Two-Speed Fan Control Using Dual Contactors

Many greenhouse ventilation fans support multiple speeds for better climate control and energy efficiency. This guide explains how to control a two-speed fan using your Advancing Alternatives environmental controller with dual contactors housed in an interface box.

How It Works

Your environmental controller outputs low-voltage control signals—it does not directly switch the high-voltage AC power required by fans. Instead, these control signals activate contactors inside an interface box (such as our ECO1150), which then switch the high-voltage power to your fan.

For two-speed operation, you’ll use two separate contactors:

• Contactor 1 controls low speed
• Contactor 2 controls high speed

Each contactor is triggered by its own output from your controller, allowing you to set different temperature thresholds for each speed.

Example Operation

Let’s say you configure your controller as follows:

• Fan 1 (Low Speed): Activates at 70°F
• Fan 2 (High Speed): Activates at 80°F

At 70°F: The controller sends a signal to Contactor 1, energizing the fan’s low-speed winding.

At 80°F: The controller sends a signal to Contactor 2, energizing the fan’s high-speed winding. At this point, both signals are active, but your wiring ensures only the high speed operates.

Below 70°F: Both contactors are off, and the fan is off.

Components Needed

1. Environmental Controller – Must have at least two relay or output channels (one for each fan speed)
2. Interface Box – Houses the contactors and wiring (e.g., ECo1150 or similar enclosure)
3. Two Contactors – Rated for your fan’s voltage and amperage
   • Coils must match your controller’s output voltage (typically 24V AC or DC)
   • Contacts must be rated for fan motor load (check fan nameplate)
4. Two-Speed Fan – With separate windings or taps for low/high speed
5. Control Wire – Low-voltage wire from controller to interface box
6. High-Voltage Wiring – Properly rated wire from contactors to fan (per NEC and local codes)

Wiring Overview

IMPORTANT: All electrical installation must be performed by a qualified, licensed electrician in accordance with the National Electrical Code (NEC) and all local codes.

Low-Voltage Control Side (Controller to Contactors):

• Controller Fan 1 Output → Contactor 1 coil
• Controller Fan 2 Output → Contactor 2 coil
• Common return wire completes the circuit

High-Voltage Power Side (Contactors to Fan):

The exact wiring depends on your specific fan model. Common configurations include:

Option A: Separate Speed Windings

• Contactor 1 switches power to the low-speed winding
• Contactor 2 switches power to the high-speed winding
• When both signals are active, high speed takes priority

Option B: Tapped Winding

• Contactor 1 connects to the low-speed tap
• Contactor 2 connects to the high-speed tap
• Common wire and neutral complete the circuit

Always consult your fan’s wiring diagram to determine the correct configuration.

Controller Setup

Configure your controller with two separate fan outputs:

1. Output 1 (Low Speed):
   • Device: Fan 1 or Stage 1
   • Activation temperature: 70°F (example)
2. Output 2 (High Speed):
   • Device: Fan 2 or Stage 2
   • Activation temperature: 80°F (example)

Set your differential (hysteresis) appropriately to prevent rapid cycling.

Safety and Installation Notes

• Qualified Electrician Required: All high-voltage wiring must be installed by a licensed professional
• Proper Enclosure: Contactors must be housed in an enclosure rated for the environment
• Overcurrent Protection: Install appropriate fusing or circuit breakers for the fan circuit
• Motor Overload Protection: Consider thermal overload protection for the fan motor
• Interlocking (Optional): Some installations use mechanical or electrical interlocking to prevent both contactors from closing simultaneously, though proper controller setup typically handles this
• Grounding: Ensure all equipment is properly grounded per NEC requirements

Contactor Sizing

Select contactors rated for:

• Voltage: Match your line voltage (120V, 208V, 240V, etc.)
• Current: At least 125% of the fan’s full-load amperage (check fan nameplate)
• Motor Load Rating: Contactors have specific ratings for motor loads (resistive vs. inductive) – use the motor rating
• Coil Voltage: Must match your controller output (typically 24V AC or 24V DC)

Troubleshooting

Fan doesn’t run at low speed:

• Check controller output signal for Fan 1
• Verify Contactor 1 coil is energizing (listen for click, check LED if equipped)
• Verify power is reaching the low-speed winding when Contactor 1 is closed

Fan doesn’t switch to high speed:

• Check controller output signal for Fan 2
• Verify Contactor 2 coil is energizing
• Verify high-speed winding is receiving power when Contactor 2 is closed

Both contactors are engaging simultaneously:

• Check contactor/coil wiring – ensure low and high speeds are properly isolated

Fan cycles rapidly:

• Increase differential/hysteresis settings in controller
• Check for adequate contactor ratings (undersized contactors can cause issues)

Additional Considerations

• Energy Efficiency: Two-speed operation allows the fan to run at lower power during mild conditions, saving energy
• Noise Reduction: Low speed is typically quieter, beneficial during nighttime or in noise-sensitive areas
• Equipment Longevity: Reduced speed operation can extend fan motor life

Wiring Diagrams

The two diagrams below provide generic examples of how contactors can be configured in an interface box for two-speed fan control. These are illustrative examples only.

IMPORTANT: Every fan manufacturer has different wiring configurations for multi-speed operation. You must consult your specific fan’s wiring diagram and installation manual to determine the correct wiring method for your application. The diagrams provided are starting points—your actual installation may differ significantly based on your fan model, voltage, and speed configuration.

If you are unsure how to wire your specific fan for two-speed operation, contact your fan manufacturer for technical support before proceeding.

Click: 41-AegisTECP – Two Speed Fan Example

Click: AegisTEC Plus to CIBAC-2F2L for Two Speed Fan

Click: Two Speed Fan Example