Prime movers for fans
The majority of fans are driven by an electric motor, the squirrel cage induction type being the most popular, except in the smaller sizes. This Chapter points the user to the selection of appropriate types of prime movers for fans, and also describes starting and running characteristics.
Just as important to the selection of the correct motor type is a knowledge of how the power absorbed by the fan varies with time, temperature and barometric pressure. The inertia of the impeller may be significant and will affect both the motor type and its control.
The majority of fans are driven by a separate electric motor. There are some exceptions to this general statement e. g. so called “inside out” electric motors may incorporate the fan impeller within their overall construction. It would then be difficult to separate the fan impeller from the (rotating) motor stator without a major de-construction.
Furthermore, fans driven by internal combustion engines are not unknown in the agricultural and marine industries. The public utilities, especially, use fans driven by steam turbines.
The type of fan and the energy sources available can have an important influence on the choice of driver. Fans can vary from very slow speeds (e. g. forward curved centrifugals) to very high speeds (e. g. narrow backward bladed high pressure fans). To develop any worthwhile pressure, axial fans also need to run at high peripheral speeds.
The most efficient fan and control systems will be directly driven, obviating any transmission losses, but this assumes that the operating conditions can be correctly calculated. As the demand for energy saving increases, variable speed transmissions become ever more popular in a successful fan system.
For mains-fed motor applications, induction motors and electronically commutated (EC) motors mainly are used. Switched reluctance motors have not been used in the past because of their poor noise behaviour. However, significant improvements are now being made.
Universal motors are series commutator motors able to work from AC and DC supply. The commutator and the carbon brushes produce electrical interference, acoustic noise and limit motor life expectancy significantly. Therefore, this type of motor has not been used in a large number of applications.
Squirrel-cage induction motors, as well as EC motors, have only the bearings as a wearing part. They therefore have a high lifetime expectancy.
EC motors have some important technical advantages: wide speed range, easy speed controllability and high efficiency. However, because of the higher price of mains-fed EC motors, AC induction motors will remain a considerable part of the market, where low cost positioning is important.
For higher power, 3-phase induction motors are often used. For single phase supply, shaded-pole motors and capacitor-run motors can be utilized. An induction motor with only one phase winding does not have a rotating magnetic field. The single winding, fed with AC, simply produces a pulsating flux in the air gap.
The motor will not start from rest. The start can be achieved by using the principle of shaded-pole motor or with an auxiliary winding. The stator of a shaded-pole motor is slotted to receive the shaded ring which is a single short-circuited turn if thick copper or aluminium. The time variant stator flux induces a voltage which causes a current in the ring. The phase-lagged magnetic field of this current produces together with the main flux of the motor a starting torque.
Capacitor-run or also called permanent split capacitor (PSC) type induction motors are squirrel-cage induction motors with two windings. The current in the second (“auxiliary”) winding is supplied from the same single-phase source as the main winding, but a series capacitor caused to have a phase-lag. In that way, a rotating magnetic field is generated which makes possible an adequate starting torque and a higher efficiency.
Single-phase induction motors are robust and reliable; especially shaded-pole motors are very inexpensive. However, shaded-pole motors tend to have low power density and poor efficiency because part of the active pole is permanently short-circuited. For example, a shaded-pole motor with 10 W nominal output power only has an efficiency of 24%. Capacitor-run motors are more efficient (35-40% at the same output power). Further advantages are favourable acoustic behaviour and a power factor (cos tp) approaching unity (1.0).
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