Re: (ET) Elec-trak Digest, Vol 19, Issue 88

["David Roden" <etpost drmm net>]

On 18 Oct 2021 at 13:17, Chris Zach via Elec-trak wrote:

> OK. Let's say you are blowing snow. You have to have a certain amount of 
> speed
> to do so properly without the chute clogging. When you start with a 
> happy full
> battery the current is lower than when you're coming towards the end 
> with an
> empty battery. Power is what moves the snow.

Correct, power moves the snow.  But in a DC brushed motor, speed is 
proportional to the applied voltage.  As voltage falls, The auger turns 
slower, throwing the snow less far, and *using less power*.  

> if you mow in say L with speed 4 locked in on a level ground the mower
> and the tractor will slow down together so amps will probably remain
> constant as power is reduced overall. 

Same here.  You're going slower, so you're *using less power*.

> But if you start flooring it to speed 5 you'll pull more amps.  

Now that's a different case.  There you're 100% correct.  To maintain the 
same power at a lower voltage, you need more current.  

What I'm arguing is that in most cases you probably don't even try to 
maintain the same power - and with a fixed-speed motor such as on a mower 
or snow thrower, you don't have that choice.  It produces the maximum 
power 
it can against a given load with a given voltage.

-----

I think part of the confusion here results from reading motor nameplates.  
For example the nameplate current of a 120 volt 1/2 hp motor will 
typically 
be about double that of a comparable 240 volt 1/2 hp motor.

But those motors are *designed for their specific nameplate voltages*.  If 
you run that 240 volt motor at 120 volts, it won't use twice the current. 
It will use *less* current, though possibly more than half.  

It's possible to model motor behavior mathematically.  I don't have the 
engineering chops to do it off the top of my head, though some of you 
probably do.  I could look it up but, eh, erm, cough, that will be left as 
an exercise for the reader.  :-)

I did a quick web search and found a page that illustrates this rather 
nicely with a simple, straightforward example:

https://hvacrschool.com/does-a-motor-draw-more-or-less-at-lower-voltage/

That page is examining an AC induction motor, and other factors such as 
power factor come into play, but the principle is similar.


David Roden - Akron, Ohio, USA

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