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Transformer question; safety

That is a good way to understand it. Another way is to solve a simple circuit containing at least two inductors with a high coupling coefficient. Just a voltage source across the primary and a resistor across the secondary is enough for that purpose.

The fuse should blow for the reasons given in that text you have. The limit is affected by the circuit resistances or impedances in series with the primary and secondary.

An isolation transformer is safer because it reduces the chances of a person completing a circuit carrying the 5-10 mA normally required for a dangerous shock (in healty people). It does not limit the current to a degree effecting safety.

Voltage sufficient to electrocute with no more help that a person completing a circuit is usually considered more dangerous than current which requires a good conductor (not a person) to flow and whose main danger is a burn from the short created by that conductor.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield
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I'm trying to get a better intuitive understanding of transformers.

One text I have explains that an increase of current in the secondary causes an increase in back emf (of the secondary) which acts to reduce the back emf of the primary, which increases the current in the primary.

Since power supplies frequently fuse the primary side, it seems that the secondary side can cause excessive currents in the primary. But what is the limit to which the primary current can be increased? In other words, what happens when the secondary is shorted?

I have seen comments regarding isolation transformers - that they limit the current that can be delivered, and thus are safer. If output power is close to input power where does the energy go? Heat? Impedence from the coils?

I have also seen things saying that a small voltage secondary is less dangerous (than high voltage), but isn't that low voltage secondary capable of delivering high current, which is usually described as more dangerous than voltage?

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Reply to
ChrisG

I never hear the term "back emf" applied to a transformer only rotating machines such as motors. Please explain what back emf is. And, if it is real, how do you measure it?

Yes increasing the secondary current increases the primary current. The maximum short circuit current is dependant on the resistance of both windings. The secondary resistance is reflected back into the primary by the turns ratio squared. The total resistance "seen" by the primary voltage determines the primary current under shorted conditions. This current causes the resistance of the windings to heat and will usually destroy the transformer if on too long. Any transformer can take overloads for a short duration determined by heating.

Some small transformers and motors are deliberately wound with a high resistance and high inductance primary winding. These are called "impedance protected" and cannot draw enough current to self destruct even when the secondary is shorted or rotor stalled in the case of a motor.

Some transformers are protected by internal, non-replaceable fuses. These will blow keeping the transformer from overheating and causing a fire when over loaded or shorted. Many isolation and modern power transformers do this. The rule is that they must not blow the line circuit breaker when shorted but must "self destruct" without fire. Furthermore, the fuse must be internal so it cannot be tampered with.

It takes voltage to "push" current through the resistance of a person. High current capability alone won't do it. That voltage is what makes "high voltage" dangerous not current. Bob

Reply to
Bob Eldred

Chris, The fuse is there to protect the integrity of the *supply* feeding the transformer and everything else - from non-behaving equipment connected to the mains distribution network. The fact that, in this case, it will rupture (blow) when a shorting or an excessive current fault affects the secondary, and therefore the primary, is beneficial to and noticed by the user when it halts further powering! It is not unusual to have another, anti-surge, fuse protecting a secondary.

In this country, half-voltage temporary site or safety transformers handling about 2kVA deliver 115V at their isolated secondary. They often have a line-to-neutral leakage-sensing, fast-acting RCD resettable trip added. And I have seen versions with a balanced secondary, ie, centre-tap to ground/earth, giving only a 57-V potential if one line is accidentally touched. Jim

Reply to
Jim Gregory

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Wow, you guys are a great source of information.

The actual term used was 'counter EMF' (this is from a HeathKit AC manual) which I believe another way of refering to inductive reactance.

So with regard to safety, while it is true that a step down secondary will have a higher current capability, the voltage is lower and the 'load' resistance is unchanged ( the large 200 lb salty-water bag variety of resistor) so the current will actually be less thru 'the load', by Ohms law. Makes sense.

Thanks all.

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Reply to
ChrisG

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