Basic scheme of fluid coupling Pump Runner
Ws Primary shaft
Wp
Secondary shaft
Turbine Runner Flow of oil
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To better understand what benefit a fluid coupling provides when connected between an electric motor and gear train, the speed and torque profile of the electric motor must be considered. During start-up, an across-the-line started motor transmits tor ue to the drive s stem com onents. As shown in the graph, these values can range anywhere from 180 percent starting torque to 250 percent breakdown torque based on full load. • Severe damage may result to the connected equipment if less than 180 drive train must absorb the additional load. Any number of components, from belts on a conveyor to bearings or rotating shafts and more, could fail as a result of "over-torquing." , the motor will fail to start. This is when the benefits of a fluid coupling become evident. The fluid coupling controls the motor's output characteristics to match load requirements. , flowed between the impeller and runner. The only load imposed on the motor is the inertia of the casing and impeller. As the motor accelerates, the impeller begins to pump oil to the runner and . , torque build-up is smooth and gradual. Once the torque build-up has matched the required breakaway value, the runner will begin to rotate and accelerate the driven load. The electric motor is now running at full-load " " . coupling is directly related to the amount of oil circulating between the impeller and runner. Adjustment of the coupling's fill can provide a wide range of torque values. More oil in a fluid coupling provides higher starting
M=
NM = motor speed
K =
NL =Load speed
L
=
N=
U =transition point
• he operating characteristics of a coupling type T
with two different machine characteristics (constant
secondary coupling characteristics, the primary characteristic as a function of motor loading can be , be derived from the operating data of the machine being driven. ese sys em c arac er s cs c ear y s ow e almost parabolic torque buildbuild-up as the motor runs up to speed, after which the coupling characteristic epen s on y on ts es gn. e torque transm tte by the coupling to each machine differs only with respect to the initial breakaway torque, and the transition point in the coupling characteristic depending on the moment of inertia of the machine in uestion.
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Fluid media
• Power is transmitted in hydrodynamic couplings as specific kinetic energy of fluid flow. This depends primarily on the physical properties of the fluid – density and viscosity– and also demands the efficient removal of heat due to losses. The fluid characteristics required in practice vary widely, depending on the kind of coupling and how it is integrated in the drive system. As far as energy ransm ss on s concerne , wa er s even e er an m nera o or synthetic fluids. With regard to safety and availability, water is excellent. However, river or seawater in particular is unsuitable with regard to , , .
• Although Föttinger carried out tests with seawater, it turned out to be unsuitable in practice. The fluids mainly used comprise mineral oils, which . long-term operating characteristics, mineral oils also meet control and lubrication requirements. The majority of these oils comprise paraffin-based required. Mineral oils with low viscosity are preferable, since they reduce flow friction losses through the blading channels and thus increase power transmission efficienc .
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If hydraulic oil is also used for gear lubrication, properties must be carefully balanced.In order to keep oil supply aggregates as compact as possible, ood deliver ca acit is also im ortant.
• let us assume p is input speed s be secondary speed i.e angular velocity so power input to the primary wheel power output in terms of torque(T) and angular velocity() • n = p.p out = s.s • Therefore efficiency of transmission = Ts.s p.p an as t ere s no parts s n etween to provide torque reaction so the input and the minor loses. Tp=Ts so =s/p • [s = (p -- p)/ p •
U3
Vr3 U2 > U3 Vr2
Vf Turbine half
Vw2 pump half
Wp
Vw4 Vf=constant
Vw1
Vf
V1 U1
Vr1
Vw4=Vw1 U1 >U4
4
Vf
V4
Vr4
5
• Let us draw velocity triangle Assumption fluid enter & leaves impeller& runner with same tangential velocity component i.e zero whirl slip Vw2=Vw3 , Vw4=Vw1 Now By Euler’s eq. the work done by the primary per unit mass Ep/m=Yp=(Vw1.U2-Vw2.U1)/g As U2= p.r a
U1=p.r i
from velocity triangle
Vw2=U2= p.r a Vw1=Vw4=U4=s.r i So we get Ep/m=Yp=(p.r a2 p.s.r i2) similarly, Es/m=Ys=(p.s.r a2-s2.r i2) So the energy dissipation may be obtained dY=Yp-Ys=(p-s)(p.r a2-s.r i2)/g it is also assumed that dy=kQ2
• The power transmission characteristic of a hydrodynamic coupling can be shown graphically by plotting the relation = f (). The entire characteristic field is described b a series of curves as a function of the filling level V. • The basic characteristic of hydrodynamic couplings exhibits a slip), beginning at the startup point A. • Hydrodynamic couplings are selected according to their power transmitted at lowest possible nominal slip sN. The other characteristics can then be developed according to . safety couplings, the flattest possible characteristic is usually required over the entire startup range, with relatively low torque pea max. • For variable-speed and fill-controlled couplings, consistently reducing power factor characteristics are requiredfrom = 0. This ensures stable operating points with various kinds of speed-regulated machinery.
• Dimensional analysis on law of geometric similarity • Let mass flow m fluid density profile Dia Dp filling volume Vol • . speed characteristic Vn = ws/wp Kinetic viscosity Reynolds no. Re=w.D 2/ • Geometrical similarity • Euler number Eu= dP/(c2) • • • • • •
• • • • • ..
Parameter Streaming pressure Streaming force Volumetric flow Mass flow Hydraulic torque
Equation Eu = const. F = p · dA V=c·A m=·V T=F·r .
·
Model law p ~ D2 · 2 F ~ D4 · 2 V ~ D3 · m ~ D3 · T D5 · 2 ·
Basic dimensions: Length r, l D Surface area A D2 Velocities u, w, c D · Hydraulic similarity Reynolds number Re= · D2/
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Im ortant Do’s & don’ts Never do painting on the FCU surface as heat dissipation will be hampered as well as performance Always check the recommended level of filling
In case of scoop control coupling after stopping motor always put the scoop in 100% position to have extended time & quantity of lub oil supply to the internal component of coupling . Generally FCU are bi-rotational and of straight radial blading. So reversal of motor rotation causes server o/L due to output gear box
• Only one VFD was supplied experimentally of BFP and this was never repeated • , almost equally shared between Voith h draulic cou lin s and VFDs de endin on the experience of the end users. • There are of course a few customers who have opted for VFDs for ID fan drives • But there are also customers who have reverted to hydraulic couplings in their extension units after using VFDs in the earlier ones.
• Torque curve
E U Q R T
e u q r o t g n i t r a t
e u q r o p u l l u P
u q r o t y a w a k a r B
o r q t o r t o M
c u r v e
e u q r o t g t i n e r e u c c c e A u q r t o p P u m
SPEED
e u q r o t d a o l l l u F
e u q r o t t u o l l u P