Electric Motor Current - RC Groups
Feb 13, 2003, 05:42 AM
Henry Springer
Henry Springer
Guest

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
Feb 13, 2003, 06:12 AM
Steve Ritchie
Steve Ritchie
Guest

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?

Feb 20, 2003, 10:42 AM
Kevin Russell
Kevin Russell
Guest

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

 Feb 20, 2003, 03:10 PM Registered User 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.
Feb 21, 2003, 04:22 PM
Sigurd Ruschkowski
Sigurd Ruschkowski
Guest

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

 Feb 24, 2003, 03:46 PM Registered User 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
 Feb 24, 2003, 09:42 PM Registered User 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
 Feb 25, 2003, 09:08 AM Registered User Any chance you were able to record the amp draw at point of impact?