# Transformer question; safety

C

#### ChrisG

Jan 1, 1970
0
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?

L

#### Larry Brasfield

Jan 1, 1970
0
ChrisG said:
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.

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.
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?

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.
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?

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.
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?

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.

J

#### Jim Gregory

Jan 1, 1970
0
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

B

#### Bob Eldred

Jan 1, 1970
0
ChrisG said:
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?

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

C

#### ChrisG

Jan 1, 1970
0
Bob Eldred said:
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

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.