Bad Transformer?

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mhanson

Member
Location
Redwood City, CA
Occupation
General Journeyman Electrician
Good Evening All,
How do I determine if my 30kva transformer has gon bad.?
ACME 30kva General Purpouse transformer 480v 3ph primary, 208/120v 3ph secondary (only Two phases are being used).
primary fuses are 60amp fast acting (two of which have blown)
I purchased Two new fuses to match the existing
I turned OFF the secondary fused disconnect to eliminate any possbile problems on the load side.
I installed the two new primary fuses and they blew imediately.
At the transformer I disconnected the three primary conductors and the three secondary conductors.
my Fluke DMM showes no resistance between H1, H2, & H3 I'm not sure what I should be seeing here.
another thought is I may have incorrect fuse type on the primary, FAST ACTING dosn't sound right to me for some reason.:?
any input would be appreciated.

Thank You
Michael Hanson
Journeyman Electrician
Redwood City, CA.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Fast acting fuses are generally not recommended for feeding transformers.

Primary inrush current is almost independent from any secondary load. It does not really matter if the secondary load is connected, if it did we would have lots of fuses blowing every time the utility power flicks off and on during a storm. There is some anecdotal evidence that a secondary load can reduce the inrush.
 

mhanson

Member
Location
Redwood City, CA
Occupation
General Journeyman Electrician
Fast acting fuses are generally not recommended for feeding transformers.

Primary inrush current is almost independent from any secondary load. It does not really matter if the secondary load is connected, if it did we would have lots of fuses blowing every time the utility power flicks off and on during a storm. There is some anecdotal evidence that a secondary load can reduce the inrush.

Thank You Jim,
so there's a chance the transformer isn't bad.
How do I properly size the primary overcurrent protection.?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Thank You Jim,
so there's a chance the transformer isn't bad.
How do I properly size the primary overcurrent protection.?

Not Jim, but.................
The answer as to what size has a lot of variables which are unknown to us.
What size conductors, what size secondary overcurrent fuses are two items that should be taken into consideration.
Transformer protection must be taken into consideration per 450.3 and conductor protection must be taken into account per Art 240.
Your primary fuses could be required to be as small as 50 amp or as large as 90 amp depending on your secondary protection.
For testing a 50 amp time delay fuse should hold.
Keep in mind if you are only using 2 phases of the secondary you do not have the full 30 kva capacity. What size is your secondary protection ? and do you know the load ?
Did you check resistance between H1,H2 & H3 to ground ?
 
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zog

Senior Member
Location
Charlotte, NC
Good Evening All,
How do I determine if my 30kva transformer has gon bad.?
ACME 30kva General Purpouse transformer 480v 3ph primary, 208/120v 3ph secondary (only Two phases are being used).
primary fuses are 60amp fast acting (two of which have blown)
I purchased Two new fuses to match the existing
I turned OFF the secondary fused disconnect to eliminate any possbile problems on the load side.
I installed the two new primary fuses and they blew imediately.
At the transformer I disconnected the three primary conductors and the three secondary conductors.
my Fluke DMM showes no resistance between H1, H2, & H3 I'm not sure what I should be seeing here.
another thought is I may have incorrect fuse type on the primary, FAST ACTING dosn't sound right to me for some reason.:?
any input would be appreciated.

Thank You
Michael Hanson
Journeyman Electrician
Redwood City, CA.

Winding resistance test
Turns ratio test
Insulation resistance test (DAR/PI)
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
...At the transformer I disconnected the three primary conductors and the three secondary conductors.
my Fluke DMM showes no resistance between H1, H2, & H3 ....

Just curious:
When you say "no resistance" do you mean near zero ohms or open circuit?

ice
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
On a transformer that size, you'd expect winding resistance to be very low, but on the lowest scale you should still see some difference between the winding resistance and a dead short. Readings to ground on primary and secondary windings should read infinity on a DMM. If not, you may have an insulation problem. I knida wonder if I should even suggest this, but since you're supposed to be a professional if using this site, I will. If you do not have access to a TTR (Transformer Turns Ratiometer) or even a megger, there's one thing you could try. If you put 120 line to line on one phase of the the primary, you should see 30V secondary line to neutral on one phase of the secondary. Make sure you have no grounds on the primary windings first, though. To be safe, you might hook up the test cord and plug it in with the breaker for the circuit off, then use the breaker to give you power. Understand, that this is only a reasonable test if you know what you're doing. Not for an amateur. As was stated above, for a real test you need a ratiometer and a megger. My bet is on the wrong fuses, though. Fast acting and inrush are a bad combination.
 
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iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Takes about one hour or less

z - All this for an hour's labor? You sure? $250 would cover it?

I'm not saying you are wrong. Just seems like a bit of overkill for a 30 kva.

I'm thinking an inexpensive megger -- then try with the right fuses.

ice
 

zog

Senior Member
Location
Charlotte, NC
z - All this for an hour's labor? You sure? $250 would cover it?

I'm not saying you are wrong. Just seems like a bit of overkill for a 30 kva.

I'm thinking an inexpensive megger -- then try with the right fuses.

ice

If you are testing several they take about 20 minutes. Overkill maybe but that is per NETA/ANSI
 
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