Smoking....

[Dickw]

Posted by Roger Dyke on 18/05/2020 06:39:55:

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I like your analysis reference the motor dissipation, although I'm not too sure about by increasing the voltage (3 cells to 4) with the current remaining the same (with a smaller prop) that the heat generated in the motor wouldn't rise (V squared over R).

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Yes, "V squared over R is the heat generated in the motor windings, but you are thinking of the wrong V. Have a look back at Pat's post - he mentions "back emf".

Think about it. If it is the same motor R must be the same, and if you prop it for the same current (I) the voltage must be the same because V=IR.

What is happening is that as the rpm rises the back emf also rises, and the voltage you need for your "V squared over R" is supply volts (battery volts) minus the back emf volts.

Just wanted to offer a second opinion so you know Pat is not making this up on his own

People often forget about back emf, but it is a very important factor in the operation of our motors.

Dick

Edited By Dickw on 18/05/2020 10:14:44

[Roger Dyke]

Hi Dick,

Ah..yes, of course it is. You are right and thank you for opening my eyes. I was taking the simplistic approach and not thinking (normal for me). I was just taking the applied voltage and applying a straight DC ohms law type formula but of course when we involve coils and frequency we are into a whole different ball game. I remember from the dim and distant past my involvement with back EMF's regarding loudspeaker driving and transformer manufacture etc.

Anyway, to yourself and Pat, Thank you so much for your expertise and valued contributions.

I really am now out of this thread.

Thanks again

Roger