single phase transformer question on load. Both primary and secondary.

[Turn10]

Let say i have a 277volt 20 amp (emergency) lighting circuit that currently has a 7 amp load on it. My question is if I put a 277 to 120 step down 750 va transformer on this 20 amp emergency lighting circuit.

What is the max amp load draw from the secondary of the transformer? 10 amps?
Does the amp draw on the secondary raise the current 7 amp lighting load equal to what is being drawn from the secondary?

Thanks!

 

[augie47]

For practical purposes (Ignoring things like current loss in transformer) it all goes back to ohms law. In this case I=P/E. For a 750va transformer that would mean a little over 6 amps of 120v and
a lillte less than 3 amps 277 volt load,.

 

[gar]

170217-1519 EST

Turn10:

I will try to answer part of your question, and if that is understandable, then we can continue from there.

We make many assumptions in describing electrical circuits because it makes problems easier to solve. So my following statements should be considered to be approximations to a real circuit. But in many cases good to possibly 5%.

The secondary load on a transformer can be considered to be reflected to the primary by some factor of the turns ratio.

If I build a transformer with tight magnetic coupling between the primary and secondary, then if X = primary to secondary turns ratio the (secondary voltage) = (primary voltage) / X.

In your example X = 277 / 120 = 2.308 .

Ideally a transformer is lossless, meaning no power is dissipated in the transformer. If a 100 W load is placed on the secondary, then the secondary current is 100/120 = 0.833 A.

Since the transformer is lossless and and energy is neither created or destroyed, then when looking into the primary we see a load power of 100 W. But the input voltage is 277 V and the input current must be 100 / 277 = 0.361 A. We can get this same answer by dividing the secondary current by the turns ratio. For your example 0.833 / 2.308 = 0.3609 .

.

 

[Ingenieur]

If I understand the question
current 277 vac at 7 A
how many amps at 120 (assuming the load can be wired from 277 to 120 and the va load remains the same at both voltage)

power in = power out = vp x ip = vs x is
Rearrange: vp/vs = is/ip
Rearrange: is = vp/vs x ip = 277/120 x 7 = 16.2 A

transformer max vp x ip = vs x is = 750 va
use vs x is = 750
is = 750/vs = 750/120 = 6.25 A

load > capacity or 16.2 > 6.25
no go
need at least 120 x 16.2 (vs x is) = 1944 va