Control Systems In Draft Fans

Control system is applied in all kinds of mechanical devices in order to obtain the required output from the system. In case of draft fans, control systems include motor controllers, belt drives and adjustable speed drives. Among the above three motor controllers, the kind of drive used is very important. These controllers will decide the overall efficiency of the system.

Motor Controllers

• The motor controllers are switching mechanisms which take signals from a low power circuit such as on or off switch or a low, medium or high start selector switch, and configure it for energizing the motor windings to drive the motor at a required speed.
• If we consider a consequent pole motor with a single set of windings, it is equipped with a switch that energizes two pole configuration or a four pole configuration.
• In the case of multiple speed motor, the use of multiple windings and appropriate switching between 2-4-6 or 8 poles, provides variable speed levels like full, half, one-third or quarter speeds.
• The multiple speed motors are a bit expensive and are less efficient than the single speed motors.

Belt Drives

• These are used to reduce the rotational speed of the fans by using an appropriate ratio of pulley diameter to sheave for achieving the required fan speed.
• When a belt transfers the power from the motor to the fan, change in fan speed is obtained by changing the pulley ratio.
• The complete energy transfer to the fans by the motor is done by belts. The operation condition of the belt with the sheave and pulley, results in the efficiency of the speed change and power transfer.
• There are several types of belt drives. For example Flat belts, V-belts, cogged V-belts and synchronous belts.
• In belt drive system, there is a common problem of slippage which results in reduction of fan rotational speed causing an energy loss in the system, that finally leads to the reduction of over all air delivery.
• If the belt drive is over tighten, it results in early bearing failure as well as early belt failure.
• Some factors are taken into consideration while selecting the belt drive. They are belt size, maximum practical speed ratio, alignment of pulleys, variable pitch fan blades and adjustable speed drives.
• In the belt size there are again four factors that are to be considered, they are horsepower required by driven load, the site operating temperature, service factor and the arc contact between the belt and the sheave or pulley.
• Speed ratios of fan belts are limited to 4:1 which is the ratio of motor speed to the fan speed.
• The reason for having a higher speed ratio is having less area of contact between the pulley and the belt, which results in lower friction loss.

The control systems will also include other mechanical systems which make the system run more effectively and efficiently. Sometimes, common control systems use electronic circuits that help them to improve the performance and productivity.