Which inverter replaces a Control Techniques Unidrive on an AHU extract fan?

A Danfoss Vacon NXP 7.5 kW inverter replaces the Control Techniques Unidrive on the AHU-2 extract fan. It takes a 400V 3-phase input and accepts a 0-10V analog input with digital run/stop, maintaining BMS integration from the Trend IQ4.

How do you wire a BMS 0-10V signal to a Vacon NXP inverter?

Connect the BMS 0-10V signal to Vacon AI1+ on Terminal 6 and the BMS 0V common to Vacon AGND on Terminal 7. Optionally, connect a BMS digital output to Vacon DI1 on Terminal 9 for run/stop control.

What parameters do you set on a Vacon NXP driving a 7.5 kW extract fan?

Set Motor Nominal Voltage to 400V, Motor Nominal Current to 14.5A, Motor Nominal Frequency to 50Hz, and Motor Nominal Speed to 1450 RPM. Set AI1 as the speed reference with 0-10V mapping to 0-50Hz, minimum frequency 10Hz, maximum frequency 50Hz, and acceleration and deceleration times of 10 seconds each.

How do you safely isolate an AHU fan inverter before replacement?

Isolate power at the local isolation switch, then lock out and tag it. Use a voltage tester to confirm zero voltage at the inverter terminals, then discharge the DC bus capacitors by waiting 10 minutes after isolation.

Why does a fan run at full speed after an inverter replacement?

A fan running at full speed regardless of the 0-10V signal usually means the analog input is not configured. Check that parameter AI1 is set as the speed reference. If the fan does not respond at all, the 0-10V wiring may be incorrect or loose at AI1+ and AGND.

AHU-2 Extract Fan Inverter Replacement: Unidrive to Vacon NXP

  To replace a Control Techniques Unidrive with a Danfoss Vacon NXP inverter on an AHU extract fan, isolate and remove the old drive, mount and wire the 7.5 kW Vacon NXP, connect the BMS 0-10V signal to AI1, set the motor nameplate parameters, then commission against the Trend IQ4 output.

  Why would you replace an AHU fan inverter?
  An inverter (VSD) is replaced when it can no longer do its job:
  
    Component failure: Inverter no longer powers on or trips frequently
Obsolescence: No spare parts or support available
Energy efficiency: Modern inverters offer better motor control and energy savings
BMS integration: Newer models support improved communication protocols
  

  What does the AHU-2 extract fan setup involve?
  This job replaces a Control Techniques Unidrive with a Danfoss Vacon NXP while keeping BMS integration via a 0-10V analog signal. The extract fan is 7.5 kW, 3-phase at 400V, driven from a Trend IQ4 BMS and protected by a contactor with overload relay. The replacement Vacon NXP takes 400V 3-phase input and accepts 0-10V analog with digital run/stop.

  What checks come before swapping the inverter?
  
    Record existing inverter parameters (speed reference, ramp times, minimum/maximum frequency)
Document wiring connections (power, control, and earth)
Confirm motor nameplate details match new inverter rating
Check that isolation switch is accessible and functional

`How do you safely isolate the existing inverter?`


  Treat the drive as live until proven dead:
  
    Isolate power to the existing inverter using the local isolation switch
Lock out and tag the isolation switch
Use a voltage tester to confirm zero voltage at inverter terminals
Discharge DC bus capacitors (wait 10 minutes after isolation)
  

  How do you remove the old Unidrive?
  Label everything before pulling a wire on the Control Techniques Unidrive:
  
    Label all wiring before disconnection
Disconnect power cables (L1, L2, L3, Earth)
Disconnect motor cables (U, V, W)
Disconnect control wiring (0-10V, digital inputs, earth)
Remove inverter from mounting bracket
  

  How do you install and power-wire the Vacon NXP?
  
    Mount inverter on DIN rail or panel (ensure adequate ventilation)
Connect power input cables to L1, L2, L3 terminals
Connect motor cables to U, V, W terminals
Connect protective earth to PE terminal
  

  How do you wire the BMS 0-10V control signal to the Vacon?
  
    BMS 0-10V signal → Vacon AI1+ (Terminal 6)
BMS 0V common → Vacon AGND (Terminal 7)
Optional: BMS digital output → Vacon DI1 (Terminal 9) for run/stop control
  

  What parameters do you set on the Vacon NXP?
  
    Motor Nominal Voltage: 400V (match nameplate)
Motor Nominal Current: 14.5A (match nameplate)
Motor Nominal Frequency: 50Hz (UK standard)
Motor Nominal Speed: 1450 RPM (match nameplate)
Analog Input 1 (AI1): Speed reference, 0-10V = 0-50Hz
Minimum Frequency: 10Hz (prevents motor stall)
Maximum Frequency: 50Hz (UK standard)
Acceleration / Deceleration: 10 seconds each (smooth ramp)
  

  How do you commission the replacement inverter?
  Bring the drive up slowly and confirm it follows the BMS:
  
    Re-energise the inverter and check for fault codes
Manually set BMS analog output to 2V (20% speed)
Enable fan via BMS and confirm motor starts smoothly
Gradually increase BMS output to 10V and observe fan response
Check motor current and compare against nameplate rating
Verify fan direction (reverse if needed by swapping two motor phases)
Test ramp-up and ramp-down times
  

  What are the common faults after an inverter swap?
  Most post-swap problems trace back to motor parameters or 0-10V wiring:
  
    Trips on overcurrent: Motor parameters incorrect — re-enter nameplate data and run auto-tune
Fan does not respond to BMS signal: 0-10V wiring incorrect or loose — check AI1+ and AGND connections
Fan runs at full speed regardless of 0-10V signal: Analog input not configured — check AI1 is set as speed reference
Fan rotates in wrong direction: Incorrect phase sequence — swap any two motor phase wires (U, V, W)
  

  Is replacing an obsolete inverter worth it?
  Replacing a failed or obsolete inverter is straightforward if documented and tested. The Danfoss Vacon NXP offers improved reliability, energy efficiency, and BMS integration compared to older Control Techniques Unidrive models.
Need help with inverter replacements or BMS upgrades? Contact Alpha Controls for expert support, or request a quote for your AHU project.