Electrical isolation
Aside from the ability to easily convert
between different levels of voltage and current in AC and DC circuits,
transformers also provide an extremely useful feature called
isolation, which is the ability to couple one circuit to another
without the use of direct wire connections. We can demonstrate an
application of this effect with another SPICE simulation: this time
showing "ground" connections for the two circuits, imposing a high DC
voltage between one circuit and ground through the use of an additional
voltage source:
v1 1 0 ac 10 sin
rbogus1 1 2 1e-12
v2 5 0 dc 250
l1 2 0 10000
l2 3 5 100
k l1 l2 0.999
vi1 3 4 ac 0
rload 4 5 1k
.ac lin 1 60 60
.print ac v(2,0) i(v1)
.print ac v(3,5) i(vi1)
.end
DC voltages referenced to ground (node 0):
(1) 0.0000 (2) 0.0000 (3) 250.0000
(4) 250.0000 (5) 250.0000
AC voltages:
freq v(2) i(v1)
6.000E+01 1.000E+01 9.975E-05 Primary winding
freq v(3,5) i(vi1)
6.000E+01 9.962E-01 9.962E-04 Secondary winding
SPICE shows the 250 volts DC being
impressed upon the secondary circuit elements with respect to ground,
but as you can see there is no effect on the primary circuit (zero DC
voltage) at nodes 1 and 2, and the transformation of AC power from
primary to secondary circuits remains the same as before. The impressed
voltage in this example is often called a common-mode voltage
because it is seen at more than one point in the circuit with reference
to the common point of ground. The transformer isolates the common-mode
voltage so that it is not impressed upon the primary circuit at all, but
rather isolated to the secondary side. For the record, it does not
matter that the common-mode voltage is DC, either. It could be AC, even
at a different frequency, and the transformer would isolate it from the
primary circuit all the same.
There are applications where electrical
isolation is needed between two AC circuit without any transformation of
voltage or current levels. In these instances, transformers called
isolation transformers having 1:1 transformation ratios are used. A
benchtop isolation transformer is shown in the following photograph:
- REVIEW:
- By being able to transfer power from
one circuit to another without the use of interconnecting conductors
between the two circuits, transformers provide the useful feature of
electrical isolation.
- Transformers designed to provide
electrical isolation without stepping voltage and current either up or
down are called isolation transformers.
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