tranformer overload

[new guy]

I understand a Tranformer's primary and secondary sides are not phisically connected. So can a short circuit or an overload in a circuit connected to the secondary side cause the primary's side circuit breaker to trip? And if so, how does this happen?

[Robert Reed]

If the circuit is properly designed, the transformer is "transparent to power". It has nothing to do with stepup,stepdown ratios. For example 100 watts in = 100 watts out (or very close depending on transformer efficiency) So the load on the secondary is directly seen by the primary. And an overload will work the same way. If the primary is fused for 100 watts of power, then when the secondary reached this point it would blow the primarys fuse.I am using very loose figures here for simplification. In reality Primary would see 100 watts slight before secondary and fusing would be taken up a level.

[Enzo]

Here is an analogy. On your bicycle, the chain and gears transform the motion of your feet into the motion of the tire on the wheel. If you pedal hard it turns the wheel. If I grab the wheel, you will feel it in your foot pedals. My resistance to your motion is transformed back into resistance to your feet.<p>A transformer translated voltage and current. The high voltage and low current of the primary is transformed into the low voltage and high current of the secondary. As Robert said, power is power. Whatever goes on on one side is transformed to the other side.

[jollyrgr]

Yes, a short in the secondary will show up on the primary. But you won't see this "short" with an Ohm meter. You WILL see the reflected impedance. Thus the AC line signal (or what ever input is on the primary) will see the short on the secondary. The reflected impedance, Zr, is the impedance on the input divided by the square of the turns ratio.

[Dean Huster]

An interesting application of this short-circuit quirk that I've seen is putting the primary of a 120vac transformer in series with a high-power load, such as a motor. The secondary runs to a switch. When the switch closes, shorting the secondary, the effect is a "shorted" primary and the motor runs. It's an effective way of using low, isolated voltage to switch a line-operated device. The down side is that the "off" position is never really and truly fully "OFF".<p>Dean<p>[ October 18, 2005: Message edited by: Dean Huster ]</p>

[Robert Reed]

Dean
When that switch is closed and full motor current is runnig thru the primary relatively unimpeded, I wonder whats going on in the secondary--Mucho current ?

[jollyrgr]

Dean or anyone else....<p>I would think that the device in series with the transformer, namely the motor, would have enough current to run, although at a reduced speed. If the motor actually stopped, wouldn't the idle current overheat the windings of this "locked" rotor motor? <p>If the motor where a "universal" type, would this burn out the brushes? In an even more interesting idea, would a lower power resistor or small power transistor on the secondary make a variable short to control speed?

[Dean Huster]

Actually, I don't remember all the details of the application. As I recall, it was one of those "Ideas for Design" articles in Electronic Design magazine where I saw it. The idea was that the transformer gave you the line isolation and from the secondary, you could use a switch or a solid-state device to actuate the load. And yes, it mention that you had to watch out for a healthy secondary current. I've not gone over all the possible implications (e.g, is this a really inefficient switch) of the design ... I'm more presenting it as an observation here ... you know, one of those topics that gets buried in a thread of a different color! And I would think that with an open secondary, the residual current available to a higher-power load would be minimal. I wouldn't try this idea with a low-power control app.<p>Dean

[Michael J]

An "Auto Transformer" mainly used as a Mains
Stepup/Stepdown can have a common connected
terminal (primary to secondary).

[dyarker]

A transformer primary in series, with the secondary open, is just an inductor. "Off" current would depend on impedance.<p>Sounds like an interesting way to control small 120VAC loads. But:<p>If a 120VAC motor draws 10A under load, and a 120 to 12VAC transformer is used; then the secondary current is 100A! Primary would have to be like 16AWG, and secondary like 4AWG or 2AWG; with a 20lb. core.<p>beginner,
The windings are not electrically connected, but they are magnetically connected.<p>For a simple mental picture, hold a strong magnet in your hand. Move your hand up and down. You only feel the weight of the magnet. This is similar to a tranformer with no load on the secondary.<p>Now touch the magnet to a hammer. It is now harder to move your hand up and down, even though you are not connected to the hammer. You are connected to the magnet which is connected to the hammer. The change in weight on the other side of the magnet affects you (reflected).<p>On overload might be like touching the magnet to a car. Your hand will not go up and down any more. Something has to give. You let go of the magnet before you break your arm. With the transformer, the fuse blows or circuit breaker trips (if the builder put one there), else the transformer burns out.<p>Cheers,