Lower the Volts, higher the Amps, and vice versa?
- Thread starter zappy
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- New Jersey
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- Journeyman Electrician
Resistive loads like a heater lower the voltage lower the current (directly proportional) . Inductive loads like a motor lower the voltage increase the current (inversely proportional).
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
That's true for constant power, i.e., I = P/V. For constant resistance I = V/R, so not in that case.
An assumption most often made is that since low voltage increases the amp draw on motors, then high voltage must reduce the amp draw of the motor.
That’s not entirely true.
Higher voltages than expected on a motor will push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron past the point that magnetizing is practical.
So short answer.. it depends
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
There's a good graph that gets posted occasionally that has motor amps at varying voltages. Maybe someone can find it and post it
There is a good one on page one. Not sure of the one you’re referring to.
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- New Jersey
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- Journeyman Electrician
There are also the tables right out of the NEC for example 430.248 or 430.250.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
Don't get confused by the difference between applying a different voltage to a given piece of equipment
. . . and . . .
Equipment designed to produce (or use) a given amount of power when supplied by a different voltage.
It's true when the equipment is built to match the voltage. When talking about delivering a given power, the P = E x I math applies. To deliver a given power, current varies inversely with voltage. Also, loads like motors use less current when the voltage increases (within its design voltage range).
When talking about applying a different voltage to equipment with no change to the equipment, then the I = E / R Ohm's Law math applies. The current varies proportionately with applied voltage. This describes most electrical loads which behave as resistive loads do. If the voltage increases, so does the current.
. . . and . . .
Equipment designed to produce (or use) a given amount of power when supplied by a different voltage.
It's true when the equipment is built to match the voltage. When talking about delivering a given power, the P = E x I math applies. To deliver a given power, current varies inversely with voltage. Also, loads like motors use less current when the voltage increases (within its design voltage range).
When talking about applying a different voltage to equipment with no change to the equipment, then the I = E / R Ohm's Law math applies. The current varies proportionately with applied voltage. This describes most electrical loads which behave as resistive loads do. If the voltage increases, so does the current.
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- Retired PV System Designer
<rant>
The partially inductive nature of a motor load has absolutely nothing to do with the inverse relationship between voltage and power.
A load made up of resistance, inductance and capacitance in any combination is still a linear load, with current directly proportional to voltage (with an optional phase shift).
A typical motor load is at least approximately a constant power load. A regulated power supply can be another example of a constant power load, and is far from linear.
I just grates when I hear a motor load called an inductive load as if that explained how current is related to voltage in a motor.
Just calling it non-linear does not work either, since the current into a non-linear (and potentially time varying) load can either increase or decrease with voltage in any arbitrary proportion.
</rant>
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
To add: Yes, that's what the math says when you get to choose which values are the constants and which values are the variables.
"the lower the Volts, the higher the Amps" is what applies when you alter the load to consume the same power on a new voltage. Here, the constant (as a desired result) is the power, the applied voltage and the load impedance are variables, and the new current is the result. The resultant current varies inversely with the applied voltage.
That's very different from merely applying a new voltage, as the variable, to a constant load. The resultant current varies proportionately with the voltage.
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
It is the case when considering a transformer. I believe 'transformer' is the only general name for a piece of equipment that can be counted upon to raise or lower voltage while doing the opposite to amps, proportionally.
It is generally not the case when changing the voltage applied to any other equipment, but it really depends on the equipment. It will be the case for some equipment that is designed electronically to draw constant power at various voltages, such as some computer power supplies. (A grid-tied solar inverter is another example, where it's designed to *output* the same power at a varying AC voltage. Again, controlled electronically.) But it is generally not the case for anything not deliberately designed to do that, especially simple resistive loads or components, which will more amps, not less, at higher voltage.
As mentioned above, mathematically if the power (and power factor) are given, and voltage and current are variables, then it will be the case. But as for whether that is a useful model for equipment, see previous paragraph.
drcampbell
Senior Member
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- The Motor City, Michigan USA
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- Registered Professional Engineer
Like so many other rules of thumb, this is applicable only in a few specific circumstances when several specific criteria are met.
It would probably be best to just assume that this is false. Most of the time, it will be.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Isn't that what I said in post #3?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
To us who understand it, yes. To someone who needs to ask, maybe not.Isn't that what I said in post #3?
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Interesting that according to that graph, the common premise of the volts/amps inverse relationship is not really true. It's more true for when voltage goes down but not really for when voltage rises. Another interesting thing to note is that when voltage rises beyond a few percent, amperage also rises, but the power factor is decreasing so it's (roughly) still constant power.
I’m guessing you didn’t check out post 14 with the all bold and larger font...
I know what you were saying, just got a chuckle out of the wording.