Model Boat Mayhem
Technical, Techniques, Hints, and Tips => DC Motors (Brushed) and Speed Controllers => Topic started by: gyronuts on September 13, 2010, 04:07:40 pm
-
HI, MY FRIEND HAS THE MOTOR SHOWN IN THE PICTURE. IT IS A MODEL MOTORS DIRECT UK MOTOR. IT IS FOR 12V,18V OR 24VOLT. IT IS 75MM ( 2 1/2 ") LONG WITH A SHAFT AT EACH END AND IS 45MM ( 1 3/4") DIAMETER. IT IS ALSO VERY HEAVY.
HE WAS HOPING TO FIT IT INTO A TUG.
QUESTION; WHAT BATTERY,PROP COMBINATION AND WHAT MODEL WOULD THIS MOTOR BE SUITABLE FOR?
THANKS
BILL
-
No idea as Graham sells several different Buhler motors with different windings giving different speeds.
What size of tug and what size is the Prop.
As a general rule you will need 3-5000 rpm.
Bob
-
Thats what i need to know. What type of boat and prop does my friend need. He is hoping to fit it into a tug.
how do i contact graham? Bill
-
Bill,
The size is the most important factor as it takes a different amount of thrust to power an 18 inch tug to a 5 foot one.
It is a bit like me saying " I have a submarine, what motor, prop and tank do I need?"
Bob
-
Bob,
Thanks for the reply.
The chap has a kit of the Deans marine models Grampian Pride . Length 830mm Beam 200mm
However, the problem is ...will this motor do the job?
What voltage should he plan to use? (Most DM models use a 6V battery. )
Does the motor have a specification?
Without some data on the motor then I agree it is impossible to quantify prop,voltage ,current or performance.
There is no detail on the motor other than that shown in the photo.
Graham's website is not yet up and running.
Bill
-
Just because DM recommend using 6V batteries in their designs, it does not follow that 6V is the only possible system. The motor needs what it needs, and a 12-24 volt motor will be downright anaemic on 6 volts. Presumably the kit will have suggestions as to type of motor in the instructions. There should be a prop, to give some idea regarding the motor requirements. All that remains then is to figure what the motor is capable of, and any given size of motor case can have a huge number of possible permutations about what is on the inside, and the insides is what determines performance.
Provided it can rev appropriately when loaded with the prop in water, it will be OK, this can only be determined by testing. Thats a bit like say it will work if it works.
-
You can, with some data and a bit of empirical testing, decide if the motor you have is adequate for the task in hand.
You need the following information.
Unloaded RPM of the motor at the voltage you wish to run at.
Pitch and diameter of the propeller you wish to use on the boat.
You can find out the RPM of any motor by using a rev counter. Optical ones are available at fairly low cost- these tend to be aimed at aeroplane use, but work just as well with boats. Once you have the unloaded RPM of the motor, you should aim for 60-75% of that figure when under load. If it's dropping to significantly under that, then you need either reduction, a smaller pitch/diameter prop, or a slower revving motor wound for more torque.
Another method is to use an ammeter. Connect this up in series with the motor and battery and measure the stall current of the motor (grip the shaft with something firm, be careful), measure the unloaded current of the motor e.g. free running. Subtract this from the stall current, the figure you are left with should be used as your reference. If you are pulling more than about 50% of the measured current bandwidth, then you need to look at the previously mentioned solutions.
The pitch of a prop can be a little more difficult to suss generally. Prop shop give excellent data with their range. Most scale props tend to be between 1.2-1.4 times the diameter of the prop.
-
Malcolm,
I accept that wholeheartedly.
On my brushed and brushless motors used in my electric flight model they are classified ,such as AXI 2820/10 ,so that I can derive the useful number of watts and therby match motor, current draw and prop size dependent on what performance I require. Duration , performance or a balance of both.
Surely boat modellers have a similar system to end up with a balanced setup to ensure some level of success other than try it and see.
I was hoping someone would say that they had the same motor and what setup was being used and suggested performance.
Bill
Is the red spot on the motor significant? Having searched the website it does not appear to be a buhler.
-
Bill,
If you go for a scale size prop you will be looking for about 3000 rpm.
This gives you the option of a small motor geared down or your motor on a suitable voltage to give 3000 rpm.
As I have said I cannot be more specific as I don't have the operating parameters for this motor, but suspect it will be at least 12 volt.
Bob
-
Deffo a Buhler. Can you find it via its dimensions under the "motors" link here?
Buhler (http://www.buehlermotor.com/C12572C600247071/CurrentBaseLink/W273RD58524WEBREN)
Andy
-
nope tried that website. thanks for the link. bill