1. Fan Load Characteristics and Starting Method Selection (Comparison of 3 Configurations)
Fans arevariable-torque loads(power ∝ speed³), with low starting torque but high inertia. Three starting methods:
| Configuration | Applicable Power | Starting Current | Advantages | Drawbacks |
|---|---|---|---|---|
| Direct-on-line starting | ≤7.5kW | 4-7x Ie | Simple, low cost | High inrush |
| Star-delta starting | 7.5~55kW | 1/3 of direct-on-line starting current | Moderate cost | Switching inrush |
| VFD speed control | 11~250kW+ | Adjustable from 0 to rated | 30-50% energy savings | Highest cost |
2. Star-Delta Fan Control Cabinet Wiring Diagram (Main Circuit + Time Relay Switching)
The star-delta starting wiring diagram for fans is similar to that for pumps, but thetime relay setting is longer(High fan inertia, long acceleration time):
- Main circuit: QF breaker → KM1 main contactor → KM2 star contactor (start) → KM3 delta contactor (run) → thermal overload relay FR → motor (delta connection, 6 terminals U1/V1/W1/U2/V2/W2).
- Control circuit: Start button SB2 → KM1 self-hold + KM2 energized + KT energized. After the KT delay (typically 15–30 s), KM2 opens → KM3 energizes → motor switches from Y to delta run.
- Interlock: KM2 and KM3 normally closed auxiliary contacts interlock to prevent simultaneous pull-in and short circuit.
- Damper interlock: Large fans require the damper closed before start (to reduce starting load); KM1 auxiliary contact interlocks the damper actuator.
3. VFD Fan Control Panel Wiring Diagram (VFD + PID + temperature control)
Compared with pump panels, the VFD fan control panel addsClosed-loop temperature control:
| Signal | Source | Wiring | Function |
|---|---|---|---|
| Temperature signal | Temperature sensor (PT100 / 4–20 mA) | VFD AI0 | PID feedback, adjusts speed by temperature |
| Frequency reference | PLC/DCS analog output | VFD AI1 | Remote target speed setting |
| Start/Stop | Pushbutton or BA system | DI0/DI1 | Start/stop control |
| Fault output | VFD relay | Intermediate relay → BA system | Fault alarm |
| Run feedback | VFD AO | BA system | Operating frequency feedback |
4. Two-Speed Fan Control Panel Wiring Diagram (high/low speed switching)
Two-speed motors havetwo winding sets(e.g. 4-pole/8-pole, corresponding to 1500/750 rpm); the control panel switches the winding connection through two contactor groups:
- Low-speed run: KM_L contactor closes → motor low-speed winding connected in Y or delta (per nameplate) → motor runs at low speed.
- High-speed run: KM_H contactor closes → motor high-speed winding connected in YY (double star) → motor runs at high speed.
- Interlock: KM_L and KM_H normally closed auxiliary contacts interlock to prevent simultaneous pull-in and winding burnout.
- Switching logic: High speed to low speed → open KM_H first → delay 0.5s → close KM_L; low speed to high speed → open KM_L first → delay 0.5s → close KM_H. The delay prevents arc short circuits.
When switching the motor from high speed to low speed, the rotor still rotates at close to high speed due to inertia, while the low-speed winding, once energized, has a low synchronous speed, so the motor enters aregenerative braking state, and the inrush current is large. For high-to-low switching, we recommend adding a 2-3s delay to let the motor slow down before closing the low-speed contactor, or using a VFD for soft switching.
5. Common mistakes in fan control cabinets (4 red lines)
① Star-delta transition time too short → the fan switches to delta before reaching rated speed, causing high inrush current; ② VFD-to-fan motor cable >100m without an output reactor → high-frequency pulses break down the motor insulation; ③ high- and low-speed contactors for a dual-speed fan not interlocked → both close at once and short the winding; ④ large fans started without closing the damper first → starting load too high and the thermal overload relay trips.
FAQ FAQ
What is the difference between fan VFD and pump VFD parameters?
Mainly theV/F curve settings. Fans and pumps are both variable-torque loads, so set the V/F mode to asquare-law decreasing curve(P1300=2 or 4); at low speed the voltage drops automatically to save energy. However, fans need longer acceleration/deceleration times than pumps (larger inertia), typically 20-60s acceleration and 30-120s deceleration.
Must dual-speed fan high/low speed switching be delayed?
A delay is required. When switching from high speed to low speed, the motor is in a regenerative braking state, and closing the low-speed contactor immediately causes a large inrush current. Delay 2-3s until the motor speed drops near the low-speed synchronous speed before closing. Low-to-high switching can be done directly (motoring state, no inrush).
What star-delta starting time should be set for a fan control cabinet?
Fans have large inertia and long acceleration times. Rule of thumb: t ≈ 6 × √P (50% longer than for pumps). For a 22kW fan, t ≈ 6×√22 ≈ 28s. During actual commissioning, listen to the sound — the best switch point is when the motor changes from a humming sound to smooth running.
How much energy can a VFD fan save?
30-50%. For fans, per the affinity laws, flow ∝ speed and power ∝ speed³. Reducing speed by 10% reduces flow by 10% but reduces power by 27% (1-0.9³=0.27). Actual savings depend on damper opening and operating conditions — the smaller the original damper opening (the greater the throttling loss), the more energy is saved after a VFD retrofit.
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