FAQ's
Find answers to the most common questions about Transformers.
A transformer is a device that transfers electrical energy from one
circuit to another through inductively coupled conductors — the
transformer's coils or "windings". Except for air-core transformers,
the conductors are commonly wound around a single iron-rich core, or
around separate but magnetically-coupled cores. A varying current in
the input or "primary" winding creates a varying magnetic field in
the core(s) of the transformer. This varying magnetic field induces
a varying electromotive force or "voltage" in the output or
"secondary" winding. If a load is connected to the secondary, an
electric current will flow in the secondary winding and electrical
energy will flow from the primary circuit through the transformer to
the load. In an ideal transformer, the induced voltage in the
secondary winding is in proportion to the primary voltage, and is
given by the ratio of the number of turns in the secondary to the
number of turns in the primary.
- Industrial Control and General Purpose
- Buck-Boost Transformers
- Toroids
- High Frequency/Ferrite Core
- Reactors
- Ferro-resonant
- Isolation Transformers
- Autotransformers
- Low-Voltage Transformers
- Lighting Transformers
- Class 2 Transformers
- Energy Efficient Transformers
- Encapsulated/Potted Transformers
Simply stated, a toroid transformer is one that uses a toroid, or
doughnut-shaped, core. Toroid cores may be made from long wound
strips of steel for low frequency transformers, or made of ferrite
materials for high frequency transformers. The round shape of a
toroid core means there are no gaps, or breaks, in the magnetic flux
line path, and therefore fewer magnetic losses. This is a distinct
advantage in certain applications. The toroid cores themselves, as
well as the specialized winding and assembly methods often render
toroid transformers slightly more costly than other types.
An isolation transformer does not have a direct electrical path from
the input side to the output side. Although any transformer with a
separate primary and secondary winding can be called an isolation
transformer, the term is usually used to denote a transformer built
just for that purpose. These transformers are used to reduce the
risk of electric shock hazard, and may have equal input and output
voltages, and are therefore used strictly for the safety isolation
they provide.
An auto transformer has only a single winding with two end
terminals, plus a third at an intermediate tap point. The primary
voltage is applied across two of the terminals, and the secondary
voltage taken from one of these and the third terminal. The primary
and secondary circuits therefore have a number of windings turns in
common. This often allows the transformer to be slightly smaller,
less costly, and often more efficient, than the isolation
transformer counterpart of the same power rating, but it lacks the
safety of an isolation transformer.
Yes. “Line frequency” transformers are designed for use at 50 Hz
and/or 60 Hz, but “high frequency” transformers are designed to
operate at higher frequencies – kHz, MHz, and beyond. High frequency
transformers can be made smaller than their 60Hz counterparts of the
same power level, but they introduce electromagnetic interference
(EMI) considerations that are largely be ignored at lower
frequencies.
Regulation compares the difference in output voltage WITHOUT the
load current applied to the output voltage WITH the load current
applied. It is usually expressed as a percentage change. The higher
the transformer’s efficiency, the less the voltage will change.
Therefore, “better” regulation means less change in voltage, and
therefore a lower percentage value.
A ferrite core transformer is required if the operating frequency is in
the kHz or MHz range.
Certainly, as long as the combination of the ambient temperature and the
temperature generated by the transformer itself does not exceed the
applicable temperature limits. The limits may be set by regulatory
standards, or, in the absence of such standards, simply by the
temperature ratings of the insulating materials.
Duty cycle, in simplistic terms, is the percentage of time a transformer
is active, or energized and loaded according to its ratings. If it is
always “on”, then it is said to have a 100% duty cycle, or rated for
“continuous duty”. Average, effective, or equivalent duty cycle must be
calculated for transformers whose loads vary over the course of a
typical cycle.
Typically, the most costly components in a transformer are the magnetic
core material and the copper wire or foil. On occasion, specialized
insulation materials (high voltage and/or high temperature) and
protective devices (fuses, circuit breakers, thermal cutoffs, etc.) can
also add substantial cost.
It is difficult, and arguably reckless, to make a blanket statement of
what the minimum safety requirements should be for transformers.
Requirements vary depending on voltage and power levels, the regulatory
standards for the specific applications, and in which global markets the
transformers will be used.
There are many standards for medical/dental applications, but most use
one or more sections of UL/EN 60601-1.
