Motors and batteries

[learner johnParticipant @learnerjohn78074]

 Hi! It’s me again…Could someone please recommend a book on motors and batteries which will explain in very simple terms what it’s all about. I’ll explain……I made a small boat for the grandson ,inbetween my main project ,based on some freebee plans from the magazine.I found a small motor and when the boat was completed I connected the motor to 4 1.5 v batteries and tried it out on my bench and found that even after1/2 an hour it was still running …so I thought that will do because by then the said grandson will be thinking of ways to sink it!! Now comes the best bit…..This was the first time that I had put a boat into the water and switched on….it went like a dream this way and that way..I was over the moon to say the least….then it became uncontrollable after about 5 mins.on taking out of the pool I found that the motor was still going and also the rudder but once back into the water the rudder didn’t want to know. I presume that I havn’t got enough battery power…sooo before I get too involved in my main project and cock it up on the power front…can someone help?Please!

[60wattParticipant @60watt]

Hello John,

Here is a hyperlink to the data sheet of a 380 motorwhich will be similar to yours.It contains tabular and graphical data but it is the Current Vs Torque graph which explains all really.

http://docs-europe.electrocomponents.com/webdocs/0273/0900766b80273327.pdf

The motor is maintained at a constant voltage and the graphs show what happens to efficiency,rotational speed and current drain as the load torque  increases.

The graph is missing the output power curve which ais parabolic and peaks at the 50% efficiency point.

When you are out of water the motor is rotating at high speed but it is not drawing a lot of current from the batteries

When the prop is immersed in more viscous water the lorque on the motor increases so its rotational speed drops and the current rises.

In water,you have drained the batteries of power so quickly that their voltage has dropped to a level where control circuitry is unable to function.

Tom

[60wattParticipant @60watt]

Hello John,

 I don’t know any book on batteries but I’m sure you may find http://www.batteryuniversity.com  informative.

Tom

[learner johnParticipant @learnerjohn78074]

Once again many thanks for the advice and help.

[ashley needhamParticipant @ashleyneedham69188]

John, I presume you were not using rechargeable batteries ?? You dont say what size of battery either, and this has a lot to do with the running time. Nowadays you can get 3.7 Ahr "c" size rechargeables, and even 2.7 Ahr "aa" batteries, and on a small motor (do you know what type it is?? dimensions may tell us) these might propel a boat in the water for an hour or more no problem. If rechargeables were being used, an unfortunate vice of such items is that they go flat really quickly…that is, your boat is chugging along nicely one minute, and the next, its dead in the water. Duracell types just tend to go slower, and slower…etc. This is also a problem if using main battery power to power the reciever, rechargeables will drop the voltage fairly suddenly (comparitively) and again all of a sudden the reciever is dead.This is made worse when the motor is pulling the last dregs of juice out of the accumulators, the voltage will drop due to the load.   Ashley, clear as mud.

[learner johnParticipant @learnerjohn78074]

Well ! What can i say…….Join a club…..A pal of mine had a similar problem ( and he has just told me!!) and it was caused by the plastic sleeve joining the motor to the prop shaft slipping!!!! Anyway another thing to look at…..but I will certainly mug up on the batteries and motors,,,once again many thanks.

  

[learner johnParticipant @learnerjohn78074]

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