General
VTs should be specified as
follows :
RATIO: input / output voltage ratio
TAPPING'S: all taps required on each winding
VA: total burden of each winding or tapping
FREQUENCY: If different from 50Hz
DIMENSIONS : maximum limits
MOUNTING: required terminations and mounting
TEMP. CLASS : If other than Class B
The following factors affect Transformer
prices :
 |
Specifying a higher VA then necessary |
|
Rated
Voltage. |
|
Temperature rise other than 80º
(40ºC ambient) |
|
The
number of windings required. |
|
Frequency. |
|
Regulation lower than those on our listings.
|
|
|
Multi - Tapped Primary
If the range of primary taps is limited,
say within
+/- 15% of the median voltage, the VA is increased by the factor:
0.5 (1 +(Vmax/Vmin))
This alters if the range is larger
than 15% as graded copper sections are required.
Call our design department for more information.
D.C. Loads
The effective VA of a transformer winding
supplying D.C. loads depends on the type of rectifier and filter
used.
If D.C. values are specified this information must be given. |
|
Regulation
This is the inherent voltage drop due to the impedance of the
transformer expressed as a percentage of the output voltage. If
%R is the inherent resistive voltage drop and %X is the
inherent reactive voltage drop, both expressed as a percentage of
the output voltage then:
%REG. = %R + (%X/200)
This applies as rated output, unity
power factor only.
Intermittent Overload
This is permissible, if of short
duration and followed by an underload. If a transformer is subjected
to a regular overload current Ii
(i.e. contactor inrush current) for ti seconds,
followed by a regular underload Is
(i.e. contactor seal current) for ts
seconds, the equivalent continuous current is :
| IEQ
|
= |
√ |
Ii²
ti + Is²
ts |
(Amps) |
| ti + ts |
The transformer should not overheat,
provided IEQ does
not exceed the rated current and provided ti
does not exceed a few minutes duration. |
|
Auto Transformers
Where electrical isolating is not
required, auto transformers may offer considerable advantages in
economy & space. The nearer the input/output ratio approaches
unity, the more marked the reduction in size.
For a simple step-up or step-down auto transformer:
Frame VA = Output VA (Vmax
- Vmin)/Vmax
Where there are a number of input and/or output tapping's, each load condition must be considered and
the above formula would not apply.
Auto transformers also provide advantages in regulation.
Metering Instrument VTs
In general the following apply:
Class
|
0.1 or 0.2 for precision
instruments |
|
0.5 for high-grade meters |
|
1.0 for commercial-grade meters |
|
1.0 or 3 for general industrial
measurements |
|
3 for approximate measurements |
Protection Instrument VTs
In addition to the general specification required for VT design,
protection VTs require a Rated Voltage Factor (VF). This is the
multiple of rated voltage up to which the VT will operate while
complying with the accuracy class standards. |
