Fan speed is controlled with thyristor or transformer speed controllers.
Thyristor speed control.
Thyristor speed controllers provide smooth manual motor speed control and air flow control respectively. Operation of the thyristor speed controllers is based on output voltage control with a triac voltage regulator.
Several fans may be connected to one controller if their total current does not exceed the maximum permissible controller current.
Thyristor controllers are featured with high control efficiency and accuracy. When operating in low-speed mode the fans with thyristor speed control may generate unusual noise, so the thyristor speed controllers are not recommended for low-speed applications. Low-voltage motor application results in reducing bearings service life. The recommended speed control range is 60% till 100%.
Transformer speed control.
Transformer speed controller operation is based on a five-step power transformer that regulates power supply voltage to the fan motors with permanent voltage frequency.
Transformer controllers are designed for voltage-controlled motors. Several fans may be connected to one power transformer if their total current does not exceed the maximum permissible controller current.
When operating in low-speed mode the fans with transformer speed control generate no unusual noise. However the motor bearings service life can be reduced as a result of continuous low-voltage operation mode (speed 1 or 2).