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Small kVA 3 Phase Transformers Guide

Isolation Transformer

See also;   AMVECO Medical Grade Isolation Transformers, ACME Harmonic Mitigating Transformers, ACME Constant Voltage Regulators & Line Power Conditioner Transformers, TEMCo Isolation Transformers, ACME Drive Isolation Transformers, Federal Pacific Motor Drive Isolation Transformers, Marcus Drive Isolation Transformers, HAMMOND Drive Isolation Transformers & Jefferson Drive Isolation Transformers

Isolation TransformerIsolation Transformers have primary and secondary windings that are physically separated from each other. Sometimes isolation transformers are referred to as "insulated".

To Order Isolation Transformers We Recommend TEMCo
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Link: Isolation Transformers

This is because the windings are insulated from each other. In an isolation transformer the output winding will be isolated, or floating from earth ground unless bonded at the time of installation. Secondary neutral to ground bonding virtually eliminates common mode noise, providing an isolated neutral-ground reference for sensitive equipment and an inexpensive alternative to the installation of dedicated circuits and site electrical upgrades.

An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. They also block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments. Isolation transformers are different from auto transformers in which the primary and secondary share a common winding.

Isolation transformers can accomplish a number of tasks:

  1. The primary and secondary windings may be constructed to step-up or step-down the output voltage. For example, the transformer can accomplish voltage matching between a 120 V load and an electrical system that measures 208 V.
  2. Isolation transformers constructed with Faraday shields, will improve power quality by attenuating higher frequency noise currents.
  3. Isolation transformers provide better impendence matching of a critical load to an electrical circuit . Internal low-impedance isolation transformer component offers 100% isolation from the input AC line.
  4. Hospital Grade Isolation transformers  is ideal for the protection of sensitive electronic equipment in patient-care areas.
  5.  Isolation transformer with Faraday shield reduces the cumulative leakage current of the Isolator and connected equipment to levels below 300 microamps.
  6. Surge suppression components placed at the line input and output combined with full line isolation offers continuous filtering of a full range of power line noise in all modes. Active transformer filtering offers continuous common-mode noise rejection with no wearable parts, uniquely able to reduce surges in the worst of power environments to harmless levels.
  7. isolation transformer provides a "code-legal" method of re-bonding the electrical system safety ground to the neutral conductor on the transformer secondary. Doing so eliminates neutral-to-ground voltage and noise, which is  major cause of reliability problems for microprocessor-based electronics. 
  8. In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.

Isolation transformers are commonly designed with careful attention to capacitive coupling between the two windings. This is necessary because excessive capacitance could also couple AC current from the primary to the secondary. A grounded shield is commonly interposed between the primary and the secondary. Any remaining capacitive coupling between the secondary and ground simply causes the secondary to become balanced about the ground potential.

All transformers provide isolation. They are constructed with a primary and secondary winding closely wrapped around the same ferrous core. Commercial transformers incorporate a single Faraday shield between the primary and secondary windings to divert noise, which would normally be electrically coupled between the primary and secondary windings to ground . The method  through which this electrical coupling of noise occurs is the capacitance between the coils of the primary and secondary windings of the transformer, which does not include a Faraday shield. This same capacitance limits the upper frequency band pass of the transformer in the same manner as the mutual and self-inductances of the device determine its low frequency cutoff. As the frequency of the exciting currents increases, the reactance caused by the capacitance between the windings,  tends to shunt these currents, thereby limiting high frequency performance.

The single Faraday shield controls all manner of evils which could be attributed to the electric coupling of noise through a transformer. However, the problem with a single shield arises when it is bonded to the ground of either the primary or secondary side of the transformer. The enclosure of a Faraday shield between the primary and secondary windings eliminates inter-capacitance, but it also establishes two new capacitances between the shield and both windings. These two capabilities allow high frequency currents to flow in the grounding systems of both the primary and secondary. Bonding the transformer shield to either the primary or secondary ground establishes current paths for high frequency noise in the reference conductor of the circuit to be isolated. The particular choice of ground for connection of the shield only provides selection of the quieter of the primary and secondary circuits. In many applications, this current path defeats any isolating effect, which a transformer might provide.

