T-Pain Is On A Boat: [Electric Boats] Motor Efficiency v Voltage v Current and Controller efficiency

Sunday, June 1, 2014

[Electric Boats] Motor Efficiency v Voltage v Current and Controller efficiency

Hi from Scottish waters,

With my new off the shelf 12 x 14 3 blade prop with 55% BAR and my more recent motor of 45 RPM no load voltage constant and motor torque constant of 0.185 nm/amp I have been looking at how direct drive compares to geared. Indeed just how matching the prop with or without gearing to your motor and system voltage and continuous current ratings, affects where on the efficiency curve you spend most of your time and what max speed system and motor efficiency looks like. Immediately it looks as though we should be running controllable pitch props with motors at constant speed, but as we know that is only part of the story.

Whilst a proper efficiency map such as the one at the bottom of this page http://www.yasamotors.com/products/yasa-400/ would be ideal, I'm stuck with graphs.

I have made a compound image posted below of a motor running at 12V, 24V and 36 V at the controller with the blue shading showing where current relates to efficiency. Note the RPM in this motor is similar but 50 rpm no load/volt

I did a reading yesterday on my boat and at the controller and it was 12.5V and 34 Amps. If that were a geared motor of say 48V nominal running at a comparable 23V and 17 Amps using 2:1 reduction then you can see using the curves that the geared motor would be slightly less efficient in that comparison for the same wattage output. Say if direct the motor alone would be around 88% and geared 86% plus any efficiency losses due to gearing.

I would be interested to hear of others comments on this.

I am happy with the matching of my voltage and current also at peak ou put for my nominal 24V system with the prop I now have creating and absorbing the power. I get 22.7 Volts at the controller and 104 Amps. The continuous rating of my motor at 24V being around 105 Amps.

At the battery this equates to 2.7kW and at the controller 2.3 kW output which equates to 85% controller/cable efficiency.

At the lower V and A setting of 12.5V and 34 Amps= 425 Watts = 85% efficiency of Controller. At the battery I had 27.4V and 21.3 Amps less 35Watts overhead of instruments = 492 Watts.

I have yet to confirm voltage at motor terminals using a meter and a clamp meter on the motor cables, as I'm just using the programmer output screen of the controller. Regardless this looks like a combined controller and motor efficiency of .85 x .88 = 75% efficient combined.

Anyone else got figures?

My 425 Watts at the motor for an 8,000 lb loaded displacement boat equated to 3 knots yesterday but conditions were around 10 knots wind and a little current so I could not get flat water, no current/tide perfect type readings as I would on a canal. Last year with the 2 blade I took about 400 Watts at 3 Knots and around 800 at 4 knots, so it all looks comparable but my top speed has now gone from 4.5 knots to 5.3 it seems, although I have to do more accurate tests.This equates to n85% hull speed in calm conditions.

The maximum controller temp I saw yesterday working the system hard was 59 C. The controller without a heatsink is rated at 85C max.My controller could be sized larger as it is 80 Amps continuous but I'm OK with that as I shall never run for long periods above that.Burst speeds are max 250 Amps for the controller so I figure short runs at 100 Amps using a heat sink on my controller is fine. I would have got a higher spec controller but finance and economy of build has always been my goal.

Max bollard pull tied to the dock at 22.7 Volts at the controller was 140 Amps ahead and 160 astern. I shall limit astern throttle to 85% of 100% in software to reduce the astern bollard pull current which effectively relates to hard acceleration and braking at 100% throttle from standstill for my boat.

Overall I am pleased with my predicted calculation of my new prop matching, with actual at full throttle (once accelerated to max speed) being 22.7V at the controlle,r and 104 Amps there too. The actual figures have come out to within 5% of my predicted on this nominal 24V/110A system i.e I get 2.36 kW going into the motor from contrller from a battery Volts/amps nominal of 24/110=2.5kW

Other readings were battery temp varying from 17C at start of tests and finishing around 30C after full on tests. Ambient engine space temp at start was around 17C and settled in the afternoon to 25C.

I wonder what my overall system efficiency is. I'd like my controller to be higher but not knowing my prop efficiency I'm guessing my overall system is around 45-50% efficient which kinda makes sense as if I run my 1kW Honda genny I use around 1/3 rd of the fuel when compared to the old 10bhp diesel. The old diesel used around 1.8l/hr to go at 4knots and the Honda petrol uses around 0.6l/hr but there is a slight difference in calorific value of diesel to petrol.I guess my old diesel system was around 15 to 20% efficient overall.

Like I say I welcome comments and corrections to my reasoning?

I hope to carry out better tests in the future on a fresh water canal where conditions (disregarding the relative densities of sea water and fresh) are more optimal than at sea.

PS my boat is long keel with the prop in an aperture so disturbed water flow is causing some ventilation/cavitation at higher speeds. I estimate my loaded motor RPM at full throttle to be 970 to 1010 RPM. I'll say 1,000 RPM max. I expect to cruise between 500 to 650 RPM shaft speed which should be around 3 to 4 knots. Hull speed based on 1.34 square root etc would be 6.2 knots.

I've really enjoyed building my system. Last season I did 600 miles and so far this year 70 miles. Electric is fun.

PPS Last week I also took my batteries below 50% SOC and it was interesting to see voltage drop.

Due to cost my propulsion is 2 x 260 AH AGMs in series. 8 D size. If space and money ahd permnitted I would have had two separate series banks of that size so you have a main and a reserve. When the voltage drops on one pack you switch to the other giving the other pack time to recover and/or charge plus if a battery goes down in a 2 unit string it is cheaper than replacing those in a 4 unit string.

Due to my boat size I could not do this so my work around is I have the 2 x 260s and also 2 x 130AH AGMs which are my inverter bank which can as a silent reserve pack charge the main pack via inverter. This is handy if the main propulsion pack is voltage depleted where you can no longer get full power but you can put in some charge from fresh batteries via the inverter bank. I know this is not an efficient way to charge (maybe 65% efficiency overall) but it is a silent way to top up a propulsion bank at a quiet anchorage for a morning getaway and also for peace of mind in narrow channels where there is current. I used it that way last week and it works a treat. Put on the genny too and I can charge both banks at the same time and keep voltages/SOC up if need be in tight situations.

Enough from me...over to you ;)

John

http://john.rushworth.com/