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Here are some of the big questions.


1. How do we compensate for altitude and why?

Transformers are derated when used at altitudes above 3300 feet. ANSI C.57.12.01 calls for a 0.3% derating factor for each 330 ft. above 3300 ft. Since convection air currents carry away most of the heat from dry type transformers and air density is reduced at higher altitudes, derating is necessary.

In the design of transformers where air is in whole or part used for insulation, a derating factor must also be used for dielectric strength. This derating factor is also specified in the above ANSI specification.


2. How high does temperature get? How does it effect transformer life?

The most common insulation system allows for a maximum temperature of 220C. Modern transformers are generally expected to have a 20-year life when operated 100%, 24 hours every day. In organic insulation, the eventual failure is due to a chemical reaction. This aging reaction is accelerated as temperature increases. 


3. What is the 'K' factor?

K factor is a method of quantifying the harmonic content of a current waveform. The factor is thoroughly described in ANSI C.57.110. It allows coordination of the expected current with a given transformer. One of the components of a transformer's losses are those do to eddy currents. Losses from this component are proportional to the square of the current and the square of the frequency. Extreme care is necessary to ensure that a transformer carrying current with high harmonics does not overheat.


4. What is the effect of under, or over voltage on the ratings of a transformer?

If a transformer has lower than rated voltage applied, the KVA is reduced by the same percentage as the voltage is reduced. 1 or 2 percent overage will generally be tolerated by most transformers without reduction in KVA. Voltage in excess of 2% should not be applied unless overvoltage taps are provided.


5. What is inrush current?

Inrush is a transient characteristic of a transformer's magnetizing current. The steel core allows the flux to support the applied voltage with a relatively low "exciting current". Depending on the magnetic state of the core and where on the voltage waveform the transformer is switched in at, super-saturation can be achieved. Peak currents as high as 15 times rated can be experienced. The current is composed of the normal exciting and a large exponentially decaying D.C. component. Normal conditions are normally reached in less then 30 cycles.


6. What is the difference between a 6-pulse and 12-pulse transformer?

A 6-pulse transformer has a single 3 phase output voltage. Generally it is Wye connected and the primary is Delta connected.

A 12-pulse transformer has two 3 phase secondary windings. They are generally phase shifted 30º which allows for currents that combine in the primary winding, cancelling out much of the 5th, 7th and 11th etc. harmonics.


7. How long can I overload a transformer?

In general, unless previously specified, transformers should not be overloaded. At specific ambient temperature and load conditions, overloading may be allowed. See the question on duty cycle. Contact factory for additional information.


8. Why are some transformers wound with aluminum instead of copper?

Aluminum is used on transformers for economic reasons. Equivalent resistance aluminum conductors are larger (by about 2 AWG sizes). The aluminum transformer tends to therefore be slightly larger. Modern termination methods eliminate any aluminum oxide problems. Generally, performance and life expectancy are the same for copper and aluminum transformers.


9. When can a line reactor replace a drive isolation transformer?

A line reactor can be used to replace a drive transformer when the drive does not need to be electrically isolated from the incoming line.


10. How much power does a transformer consume when idle?

All transformers consume power when they are connected to a line and there is no load connected to the secondary. Small in reference to the KVA rating of the transformer, these "no load losses" are approximately 1% or less for small and medium size transformers. The losses appear in the form of heat in the iron or laminations of the transformer. The losses are in relation to the level of exciting current necessary to make the transformer operate.


11. What is the efficiency of a transformer when fully loaded or less?

Transformers from 1KVA through 1000KVA have full load efficiencies between 95% to 98.5%. Transformer losses appear in the core and windings. The core accounts for 1% or less of the name plate rating and the wire resistance losses account for 1% to 3% of the full-load rating. These losses are dissipated as heat. Copper and aluminum wire are used as they are the best commercial metals for conducting electricity.


12. What is the sound level of a dry type transformer, what causes such sound?

Sound is produced in transformers as a result of flux variations in the iron core. Sound is caused by physical movement of pieces in the core, as well as elongation and contraction of iron during each AC cycle. The National Electrical Manufacturers Association (NEMA) has set up the following audible sound level limits for dry type transformers.













13. What is an auto transformer and how is it different?

Auto transformers have their primary and secondary windings connected to each other electrically. The grounds of these transformers are not isolated from one circuit to the other. One ground may be at a considerably higher voltage than the ground in another section of the same circuit. For this reason, local inspectors and utility companies should be consulted before installing auto transformers. Where appropriate, the auto transformer represents a cost savings over a two-winding transformer. This savings varies as the ratio of windings change. After ratio windings reach approximately 4:1 or 5:1 there is little economy in using an auto transformer. Auto transformers can be single or three phase and are best used where a small percentage of voltage raising or lowering is required and isolation between two circuits is not required.


