Marine propulsion – Means to control the supply of energy responsive to a sensed...
Reexamination Certificate
2002-11-12
2004-06-22
Morano, S. Joseph (Department: 3617)
Marine propulsion
Means to control the supply of energy responsive to a sensed...
Reexamination Certificate
active
06752670
ABSTRACT:
FIELD OF THE INVENTION
The present application generally relates to ship propulsion systems.
BACKGROUND OF THE INVENTION
Propulsion devices for ship propellers with an electric propeller motor use rotation speed regulators for closed-loop control. A rotation speed nominal value is preset using the control lever on the bridge. Upstream of the input to the regulator, the rotation speed nominal value (reference variable) is compared with the rotation speed value at that time in order to determine from this a control error, which is supplied to the regulator. The output signal from the regulator is passed as a controlled variable to an actuating device, via which the propeller motor is connected to the current source.
When synchronous machines are used for propulsion, the actuating device is in the form of a frequency changer/converter, which uses the generator voltage from the diesel generator system to produce a suitable polyphase, variable frequency supply voltage. The converter circuit is designed such that the interconnection of the converter and synchronous machine results in a similar response to that from a DC machine whose current is controlled via a DC controller. The signal which is passed to the control input of the DC controller governs the current drawn by the DC machine. In the same way, the control signal from the regulator governs the current used to operate the synchronous machine. Asynchronous machines can also be supplied with electrical power, and can be used for ship propulsion, in the same way. It has now been found that propulsion systems of this type are relatively stiff, that is to say they are also able to regulate out minor rotation speed fluctuations which are within one propeller revolution.
The reason for rotation speed fluctuations and/or angular velocity changes is the behavior of the ship's propeller in the water which is flowing past the hull while the ship is in motion and whose speed profile is not three-dimensionally uniform. During their rotational movement, the propeller blades in some places move through the skeg or propeller-shaft stay on the ship's hull while, in the rest of their rotational movement, different water flow speeds impinge on them.
From the hydrodynamic point of view, the change in the load on the ship's propeller with time can be described by its wake field. The fluctuation in this load which is caused by the skeg or propeller-shaft stay on the ship's hull is once again evident in the inhomogeneity of the wake field of the propeller, which in turn results in a fluctuating angle of advance during revolution of the propeller blade.
Thus, the torque fluctuates cyclically, resulting in the ship's propeller having a fluctuating angular velocity which is regulated out by the rotation speed regulator, or by the current regulator that is subordinate to it, in order to keep the rotation speed of the ship's screw as exactly constant as possible at the preselected nominal rotation speed value. The frequency of the torque fluctuations corresponds to the shaft rotation speed multiplied by the number of blades on the propeller. The torque fluctuation is transmitted from the propulsion motor to its anchorage, and thus to the ship's hull. A torque reaction also occurs on the diesel generator system. In consequence, parts of the ship structure are caused to oscillate at the fundamental frequency of this pulsating torque and, as a result of the mechanical characteristics, the resonance of the ship's hull is not negligible at the relevant frequency. The vibration that this results in is not only annoying to those on the ship, but also results in a considerable load on the entire structure of the ship and its cargo, and should thus be avoided
In the past, attempts have been made to calculate the weak points for such oscillations using the so-called finite element method and to reinforce the critical areas determined in this way by the use of tons of steel. This method is on the one hand expensive and on the other hand reduces the maximum permissible cargo weight and the useful cargo area of the ship, while increasing the fuel consumption and, furthermore, although it can reduce those effects of the oscillations produced by the propulsion that destroy material, it does not eliminate the cause, however.
Closed-loop rotation speed control, which keeps the rotation speed of the ship's propeller at the preselected nominal rotation speed value as exactly as possible, leads to a further negative effect.
Since the inhomogeneity of the wake field fully reflects the fluctuation in the angle of advance of the propeller, the cavitation safety margin of the propeller is reduced, since the operating point of a propeller becomes closer to its cavitation limit, or may even exceed it. Particularly in the region of a skeg or propeller-shaft stay on the ship's hull, the operating point of the propeller may reach or exceed the cavitation limit and thus initiate cavitation, which can then lead to considerable damage to the ship and, in particular, to the propeller. Cavitation also leads to unacceptable pressure fluctuations and noise, which considerably reduce, in particular, the useful value and comfort of passenger, research and naval ships.
The rotation speed of ship's propellers which are driven via electric motors can be adjusted very quickly. Rapid adjustment of the rotation speed also leads, inter alia, to cavitation on the propeller blades. In this case, the rate at which the rotation speed is adjusted depends on the speed of motion of the ship, that is to say on the incidence speed at which the water strikes the propeller.
For this reason, the ramp-up transmitters are provided, which, from the control engineering point of view, are located between the control lever and the nominal value input to the regulator.
When the actual rotation speeds of the ship's propeller increase, its dynamic response changes considerably. Since the family of curves for the propeller (transition from the towing curve to the free drive curve) follow a square law, the maximum permissible dynamic response of the ship's propeller decreases more than proportionally as the actual rotation speeds rise.
In the case of ship's propeller propulsion devices which are known from the prior art, the ramp-up time which is governed by the ramp-up transmitter is increased in one to three stages as the rotation speed of the propulsion motor for the propeller increases, in order to keep the excess rotation speed within the maximum permissible range of the propeller curve.
Furthermore, with regard to the power requirement, the electrical propulsion system also has to take account of the generator excitation. Its time response is slower than the possible dynamic response of the electrical machine for the ship's propeller.
Taking account of these two boundary conditions, the ramp-up transmitter from the prior art is designed as follows:
Starting from a rotation speed of zero, the propeller motor first of all accelerates without any restriction, that is to say optimally. The power consumed by the propeller rises more quickly while ramping up with a constant ramp-up time, and finally reaches a current limit in the rotation speed regulator, in order to avoid overloading the diesel generator system. At the end of the first stage of the ramp-up transmitter, a change is made to a different ramp-up time. The acceleration power which is available from the electrical propulsion decreases to virtually zero. This results in a sudden change in the power consumption from the diesel generator system, which it must, but cannot necessarily, regulate out. This leads to frequency and/or voltage fluctuations in the on-board power supply network.
At least in the first phase of the ramp-up time, the propulsion device draws electrical power from the diesel generator system, which in some circumstances leads to failure of the supply to the rest of the on-board power supply network.
When changing from the first ramp-up phase to the second ramp-up ph
Harness & Dickey & Pierce P.L.C.
Morano S. Joseph
Olson Lars A.
Siemens Aktiengesellschaft
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