Running transformers in reverse?

[fastline]

I seem to recall some literature indicating larger transformers need to run as designed regarding the load side. However, I'm wondering about smaller 3-7.5kva dry types? And while on the subject, do you need to honor the nameplate voltage? I've seen more than one 480V transformer running on 240V.

 

[jim dungar]

When they say small they are usually referring to those sized in VA not kVA.

Running transformers at less than nameplate voltage(s) can have poor overall perfomance.

 

[fastline]

Just to walk some simple math for clarity. If we assume a 240v pri, 120v sec, 5%Z, assuming actual load is full FLA of 20.8A, that is a Vdrop of 12V on the primary? Or 24V seen on the 120V side? Or?

The Z on the secondary is Z*4 or 20%, so back feeding presents .20*120V = 24V? Or 12V on the 240V side?

I guess asking for a math check, and/or confirm if I am missing something?

 

[electrofelon]

Just to walk some simple math for clarity. If we assume a 240v pri, 120v sec, 5%Z, assuming actual load is full FLA of 20.8A, that is a Vdrop of 12V on the primary? Or 24V seen on the 120V side? Or?

The Z on the secondary is Z*4 or 20%, so back feeding presents .20*120V = 24V? Or 12V on the 240V side?

I guess asking for a math check, and/or confirm if I am missing something?

Note "impedance" in regards to transformers is a special use of the word (you did correctly use %Z not just Z). It is the ratio of the primary voltage applied, with the secondary shorted, that results in rated current, to the rated primary voltage. Voltage drop is more complex, you would need the X/R ratio and the power factor. Here is a calculator:

Transformer Calculator – Voltage Disturbance