14. When can transformers be used in parallel?

Transformers in parallel must be of equal voltage. If the voltages are not equal, the difference between the voltages will result in a net voltage, which will cause current to circulate on the closed network between the two transformers. This will cause false loading and, if there is enough difference between the two voltages, the transformers may actually burn out without any useful load being connected to them. In order to have transformers with like voltages share the load proportionately, their impedance must be similar. A 10% tolerance of impedance is permissible when transformers are installed in parallel. In regard to three phase transformers, the phasing must be the same on each transformer.


15. What does "Scott T Connection" mean?

Named after its inventor, Charles F. Scott, the "Scott T" connection is a method used for changing 3 phase to 2 phase or vice versa. Two transformers are connected: one with a center tap called a main and one with an 86.6% tap called a teaser. The teaser is connected to the center tap of the main, and the start, finish of the main and 86.6% tap are all connected to the 3 wires of a 3 phase supply. The secondary on the teaser and the secondary on the main each produce a voltage, which is 90 degrees electrically from each other. These two voltages make up the two phase supply. Although there are other connections that accomplish this, the Scott T connection is the most economical. Auto and isolating types can be built as phase changing transformers.


16. What is a zigzag transformer?

A zigzag transformer is used in connection with 3 phase and is made up of 6 coils connected in a "Y" manner. Each leg of the "Y" is made up of a coil on a different phase leg of the transformer. The neutral formed by the zigzag connection is very stable. This type of transformer, which can be an auto transformer, lends itself very well for establishing a neutral for an ungrounded 3 phase system.


17. How do you select transformer ratings for various motor loads?

The most accurate method for determining the transformer requirements is to obtain the ampere rating from the motor nameplate and multiply this by its rated voltage. This will give the true KVA required. This applies to both single and three phase motors. The requirements can be estimated for fractional HP motors at 2.2KVA of transformer capacity per horsepower. Motors of approximately 5 HP and above can be estimated as requiring 1 ¼ KVA per HP.


18. Can any air-cooled transformer be used outdoors?

Not all air-cooled transformers are designed or manufactured for outdoor installations and should not be installed outdoors unless specifically advised by the manufacturer. The two main reasons for limitations on the installation of air-cooled transformers outdoors are moisture absorption in the windings and flashovers due to lightning. Outdoor use generally requires the equivalent of a NEMA 12 waterproof or a NEMA 3R enclosure.


19. How is duty cycle taken into consideration?

In many applications, a transformer will not be called upon to provide the maximum amount of current required by a system on a continuous basis. Just like motors, which are rated by a duty cycle calculation, a transformer rated for continuous duty will be bigger and more expensive than one rated for partial duty. In typical machine tool application, for example, a duty cycle of 50% is assumed. By calculating the percentage of full-load use required, you can properly size both the transformer and motor in a system. The square root of the duty cycle times the full-load KVA require-ment gives the effective KVA rating required for the transformer. For example, take a 50% duty cycle for which the square root is .71. Thus, the transformer will have to be 71% of the full-load rating. 


20. What specific information is required for specifying a choke or inductor?

In general, a choke is a device used with a DC motor drive and an inductor is used for an AC motor or load. The following are items to consider:


DC current rating
Peak operating current
Ripple voltage
Ripple frequency
Linearity Surge current


AC Current

AC Voltage Rating



Temperature Class

Temperature Class Linearity

21. How do harmonic filter inductors differ from standard inductors and reactors?

There are three major differences between harmonic filter inductors and standard inductors or line reactors. These differences are:


A. Tighter Inductance Tolerance
When the harmonic filter inductor is used in a "detuned" circuit, or a filter circuit, the inductance value is very critical to the correct operation of the system. Harmonic filter inductors need inductance tolerances no greater than +/- 5%, and under some instances as tight as +/-2% or less. Standard inductors will have inductance tolerances of +/- 10%.


B. Multiple Frequency Current Spectrum Ratings
Typically, harmonic filter inductors have multiple frequencies of current flowing through them simultaneously. Each of these currents, at their respective frequency, contribute to the heating effects on the inductor. A standard inductor such as a line reactor, has a current rating assigned to it that is determined at a single frequency only. More often than not, the heating effect of the multiple frequency currents, is much greater than the heating effect of a single frequency current. The harmonic current responsible for heating (i.e., thermal current) is calculated by using the square root of the sum of the squares. These currents are determined from the harmonic current spectrum. Typically harmonic currents can be 50% or more than a single frequency current seen by a standard line reactor. A single frequency thermal current rating, even though equal to a thermal current rating by the sum of the squares method, has significantly less heating characteristics.


C. Longer Designed Life Expectancy
Longer life expectancy is both a system characteristic and a customer requirement in harmonics suppression. Due to the heating characteristics of harmonic currents, it is necessary to be able to design and manufacture equipment targeted at the specific application and harmonic current frequencies generated. When designed correctly, a 20-25 year life can be expected.

Transformer Rating (KVA)

  • 0 - 9

  • 10 - 50

  • 51 - 150

  • 151 - 300

  • 301 - 500

Average Sound Levels (Decibels)

  • 40

  • 45

  • 50

  • 55

  • 60

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