An isolation transformer is designed to address the problems associated with referencing its internal shields to ground. It is constructed with two isolated Faraday shields between the primary and secondary windings. When properly installed, the shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated. The use of two shields in the construction of the isolation transformer diverts high frequency noise, which would normally be coupled across the transformer to the grounds of the circuit in which they occur. The two shields provide more effective isolation of the primary and secondary circuits by also isolating their grounds. The isolation transformer adds a third capacitance between the two Faraday shields, which may allow coupling of high frequency noise between the system grounds. However, increasing the separation between the two Faraday shields normally minimizes this third capacitance. Additionally, the dielectric effect of the shields plus the increased separation of the  windings significantly reduce the inter-capacitance between the windings.

Generally, a conductive foil completely enclosing the windings will provide a ground path for primary circuit noise and has the advantage that a very much smaller capacitance exists between primary and secondary coils than in the case of a simple Faraday shield. The Faraday shield is simply a grounded single turn of conductive nonferrous foil placed between coils to divert primary noise to ground. The enclosing shield, if grounded properly, will not re-radiate the noise signal, and will provide effective electromagnetic noise reduction. Typically, according to Topaz  at a distance of 18 inches from a transformer's geometric center, the field strength will be less than 0.1 gauss, and will roughly follow inverse cube laws.

Since inter-winding capacitances are the primary path by which significant power line and transient related noise couples to the system, more information is needed to describe what occurs. During the time power is being transferred between transformer windings, noise potentials between the primary circuits and ground is similarly coupled to the secondary through both capacitive and resistive paths. This noise appears in three forms normally in a transformer circuit: common-mode, transverse mode, and electromagnetic.

Common - Mode Noise

This noise appears between both sides of a power line and ground.  Since this noise is referenced to the power system ground, the most obvious method of eliminating this noise is by grounding the transformer center tap to the system ground via the lowest impedance path possible. Internal transformer designs, which separate the coils to reduce capacitive coupling, have some advantage, but it also increases leakage inductance and reduces the power transfer.

Transverse - Mode

Transverse-mode noise is much more difficult to eliminate than common-mode noise. The key here is to differentiate between power and noise, and then reduce the noise.

Noise and power are separated by the difference in their frequencies. The most effective transformer would be a design exactly opposite to a audio transformer. The purpose is to transfer the power required by the load at the fundamental power frequency and to eliminate all higher and lower frequencies. Sub-harmonic frequencies are attenuated by operating the transformer at relatively high flux density, which is effective in reducing or eliminating them. Above the fundamental frequency, noise is reduced by introducing as much leakage inductance as possible, consistent with good power transfer to the secondary.

Transverse-mode noise appears as a voltage across both the primary and secondary windings of an isolation transformer. It occurs when a common-mode noise signal causes current to flow in the primary winding (or secondary winding), and from there to ground via capacitance to a grounded shield. Common-mode noise can also be transformed into 'transverse-mode noise, and thereby, through magnetic coupling, contaminate the secondary of an isolation transformer. Normally, by the proper selection of core loss verses primary winding inductance, a well-designed isolation transformer will eliminate the majority of this type of noise. Here again, grounding the transformer shield to the lowest impedance path available, will result in noise currents using this return path rather than some other higher impedance path to the noise source ground.

Electromagnetic Noise

Electromagnetic noise does not constitute a major problem in most applications, but is sometimes critical in some recording or digital data systems, and in making electromagnetic interference measurements.

Box Level Applications

Isolation transformers are often used to protect high gain circuits, or prevent noisy ground paths in instrumentation. Shielding at the instrument level is difficult and often ineffective. Since most commercial instrumentation has single shielding in its power transformer, designers sometimes hope that by adding a isolation transformer ground problems can be eliminated. This approach often results in no benefits to the system unless all other ground paths in the instrument can be totally isolated. An isolation transformer is not a substitute for the proper shielding or grounding of individual instruments.

. The amount of ground isolation provided by the transformer at the box level is limited by the use of a single chassis shield enclosing the box. High frequency noise currents generated by the box circuitry can be coupled onto the circuit reference conductors through the connection of both transformers' shields to the circuit reference. Additionally, any potential difference between the power system ground at the isolation transformer primary input and the power system ground at the equipment and the power system ground at the equipment chassis will cause currents to flow in the reference conductor of circuitry.

