Abstract Where the control of ventilation rate in livestock buildings is achieved by controlling the voltage to the fan motor to adjust its speed, it is necessary to understand the motor speed-control characteristics. This paper evaluates the speed control characteristics of a number of fans commonly available in the UK and shows that a wide range of capabilities exist. Sensitivity of control as shown by the speed/control voltage characteristic and stability of control as shown by the main winding current/control voltage characteristic are discussed. The merits of controlling the voltage to the main winding only whilst supplying the auxiliary winding at full voltage (three-wire control) are shown. Two-wire control should not be used. Considerations in the use of transformer and triac speed controllers are outlined.