# Very simple per unit question

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1. Nov 9, 2015

### tim9000

If you took: VBase / ZBase

does that give you IActual

or IBase or what?

Because it seemed to give me IActual.

Cheers

2. Nov 10, 2015

### Simon Bridge

What is the difference?

3. Nov 10, 2015

### tim9000

Like if Iactual = Ibase*Iper unit

4. Nov 10, 2015

### Simon Bridge

OK... so do that for the other "base" units.
Remember what the base and per unit parts mean.
What is the relationship between actual things?

5. Nov 10, 2015

### tim9000

Well I know that Vbase / Zbase = (Vactual/Vpu) / (Zactual/Zpu)
=Vactual * Zpu / Vpu*Zactual = Iactual * (Zpu/Zactual)
But I'm not sure what that means for the last part of the last term

6. Nov 10, 2015

Sorry, the final term is wrong here:

Ibase=Vbase / Zbase = (Vactual/Vpu) / (Zactual/Zpu) =Vactual * Zpu / (Vpu*Zactual) = Iactual * (Zpu/Vpu)

Iactual * (Zpu/Vpu)=Iactual*Ipu

7. Nov 10, 2015

### tim9000

Oh right.
but Isn't (Zpu/Vpu) = 1 / Ipu

so Vbase / Zbase = Iactual / Ipu ?

Also, side question, say you were given the rated power and voltage for a system, so then you can work out the rated base impedance. But does that mean that
Ibase = √(Sbase - Zbase) ?

8. Nov 12, 2015