Rack Level Applications

The most effective application of isolation transformers is with racks of equipment. A rack acts as an outer shield for internal instruments, while serving as the zero-signal reference for system output signals. Isolation transformers are used to control shield currents, and to break up the mutual capacitance between rack instrumentation and an unknown power ground.

 The main benefit of using an isolation transformer with a rack of equipment is the enhanced control of currents in the equipment shields. Any potential differences between the utility power ground and the rack's ground will cause currents to flow in the loop. The isolation transformer allows these "ground" currents to be directed through a portion of the rack's shielding which will not effect the operation of sensitive circuits and completely isolates these currents from the internal equipment reference conductors.

Room Level Applications

It is often necessary to isolate EMC test enclosures from noisy building grounds. Not only can isolation transformers be used to effectively decouple building power, but also since they also act as tuned circuits; they reduce the differential noise from external equipment, which reaches your screen room. While it is recognized as a second isolation transformer inside the test room will greatly reduce power line ambient, this section will only consider using transformers on the power lines to a typical screen room.

As with any transformer, isolation transformers radiate magnetic fields. Physically locating the transformer adjacent to, or connected to, a screen room may increase rather than decrease ambient noise. Since the physical case of a transformer, as well as the primary winding shield, are normally connected to the third-wire power ground of the supplied power, the secondary winding shield must be isolated from the transformer case and connected only to the conduit shield going to the shielded room to achieve proper ground isolation. The conduit acts as an RF shield for the room's power and completes the connection between the shielded room and the secondary winding shield in the transformer.

If the transformer is three phase and supplies more than one room, the best application for isolation between rooms is to use only one phase for each room, with a limit of three rooms per transformer. With this approach, power line filters will effectively isolate the room while providing practical noise attenuation.

Proper transformer design, wiring, and, above all, grounding, are the only effective means of reducing the three types of noise problems. Grounding should be controlled and use the lowest impedance path possible (i.e., bonding) to the central reference ground system to insure maximum attenuation of noise sources. To achieve the maximum protection from a transformer, not only must it be applied properly, but also the transformer should be one specially designed for isolation usage.

Three Phase Isolation Transformers

Three phase Isolation transformers are used for many applications ranging from grain dryer, saw mills, conveyer belt systems, refrigeration and air conditioning. Three phase have 3 primary and 3 secondary windings that are physically separated from each other. Each of these windings are insulated from each other. The output windings will be isolated, or floating from earth ground unless bonded at the time of installation .

The Shielded three phase isolation transformers have all the feature of the standard 3 phase plus they also incorporate a full metallic shield (usually copper or aluminum) between the 3 phase primary and 3 phase secondary windings.  This electrostatic shield  or Faraday Shield, is connected to earth ground and performs two functions:

  • Its attenuates (filters) voltage transients (voltage spikes). These shielded 3 phase isolation transformers have an attenuation ratio of 100 to 1.

  • It filters common mode noise, Attenuation of approximately 30 decibels.

The shield three phase isolation transformer is preferred over the standard three phase isolation transformer because it provides protection to sensitive and critical equipment. When more that one shielded 3 phase isolation transformer is used between the source and the load, it is referred to as a " cascading" and greatly improves power quality.

Power Transformer Sources: Power Transformer * Step Up Transformer * Step Down Transformer * Electrical Transformer * Isolation Transformer * Toroidal Transformer * Acme Transformer * High Voltage Transformer * Distribution Transformer * Transformer Manufacturer * Three Phase Transformer * Dry Type Transformer * 3 Phase Transformer * Cast Transformer * Voltage Transformers * Variac * Voltage Stabilizer * Voltage Regulators

 Power Transformer Information:

