The rule of thumb is when the HP exceeds the voltage, go to a higher voltage.
But there are exceptions.
If you have a Volt system in place and are already using Volt motors, then strongly consider going to Volts.
If you do not presently have Volts available, it may be more reasonable to use a 460 Volt motor.
Your present maintenance crew may not be qualified to service Volt or Volt equipment.
When you compare the installed cost estimates of the two solutions the answer may be readily apparent.
Bill
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"Why not the best?"
Jimmy Carter
400HP is really at the breakpoint between voltage levels, so as Bill says you could choose either option and not be 'wrong' , but the decision may be made by other factors such as availability of an existing MV network. You almost certainly wouldn't put in a complete MV switchboard just for one motor.
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.
That's true for constant power, i.e., I = P/V. For constant resistance I = V/R, so not in that case.
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.
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
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
There's a good graph that gets posted occasionally that has motor amps at varying voltages. Maybe someone can find it and post it
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.
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).
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.
I always thought the lower the Volts, the higher the Amps.
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.
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.
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.
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.
Isn't that what I said in post #3?