SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
 
Thread Tools
Old Feb 13, 2003, 04:42 AM
Henry Springer
Guest
n/a Posts
Electric Motor Current

I have been checking the current (amps) taken by motors on my boats
but can only do so when the boat is stationary in the water. Anyone
know how the current changes when the boat is under way, my gut
feeling is that it will drop but I cant find any authority on this.

Henry
Old Feb 13, 2003, 05:12 AM
Steve Ritchie
Guest
n/a Posts
Re: Electric Motor Current

"Henry Springer" wrote in message ...
> I have been checking the current (amps) taken by motors on my boats
> but can only do so when the boat is stationary in the water. Anyone
> know how the current changes when the boat is under way?


If you've got a *proportional* control lever, the current draw will be the
same for any given angle of deflection of the contol whether the boat is
stopped or under way. Can you make measurements at various control
deflections if it really is that important to know the *exact* current draw?


Old Feb 20, 2003, 09:42 AM
Kevin Russell
Guest
n/a Posts
Re: Electric Motor Current

"Steve Ritchie" <stevie_ritchie@hotmail.com> wrote in message
news:uZK2a.2513$7T1.22428871@news-text.cableinet.net...
> "Henry Springer" wrote in message ...
> > I have been checking the current (amps) taken by motors on my boats
> > but can only do so when the boat is stationary in the water. Anyone
> > know how the current changes when the boat is under way?

>
> If you've got a *proportional* control lever, the current draw will be the
> same for any given angle of deflection of the contol whether the boat is
> stopped or under way. Can you make measurements at various control
> deflections if it really is that important to know the *exact* current

draw?
>

I'm not a expert in this field at all but if the current drawn at rest is
the same as full speed i.e. the same power torque then why do full size
power boats use a gearbox?? I can only surmise that a moving boat uses less
power to maintain the same prop speed
There is a program for planes called motorcalc that will predict the actual
current taken by an electric motor in flight which is less than the static
amount and as air acts like a fluid it must be the same for a boat
as for measuring current at a set stick position you have to take into
account the loading on the prop other wise it would take the same current
running in air running in water or even stalled the more load the bigger the
current drain
Kevin


Old Feb 20, 2003, 02:10 PM
Registered User
Woodbury, MN
Joined May 2002
41 Posts
The amp draw will drop considerably when the boat is at speed. It will drop even more if it is on plane, or other steps taken to reduce drag will also results in less amp draw.

A simple explanation would be that the electric motors we use draw less amps at higher rpm. When the motor is under load, the amp draw will increase as the load is increased. A good example would be the guy that tried to drive his boat back to shore with weeds wrapped around the prop and fried a motor or speed controller on the way in. The amp draw can be incredible! 50-60-70-100 amps are not unheard of!

You could see the load drop in half on plane as compared to a boat held in hand to test at the waters edge.
avalentine is offline Find More Posts by avalentine
Old Feb 21, 2003, 03:22 PM
Sigurd Ruschkowski
Guest
n/a Posts
Re: Electric Motor Current

Hi,

the current will drop consideraly. It is impossible to say how much as that
depends on how wet your boat will
run in the water at speed. At plane it will be the lowest.

Sigurd Ruschkowski


"Henry Springer" <henry.springer@virgin.net> skrev i meddelandet
news:knsm4vgqnfajtsaoa4r0iotu64oall4nia@4ax.com...
> I have been checking the current (amps) taken by motors on my boats
> but can only do so when the boat is stationary in the water. Anyone
> know how the current changes when the boat is under way, my gut
> feeling is that it will drop but I cant find any authority on this.
>
> Henry



Old Feb 24, 2003, 02:46 PM
Registered User
Graham, WA
Joined Feb 2003
28 Posts
Hey sigge,
I heard of a guy who tried a test like that in a bathtub once.
It's still one of my favorite stories. Lesson learned, use the bathub for float tests only!

See ya,
Dick
Dick Crowe is offline Find More Posts by Dick Crowe
Old Feb 24, 2003, 08:42 PM
Registered User
Lemon Tree Passage NSW Australia
Joined Sep 2001
378 Posts
Sigge

my first boat a 380 powered balsa micro rigger was tested in the bathtub

unfortunately it slipped from my hand . The acceleration was phenomenal - right up to the point where the motor hit the other end of the tub!


Motor current draw

holding the baot still in the bath you are likely to see about 30-40% more current than when the motor is un loaded

a motor draw peak current at

o% rpm - the stall current at this point the motor will generate maximum torque

50% rpm - it will draw about 40% of the peak but will do so with poor eficiency - the lower the motor technology the more innefficiently it will do so so it will run hot but will also prouce its maximum ouput in watts

75-90% of maximum rpm the motor reaches maximum efficiency and drws 60-90% of the current drawn at maximum power.

Althought this graph is for the Trinity D4 and P94 which are high tech brushed motors the basic parameters are clear and the trends are similar for motor motors

Also see

http://www.johnsonmotor.com/index.htm or http://www.mabuchi-motor.co.jp/eng/index.html

motor parameters are displayed in a slightly different way but you may find your motor here

Andrew Gilchrist
www.fastelectrics.com
supporting model boaters
Andrew Gilchrist is offline Find More Posts by Andrew Gilchrist
Old Feb 25, 2003, 08:08 AM
Registered User
Woodbury, MN
Joined May 2002
41 Posts
Any chance you were able to record the amp draw at point of impact?
avalentine is offline Find More Posts by avalentine
 


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Electric motor break in DaleM Power Systems 12 Jun 13, 2014 03:06 AM
Comm Drops on electric motor e-sailpilot86 Beginner Training Area (Aircraft-Electric) 11 Apr 21, 2009 12:32 AM
electric motor for 13 pound model? Steve_JR Electric Plane Talk 5 Oct 29, 2001 10:50 AM
electric motor direction foamman01 Electric Plane Talk 1 Oct 11, 2001 09:30 PM
Electric motor & prop for a HLG Butch B Electric Sailplanes 3 May 30, 2001 12:58 AM