voltage vs current with resistive loading

[ron]

I should augment my post by saying that when the NYC utility company drops voltage below 10%, many of the buildings that I work with, drop off the grid entirely onto standby generators resulting in a reduction of utility power consumption whether it be resistive or not.

 

[tallgirl]

I should augment my post by saying that when the NYC utility company drops voltage below 10%, many of the buildings that I work with, drop off the grid entirely onto standby generators resulting in a reduction of utility power consumption whether it be resistive or not.

Do they actually lower the voltage or is it a load related voltage sag? There are industry standards that say the voltage at the service must be between 114VAC and 126VAC (multiply as needed for other than 120/240 services).

I was doing research for some patent filings I have in process and what I saw was that increases (and decreases) in grid voltage are more controlled by load and long times to reach thermal equilibrium on large generators than someone turning down the voltage knob. I see a day-to-day change of around 5% (for the one day I'm looking at the range was 244v to 256v -- it's been as high as 275v, at which point my house decided it had enough and disconnected ...) -- I'm thinking a 10% drop is probably poor grid management and not someone hoping buildings will drop themselves off the grid and reduce load.

 

[ron]

Julie,
I believe that in these extreme cases, they purposefully reduce the grid voltage. Surely it is sagging anyway due to load and the associated voltage drop.

 

[mull982]

So can it be generally assumed that with a purley resistive load the current will increase with an increase in voltage and vise-versa, and for an incudtive or capacitive load (Reactive) the current will decrease with an increae in voltage and vise-versa?

 

[drbond24]

So can it be generally assumed that with a purley resistive load the current will increase with an increase in voltage and vise-versa, and for an incudtive or capacitive load (Reactive) the current will decrease with an increae in voltage and vise-versa?

Not exactly. With a resistive load, current increases linearly with respect to voltage. With a reactive load, an increase in voltage still increases current, just not linearly. A capacitor requires less and less current as it charges up, and an inductor requires more and more current as it charges up. You end up with a current curve instead of a straight line on the graph.

 

[rattus]

Not exactly. With a resistive load, current increases linearly with respect to voltage. With a reactive load, an increase in voltage still increases current, just not linearly. A capacitor requires less and less current as it charges up, and an inductor requires more and more current as it charges up. You end up with a current curve instead of a straight line on the graph.

What you say is true for DC, but we are talking AC here, no? Then the current is determined by the impedance, that is,

Irms = Vrms/Z

 

[winnie]

So can it be generally assumed that with a purley resistive load the current will increase with an increase in voltage and vise-versa, and for an incudtive or capacitive load (Reactive) the current will decrease with an increae in voltage and vise-versa?

For a pure resistor, inductor, or capacitor, current flowing through the load will _increase_ if the applied voltage is increased.

However when we are talking about loads connected to the AC power distribution system, _many_ loads characterized as inductive are also somewhat constant power, at least over a reasonably small voltage range.

In particular we are talking about electric motors.

Electric motors are generally inductive loads, but clearly they are not pure inductors. When you consider the motor and its connected mechanical load, it is approximately a constant power load over a range of supply voltage around its normal operating voltage. If the supply voltage is too high, then you will see magnetic saturation and poor efficiency; if the supply voltage is too low the motor will stall, again with poor efficiency. But for reasonable voltage changes, the motor will continue to run, continue to produce approximately the same mechanical output, and have roughly the same high efficiency.

For a _constant power_ load, as supply voltage increases, the current drawn by the load will decrease.

-Jon

 

[LarryFine]

When you consider the motor and its connected mechanical load, it is approximately a constant power load over a range of supply voltage around its normal operating voltage.

That's why most 3-phase motors will run on 208v or 240v without any connection changes.