Chris
Thanks for that reference – ME were using it too.
at 2.57 ratio and 856 prop speed the diesel was turning 2200rpm. Perkins spec sheets indicate at those rpm the motor could put out up to 55 shaft hp – and estimated torque of 131*2.57 = 337ft pounds. At part throttle who knows...
Your boat displacement how does that compare to Erics? and the prop what are its diam pitch and number of blades?
I think you may need way more power - you are (if memory serves) trying to propel twice the displacement (needs twice the torque) at twice the velocity (usually needs 8 times the power and commensurate torque)
My thought is you need a larger motor with much higher torque per/ amp yield and a larger diameter at least and keep the g/box you have that ratio will come in handy.
Need to know a bit more about throttle setting at 7 knots - if possible how fast at 100% throttle and prop dimensions, blade shape helps
Andrew
Andrew &Eric,
I found a more informative datasheet for the ME-0913 that shows more than just the graph. I've attached it here in case you guys have not seen it. For me it was pretty enlightening. I think I've solved my puzzle but please let me know if I've gone astray.
The particulars on my boat: 27,000 lb displacement, 34' LWL. Engine 62 HP Perkins with a 2.57:1 reduction. A reputable electric drive mfg predicted it would take 17 SHP to push my boat at 7 knots. This puts me deep into the 2 motor realm. In fact this pushes each motor to nearly 180 amps into each controller which is max. Before pulling the engine out I ran the boat at 7 knots and measured 856 shaft RPM.
Motenergy states that the torque constant for the ME-0913 is 1.6 lb-in/amp into the motor controller. Taking the torque constant (1.6 lb-in X 360 amps) / 12 gives 48 lb-ft. Plugging 48 lb-ft and 17 HP into the torque formula and solving for RPM yields 1860 motor RPM. 1860/856 gives a gear ratio of 2.17:1.
Your Thoughts?
--- On Tue, 2/28/12, Andrew Gilchrist <andrew@fastelectrics.com> wrote:
From: Andrew Gilchrist <andrew@fastelectrics.com>
Subject: Re: [Electric Boats] Re: PMAC gear reduction
To: electricboats@yahoogroups.com
Date: Tuesday, February 28, 2012, 8:03 AM
Chris
the ratio - depends
Eric reports using a 48v supply the ME0913 turns 1900 rpm and draws 106 ampsat full throttle while spinnign a 13*15.5 prop through a 2.55 reduction box. (Eric is it 2 or three blade - do you have a brand or blade style)
Another way of putting it is on 48v the motor is making 16.56Nm and using 4.172kw at 1900 rpm.
Through a 2.55:1 box the prop is being turned by 42.3Nm and 4.172 kw at 750 rpm with 35% slip
Thats a reference point.
What engine is in there now, what boix ratio is used and whats your prop? what do you want in terms crusing speed? is the battery size limited by weight (say the same as the current engine and half a tank of fuel) or other factors?
On 2/28/2012 11:29 AM, Chris Hudson 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?
--- On Mon, 2/27/12, Eric <ewdysar@yahoo.com> wrote:
From: Eric <ewdysar@yahoo.com>
Subject: [Electric Boats] Re: PMAC gear reduction
To: electricboats@yahoogroups.com
Date: Monday, February 27, 2012, 5:14 PM
Hi Christopher,
Your optimum gear ratio will depend on a number of factors. What kind of boat (how much load)? What kind of controller (target RPM)? What performance (target speed)? What prop specs (max diameter, pitch, etc.)?
For an auxilary sailboat in the 27'-32' range, you can use a lot of the specific info that has been posted in this list by other people using 48V ME0913 systems (including me) like Electric Yacht, Propulsion Marine and others.
In my boat, the max motor RPM is 1900 and the max amps are 106, well within the continuous rating for the motor. My controller is a Sevcon Gen4, my prop is 13x15.5 and I use a 2.55:1 reduction. You can see a photo of the prop in this group. http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/1689340396/view?picmode=&mode=tn&order=ordinal&start=1&count=20&dir=asc With a less aggressive prop, I would need to move to a lower gear ratio (higher prop speed) to hit the same balance. Likewise, if I changed to a lower gear ratio while using this prop, the motor be turning slower and it would pull more amps to hit the same prop RPM (boat speed). If I switched to a higher gear ratio, I would run out of motor RPM at a lower boat speed and my max amps would be correspondingly lower too. As it is, I'm getting the amps that I want at the speed that I want, so it's good enough for me.
For other types of boats, you need to describe more of what you're trying to do.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "ChristopherH" <clh5_98@...> wrote:
>
> Hi Andrew, thanks for the reply. To clarify, I'm looking at the ME-0913 motor. The mfg website states 92% efficiency between 24-96V. They give the torque constant at .15 N/Amp. I don't think that gives me the data I need. I plan to run a 48VDC system, how can I figure out what gear ratio I need for a PMAC motor such as this?
> Thanks,
> Chris
>
> --- In electricboats@yahoogroups.com, Andrew Gilchrist <andrew@> wrote:
> >
> > Chris dont complicate it
> >
> > get the manufacturers spec sheet
> >
> > Find out the amps at max efficincy - this will be at a particualr
> > amperage the three are locked together at a given voltage on the same motor.
> >
> > For the voltage to be used, the recommended or a little more voltage is
> > usually the preferrable range
> >
> > Then gear to ensure the motor is running at the maximum efficeincy
> > amps/rpm. That way you will have least heating for the power generated.
> >
> > Nontheless the motor life may benefit from a cooling jacket.
> >
> > Andrew Gilchrist
> >
> >
> >
> > On 2/27/2012 2:31 AM, ChristopherH wrote:
> > >
> > > I understand how to calculate gear reduction for PM DC motors based on
> > > the manufacturer stated motor RPM/V X System Volts/desired max prop
> > > speed. I'd like to understand how to calculate gear reduction when
> > > using a PM AC motor.
> > >
> > > Thanks,
> > > Chris
> > >
-- AJ Gilchrist Fastelectrics 0419 429 201
-- AJ Gilchrist Fastelectrics 0419 429 201
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