SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
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insulation class 130°C. Compound filled transformers use
insulation class 180°C. Larger ventilated transformers are
designed to use 220°C insulation. All of these insulation
systems will normally have the same number of years
operating life. A well designed transformer, observing these
temperature limits, will have a life expectancy of 20-25
years.
17. Why should Dry-Type Transformers never be over-
loaded?
Overloading of a transformer results in excessive
temperature. This excessive temperature causes
overheating which will result in rapid deterioration of the
insulation and cause complete failure of the transformer coils.
18. Are temperature rise and actual surface
temperature related?
No.
This can be compared with an
ordinary light bulb. The filament temperature of a light bulb
can exceed 2000 degrees, yet the surface temperature of
the bulb is low enough to permit touching with bare hands.
19. What is meant by “impedance” in transformers?
Impedance is the current limiting characteristic of a
transformer and is expressed in percentage.
20. Why is impedance important?
It is used for determining
the interrupting capacity of a circuit breaker or fuse employed
to protect the primary of a transformer.
Example:
Determine
a minimum circuit breaker trip rating and interrupting capacity
for a 10 kVA single phase transformer with 4% impedance,
to be operated from a 480 volt 60 Hz source.
Calculate as follows:
Normal Full Load Current =
Nameplate Volt Amps
=
10,000 VA
=
Line Volts
480 V
20.8 Amperes
Maximum Short Circuit Amps =
Full Load Amps
20.8 Amps
4%
=
4%
=
520 Amps
The breaker or fuse would have a minimum interrupting rating
of 520 amps at 480 volts.
load is 40 amperes, such as a motor, and the secondary
voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10
kVA (10,000volt-amperes) transformer is required. ALWAYS
SELECT THE TRANSFORMER LARGER THAN THE ACTUAL
LOAD. This is done for safety purposes and allows for
expansion, in case more load is added at a later date. For 3
phase kVA, multiply rated volts x load amps x 1.73 (square
root of 3) then divide by 1000.
(4)
Determine whether taps are required. Taps are
usually specified on larger transformers.
(5)
Use the selection charts in Section I.
11. What terminations are provided?
Primary and
Secondary Terminations are provided on ACME Dry-Type
Transformers as follows:
No lugs—lead type connection on
0-25 kVA single phase
0-15 kVA three phase
Bus-bar terminations
(drilled to NEMA standards)
37.5 -250 kVA single phase
25-500 kVA three phase
12.Can 60 Hz transformers be used at higher frequencies?
ACME transformers can be used at frequencies above 60 Hz
up through 400 Hz with no limitations provided nameplate
voltages are not exceeded. However, 60 Hz transformers will
have less voltage regulation at 400 Hz than 60 Hz.
13. What ismeant by regulation in a transformer?
Voltage regulation in transformers is the difference between
the no load voltage and the full load voltage. This is usually
expressed in terms of percentage. For example: A transformer
delivers 100 volts at no load and the voltage drops to 95
volts at full load, the regulation would be 5%. ACME dry-
type distribution transformers generally have regulation
from 2% to 4%, depending on the size and the application
for which they are used.
14. What is temperature rise in a transformer?
Temperature rise in a transformer is the temperature of
the windings and insulation above the existing ambient or
surrounding temperature.
15. What is “Class” in insulation?
Insulation class
was the original method used to distinguish insulating
materials operating at different temperature levels. Letters
were used for different designations. Letter classifications
have been replaced by insulation system temperatures
in degrees Celsius. The system temperature is the
maximum temperature at the hottest spot in the winding
(coil). Graphical representations of six insulation systems
recognized by Underwriters’ Laboratories, Inc. are shown in
Figure A. These systems are used by Acme for a large part
of the product line.
16.Is one insulation systembetter than another?
Not necessarily. It depends on the application and the
cost benefit to be realized. Higher temperature class
insulation systems cost more and larger transformers are more
expensive to build. Therefore, the more expensive insulation
systems are more likely to be found in the larger kVA units.
Referring to Figure A, small fractional kVA transformers use
10
Total Winding Temperature °C
COIL HOT SPOT
DIFFERENTIAL
AV. WINDING
RISE
AMBIENT
105
130
180
220
30
25
10
55
80
115
150
AGENCY: UL/ANSI 1561 MARCH 1987
40
40
40
40
Figure A
