[thread name should be "Can i connect the ends of a transformer"]

If I connect the primary coil of a transformer to a regular (120v 60hz) outlet, what will happen if i connect the ends of the secondary coil together, with nothing else (making a complete closed loop)?

 

Since you do not specify the type of transformer, I will make the assumption you are talking about a standard step-down power transformer.

1) the magnetizing current in the "primary" (normally the secondary) with be high.
The winding has fewer turns - less inductance, therefor greater magnetizing current.
The inrush current may be high enough to blow fuses.

2) shorting the "secondary" (normally the primary) will cause sparks - large uncontrolled current leading to destruction of something.
You are basically shorting out the transformer.

This is not a good idea.

 

You will blow up your transformer!!

 

BANG !!!

Simon sez do not do this.

 

You will blow up your transformer!!

Not always. Some transformers are designed to allow for a shorted output. One example is a doorbell transformer.

One UL requirement is that a transformer can not get to an unsafe high temperature after 4 hours with the secondary shorted. This can be accomplished several ways such as a thermal fuse built into the transformer, a transformer that limits maximum current by design or fuses in the power line or secondary windings.

p.s. I have done the UL test on a small power transformer (a few VA ratinf) and it really does take hours for the transformer to reach maximum temperature. I was quite surprised.

 

what will happen if i connect the ends of the secondary coil together, with nothing else (making a complete closed loop)?

For one thing, you'll exceed the current rating of the secondary and likely damage the transformer. If you want to generate heat, there are better, safer methods.

 

So how can I measure the power output with a clamp ammeter - does it need a load?

 

From what I remember of AC circuits, the theory goes, A purely inductive load draws no current in an AC circuit. In practice, the DC resistance of the load is still there so some heating will occur. I have not tried it myself but, according to theory, there will be no "boom" but heating should be fit any non-ideal inductor (I.e. DC resistance) power loss and heating.

Good luck and be safe. Theory should be tested for your specific application with great care.

 

So how can I measure the power output with a clamp ammeter - does it need a load?

Yes. It needs a load.

You should measure the output voltage drop across the load as you test bigger and bigger loads. Current should be calculated from the known resistance of the load and the voltage drop across the load.

 

From what I remember of AC circuits, the theory goes, A purely inductive load draws no current in an AC circuit..................

What inductive load?
The OP is applying a short.
The only inductance in the circuit limiting the current would be the leakage inductance, which is very small.

 

So how can I measure the power output with a clamp ammeter - does it need a load?

Are you trying to determine the maximum rated output power?
That's not a straightforward test since a typical power transformer will output much more power with a low resistance load than it can safely continuously deliver.
The easiest way to determine a transformer's power capability is to look at other transformers with a similar core size and note their power rating.

 

What inductive load?
The OP is applying a short.
The only inductance in the circuit limiting the current would be the leakage inductance, which is very small.

Ok, I'm wrong.

Here is a daring Russian experiment or showing the wrath of electrical power on a poor block if iron and copper to prove how wrong I am.