Specifications

T4.32
GE Power Controls
Comfort Functions
T4
Series T
Transformers
Function and range
Transformers are mainly used for 2 reasons:
- To galvanically separate one circuit from the other
and / or
- To step down the energy supplier network voltage
in order to supply low voltage circuits.
Two main different subfamilies exist in the complete
Series T range of transformers:
- Bell transformers and
- Safety transformers.
For the range of the bell transformers, devices with
5, 10, 15 and 25VA output power are available,
some with and some others without short-circuit
protection, some with one combined secondary
winding of multiple voltages 12/24V, others with two
separate secondary windings for 8/12V.
This range also includes an 8VA/8V bell transformer
with integrated on-off switch.
For the range of the safety transformers, the output-
power covers the range of 15 up to 63VA, all have
two separate secondary windings for 2 voltages
(12/24V) and all are short-circuit proof.
All bell as well as safety transformers have double
isolation.
Terminology
For more detailed information, please refer to the
standard IEC 61558-2-6 (issued in 1997) which served
as base for the definition of the below terminology.
Safety transformer
All Series T transformers have an output power
below or equal to 63VA.
According to the above mentioned standard, the
ratio between the output voltage at no-load and at
rated output can be as high as 100%, at rated
frequency and rated ambient temperature.
This means that with a nominal output voltage of
12V (at nominal load), the output voltage at no load
is allowed to be as high as 24V.
However, for all Series T safety transformers, this
ratio is limited to 105%.
Also, the real output voltage of the highest voltage
output at rated output power, at rated supply
voltage, at rated frequency and below or equal to
the rated ambient temperature, is guaranteed not to
differ more than 5% from the rated output voltage
(above or below).
Bell transformer
Completely the same explanation as for the safety
transformers can be given here except for the ratio
between the output voltage at no-load and at rated
output, which is limited to 150% in the case of the
Series T bell transformers.
Short-circuit proof
Transformers can be short-circuit proof by construction
or by integrating a PTC in the primary of the transformer.
Short-circuit protection by construction is achieved
through the geometry of and material used in the
transformer. In this case, the transformer saturates
when trying to pull more secondary current than
allowed. However this causes the transformer to
excessively heat up.
A better way of protecting a transformer against
overloads or even against destructive secondary
short-circuits, is to include a PTC-resistance in the
primary of the transformer (see fig.1).
In this way, an excessive high secondary current will
‘ask’ for an excessive high primary current. This
high primary current will heat up the PTC, which in
its turn will increase its resistance, limiting herewith
the primary current.
All safety transformers and some bell transformers
are protected against secondary shorts by means of
a PTC in the primary winding of the transformer.
Double isolation
Double isolated transformers have two different
isolations between their primary and secondary
windings: the first being the wire-isolation, the
second being the isolation formed by the resin-cast
that is completely encapsulating the transformer.
Both the symbol used to indicate double isolation
as well as the schematic representation of a double
isolated transformer are given in figure 2.
fig.1
fig.2