FAQ - Question about Control Transformers, Class 2 Transformers and various other Transformers?


Question about Control Transformers, Custom Transformers, Class 2 Transformers and Various Other Transformers? Come Visit Us.

What is a transformer?

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.

Different Types of Transformers:

  • 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

What is an isolation transformer?

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.

What is an auto-transformer?

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.

Can a transformer be used at higher frequencies?

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.

What is regulation?

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.

What is a toroid transformer?

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.

What is a ferrite core transformer used for?

A ferrite core transformer is required if the operating frequency is in the kHz or MHz range.

Can transformers be used at ambient temperatures other than 40oC?

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.

What is a duty cycle?

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.

What are the main contributors to the cost of a transformer?

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.

What is the minimum safety requirement for general purpose use of my transformer?

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.

What approvals are required for medical/dental applications?

There are many standards for medical/dental applications, but most use one or more sections of UL/EN 60601-1.

How can I get a transformer approved for medical/dental, or any other specialty application?

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.

What is the effect of altitude on a transformer?

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)

What is the effect of load on a transformer?

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.

What is temperature class?

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).

What is temperature rise?

Temperature rise is the difference between the average temperature of the transformer windings and the ambient temperature.

What is a class 2 transformer?

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 Current
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

Note: As the voltage increases, the power limit is decreased.

When would I use a Class 2 transformer?

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.

What is Class 3 transformer used for?

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.

What is design for manufacturing?

(DFM) is the general engineering principle of designing products in such a way that they are easy to manufacture to insure fit and function.

What is a control transformer?

A control transformer is an isolation transformer designed to provide a high degree of secondary regulation during inrush current.

Can a transformer be reverse connected?

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.

Can a single phase transformer be used with a three phase source?

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.

What is hot spot?

Hot spot is the highest temperature inside the transformer coil.

Why would I want a potted transformer?

Potting or encapsulating will help protect the transformer from moisture, dust, dirt and other contamination.

Is one insulation system better than another?

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.

Why is a transformer needed?

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.

Are control transformers current limiting?

No control transformers are not current limiting. They allow all the current required by the load.

Will control transformers regulate output voltage?

Control transformers do not regulate the output voltage. Variations in input voltage will be proportionately reflected in the output voltage.

What is the effect of encapsulation in control transformers?

Encapsulation, or potting, protects the transformer coil from industrial contamination, and moisture. It also makes transformer run cooler under load and no load.

What effect does a control transformer have on electrical disturbances found on the line?

A control transformer is not a power conditioning component, however it may diminish electrical noise, spikes, surges and transients.

What is Wabash’s CAGE Code?

Our CAGE Code number is 5GQ82 (Parent company PowerVolt is 4SMM8).

What is Wabash’s ECCN Number?

Export Control Classification Number (ECCN) is EAR99.

Does Wabash Transformers meet Level VI efficiency?

Yes. Please notify us during the design and quotation stage so we can ensure the correct part number is selected.




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