Lower the Volts, higher the Amps, and vice versa?

[Jraef]

I always thought the lower the Volts, the higher the Amps. I found out that that's not always the case. So my question is when is it the case, and when is it not the case. Thank you for your help.

So maybe this will clear it up for you.

There is a difference between DESIGN DECISIONS, and DEVIATIONS FROM EXISTING.

In the design decision phase of a project, that maxim is true, in that if, for example, you are choosing between 480V or 240V for a given load, the higher voltage will result in lower amps and vice versa. That is true.

But if you HAVE a piece of equipment already CONNECTED to a voltage source, and that voltage DEVIATES from normal, the effect of that deviation is totally dependent upon the nature of the equipment. Fixed resistance, like a heater, will result in current DROPPING with voltage drop. Motors, current INCREASES with voltage drop, but ALSO with voltage increase! Just at different rates (see above chart). So that maxim does not hold in this case.

 

[wwhitney]

Here it is without the accompanying white paper.

Do you know if the "Percent Voltage Variation" on that graph is relative to supply voltage standards (240V, say), or utilization voltage standards (230V correspondingly)?

Cheers, Wayne

 

[Jraef]

Do you know if the "Percent Voltage Variation" on that graph is relative to supply voltage standards (240V, say), or utilization voltage standards (230V correspondingly)?

Cheers, Wayne

I've always taken it to mean utilization voltage (motor nameplate voltage rating). I don't remember where I got that now, I saved that image over 2 decades ago because it comes up with some regularity in motor troubleshooting discussions.

Here is another page using a similar graph, this person specifically says it is motor nameplate voltage rating.

 

[Besoeker3]

For what ever reason the bold and larger font got changed. It has since changed to regular font.

 

[Hv&Lv]

Do you know if the "Percent Voltage Variation" on that graph is relative to supply voltage standards (240V, say), or utilization voltage standards (230V correspondingly)?

Cheers, Wayne

Per the first page of the white paper, its nameplate rating

 

[Hv&Lv]

Per the first page of the white paper, its nameplate rating

What’s interesting on this chart is a 230 motor on what is generally supplied with a utility ~242 volts, with voltage drop added to the feeders to the motor looks to run extremely well.
On the other hand, as many times as it has been posted here about using a 230 motor on 208 system, one would think the poor performance would justify buying a 200V rated motor.

 

[kwired]

Lower the volts the higher the amps when determining what is needed for a specific power output.

Simply changing input volts without changing any thing about the actual load usually doesn't work that way.

Then there is things like electronic driven power supplies that can take a range of input volts and still end up making needed adjustments to derive same output over the designed input input range. We see this often with 120-277 volt ballasts/drivers for lighting, but there is other things that tend to be this way as well.

 

[Besoeker3]

Lower the volts the higher the amps when determining what is needed for a specific power output.

For a fixed voltage any current can be any rating.

 

[powerpete69]

IF you are referring to power transmission for example, then yes. P=I*V
Running 500,000 volts down power lines is fantastic for lowering amps to deliver the same power for example. The lower the amps, the smaller the electric lines need to be running down the roads.
Smaller electric lines equals less money spent. Multiply that by millions of miles of power lines and you have quite a savings....a savings that is passed on to you in a relatively low monthly electric bill. And of course, this was all made possible from the invention of AC and the most wonderful transformer. Transformers short in DC......(JWL) W=0 in DC. Edison was wrong....Tesla was right.