What Is Star-Delta Connection? Switch Selection Guide
Star-delta reduces the starting current to one third by connecting the motor first in star and then in delta; check current, size and enclosure when selecting a switch.
What is star-delta and where is it used
Star-delta is a starting method that runs a three-phase induction motor first in star connection and, after it approaches its speed, in delta connection, reducing the starting current and starting torque to about one third of direct-on-line starting. Manufacturer catalogs consider it suitable for starts with no load or low moment of inertia; machine tools, centrifugal compressors and woodworking machines are given as examples.
The method works as follows: the motor windings are initially connected in star, and each winding sees 1/√3 of the mains voltage. When the motor approaches its speed, the windings are switched to delta and the motor runs at normal voltage. This transition can be made automatically with three contactors and a timer relay. A star-delta cam switch, whose position is selected with the shaft, makes the same transition manually; see the manufacturer's diagram for the position sequence.
The method cannot be applied to every motor. All winding ends of the motor must be brought out to the terminal box (at least six ends), and the motor must be built for delta connection at the mains voltage; for example, on a 380 V mains a motor labeled 380/660 V is required. Because torque also drops in star, the motor may not be able to accelerate if the load torque is high. High transient currents can occur during the switch from star to delta.
What to look at when choosing a star-delta switch
- Current: Selected according to the motor nameplate current and the manufacturer data. In this category, values such as 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A and 80 A are listed. Models of 100 A also appear in product names.
- Phase type: All products that state a phase type are 3-phase.
- Size type: Products that state a size type have values of 66x66 and 90x90. Match this dimension with the manufacturer data on the product page for your panel door cutout and for replacing an existing switch.
- Position diagram: Some product names include the term "Center 0". Compare the diagram of the existing machine with the manufacturer's position diagram.
- Enclosure: There are models whose product names include the terms "enclosed" and "motor switch"; check the enclosure status on the product page. If a separate enclosure is needed, see the Switch Enclosures category.
A cam switch is not a short-circuit breaking device. The motor circuit must also be protected with a fuse, a motor protection circuit breaker or a thermal overload relay; values are determined according to the project and the manufacturer data. For options you can use the Motor Protection Circuit Breakers and Thermal Overload Relays pages. You can check the supply cable with the cable current carrying capacity calculation tool.
Difference between a manual switch and automatic star-delta
A star-delta cam switch leaves the transition to the operator's handle movement. In an automatic arrangement, three contactors and a star-delta timer relay make the transition time-based.
For two-speed motors, Two-Speed Dahlander Switches are a separate category. If only opening and closing the circuit is enough, On-Off Switches may be sufficient.
Brands in this category
- Opaş: 3-phase star-delta switches; "Center 0" models in 66x66 and 90x90 size types, with values of 20 A, 25 A, 32 A, 40 A, 63 A and 80 A in the example names.
- Metop: star-delta motor switch and enclosed switch models; values of 20 A, 25 A, 32 A, 50 A, 80 A and 100 A are seen in the example names.
Order notes
Products are sold by the piece. Some products are listed as "lead-time" items; for these, the supply time is communicated separately after the order. You can search by stock code.
Comparison and selection table
| Selection Criterion | Manual Star-Delta Rotary Cam Switch (0 - Y - Δ) | Automatic Contactor Star-Delta Starter | Star-Delta Timer Relay Selection |
|---|---|---|---|
| Operating Sequence | Operator turns handle to Star (Y), then shifts to Delta (Δ) at speed | Starts on Main + Star contactors, automatically transitions to Delta | Inserts a mandatory 30–100 ms pause (dwell time) between Y and Δ |
| Motor Nameplate Rule | Motor must be rated Δ 380/400 V (400/690 V) for 400 V mains | Requires 6 motor leads (U1-V1-W1 & U2-V2-W2) wired to panel | Prevents phase-to-phase short circuit while Star arc extinguishes |
| Current Sizing (AC-3) | Cam switch sized to full motor kW & AC-3 current rating | Main & Delta contactors sized to 0.58 × In; Star contactor 0.33 × In | Thermal overload relay in phase loop is set to 0.58 × In |
| Typical Application | Manual workshop saws, blowers, mills & compressors | Automated pumps, HVAC fans, compressors & hydraulic power packs | Always pair automatic starters with mechanical & electrical interlocks |
Frequently asked questions
What is a star-delta connection, and how much does it reduce the starting current?
Star-delta connection is a starting method that first connects the motor in star and switches it to delta when it approaches its speed. According to the ABB catalog, the starting current and torque drop to one third of direct-on-line starting. The actual value depends on the motor and the load; for selection, the motor and manufacturer data should be consulted.
Which motors can be started star-delta?
It is applied to three-phase motors whose winding ends are all brought out to the terminal box (at least six ends) and which are built for delta connection at the mains voltage. The voltage on the motor nameplate shows this, such as 380/660 V for a 380 V mains. The load must be able to accelerate with the low torque in star.
What changes between the star and delta positions in a star-delta connection?
In the star position, each winding sees 1/√3 of the mains voltage, so current and torque drop. In the delta position, the windings operate under full mains voltage. In a cam switch, the position diagram provides this transition; use the project and manufacturer diagram for connection.
What is the difference between a star-delta switch and a star-delta with contactors?
In a star-delta cam switch, the operator makes the transition manually by turning the handle. In an arrangement with contactors, three contactors and a timer relay make the transition automatically and time-based. With the manual transition, the operator's timing is decisive; the automatic arrangement requires a control circuit and additional panel space.
Does a star-delta cam switch protect the motor?
No. A cam switch is not a short-circuit breaking device and does not protect the motor against overcurrent. The motor circuit must also be protected with a fuse, a motor protection circuit breaker or a thermal overload relay. The value of the protective device is determined according to the motor nameplate, the project and the manufacturer data.
What do 66x66 and 90x90 mean when choosing a star-delta switch?
These are values listed as Size Type on the products. The panel door cutout, the dimensions of the product that will replace the existing switch, and the current value are evaluated together. Check the manufacturer data on the product page for what the size type measures.