1. Motor Forward/Reverse Control Wiring Diagram (Dual Interlock + Push Button Interlock)
The core of motor forward/reverse control isphase reversal— swapping any two of the three phases. The forward contactor KM1 connects L1-L2-L3, and the reverse contactor KM2 connects L1-L3-L2 (swapping L2 and L3). The control circuit usesdual interlock:
- Contactor interlock: the KM1 normally closed auxiliary contact is wired in series in the KM2 coil circuit, and the KM2 normally closed auxiliary contact is wired in series in the KM1 coil circuit. When KM1 is energized, KM2 cannot energize, and vice versa.
- Pushbutton interlock: the normally closed contact of the forward push button SB2 is wired in series in the reverse circuit, and the normally closed contact of the reverse push button SB3 is wired in series in the forward circuit. Pressing the forward button first breaks the reverse circuit, then closes the forward circuit.
- Stop button SB1: wired in series in the main control circuit, normally closed contact, pressing it breaks all circuits.
- Thermal relay FR: normally closed contact wired in series at the very front of the control circuit, breaking the entire control circuit on overload.
2. Star-Delta Startup Wiring Diagram (Main Circuit + Control Circuit)
Star-delta startup is used forhigher-power motors (typically ≥18.5kW)reduced-voltage starting, cutting starting current to1/3of direct-on-line starting. Main circuit has 3 contactors:
| Contactor | Code | Function | Energizing sequence |
|---|---|---|---|
| Main contactor | KM1 | connects main power | Held in on startup |
| Star contactor | KM2 | Motor in Y connection (supply to the first ends, tail ends shorted) | Held in during the start phase |
| Delta contactor | KM3 | Motor in Δ connection (U1-W2 / V1-U2 / W1-V2) | Held in during the run phase |
3. Star-Delta Control Circuit Logic (switched by time relay)
Star-delta start control circuit logic4 steps:
- Start: Press SB2 → KM1 coil energizes (self-holding) + KM2 coil energizes + time relay KT energizes → motor starts in Y connection.
- Time Delay: KT delay expires (typically 5–15 s, set according to motor power) → KT time-delay opening contact opens the KM2 coil.
- Transfer: After KM2 opens, its normally closed interlock contact resets → KM3 coil energizes → motor transfers from Y to Δ run.
- Stop: Press SB1 to open the main control circuit; all contactors drop out and the motor stops.
At the moment of transfer, KM2 opens first and KM3 closes after, with a50–100 ms intervalbetween them, to prevent both contactors from being held in at the same time and short-circuiting the main circuit. KM2 and KM3must be interlocked(normally closed auxiliary contact interlock). The time relay KT setting = the time for the motor to reach rated speed, typically by the rule of thumb t ≈ 4×√P (P = motor power in kW).
4. Component Code Quick Reference (commonly used in control boxes)
| Code | Component | Common control box models |
|---|---|---|
| QF | Circuit breaker | Schneider NSX/CVX, Chint NXM |
| KM | AC contactor | Schneider LC1D, Chint CJX2 |
| FR/FT | Thermal overload relay | Schneider LRD, Chint NR2 |
| KT | Time relay | Schneider RE17, Chint JSZ3 |
| KA | interposing relays | Omron MY2NJ, Schneider RXM |
| SB | Pushbutton | Schneider XB2, Chint NP2 |
| HL | Indicator light | Schneider XB2-LED, Chint ND16 |
5. Wiring Practice Essentials (conductor sizes and terminal specifications)
Control box wiringConductor size selection: Size the main circuit according to motor rated current (e.g., an 18.5 kW motor at ~37 A uses 6 mm² copper); use 0.75–1.0 mm² throughout the control circuit. For terminal blocks, useSpring-type terminal blocks(such as Phoenix PTFIX) are more vibration-resistant and reliable than screw-type. Thermal relay outgoing terminals are markedU1/V1/W1(motor lead end). For star-delta switching, note the jumpers at U1/W2, V1/U2, and W1/V2.
FAQ FAQ
What motor power is suitable for star-delta starting?
Typically≥18.5kWmotors use star-delta starting. Below 18.5 kW, direct-on-line starting is acceptable; above 55 kW, a soft starter or VFD is recommended (star-delta starting torque is only 1/3 of direct-on-line torque, making large motors hard to start).
Is dual interlocking mandatory for forward/reverse control?
Yes, it is required. Contactor interlock (normally closed auxiliary contact) plus pushbutton interlock (normally closed pushbutton contact) provides dual protection. With only one interlock, a single contact failure causes a short circuit — dual interlocking is the safety baseline.
How do you calculate the time relay setting?
Rule of thumb t ≈ 4 × √P(P is motor power in kW). For a 22 kW motor, t ≈ 4×√22 ≈ 19s. During actual commissioning, switch over when motor speed approaches rated speed — switching too early causes high inrush current, switching too late causes severe heating in the Y connection.
Should contactor coils in a control box have an RC snubber?
Recommended. When an AC220V coil disconnects, the reverse electromotive force can pit the pushbutton contacts and time relay contacts. An RC snubber (such as 47Ω + 0.1μF) can extend contact life by 2-3 times.
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