When connecting a 3-phase asynchronous motor to a Variable Frequency Drive (VFD), one critical step often trips up engineers and technicians: Should the motor be wired in Star (Y) or Delta (Δ)?
A common misconception—carried over from traditional direct-on-line (DOL) motor starting—is that Star is used for starting and Delta is used for running. When operating with a VFD, this is false.
With a VFD, the motor is wired in a fixed Star or Delta configuration based strictly on the VFD output voltage and the motor’s nameplate specifications.
In this guide, we break down the exact wiring rules, nameplate readings, and VFD parameter adjustments to ensure your motor operates safely without burning out or underperforming.
1. The Core Rule: Voltage Matching (Not Starting Torque)
Traditional Star-Delta starters use contactors to reduce starting current on grid power. However, a VFD already controls starting current and torque by smoothly ramping frequency (Hz) and voltage (V) together (V/f control or Vector Control).
Therefore, your choice between Star and Delta depends entirely on matching the VFD’s output voltage to the motor’s phase winding rating.
Reading the Motor Nameplate
Every standard 3-phase induction motor has a nameplate indicating two voltage levels:
230V / 400V (Δ / Y):
The internal individual winding is rated for 230V.
Wire in Delta (Δ) if supplying 230V 3-phase.
Wire in Star (Y) if supplying 400V 3-phase.
400V / 690V (Δ / Y):
The internal individual winding is rated for 400V.
Wire in Delta (Δ) if supplying 400V 3-phase.
Wire in Star (Y) if supplying 690V 3-phase.
2. Terminal Box Wiring: Jumpers Setup
Inside the motor terminal box (terminal block with U1, V1, W1 and W2, U2, V2), copper jumper plates dictate whether the motor is in Star or Delta configuration:
Star Connection (Y) – High Voltage Setup
Jumper Configuration: Link W2, U2, and V2 together with horizontal jumpers to form a central neutral point. Connect the 3 supply phases from the VFD (U, V, W) to U1, V1, and W1.
Voltage Applied: Full line voltage (e.g., 400V) is split across two windings. Each winding receives Line Voltage divided by 1.732 (approx. 230V).
Delta Connection (Δ) – Low Voltage Setup
Jumper Configuration: Place three vertical jumpers linking W2-U1, U2-V1, and V2-W1. Connect the VFD output wires (U, V, W) directly to these pairs.
Voltage Applied: Each individual winding receives the full line voltage (e.g., 230V from a 1-phase 230V input VFD).
3. Selecting the Connection Based on VFD Input / Output Power
To select the correct connection, identify your supply grid and your VFD type:
| Grid Supply / VFD Type | VFD Output Voltage | Motor Nameplate (230V Δ / 400V Y) | Motor Nameplate (400V Δ / 690V Y) |
| 1-Phase 230V Input VFD (Single-phase grid to 3-phase out) | 3-Phase 230V | Delta (Δ) | Do NOT use (Underpowered) |
| 3-Phase 400V Input VFD (Standard industrial European grid) | 3-Phase 400V | Star (Y) | Delta (Δ) |
Crucial Warning: If you wire a 230V/400V motor in Delta and connect it to a 400V 3-phase VFD output, you will over-excite the stator iron, draw excessive current, and burn out the motor windings within minutes.
4. The 87 Hz Technique: Boosting Power with Delta Wiring
Advanced automation engineers often use a trick called the 87 Hz Characteristic to extract up to 73% more power from a standard motor without overheating it.
How It Works:
Take a standard 230V / 400V (Δ / Y) motor.
Wire the motor terminal block in Delta (230V rating).
Connect it to a 400V 3-phase VFD (such as an INVT Goodrive series).
Set the VFD base frequency to 87 Hz (instead of the standard 50 Hz) and set maximum voltage to 400V.
Why It Works:
Because the motor is wired in Delta, it reaches full magnetic flux at 230V / 50Hz. By increasing the frequency up to 87 Hz, the VFD scales the voltage proportionally up to 400V (400V / 1.732 = 230V). The motor maintains its full rated torque all the way up to 87 Hz, delivering higher horsepower output at higher RPM while keeping current within safe thermal limits.
5. Summary Checklists for Installation
Before powering up your drive, run through this sanity check:
Check Motor Nameplate: Verify line-to-line voltage ratings.
Check Drive Specs: Verify actual maximum output voltage of the drive.
Inspect Jumpers: Ensure horizontal bars for Star or vertical bars for Delta are tightly bolted.
VFD Parameter Settings: Set Motor Rated Voltage, Rated Current, and Rated Frequency (P02 group in INVT drives) strictly according to the connection choice made in Step 3.
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