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Power Transformer Types

Step Up and Step Down Transformers Step Up and Step Down Transformers to Power transformers to step-up ( raise) or step-down (lower) the electrical voltage.
Isolation Transformers Isolation Transformers allows signal or power to be taken from one device and fed into another without electrically connecting the two.
Toroidal Transformers Toroidal Transformers are devices that transfer electrical energy from one electric circuit to another, without changing the frequency, by electromagnetic induction.
Custom Transformers
Custom Transformers are designed to meet certain performance specifications and size requirement that you require. There is a wide range of custom transformer types.
Buck Boost Transformers
Buck Boost Transformers is a ideal solution for changing line voltage by small amounts. Often used to buck (lower), or boost (raise) the voltage from 208v to 240v for lighting applications.
Pole Mounted Transformers
Pole Mounted Transformers are mounted to poles for overhead electrical lines. Used in various applications. Are available in single phase or three phase transformers.
Medium Voltage Transformers
Medium Voltage Transformers are used with a medium range of voltages. They come in a full range from liquid-filled, convention dry type as well as cast coil.
Pad Mounted Transformers Pad Mounted Transformers are a excellent choice for commercial and industrial such as manufacturing facilities, refineries, office buildings, schools, hospitals, restaurants, and retail stores. They come in various sizes and can be used underground as well.
High Voltage Transformers High Voltage Transformers typically these voltage transformers are used in power transmission applications. High voltage transformers are also used in microwave.

 Power Transformer Manufacturer

  • ACME Transformers - With Acme Electric being in business over 80 years, they have always believed in offering there customers superior service, quality and technical expertise in the transformer market.
  • AMVECO Transformers - AMVECO designs and manufactures toroids transformers, current transformers, and auto transformers. Most AMVECO products are custom designed utilizing their state-of-art proprietary CAD programs.  The AMVECO engineers can quickly generate designs in a matter of hours, if needed.
  • Federal Pacific TransformersFederal Pacific is a division of Electro- Mechanical Corporation, a privately held, American owned company founded in 1958. Federal pacific offers dry-type transformers from .050 KVA through 10,000 KVA single and three phase, up to 34.5 KV, 150 KV BIL with UL approval through 15 KV.
  • Marcus Transformer - Ever since they opened their doors for business a half a century ago, they have been a leader in innovative transformer design. As a family-owned company they are proud of the reputation they have earned for making quality-built transformers that deliver exceptional performance and savings.
  • Hammond TransformersHammond Manufacturing was founded in 1917 in Guelph, Ontario, Canada. In the last 3 decades it has expanded to the US and the international markets offering many types of power transformers. 
  • TEMCo Transformers - TEMCo Transformer, a family-owned business which has been manufacturing and distributing electrical products since 1968. They focus on transformers that significantly reduce power consumption over 30 percent compared to competitive makes.
  • GE Transformers - GE has been a key player in the energy industry for more than a century.  Since the installation of their first steam turbine in 1901. They have become number one provider of high-technology power generation and distribution equipment.
  • Jefferson Electric Transformers - Jefferson Electric has been a pioneer and innovator of magnetic products since 1915. Jefferson broad line of dry-type transformers are backed by quality assurance systems so stringent that each and every unit gets thoroughly tested before it goes out there door.
  • More power transformer brands - Check out more companies by clinking this link.

 Power Transformer Types

  • Distribution Transformers - Distribution transformers are generally used in electrical power distribution and transmission power. This class of transformer has the highest power, or volt-ampere ratings. and the highest continuous  voltage rating.
  • Substation Transformers - Substation Transformers are large devices which usually weigh tens of thousands of pounds.   They are filled with tens of thousands of gallons of heat transfer fluid.  Although they are typically 99.8% efficient in the transforming of electricity from one voltage to another, processing hundreds of Mega Volts-Amps of electricity force the liberation of hundreds of BTUs per second.
  • Medical Grade Isolation
     Transformer -
    Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers.
  • Drive Isolation Transformer - They are used to isolate a drive from a main power line to prevent the transmission of harmonics that the drives produce back into the power line.  They stop drive harmonics from disrupting computers and other sensitive equipment.
  • Toroidal Transformers - Toroidal Transformers are more efficient than the cheaper laminated EI types of similar power level. Some of the advantages are smaller size, lower weight, less mechanical hum, (making them superior in audio amplifier), low-off-load loss.
Capacitor for Motor Resources


