactors being closed at once and causing a phase-to-phase short circuit.
For the method to work, the motor must be rated to run in delta at the supply voltage: on a 400 V supply the rating plate reads 400/690 V and the terminal box has six terminals.
Types and models in the catalogue
There are two types: electronic relays with their own supply, and pneumatic add-on blocks that fit on the front of a contactor and need no supply.
| Model | Type | Supply | Star time | Transition time | Output |
|---|---|---|---|---|---|
| Schneider Electric RE22R1QCMU | Electronic, 22.5 mm modular | 24–240 V AC/DC | 0.05–300 s, in ranges | 50 ms | 1 C/O, 8 A |
| Schneider Electric RE10RQM7N | Electronic, Harmony | 240 V AC | Not specified | Not specified | 1 NO + 1 NO, 5 A |
| Schneider Electric LADS2 | Pneumatic add-on block, TeSys Deca | Not required | 1–30 s | About 40 ms, fixed | 1 NO + 1 NC |
| Entes SER-YU (M4091) | Electronic | 230 V AC | Not specified | Not specified | Not specified |
| Tense ERV-YU | Electronic | 220 V AC | Not specified | Not specified | Not specified |
Schneider values are from the manufacturer datasheets; the Entes and Tense supply is from the product names.
Selection criteria
- Supply voltage: The supply of an electronic relay must match the control circuit voltage. For a 24 V DC control circuit the RE22R1QCMU suits; the LADS2 needs no supply.
- Star time range: The time the motor needs to run up to speed in star must fall within the setting range. For slowly accelerating loads such as fans and flywheels, check the upper limit of the range.
- Transition time: Too short a transition can let the delta contactor close before the arc in the star contactor has cleared. The transition time is 50 ms on the RE22R1QCMU and about 40 ms on the LADS2.
- Contact arrangement and load: Single C/O and two-output models are wired differently; take the wiring diagram from the datasheet. The power drawn by the contactor coils must not exceed the relay contact rating.
- Mounting: The LADS2 takes no separate panel space but fits only on the front of compatible TeSys contactors.
Common mistakes
- Treating the relay as motor protection: The relay only times the changeover; overload protection is provided by a thermal overload relay or a motor protection circuit breaker.
- Setting the thermal overload relay to the motor rated current: If the overload relay is connected in the winding circuit after the main contactor, it carries the winding current; the setting is about 0.58 times the motor rated current (In/√3).
- Leaving interlocking to the relay alone: The star and delta contactors must also be electrically interlocked through each other's NC contacts.
- Star-delta starting a 230/400 V motor on a 400 V supply: This motor runs in star on a 400 V supply; in delta its windings see overvoltage.
Related standards
- IEC 61812-1: Time relays for industrial and residential use.
- IEC 60947-4-1: Contactors and motor starters; star-delta starters fall within the scope of this standard.
Star-delta timer relays in the catalogue
The Star-Delta Timer Relays category has 5 active products: Schneider Electric 3 (RE22R1QCMU, RE10RQM7N, LADS2), Entes 1 (SER-YU) and Tense 1 (ERV-YU). The RE10RQM7N is priced on quotation; the others can be ordered directly.
The other parts of the starter circuit are in the Contactors and Thermal Overload Relays categories, and switches for manual starting are in the Star-Delta Switches category.
Frequently asked questions
How many seconds should a star-delta timer relay be set to?
There is no fixed value. Watch the motor current on the first start: set the relay so the changeover happens when the starting current has dropped and settled in star and the motor has reached speed.
What is the difference between a star-delta timer relay and an on-delay timer relay?
An on-delay relay times a single output. A star-delta relay switches the star and delta outputs in sequence with a transition delay between them.
What is the difference between a pneumatic block and an electronic relay?
A pneumatic block such as the LADS2 fits on the front of the contactor, needs no supply and works like a timed auxiliary contact. An electronic relay mounts on a DIN rail and has its own supply; the RE22R1QCMU offers a wider, stepped setting range.