ELECTRIC MOTORS AUTONOMOUS ROBOT SHIP PROPULSION

MOTORS - Introduction

For an efficient ship, we need efficient motors. Cost and complexity too are important considerations. Before deciding what motor SolarNavigator should be fitted with, we should consider the latest advances in design. Synchronous DC motors would be an obvious choice, if it were not for the complication of the electronics - and yet several manufacturers supply these motors as drives where reliability is paramount, such as submersibles.

On the other hand there are very efficient DC motors such as the Lynch, with a very high power to weight ration and proven reliability. These motors are brushed and it is the brushes and commutator materials that determine operational life. Inevitably, brushes will need servicing. So we must look at the mission parameters to see if DC brushed motors could provide the service requirement on a world circumnavigation.

To make the most of excess energy at peak charge times and to use stored energy for the best range, it is sometimes necessary to engage a higher output motor with reduced gearing - when excess energy cannot be stored, and at other times, when running on stored energy for prolonged periods, to engage a lower output motor with higher gearing. The graph below makes this plain. A designer should aim to keep motor reduction ratio choice in the crimson band for peak efficiency. This is basic good design.

For an ultra reliable autonomous robot boat, we need back-up motors. If one motor set fails, we might engage another set. Compared to the long term energy savings, the additional setup costs are insignificant.

Where we are cruising at relatively slow speeds, there are times when more power is needed, such as in tidal conditions, to avoid collisions, when docking, or just because we are late. By having the ability to engage and disengage motor sets without mechanical losses from layshafts and bearings, the SolarNavigator can operate at peak efficiency in real time. We cannot show you how we achieve this here because the method is patentable and patent applied for.

A GRAPH USED TO ESTIMATE BEST MOTOR REDUCTION RATIOS FOR DC MOTOR COMBINATIONS

VOLTS	RPM	3:1	knots	4:1	knots	5:1	knots	6:1	knots	7:1	knots	8:1	knots	9:1	knots	Efficiency%
12	1,000	333	4	250	3	200	2.4	166	2	143	1.7	125	1.5	111	1.33	25
18	1,500	500	6	375	4.5	300	3.6	250	3	214	2.5	187	2.25	166	2	37
24	2,000	666	8	500	6	400	4.8	333	4	286	3.4	250	3	222	2.66	50
30	2,500	832	10	625	7.5	500	6	416	5	357	4.2	312	3.75	277	3.33	62
36	3,000	1000	12	750	9	600	7.2	500	6	428	5.1	375	4.5	333	4	75
42	3,500	1165	14	875	10.5	700	8.4	583	7	500	6	437	5.25	388	4.66	88
48	4,000	1332	16	1000	12	800	9.6	666	8	571	6.8	500	6	444	5.33	92
54	4,500	1498	18	1125	13.5	900	10.8	750	9	642	7.6	562	6.75	500	6	-
60	5,000	1665	20	1250	15	1000	12	833	10	714	8.5	625	7.5	555	6.66	-
66	5,500	1830	22	1375	16.5	1100	13.2	917	11	785	9.3	687	8.25	610	7.32	-
72	6,000	2000	24	1500	18	1200	14.4	1000	12	856	10.1	750	9.0	666	7.98	-

Motor/Kw Configuration		60Kw (80.4hp)		40Kw (53.6hp)		30Kw (40.2hp)		20Kw (26.8hp)		15Kw (20.1hp)		10Kw (13.3hp)		7.5Kw (10.0hp)

The configuration we are looking at requires integration with the transmission. That may also be so for synchronous motors, depending in what is available off the shelf. We do not want to get involved in development where that is not necessary.

Should any one of the main drive motors fail, the proposed design will allow several options to keep the craft powered.

The SolarNavigator team hope to be able to run tests on as many of the above and report those findings in the years ahead.