 Power Transformer Types

  • Step-Up Transformers - A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "steps up" the voltage applied to it. -
  • Step-Down Transformers - A Step-Down Transformer is  designed to reduce voltage from primary to secondary.  They can range from sizes from .05 KVA to 500 KVA
  • Isolation Transformers - An Isolation Transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics.
  • Buck Boost Transformers - Buck Boost Transformers make small adjustments to the incoming voltage. They are often used to change voltage from 208v to 240v for lighting applications.  Major advantages of Buck boost transformers include; low cost, compact size and light weight. 
  • High Voltage Transformer - There are many different types of voltage transformers. A High Voltage Transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
  • Medium Voltage Transformers - A Medium Voltage Transformer can be connected directly to a primary distribution circuit and generally has the most load diversity. These voltage transformers have installation practices that are generally in accordance with application recommendations from the Institute of Electrical and Electronic Engineers (IEEE).
  • Low Voltage Transformers - A Low Voltage Transformer is an electrical device that transforms 120 volts (line voltage) into 12 volts or 24 volts (low voltage). Some uses for low voltage transformer are in landscaping lighting.
  • Single Phase Transformers - In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
  • Three Phase Transformers - Three Phase Transformers must have 3 coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage needed while maintaining the proper phasing or polarity.
  • Custom Transformers - Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification. 
  • Industrial Control Transformers - Industrial Control Transformers are used to convert the available supply voltage to the required voltage to supply industrial control circuits and motor control loads.
  • Pad Mounted Transformers - Pad Mounted Transformers are usually single phase, or three phase, and used where safety is a main concern. Typical applications; restaurant, commercial building, shopping mall, institutional. 
  • Pole Mounted Transformers - Pole Mounted Transformers are used for distribution in areas with overhead primary lines. Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
  • Oil Filled Transformers - Oil Filled Transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.
  • Dry Type Transformers - Dry-Type Transformers are available for voltages up through 34.5 kV (although the most common upper limit is 15) and KVA ratings up through 10,000 (with 5000 as the usual limit). Dry-type use air as a coolant, lowering health and environmentally concerns.
  • Auto Transformers - An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on the local 600 V supply.
  • More power transformer types - Read further about additional transformer types and their uses.


 Power Transformer Term Definitions

  • Electrical Transformers - Electrical Transformers are devices used to raise or lower the voltage of alternating current. For instance, power is transported over long distance in high voltage power lines and then transformers lower the voltage so that the power can be used by a business or household.
  • Isolating Transformers - An Isolating Transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits.  An Isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. 
  • Transmission Power Lines - A Transmission Line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic or acoustic waves as well as electric power transmission. Components of transmission lines include wires, coaxial cables,  dielectric slabs, option fibers, electric power lines, and waveguides.
  • Transformer Voltage - The measure of the amount of force on a unit charge because of the surrounding charge.
  • Transformer Phase - Most transformer are either single phase or three phase.
  • Transformer Frequency - The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.
  • Transformer K Factor - Some transformers are now being offered with a k-factor rating. This measure the transformer's ability to withstand the heating effects of non-sinusoidal harmonic currents produced by much of today's electronic equipment and certain electrical equipment.
  • Primary Voltage - The coil winding that is directly connected to the input power.
  • Secondary Voltage - The coil winding  supplying the output voltage.
  • Harmonic Cancellation - Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st.
  • Weatherproof - Enclosed transformers come with a weatherproof standard set by NEMA.
  • Epoxy Encapsulated - A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally preferred when a unit might encounter harsh environmental conditions.
  • More power transformer terms - Such as inductor, ground fault, core saturation, current transformer, faraday shield, etc.

Related Transformer Products

  • Voltage Regulators - A Voltage Regulator is an electrical regulator designed to automatically maintain a constant voltage level.  It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
  • AC Line Reactor - AC Line Reactors is a three phase transformer used in conjunction with AC variable frequency and DC motor drive. They are a bi-directional protective filtering device.
  • Line Power Conditioners - Power or Line Conditioners regulate, filter, and suppress noise in AC power for sensitive computer and other solid state equipment.
  • DC Power Supplies - Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company to a well-regulated lower voltage DC for electronic devices.
  • Rotary Phase Converters - Rotary Phase Converters are commonly used in home or small commercial or industrial settings. Rotary phase converters convert single-phase power into three-phase power. This is a very cost-effective way to power three-phase electric motors and other three phase equipment.
  • Frequency Converters - A Frequency Changer or Frequency Converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency.
  • Voltage Converters - A Voltage Converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply.
  • Magnetic Motor Starters - Magnetic Motor Starters are essentially heavy duty relays mounted in boxes, often equipped with heater/thermal overloads matched to the motor they start.
  • Motor Starting Auto Transformers - An Auto Transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter.

For an additional resource the Best of Industry Web Directory : Electrical Power Transformer Directory section is quite useful.