The v-POD - A New, Attractive Electric Propulsion (Paper)

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New propulsion outboard drive.
  1DayPaper TUGNOLOGY   ’ 13 LONDONOrganised by The ABR Company Ltd Conference papers sponsored by21 INTRODUCTION TO PODSBrief history Electrical POD drives were developed approximately 20 years ago – the rst documented large-sized POD dates back to 1990, when ABB installed a 1.5MW POD on Seili   in Finland. 1  From 1995 onwards, various larger-sized PODs were installed on cruise vessels such as those belonging to Carnival Cruise Lines. The main manufacturers at that time were ABB, Rolls-Royce and Siemens. These POD propulsion devices all have the same characteristics – a gondola (cocoon) below the ship wherein an electric motor (E-motor) is tted, driving an outboard propeller directly through a shaft. The entire gondola can be steered in the horizontal plane, thereby delivering exible thrust in all directions. The water owing around the gondola provides cooling to the E-motor (see Figures 1 and 2)  . Although the name POD is commonly-used, the srcin of the term is not clear and can either refer to the cocoon-shaped gondola or an abbreviation of Propulsion Outboard Drive.The delivered ships with PODs have, in general, other electrical power consumers, thereby sharing the generated electrical power both for propulsion and other purposes (eg cruise vessels: hotel accommodation; tankers: pumps). The V-POD: A New, Attractive Electric Propulsion SYNOPSIS With increasing environmental demands, there is a growing need for electric propulsion, and therefore IMC, SARC and Verhaar Omega joined forces to develop a new propulsion outboard drive (POD). This 360-degree rotating electric Verhaar POD (V-POD) propulsion consists of a new arrangement of a medium-speed E-motor and a planetary gear – the main shaft runs from the propeller through the hollow E-motor shaft to the planetary gear on the opposite side. This results in a small, medium-speed E-motor and a compact POD housing. The robust planetary gear enables low propeller revolutions with high propulsion and bollard pull efficiency. The long prop shaft is supported by sturdy bearings on both sides of the POD housing and offers torsional flexibility between the propellor and the gears. This paper presents the V-POD development process, prototype testing, the first installation on an inland bunker tanker and the future developments specific for the tug market as an alternative to thrusters. Dr Markus van der Laan  (speaker/co-author), IMC Maritime System Development BV, The Netherlands Dick-Jan de Blaeij  (speaker/co-author), Verhaar Omega BV, The Netherlands Herbert Koelman  (co-author), SARC BV, The Netherlands Figure 1: ABB AziPOD CO. 2  Figure 2: Close-up internal detail of large direct-drive E-motor.  2 Direct-drive PODs All the PODs presently on the market follow the same layout as described before, and the E-motor drives the propeller directly without a gearbox. The claimed advantages of PODs are:ã Lower fuel consumption, eg ABB claims typical reductions by 5-15 per cent 3 ;ã Improved manoeuvrability;ã Reduced noise levels;ã Reduced emissions.This last advantage in particular has attracted more attention in recent years. Diesel-electric propulsion enables diesel engines to be operated at a more favourable operational point with reduced NO x  and SO x   as well as higher efciency in part-load conditions.Publications from both ABB and Rolls-Royce focus on the high efciency of the direct-driven propeller by the E-Motor, but there is another important aspect: propeller efciency in relation to the revolutions. In many ship applications, the optimal propeller is large and runs at relatively low revolutions. But this requires a high torque on the propeller shaft, and also requires a large and expensive electric motor. Therefore the following applies: Direct-drive PODs = a compromise between propeller efciency and E-motor size/cost. By increasing the propeller revolutions, the torque lowers and a smaller motor can be applied (size and cost), but the propeller efciency is also reduced. Although specic details of various ships are not available, many POD-driven ships have signicantly smaller diameter propellers than would be selected for reasons of propulsion efciency (compared to conventional diesel-shaft-driven propellers). Particularly for ships with sufcient draught, a larger propeller running at lower revolutions in design condition may improve efciency in a typical range of 5-10 per cent compared to the PODs currently offered 4 . A brief summary of the pros and cons is shown in Table 1  below. Although there are clearly certain advantages, the disadvantages have, in general, limited the broader application of PODs and they have only been applied to specic ships with other electrical power consumers. Geared POD In many conventional ship designs, a gearbox is mounted between the high(er) speed engine and the low(er) speed propeller. Consequently, a relatively small (and cheaper) diesel engine can drive a large propeller at low revolutions and at high propeller efciency. In shaft-driven propellers, the suitable gearbox is tted inside the hull and only the prop shafts are outside. The same also applies to PODs with electric motors (E-motor) and propellers. Here also a smaller (and cheaper) high(er) speed E-motor can drive a large propeller with high efciency through a gearbox. Although the gear takes some energy (typically 2.5-3 per cent), the propeller efciency gain is, in general, signicantly higher. Geared PODs = a combination of high propeller efciency and small E-motor size/cost. In PODs however, a gearbox requires a detailed analysis. Several large propulsion manufacturers have investigated various congurations of gearboxes in POD drives, but only limited details have been published. Schottel disclosed the following internal section of its POD in its patent application 5   (see Figure 3)  . Table 1: Brief summary of the pros and cons of PODs.Figure 3: Longitudinal section of geared POD.  3 The congurations are characterised by the following set-up: propeller – short shaft – planetary gear – E-motor. A planetary gearbox (PGB) has a gear ratio in a range from 1:5-1:7, so the geared E-motor can be ve to seven times smaller than the direct-drive E-motor. Since the E-motor price is closely related to the torque, the price of the smaller E-motor is also far cheaper.If, in addition, the propeller revolutions drop by 20 per cent to improve efciency and a gear ratio of 1:5 is applied, the E-motor is still four times smaller than a direct-drive motor. This not only results in a smaller and cheaper E-motor, but also in a smaller POD gondola with less water ow resistance and a lower installation size/weight, which clearly illustrates the potential of this concept.So there is great potential in the development of a geared POD and detailed research has followed in the mechanic/dynamic analysis of the whole drive train.   Without going into full detail, the mechanical design of a geared propulsion system has to meet a number of requirements:ã The propeller blades rotating in a disturbed ow eld excitate the shaft with a combination of torque vibrations, axial vibrations and bending moments; ã The shaft in return excitates the gearbox with the ‘remaining’ vibrations and bending moments;ã Inside the gearbox, the gear teeth contact each other with the ‘remaining’ vibrations and bending moments;ã Gear teeth are particularly vulnerable to vibrations and bending moments. In particular, non-parallel loading and temporary loss of contact between the teeth face should be prevented.In order to reduce vibrations and bending moments from the propeller to the gear, the propulsion train should include as much mechanical exibility as possible. However, this conicts with the concept:ã A stiff and short prop shaft is needed to meet the vibrations and bending moments; ã The short shaft introduces high radial bearing forces;ã The shaft forms a rigid part with the planetary gear drive and transfers the vibrations and bending moments;ã These loads are directly transferred to the gear teeth, limiting the operational life. This short, qualitative analysis demonstrates clearly that the mechanical design conicts with the given constraints. Although various companies have investigated the Geared POD concept in detail, this has not resulted in POD designs being applied to shipping, presumably because of the mechanical restrictions on the design. A detailed internal section of a Geared POD is shown in Figure 4  . The mechanical design is based on a stiff and short prop shaft forming an integral part with the planetary gearbox and short bearing distance. DEVELOPMENT OF THE NEW GEARED POD: THE V-POD With increasing environmental demands, there is a growing need for diesel-electric propulsion and therefore IMC, SARC and Verhaar Omega joined forces to develop a new POD. In summer of 2010, the analysis and evaluation of the Geared POD started with a simple objective:   Create mechanical fexibility between  propeller and the planetary gearbox. New conceptual design Using the IMC methodology of systematic innovation, two separate concepts were developed based on a long shaft running from the propeller end of the gondola to the opposite side: 1. A number of slender E-motors positioned around the central shaft all driving a central gear at the opposite end (see Figure 5 on next page)  .2. A single E-motor with a hollow shaft around the Figure 4: Longitudinal section of Geared POD.  4 prop shaft driving a PGB at the opposite end using a hollow sun shaft (see Figure 6)  . Figure 6: Hollow E-motor and planetary gearbox. Both options were developed into a preliminary mechanical design to validate the feasibility. After detailed consultation with E-motor and PGB manufacturers, it became clear that option 2 was more realistic, more compact in size and more cost-attractive. Therefore, this option was selected for the development of the V-POD for Verhaar Omega. Mechanical and electrical design With the basic concept xed, the engineering began. Many rather standard components were selected, but combined in a completely different setting with hollow shafts. Various components are described here in drive order: ã Motor/hollow shaft Initially, E-motor manufacturers were conservative and only allowed for small diameter openings through the rotor, but gradually, more detailed analysis disclosed that in conventional AC motors, a signicant opening can be made in the rotor shaft without losses in magnetic eld. By increasing the pole number from 4 to 6 or even 8, the opening can reach the required size. Permanent Magnet (PM) motors were also investigated, and although a higher power density is achieved than for AC motors, the higher price was not attractive. Also, the marginal efciency advantage compared to AC motors was not considered sufcient in relation to the price level (there are still questions on long-term PM efciency). ã Planetary gear A planetary gear balances the load from the sun- shaft to all three planetary gear wheels and, in order to provide exibility, a spline connection is used between the E-motor rotor shaft and the sun-shaft. As in case of the E-motor, the planetary gear manufacturers were conservative and allowed at rst for only small diameter openings through the sun shaft. Finite element analysis demonstrated, however, that with a typical gear ratio near 1:4, the opening size was possible (see Figure 7)  . Figure 7: Finite element specially-designed sun shaft. ã Propeller shaft connection to planetary gear by a spline The plan gearbox drives the prop shaft without taking axial loads and bending moment. Therefore, a spline is also used in this connection, thereby ensuring proper loading on the gear teeth. Figure 5: Four E-motors around one central shaft.Figure 8: Finite element modelling of prop shaft tapering/exibility.

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Jul 23, 2017
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