Current draw??

[Corky]

Hello everyone,

i am happy with ohms law and the concept of voltage supply and the resistance in a circuit determines how much cureent is 'Drawn' BUT...

i have recently been told that something with a set resistance can draw more current than i would initially think with the result leading to a drop in voltage from the supply, im really confused by this and have had it explained a few ways which i dont understand

i understand by ohms law something has to 'give' either less current more resistance or less voltage but how does the load decide what current it has?

I think i have worded this ok but im struggling to understand myself so any questions welcome

p.s. specifically were talking about dc motor output and when the load current increases but the resistance stays the same the voltage across the generator decreases.

 

[4pyros]

i have recently been told that something with a set resistance can draw more current than i would initially think with the result leading to a drop in voltage from the supply

That statement is wrong.
If a motor is loaded more its perceived resistance drops and it draws more current, if the current is more than a power supply can handle then its output voltage will drop.

 

[Corky]

That statement is wrong.
If a motor is loaded more its perceived resistance drops and it draws more current

Could you explain this again please, i honestly think im over thinking this now as i cant grasp it at all

 

[4pyros]

No matter what ohms law is ohms law, no exemptions to the rule. If one thing changes then something else must change too.
I think what is confusing you is motor theory.
You should look up how a motor works.

 

[MikeMl]

A Motor has two resistances, one is the static resistance you would measure with an Ohmmeter with the motor at rest, the other is dynamic resistance related to the mechanical work the motor is doing; the more work, the lower the resistance. When you combine these two resistances, and measure the current as the load on the motor varies, the current increases at the combined resistance decreases...

 

[Corky]

sorry, i have wrote DC motor when i meant DC generator and a load can reduce the output voltage of it.

 

[JonSea]

Keep in mind that every power supply, be it a battery, a lab power supply or generator, has internal resistance. As the current draw increases, the voltage drop across the internal resistance increases, reducing the output voltage.

Regulated lab power supplies attempt to maintain the same output voltage regardless of load, but even of output of these will vary because of the internal resistance. Better lab power supplies have separate sense terminals so that the actual voltage delivered to the load can be monitored and adjusted.

 

[4pyros]

Ever power source has its limits.
If you exceed its limits you will start to loose power, ether voltage or current or both.

 

[Corky]

my understanding: so the generators internal resistance rises as more current is drawn therefor a bigger Vd will occur across the generator? if this is the case i am still confused about how the load can draw more current

 

[audioguru]

I think you are confused about a start-up current surge.

A resistor will draw a steady amount of DC current as calculated with Ohm's Law. But if it has a discharged capacitor parallel with it then the moment the power supply is connected, the discharged capacitor draws a very high current surge when it begins to charge. Only the resistance of the wiring and the internal resistance of the battery or generator limits the current by reducing the voltage delivered.

A DC motor without a load draws a low steady current. But when it is stalled its current is very high. When it starts running it is stalled for a moment and causes a high current surge.

 

[JonSea]

V = IR

In this case, V is the voltage drop across the internal resistance, which is R. As I (the current) increases, V increases.

The voltage drop across the internal resistance increases, so the output voltage drops.

 

[KeepItSimpleStupid]

Here https://www.quora.com/What-is-the-difference-between-current-and-voltage is a comic that MIGHT help.

 

[Ratchit]

Hello everyone,

i am happy with ohms law and the concept of voltage supply and the resistance in a circuit determines how much cureent is 'Drawn' BUT...

i have recently been told that something with a set resistance can draw more current than i would initially think with the result leading to a drop in voltage from the supply, im really confused by this and have had it explained a few ways which i dont understand

i understand by ohms law something has to 'give' either less current more resistance or less voltage but how does the load decide what current it has?

I think i have worded this ok but im struggling to understand myself so any questions welcome

p.s. specifically were talking about dc motor output and when the load current increases but the resistance stays the same the voltage across the generator decreases.

For a DC motor, a heavy load or startup causes the back-voltage to decrease, thereby allowing a relatively high current through the motor. When the motor comes up to speed or the load is less, then more back-voltage occurs and the current becomes less.

Ratch

 

[nsaspook]

The simplified DC motor current/power draw is the electrical equivalent of a simple physics equilibrium equation.
http://large.stanford.edu/courses/2012/ph240/kramer2/

Most of the other gory details are here: **broken link removed**

 

[Corky]

thankyou everyone for the replies but i think i have explained what im after wrong, i completely understand ohms law but the cartoons were still pretty cool, ive attached the graph where this question comes from, Why does the voltage across the output terminals drop as the current goes up? cheers guys

 

[audioguru]

Why does the voltage across the output terminals drop as the current goes up?

Simple Ohm's Law explains the voltage drop at the output of the generator when it has current drawn from it.
It has an "armature reaction" drop plus an "ohmic" drop. Its wire windings have resistance that is in series with its output. Current in a resistance produces a voltage drop V= I x R.

 

[Corky]

cheers, i understand this bit now, the more current drawn the more current running through the armature windings the bigger the Vd, how does the load draw more current? is the only way by lowering its resistance?.... if so does this mean a greater powered load at the same voltage has less resistance than its less powered brother?

thanks again this is really helping my brain work

 

[audioguru]

When a DC electric motor runs without a load then it is not working hard. It is generating a "back-emf" which causes its current to be very low.
When it is loaded then the back-emf is reduced and the resistance of the motor becomes closer to the resistance of its windings which is very low. Low resistance causes high current.
When the motor is stalled or is started then it is not spinning so it is simply a piece of wire which draws a very high current.