Camara Power Problem #42-8

[riotgurl7785]

Can someone explain the solution to this problem? Specifically at the end of the solution when you find the new rotor current. I do not understand how that ratio was found. I would have thought it to be:

Inew = Iold(Eold/Enew)

Thanks

 

[Judowolf PE]

Riot, I don't have that test example, but if you give some details I'll let ya know how I'd solve it...

 

[riotgurl7785]

Here is the problem statement:

A 440V, three-phase synchronous motor is driving a 45 HP pump. Assume the motor is 93% efficient. The rotor current is adjusted until the line current is at a minimum, occuring when the rotor current is 8.2A DC. Determine the rotor current necessary to provide a leading power factor of 0.8.

 

[DK PE]

Can someone explain the solution to this problem? Specifically at the end of the solution when you find the new rotor current. I do not understand how that ratio was found. I would have thought it to be:

Inew = Iold(Eold/Enew)

Thanks

I have the EERM 5th edition (older than yours) but it still has this problem. In the section on power factor correction it states "Power factor correction can be accomplished by adjusting the rotor current since Eg is approximately proportional to rotor current"

That is why they solve for Eg in each case and then just scale the unity PF rotor current by the ratio of Eg for each case. I assume you realize that you need to increase the field (rotor) current to end up with a leading power factor, ie.overexcited gives leading PF?

 

[riotgurl7785]

Ok, thanks, that helps. To clarify, field current & rotor current are the same, and armature current & stator current are the same?

 

[DK PE]

Ok, thanks, that helps. To clarify, field current & rotor current are the same, and armature current & stator current are the same?

On a synchronous machine, yes