10 MW coupling combination with lock-up clutch
To improve the efficiency of the drives used for the large mills found in the cement industry, Voith Turbo GmbH & Co. KG has developed a powerful and slip-free coupling combination rated at 10 MW. The 1210 TPL-SYN coupling is also suitable for other drives with constant input speed, such as extruders, turbo compressors, axial fans with adjustable guide fans. Thanks to its slip-free nominal operation, an efficiency of 99.5 % can be achieved. The new TPL-Syn (Fig. 1) hydrodynamic coupling was presented on 9.12.2008 at the plant in Crailsheim (Fig. 2). The first such coupling is scheduled to be shipped to a cement manufacturer in the southern part of the USA. The new 1210 TPL-Syn is a combination of proven and reliable components offering high power transmission capability with a compact size. The coupling provides smooth acceleration of the driven machine for up to 60 seconds and reduces the forces transmitted to the driven machine.
1 Technical background
In America, synchronous motors are preferred as cement mill drives. They provide slip-free operation and offer the opportunity for power factor correction, which means a considerable saving of energy costs for the plants. But even in Europe, where high-speed squirrel cage and slip ring motors are more commonly used in ball mills, synchronous motors used in conjunction with the new Voith coupling represent an alternative to be taken seriously. The new unit then permits power factor correction for the entire plant. With an amortization period of about 1.5 years, the initially higher procurement costs can be quickly regained through significant energy savings.
To start up a ball mill requires a very high torque (Fig. 3), so that it is necessary to have a means of decoupling the mill from the synchronous motor, which is incapable of starting-up on its own under load. The operating principle employed in a hydrodynamic coupling is especially well-suited for this purpose, since the torque is transmitted as a function of the primary speed and the degree of filling of the bladed working chamber. By completely emptying the working chamber, fill-controlled couplings permit the motor to be accelerated up to speed under no-load conditions, which suits the torque characteristics of high-speed synchronous motors very well.
It is now possible to combine the exceptional characteristics offered by hydrodynamics for acceleration with the additional benefit of slip-free normal operation through use of Voith
Turbo fill-controlled fluid couplings. The energy losses that previously had to be accepted because of the physical slip
that occurs in fluid couplings are eliminated by mechanically locking the hydrodynamic circuit during normal operation. In this way, the entire range of performance capability from the motor through the gearbox to the driven machine can be fully utilized.
2 Design of the coupling
The coupling consists essentially of 3 major assemblies (Fig. 4):
– Hydrodynamic circuit components (Fig. 4a)
– Mechanical lock-up mechanism (Fig. 4b)
– Housing with integrated oil tank (Fig. 4c)
When designing this coupling type, knowledge of hydrodynamics was combined with the know-how and experience gained from similar Voith components with the objective of creating an easily installed and self-supported drive unit. The connection between the motor and driven machine is provided by commercially available connecting couplings.
The usual operating cycle (Fig. 5) proceeds in the following manner:
a) Smooth acceleration of the motor with an empty fluid coupling and physical decoupling of the load
b) Acceleration of the driven machine via increased transmission of torque as the fluid coupling is filled. The coupling must be able to provide high breakaway torque here - depending on the load characteristic, up to 1.5 times the nominal torque rating of the coupling.
c) Acceleration of the driven machine to the operating speed; if necessary, acceleration can be controlled
d) Actuation of lock-up to provide slip-free normal operation with an increase in overall efficiency (Fig. 6)
e) Shutdown of the driven machine while the motor continues to operate; power factor correction for other drives on the grid is still possible while using a synchronous motor
The power transmission capability of this new coupling type extends up to 10 MW at speeds of 1500 rpm. Detailed information on the Type 1210 TPL-SYN coupling is available on CD as an interactive presentation (startup.components@voith.com.)
3 Performance capability
To demonstrate the performance capability of the coupling, the TPL-Syn was shown in operation on the test stand. It was possible to accelerate the motor up to speed under no load and then couple it hydrodynamically to the test gearbox and attached demonstration loads. Upon actuation of lock-up, slip was reduced to 0 %. The multiple disc clutch employed for mechanical lock-up between input and output is subject to almost no wear, since lock-up occurs in an oil bath and at a speed difference of only approx. 5 %. Expected bearing life is L10 > 80 000 h.
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