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How to regulate AC fan speed?

 
In general, there are two ways to regulate AC fan speed:
 
1. Reducing the motor voltage
Reducing motor voltage reduces both motor speed and motor torque - this technology can be used with voltage controllable motors only. At startup, it is important to verify that the motor generates sufficient torque to avoid stalling. The required torque depends on the application and the type of load.
Most fans follow a quadratic torque curve - meaning that they require little torque at startup.
control motor speed Voltage reduction 
 
This is why fan speed control by reducing motor voltage is a very popular method in HVAC industry. It is simple and economical and it works fine for most HVAC applications.
 
Following fan speed controllers can be used to reduce motor voltage:
 
1.1 Fan speed control in steps
Transformer fan speed controllers are used for 5-step speed control for AC voltage controllable motors. These fan speed controllers are equipped with autotransformers. It is a simple, yet robust solution to control fan speed by adjusting the motor voltage in steps via autotransformer technology:
 5-step speed control 
In some cases, the fan speed controller can produce a humming noise due to the autotransformer technology. This noise is minimized by applying
a special impregnated coating to the autotransformers.
Thanks to the perfect sinus shape of the motor voltage, the motor will run very quietly.
  voltage reduction autotransformer
 
No configuration is required. If necessary, the minimum motor speed can be adjusted via the internal wiring to avoid motor stalling at low speed.
Transformer fan speed controllers are available for single or three phase voltage controllable motors up to 20 A.
 
 
1.2 Continuous variable fan speed control
Electronic fan speed controllers use Phase angle control (TRIAC technology) to adjust the motor voltage and to control the fan speed. The TRIAC cuts parts of the incoming voltage to reduce the overall motor voltage:
 phase angle control TRIAC
  
Thanks to this technology, the fan speed controller operates completely silently. Depending on the motor type, the motor might produce a little more noise at lower speeds (due to the imperfect sinus shape of the motor voltage). No configuration is required. If necessary, the minimum motor voltage can be adjusted to avoid motor stalling at low speed. In some cases, maximum available motor voltage can also be reduced to optimize fan speed for your ventilation system. Settings can be adjusted via internal trimmers or via Modbus RTU communication.
Available for single or three phase voltage controllable motors up to 10 A.
 
  

2. Motor speed and motor torque control
Regulating motor speed via Pulse Width Modulation or PWM technology offers continuous variable motor control. Motor speed and motor torque are controlled. PWM technology offers optimal motor control but is more expensive and more complicated to configure compared to the previous technologies. Depending on the application and operation mode, extra energy savings can be realized.
  
Fan speed control via PWM technology requires a frequency inverter or Variable Speed Drive (VSD).
Frequency inverters generate an almost perfect sinusoidal motor voltage via PWM (Pulse Width Modulating) technology using IGBTs (Insulated Gate Bipolar Transistors) that chop a DC voltage at a very high frequency. Configuration is required; all settings can be adjusted via the key button interface or via Modbus RTU communication. 
 pulse width modulation
  
It is important to verify that the motor windings can withstand the high voltage peaks generated by this technology. The ratio of voltage to frequency is kept constant, resulting in an optimal motor control and very silent operation of both motor and frequency inverter.
Available for single or three phase motors up to 46 A.
 
 
In all of these scenarios, the desired motor speed can be adjusted manually via a knob (local or remote control) or fully automatically based on CO2, air quality or another parameter (demand based). For demand based fan speed control, an HVAC sensor is connected to the fan speed controller. Fan speed is automatically optimized based on the sensor measurement. More information can be found on the Sentera solutions page.
  
We advise to contact the supplier of your fan to check which of the above mentioned  technologies are allowed for your motor.
 

  
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