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Application Notes
Indications:
The technical information laid out below should be referred to for correct use
of the transformers in various voltage fields. The differences in the construction
of the transformers are determined by the installation and equipment requirements.
We have limited ourselves to laying down the main definitions set down in the norms
CEI 96-3 EN 61558.1 and part 2. Other norms to which we refer are CEI 14-5, 14-8,
CEI 96-1, UL 506 - CSA C 22.2 n.66.
-Transformers -
- Isolating transformers: transformers suitable for limiting risks caused by contact between the earth and the live parts that can give an electric shock if
the isolation is broken or damaged.
Primary voltage: 1000V max - Secondary voltage: 500V no-load - Frequency: 500Hz max - Capacity: 25KVA if single-phase, max. 40KVA if three-phase
with double or reinforced isolation.
- Safety transformer: isolation transformers suitable for feed circuits of low safety voltage. Primary voltage: 1000V max - Secondary voltage: 50V max.
no-load - frequency 500Hz - Capacity 10KVA if single-phase , max. 16KVA if three-phase.
Type of isolating: double or reinforced.
Types of protection: the equipment has built-in safety to protect against dangerous electrical currents.
- Type I: All the metallic parts accessible from the transformer are insulated from the voltage parts by basic isolating and added safety measures
consisting of a safety conductor within the electrical system of the installation.
- Type II: All the metallic parts accessible from the transformers are separated from the voltage parts with double or reinforced isolating.
The transformer must not be connected to the earth..
- Type of isolation: normal types of isolation materials:
A (105°C) -E (120°C) -B (130°C) -F (155°C) -H (180°C)
- Nominal ambient temperature:this is the temperature in the vicinity surrounding the transformer. If not otherwise specified,
the nominal ambient temperature is 40°C. Particular care should be taken when
the temperature is higher. It should not exceed the values laid down in the table below.
The core power must be adapted to approximately the values given below::
Nominal room temperature
40°C
50°C
60°C
70°C
|
Nominal power X factor
1
0.86
0.72
0.59
|
- Temperature rise:
this is the temperature which is verified in the transformer when working normally.
In particular during winding depending on the type of isolating materials.
The values on the following table must not be exceeded:
Maximum excessive temperatures for winding with ta=40°C
A=60°C- E=75°C -B=80°C -F=100°C -H=125°C
- Nominal input voltage:
network voltage feed of the transformer. Unless otherwise stipulated,
the maximum limit displayed is a value of 1.06 volts as the nominal input
voltage, as long as that does not cause damage to the continuous running
of the transformer.
- No-load current: this is the input current of the discharged transformer and nominal frequency.
The value of the no-load current is mainly effected by the properties of the magnetic
core laminations used and to all intents and purposes these can oscillate,
even between transformers from the same production.
An oscillation of - 10%/+20% with respect to the nominal value is acceptable.
- Primary winding: particular attention should be paid if, rather than only one input
voltage, additional voltage is required, or derived. In this case a raising of the core power is necessary. For example:
| Input voltage |
Nominal power X factor= scaling of the core power |
| |
1 section |
2 sections |
230
230+400
230+500
400+440
400+440+500
|
1
1.23
1.26
1.06
1.12
|
1
1.52
1.49
1.12
1.25
|
- Nominal output voltage: this is the output voltage of the transformer at nominal frequency, to the nominal input voltage, with a nominal output current
at a nominal power factor (=1). This is obtained when running warm with a nominal room temperature. If not otherwise stipulated, a tolerance of ±5% is acceptable.
For transformers short-circuit proof for construction it is:±10%.
- No-load voltage: : this is the secondary voltage of the discharged transformer, to the primary
voltage and the nominal frequency.
This formula can be found on various types of tables:
| CdT% = |
No-load voltage - secondary nominal voltage
No-load voltage |
x100 |
Norm EN 61558.1 gives the following values for isolation and safety of transformers:
Nominal % differences between secondary no-load and loaded voltage:
Up to 10 VA
Over 10VA up to 25 VA
Over 25 VA up to 63 VA
Over 63 VA up to 250 VA
Over 250 VA up to 630 VA
Above 630 VA
|
100
50
20
15
10
5
|
NB. For insulating transformers up to 63VA:20%%
One should bear in mind that in the case of control
transformers in accordance with norm EN 61558.1 part CEI 96-3 the max. difference is 10%. |
- Output winding. By which we mean derivation of the output , the current is
calculated in reference to the highest voltage, not specified otherwise.
In the case of more winding, one should consider that, owing to the quantity
of isolation, it could be necessary to enhance the core power.
- Nominal power. This is the product of the nominal secondary voltage for the nominal secondary current.
In the case of three-phase transformers is √3 the product of the nominal secondary voltage for the nominal secondary current.
In the case of secondary winding it is the sum of the product of nominal secondary voltage for the nominal secondary current of the circuits charged simultaneously.
- Auto-transformer. This is a transformer with shared winding for inlet and outlet, without galvanic isolation between the windings.
The relative power is given in the formula:
where Papp= power (VA), Pnom= nominal power (VA),
V1=inferior voltage (V) e V2=superior voltage (V)
- Short-circuit and overload protection. It is advisable to transfer the subdivision that the norm EN 61558.1 applies to transformers short circuit proof:
a) Transformer not inerenthly short-circuit: this is a transformer in which has been designed to resist extreme temperatures using a protective
device not supplied with the said transformer.
b) Transformer short-circuit proof: this is a transformer in which the running temperature cannot
exceed the limits specified when the transformer is short circuited,
and can function normally after the removal of c.c.
b1) Transformer short-circuit proof for construction: this is a transformer which, in the absence of protective devices, should c.c. occur, does not exceed the temperature limits (e.g.small transformer with high internal resistance).
b2) Transformer short-circuit proof not for construction: this is a transformer which comes with a protective device inside which opens the primary or secondary circuit or reduces the current should c.c. occur (e.g. PTC bi-metal).
b3) 3)Fail-safe transformer: this is a transformer which after abnormal use, stops working without placing the user at risk (e.g. transformer with thermo-fuse included)
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