Other considerations in fixed output systems

Almost without exception, both fans and motors can be made more efficient in large sizes. Where a number of units have to run continuously, consideration should be given to providing a manifold or plenum to couple up the duct systems and use one common fan/motor unit. Even allowing for the additional duct resistance, power savings can often be made. Tables 19.6 and

19.7 are typical but show the improvements possible. They show how motor efficiency increases with increasing motor frame size whilst fan efficiency also increases with fan size due to “scale” effects.

Frame size

Motor kW

Rev/m in

Efficiency %

Power factor

63

0.25

2800

67

0.83

71

0.55

2800

71

0.85

80

1.1

2800

76

0.85

90L

2.2

2820

78

0.88

132S

5.5

1440

85

0.83

160M

11.0

1440

88

0.85

200L

22.0

970

90

0.80

Table 19.6 Typical motor efficiencies at rated output

Impeller diameter mm

Rev/mi n

Peak fan static efficiency %

Relative tip speed factor

630

2880

73.8

1.000

800

2270

74

0.995

1000

1815

75

0.992

1250

1440

77

0.989

1600

1125

78

0.987

Table 19.7 Typical peak fan efficiencies of varying fan sizes

Fixed speed motors

These drives are almost invariably from three phase induction motors. They should be matched to the fan to absorb as near nameplate load as possible, for both efficiency and power fac­tor are likely to fall at reduced load. Multi-pole slow speed mo­tors can often be used to eliminate mechanical reductions. Lower powerfactor can be compensated with powerfactor cor­rection devices.

Wherever continuous running is a requirement, the increased efficiency of both larger and slower units is apparent, see Table 19.8. Energy efficient designs can repay their increased first cost many times over. When allied with the power savings from high efficiency fans (often of a much higher magnitude — there are many fans with a 60% peak fan efficiency which could be re­placed by 80% efficient units), the effects on overall electrical demand could be enormous.

Perhaps of greater importance, especially where a fan works for considerable periods at reduced load, is the “flatter” motor efficiency curve (Figure 19.11).

Other considerations in fixed output systems

% Full load speed

Figure 19.11 Power required at the motor shaft by a vee belt driven fan operating on a constant orifice system

Table 19.8 emphasises how motor efficiency and power fac — tor. at full load may vary with speed for a typical manufacturer’s 45 kW motor of standard or “energy efficient design”. The in­crease in fan efficiency with reduced speed could well be larger than that for the motor.

Frame

Size

Poles

Rev/min

Full load efficiency %

Full load p. f.

Std

Energy

Eff

Std

Energy

Eff

225M

2

2955

90.0

93.9

0.85

0.91

225M

4

1470

92.0

93.8

0.86

0.87

250M

6

980

92.5

94.0

0.84

0.83

280S

8

735

92.0

94.0

0.79

0.75

280M

10

589

91.0

94.3

0.73

0.75

315S

12

490

90.5

94.8

0.73

0.75

Table 19.8 Efficiency at rated power of typical slower speed motor

Vee belt drives

The major fan manufacturers have families of designs covering a range of specific speeds and diameters. These ensure that all duties can be met by a fan running at a direct drive speed often with the impeller mounted on the motor shaft, thus eliminating the losses in vee rope drives. Greater effort should be made in the accurate determination of system resistance (how many enquiries for fans given an obviously rounded pressure of 500 Pa?). Thus the need for pulley changing will be obviated, a worthwhile reduction in first cost results, and the transmission losses will be eliminated. Where belt drives are inevitable, greater thought should be given to their selection. It should here be noted that if a vee drive is either under or over-engineered, efficiency will reduce as seen in Chapter 11, Section 11.5.7.

Flat and toothed belts do not suffer to the same degree, whilst their peak efficiency is usually higher. With very small belt drives, the difference in power transmitted between say one and two vee belts or between two or three is obviously substan­tial.