There are typically two (2) options: • The transformer manufacturer
submits the transformer to the applicable safety agencies for component
approval(s), or, • The end product manufacturer submits the transformer
for investigation along with the end product. In this case, the
transformer manufacturer provides the end product manufacturer with the
requisite transformer documentation needed by the investigating agency.
Limited air density, due to increased altitude can have an effect on the
operation performance of low-voltage components. For applications at
high altitude, some studies have been performed, (Study by Subhas Sarkar
and John K. John) but not much is known about this effect on the
operation performance of these components. Characteristics such as
dielectric voltage withstand, thermal current carrying capacity,
overload calibration, contact life, and interruption capability can be
affected by the decreased air density. Standard - A transformer may be
used at full nameplate capacity up to 3300 feet (1000 meters). Above
that altitude, the capacity of the transformer should be derated by 0.3%
for each 300 feet of elevation above 3300 feet. (Per IEC 726/ANSI
C57.12)
A control transformer is designed to provide rated output voltage at
full VA. As the load goes down, the output voltage will go up.
Conversely, increased load current will result in lower output voltages.
Typically, the smaller the VA size transformer, the greater difference
there is between no-load and full-load voltage.
Temperature class = The transformer insulation system The standard
insulation system classification, are as follows: 105(A), 130(B),
155(F), 180(H), 200(N), and 220(R).
Temperature rise is the difference between the average temperature of
the transformer windings and the ambient temperature.
Definition - Class 2 transformer: A transformer that
has 30 volt rms
[root mean squared] maximum secondary potential under any condition of
loading. The portion of the wiring system between the load side of a
Class 2 power source and the connected equipment. A Class 2 power source
is limited to the following ratings:
| Voltage | Wattage | Currents |
|---|---|---|
| 0 to 20V (Class 2 & 3) | 100 Watts | 5 Amps |
| 21 to 30V (Class 2 & 3) | 100 Watts | 3.3 Amps |
| 31 to 150V (Class 3) | 0.5 Watts | 5 MilliAmps |
These transformers are used in Class 2 circuits that need to comply with
ANSI / NFPA 70, or the Canadian Electrical code part 1, CSA C22.1,
connected to sinusoidal sources. NOT used for - Power supplies, toy
transformers, cord or plug connected, direct plug-in, for audio,
television type appliances, or other special types of transformer
covered in requirements for electrical devices or appliances. The
application / end product dictate which category of transformer may be
used. The safe use of transformers is critically dependent on the
electrical system they are installed into. The investigation to assess
safety of the system and components, is for system compatibility.
These transformers are intented to be used in circuits that need to
comply with ANSI / NFPA 70, and which are connected to sinusoidal
sources. The portion of the wiring system between the load side of a
Class 3 power source and the connected equipment. The output of a Class
3 transformer must be between 31V and 100V, if inherently limited, or
between 31V and 150V, in non-inherently limited. Like a Class 2 circuit,
it can be installed without a conduit; however, because has higher
voltage than a Class 2 circuit, the NEC has additional requirements for
safety.
(DFM) is the general engineering principle of designing products in such
a way that they are easy to manufacture to insure fit and function.
A control transformer is an isolation transformer designed to provide a
high degree of secondary regulation during inrush current.
In a perfect loss-less world, reverse-connecting a transformer would
work just fine. However, the real world has losses, and the
transformer's windings are usually adjusted to make up for the expected
losses. Therefore, although transformers can be reverse-connected, the
turn ratios may not result in the hoped-for performance.
Three single phase transformers can be connected to form a three phase
bank, their primary and secondary windings are connected in WYE or DELTA
connection.
Hot spot is the highest temperature inside the transformer coil.
Potting or encapsulating will help protect the transformer from
moisture, dust, dirt and other contamination.
The insulation system is based on various material used in group use in
designing. It provides a comparable life expectancy. The choice of
insulation system depends on application and cost.
A transformer is needed to step down or step up the voltage from an
input source. It can also provide output voltage stability for short
periods when overload inrush currents occur.
No control transformers are not current limiting. They allow all the
current required by the load.
Control transformers do not regulate the output voltage. Variations in
input voltage will be proportionately reflected in the output voltage.
Encapsulation, or potting, protects the transformer coil from industrial
contamination, and moisture. It also makes transformer run cooler under
load and no load.
A control transformer is not a power conditioning component, however it
may diminish electrical noise, spikes, surges and transients.