Droop

Stiffness of the films and sheeting can be measured as the tensile modulus of elasticity. Droop or drape tests may be used, particularly for multilayer products. The stiffness is strongly influenced by thickness (to the third power) and temperature, and is important to the processing of film in printing, coating, or end use appHcations where it affects the "hand" of the product. 

Compound-wound dc motors have both series and shunt fields. The addition of a small series field helps provide the proper amount of no-load to fuU-load speed regulation or droop. Shunt or compound-wound motors are apphed widely to many adjustable-speed drives. They are important for drives requiring accurate speed regulation and adjustment. 

A dc motors inherent speed-torque curve can be varied widely by adjusting the relative amounts of shunt and series fields. The series field may also be connected to aid or buck the shunt field. The usual practice is to connect the series field so that it adds to the shunt field (cumulative compound), which gives a stable, drooping speed with increasing load. 

Drooping torque at lower speeds. At higher speeds loo, the torque profile is variable... 

The difference in the speed of the engine at no load and full load is termed the speed droop, and is expressed as a percentage of the no-load speed, i.e. 

The droop is maintained at around 3-5% by the leading manufacturers. The lower the droop, the better will be the performance of the engine on load. Since... 

Figure 16.2 Typical speed-load characteristics of a 1500 r.p.m. engine with 4% droop...

This senses the speed of the machine and performs extremely fast and accurate adjustments in the fuel supply to the PM. In turn it regulates the speed and the output of the PM within predefined limits, depending upon the droop of the PM. The governor may be a mechanical (manual), hydraulic or electronic (automatic) device. 

This device controls the generator and maintains a steady-stale armature voltage automatically within the predefined limits. It also serves to control the reactive kVAr loading during a parallel operation or when the machine is being used as a synchronous condenser for reactive power compensation through a quadrature droop control (QDC) as noted below. 

Consider the speed-load (drooping) characteristics of the two machines as shown in Figure 16.20. For ease of illustration, the slopes have been exaggerated. Normally they are within 4% of the rated speed, as discussed earlier. When both machines are loaded equally, the total load may be defined by the load line AA, at the bus frequency,/ When the power input to PMf is increased, so that the drooping curve AO shifts to curve BO, it shifts the load line AA also to BB, so that the total load shared by the two machines will still remain the same. The load shared by G is now more than before at so that , > /, and by Gt less than before at P, so that P < Pi The generators now operate at a higher system frequency, /i,. If the... 

Figure 16.20 Drooping curves of two machines, illustrating load sharing when running in parallel...

The load sharing by the two machines can thus be varied by shifting the drooping curves of the prime movers by altering their power input. 

The load sharing of the tw o generators is therefore dependent on the speed-load (drooping) charaeieristics of the prime movers. 

To determine the load sharing between the two, draw the drooping curves as shown in Figure 16.37. 

Figure 16.37 Determining the load sharing between G, and G2 with the help of prime-movers drooping characteristics...

Therefore, the generators would share a load of 1200 kW equally. If, however, the total load is reduced to, say, 1000 kW, the loading will differ due to unequal drooping characteristics. The revised load sharing can be determined as follows. 

A slight variation in the drooping characteristics causes a variation in the load sharing. 

A machine that has a higher droop (Gj in the above case) will share a larger load than the one that has a lower droop. 

The higher the droop, the higher will be the load variation. When there is a difference in the full-load speeds the load sharing during a parallel operation will not be equal and the generators will operate underutilized. 

The receiving-end voltage rises with leading p.f.s and droops with lagging. This is illustrated with the help of phasor diagrams (Figures 24.22(a) and (b). 

Reactive control can alter the line length ( f LC) to the level at which the system will have the least possible swings. It is evident from these curves that an uncompensated line of a much shorter length may not be able, to transfer even its natural load (Pq) successfully. This is due to the steeply drooping characteristics of the voltage profile at about this load point, which may subject the... 

Figure 19-4. Droop ourves for Combined Cyole Power Plants.

A dc restoration circuit is needed following the output coupling capacitor to make the drive voltage referenced to the power switch s common. The supply voltage of the driver should be well bypassed so that its voltage does not droop during the drive pulse. 

The characteristic curves of each pump must be continuously rising (right to left) as show u for the single pump of Figure 3-35, otherwise with drooping or looped curves they may be tw o flow conditions for any one head. 

Requirements become more demanding as the pressure at the inlet rises. A governor alone has its characteristics which include a pressure droop at high throughputs and an increase above set point at very low throughputs. In many cases, there must additionally be protection against the failure of a governor. This can be provided by ... 

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