The use of larger single units is indicated, if transmission losses are to be reduced. Especial attention is drawn to the increased losses in vee belt drives when used at lower than their maxi­mum rating on variable speed units. Whilst the fan impeller power on a constant orifice or fixed system may vary as rota­tional speed, (if we assume fully turbulent flow with a constant duct friction factor — which is unlikely), the losses in the bear­ings and shaft seals will vary approximately as the speed. Drive efficiency will fall from about 94% at rated full load to around 70% at half load so that the full “cubic” saving is not achieved where vee belts are fitted (Figure 19.11). Again the preference for direct drive fans with variable speed is indicated.

Electric motor design

Whilst a very small number of fans may be driven by prime mov­ers such as steam turbines or petrol engines, the vast majority

— in excess of 98 % — are driven by electric motors. With axial flow fans, it is common for the fan impeller to be mounted di­rectly on the motor shaft extension. Centrifugal fans, may, of course, be vee belt drive, directly driven or through a flexible coupling with or without an intermediate gearbox (common in the UK on large mine ventilation fans).

Again, with the majority of fans, electric motors are of the totally enclosed squirrel cage induction form suitable for a three — phase supply. Single-phase motors are usually limited to frac­tional horsepower outputs. See Chapter 13 fora fuller descrip­tion.

The induction motor is extremely reliable and robust. In nearly all cases it may be considered symmetrical both mechanically and electrically. The windings are balanced between phases and slots. Care is taken to ensure that the rotor runs in the cor­rect position axially within the stator field, and that the air gap between the rotor and stator is the same at all axial and radial positions. However, especially with direct driven fans, there will be an end thrust due to the impeller action and this will “try” to take the rotor out of the magnetic field being resisted by the magnetic forces and also such devices as wave washers in the bearing housings.

The heart of an induction motor is its laminated iron core and the stator and rotor windings. As the core is in no way con­nected to the power supply nor is power directly removed from it, it can be considered as passive. It is, however, the path of minimum resistance for the flux generated by the m. m.f set up by the stator winding, which itself is the path of least resistance for the input current.

In summary, the power supplied to a three-phase stator winding sets up a rotating magnetic field. This induces an opposing cur­rent in the rotor winding and thus another magnetic field. Inter­action of these two fields produces a tangential force. As the ro­tor shaft is only restrained by its bearings, it has to rotate.

The AC induction motor can be easily adapted to variable speed output by inverter control and this is perhaps the most popularform of flowrate variation. According to the type of con­trol and its effect on the electrical wave form, this may be dis­torted sufficiently from the ideal sinusoidal shape that motor noise may increase and efficiency reduce as the speed is de­creased.

Selection of the relationship between voltage and frequency is of special importance particularly where the fan is of lightweight design, or where it is built onto a flimsy structure. It is usual to select the so-called fan torque variant where v oc f2. If the trans­mission efficiency also varies with speed (as in a vee belt driven fan) this may not, however, be ideal.

Effects of the European Directive 89/336/EEC on Electromag­netic Compatibility (EMC) are still being resolved. It has been subject to three amendments and at present it is due to be re­pealed by the new 2004/108/EC Directive as from 20th July 2007.

Certainly an AC induction motor on a sinusoidal supply is unaf­fected, but inverters can present a problem where screening is inadequate or cable lengths excessive. There are, however, other types of motor currently being developed for fan drives,

Such as the permanent magnet type and the switched reluc­tance drive. It remains to be seen whether, in addition to in­creased efficiencies, these drives will overcome their disadvan­tages from an EMC viewpoint.

Selection of correct motor speed and type

During the early postwar years, when energy was cheap, it be­came accepted for motors to be designed with ever smaller frames, higher temperature rises, and consequent reduced amounts of active material. Fans also were subjected to the same thinking, and they were designed for lowest capital cost. This led to reduced diameters and increased blade widths with an ability, where direct driven, to run at 2 and 4 pole motor speeds.

In the 21 st century, it is suggested that this trend ought to be re­versed. Are-emergence of larger fans, driven at slower speeds, and with a high efficiency across a greater portion of the charac­teristic, should be encouraged. The decrease in lifetime costs, where energy consumption and maintenance costs are added to the capital cost will be readily apparent. The matching of mo­tors and fans, and the choice of control system concepts to per­mit optimisation, should become a design priority.

Posted in Fans Ventilation A Practical Guide


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