Eric
ME doesn't seem to know what is on their web site
ME0193 specified in PDF at http://www.electricmotorsport.com/store/pdf-downloads/PMAC-DS.pdf
P3 shows a chart which is dated 2010 and claims to have been made with a 77vdc battery supply. The form of the graph curves are consistent with the graphs produced by a brake dynamometer.
However there are several problems with this data sheet and chart
1. The motor Kv is given as 50.
2. The free running rpm on the graph is 4420rpm - this suggests the kv is at least 57rpm/v
3. the efficiency curve indicates the claim the motor can run 700 amps is false. Stall amps perhaps. Usable amps? Only if one wants a warming device.
Also shown on the graph is that the voltage was 77v; it ranged from 70v at 135amps up to 77v when (free running. It seems consitent with Lithium manganese or lead.
Eric
Do you have an voltage, amp draw and rpm figure for your motor free running at wot? It would be interesting to see what the real figures are. It would make predictions a lot easier.
Have you run a static bollard push/pull test? I ask becasue the data is real world and free to obtain (nearly) I have found them usefull for working out the props most efficient rpm range. Two cheapish bathroom scales bolted to a board and board across the top will allow you to measure up to 240kg thrust just pushing on a solid wharf.
Recoprding volts amps rpm and thrust enables the props efficient operation range to be comapred to the potential in the current system and the cheapest change toward goodness of fit identified.
Chris
Going out on the limb of the ME motor specs; using the Ir figure (.0135 ohms is that for one or three phases - the latter seems a better bet??) and the graphed Io figure (1.35 @10v), then @ 48v the peak efficiency shoudl be apporx 82% around a 94 amp draw. On direct drive the motor delivers 4.9hp and 11ft/lbs (15Nm) of torque levels. That torque/amp seems low for 10,500lbs of boat
IMO with most displacement hulls, given the character of the hull speed presure wave it is relatively easy to to supply adequate (low) Hp/Kw to reach a sizeable (efficient) proportion of the hull speed. However adequate torque to do that efficeintly is rarely available.
Using this motor in direct drive the torque is lowto fall down or rather incur amp draws which create much more Hp than is required ie waste energy. Too little torque means more slip and/or fewer rpm and/or wasted energy .
Electrics do ok because they deliver torque in relation to the load imposed. They respond almost immediately to load and at the rpm at which the motor is running - because the magnet flux varies without much resistance.
A diesel hwoever has to move metal componets to the rpm where it makes the needed torque at least in theory. from what I have seen it is really designed to have enough torque to enable it to rev a large dia prop to the lowish rpm at which the prop can utilise the hp the engine can make. The effect of this is it needs to make more torque almost every where than is needed.
I assume Eric is using a 3 blade prop - beg borrow and steal em if you can to get some data as to how your setup performs with different props - a prop library is an extremely valuable tool - your are going to give them back so its an afforable process.
It looks like the efficiency should be good at WOT between 1800 to 2250 motor rpm. hopwever i think youcould safely drop the prop rpm to 1900 rpm peak and get benefits from the multipliction of availble torque by the reduction factor i.e. 18% extra
At 1900 rpm 1" of pitch buys you 1 knot if you can get near Erics slip rate. The lower the ratio the larger the prop or the more blades the mnotor can turn with the same or fewer amps - the torque per amp will increasein direct proportion ot the ratio.
A bollard test will be useful as above
regards Andrew
On 2/28/2012 1:33 PM, Eric wrote:
Hi Chris,
I'm kind of interested to hear how you figured the power needed to reach your target speed. I have found that a number of generally accepted methods don't always predict accurate numbers for some types of boats. Here's an excerpt from an earlier post that I made on the subject back in Nov 2011:
"Ealier this summer, there was considerable discussion about the Dave Gerr calculations and how applicable they are to our boats. Because we are operating at such slow speeds and electric motors have very different power curves compared to ICE, much of the information in Gerr's book that has been validated many times over in the ICE world doesn't always hold true for electric boats.
For example, according to Gerr's calculations, my hull would require 3.98hp (3kW) at the propeller to drive it at 3kts. I know that I only draw 500W from my battery bank to power my boat at 3kts and that hasn't included any losses for the controller, wiring, motor or gearbox yet.
In an email from Gerr, he told me that his work doesn't really cover our specific use cases, his focus has been on hulls close to hull speed and beyond.
So while Gerr's book is treated as gospel for mainstream boaters, and rightly so, we should remain open to different answers for our boats.
We can see that just about any prop will make a boat go, but some props will do the job better."
Just wondering out loud....
If your system can change reduction ratios easily, then I don't see any problem with using the prop from your ICE installation, but there are opportunities to reduce slip with more aggressive props that would be completely unsuited for ICE operations. You could start with what you've got to get back on the water as quickly and cheaply as possible, then re-optimize with another prop later.
I went to Motenergy to check the performance graphs for the ME0913 motor and it looks like they never published performance graphs for their brushless motors. So it's hard to tell what your amp load will be at what RPM so that you can match the motor to your prop. I can say that my ME0913 will run as long as there's power at 1900 RPM pulling 106A @ 48V without overheating. So maybe that's a number that you can use...
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Chris Hudson <clh5_98@...> wrote:
>
> Thanks Eric. I know the power required to reach my target speed and the prop RPM needed to get there. I'll keep my current prop for now. With this data shouldn't I be able to calculate my gear ratio?
>
-- AJ Gilchrist Fastelectrics 0419 429 201
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