Voltages applied

[relearn]

Recently sat for a exam with a question was,
240 3phase 48 Amp load, what would the amperage be if voltage applied was 480 volt 3 phase. Could not remember the formula for solving this.

 

[hillbilly1]

Double the voltage. 1/2 the amperage. That was an easy one!

 

[ptonsparky]

Double the voltage. 1/2 the amperage. That was an easy one!

I agree, unless there are other parts to the questio.

 

[hillbilly1]

I agree, unless there are other parts to the questio.

True, assuming both were delta’s.

 

[LarryFine]

Double the voltage. 1/2 the amperage. That was an easy one!

That presumes a constant power, requiring that the load is changed to suit the new voltage.

With the same load, if the voltage is doubled, the current doubles, quadrupling the power.

 

[hillbilly1]

That presumes a constant power, requiring that the load is changed to suit the new voltage.

With the same load, if the voltage is doubled, the current doubles, quadrupling the power.

Op didn’t say either whether it was resistive load, so could be true. Motor load would let out the magic smoke rather quickly! LOL!

 

[retirede]

If that’s the entirety of the question, it’s horribly written and unanswerable without making a lot of assumptions.

 

[LarryFine]

By the way, OP, welcome to the forum.

The question as asked effectively says applying the new voltage to the same given load.

 

[drcampbell]

If that’s the entirety of the question, it’s horribly written and unanswerable without making a lot of assumptions.

It's answerable, if there's time.

Just make a list of several different possible situations (resistive load, correctly-reconfigured motor, 230-volt motor shocked with 480 volts, et al.) and answer each one. Make the answer as tedious as the question is horrible, starting with "This would never actually be done in practice, but ... ".

 

[Knightryder12]

If that was the question in its entirety, you do exactly as post #2 says. You guys read way to much into a simple question.

 

[hillbilly1]

If that was the question in its entirety, you do exactly as post #2 says. You guys read way to much into a simple question.

We’ve seen too many trick questions on electrical exams! LOL!

 

[LarryFine]

If that was the question in its entirety, you do exactly as post #2 says.

240 3phase 48 Amp load, what would the amperage be if voltage applied was 480 volt 3 phase.

Double the voltage. 1/2 the amperage. That was an easy one!

Disagree. If you apply 480v to a 240v load, what prevents the current from doubling?

 

[hillbilly1]

Disagree. If you apply 480v to a 240v load, what prevents the current from doubling?

That’s the trick part, we don’t know that from the info provided.

 

[ptonsparky]

We’ve seen too many trick questions on electrical exams! LOL!

Either trick or 'I forgot to mention...'

 

[Tulsa Electrician]

Recently sat for a exam with a question was,
240 3phase 48 Amp load, what would the amperage be if voltage applied was 480 volt 3 phase. Could not remember the formula for solving this.

Could be zero after a few seconds.
If the load is not rated for 480 v.
C

In reality I would find VA and apply new voltage.

It would be half however no help in way of a formula.
240*48*1.732= 19,962.64 va
19962.64/480/1.732= 24 amps

This way if they say 208 you can find the ansawer.
208*48*1.732=VA
VA/480/1.732= new amps

Square root of 3 is 1.732 for 3 phase

Hope you passed.

 

[LarryFine]

240*48*1.732= 19,962.64 va
19962.64/480/1.732= 24 amps

How did you keep the VA the same when you doubled the voltage???

When you double the voltage across a given impedance, the resultant current doubles.

You're answering a question that wasn't asked.

 

[ptonsparky]

How did you keep the VA the same when you doubled the voltage???

When you double the voltage across a given impedance, the resultant current doubles.

You're answering a question that wasn't asked.

By reducing the amperage?

 

[LarryFine]

By reducing the amperage?

The only way to do that is to increase the impedance so it uses the same power at the new voltage.

But that requires replacing the existing load with a new one with quadruple the impedance.

The only change mentioned in the original question was the applied voltage, not the load.

 

[ptonsparky]

I was past the OPs problem.

I thoughts you were asking about Tulsa's math. Sure looked right.

 

[Knightryder12]

Disagree. If you apply 480v to a 240v load, what prevents the current from doubling?

We don't know the the type of load, so there is that. But you are correct, that can happen depending on the load type.