What is a transformer and how does it work?

A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures under normal conditions, a long and trouble-free life. It consists, in it’s simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

Why do transformers hum?

Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.

The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbor, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level.

Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear.

How can I reduce airborne noise?

Below is a list of your most effective options:

Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls can be of brick, steel, concrete, lead, or most other dense building materials.

Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the viscous material. The outer sheet should not vibrate.

Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood, concrete, brick and with dense bushes (although the latter becomes psychological)

Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean? Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations (echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply absorbent materials to the offending walls (fiberglass, wool, etc.)

What are taps and when are they used?

Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and five percent above and below the rated primary voltage.

What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers?

Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise.

Can transformers be operated at voltages other than nameplate voltages?

In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly.

Can 60 Hz transformers be operated at 50 Hz?

Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.

Can transformers be used in parallel?

Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers which will cause excess heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be within 7.5% of each other.

Can MGM Transformers be reverse connected?

MGM’s low voltage dry-type transformers (75KVA and below) are made to withstand reverse connections without a loss of KVA rating. However, reverse feeding a step-down transformer has several limitations which include but are not limited to:

1. Requirement for larger sizing on breakers
2. Increased inrush current
3. Incoming voltage tap settings will not be available

NOTE: It’s always best to purchase a specially designed and manufactured step-up transformer instead of reverse feeding a step-down unit.

Why do I need a bigger breaker when reverse feeding a transformer?

Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.

Do taps work the same when a transformer is reverse fed?

Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage.

Why may I get the wrong output voltage when installing a step up transformer?

Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals.

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

Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.

Can transformers develop three phase power from a single phase source?

No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase.

What is meant 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.

What is temperature rise in a transformer?

Temperature rise in a transformer is the average temperature of the windings and insulation above the existing ambient temperature.

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.

Is one insulation system better 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.

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.

What is meant by impedance in transformers?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage.

Why is impedance important?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary winding of a transformer. The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.
High and Low Impedance Transformers:
High impedance transformers have a lower fault or short circuit current and have no need for high AIC rated breakers. However, they will have a higher voltage drop or regulation.

Low Impedance transformers on the other hand have a lower voltage drop or regulation but have a higher fault or short circuit current and will need higher AIC rated breakers.
Why is impedance given in a percentage?
The percentage impedance of a transformer (Z%) is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.
Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit. In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal. The total impedance is the vector sum of the resistance and the reactance. The impedance is measured by shorting the low voltage terminals. With low voltage windings shorted, a voltage at the rated frequency is applied to the high voltage windings until full load current is circulated in low voltage windings. The ratio of voltage applied to circulate full load current to the primary voltage is the percentage impedance of the transformer.
The percentage impedance of the transformer is calculated as: Z%= (Impedance Voltage/Rated Voltage)*100

Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.


Can single phase transformers be used for three phase applications?

Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer.

Does MGM provide “Zig-Zag” grounding transformers?

Yes. This system can be used for either grounding or developing a fourth wire from a three phase 3 wire. (neutral)

What color are MGM dry type transformers?

ASA 61 light gray is used on all enclosed transformers from .050 to 500 KVA. Other colors are available by request.

Are MGM transformers U.L. listed?

All of the transformers are listed by Underwriters Laboratories and have met their rigorous requirements.

Is CSA certification available for MGM transformers?

All MGM transformers are certified by the Canadian Standards Association. They have been designed and tested in accordance with the latest specifications.

What is BIL and how does it apply to transformers?

BIL is an abbreviation for Basic Insulation Level. Insulation levels in electrical equipment are characterized by the withstand voltages used during the impulse test. Impulse test is a dielectric test which consists of the application of a high frequency steep wave front voltage between windings and between windings and ground. The BIL of a transformer is a method used to specify the magnitude of the voltage surge that a transformer can tolerate without any damage to the windings and live parts of the transformer. When lightning impulse over voltage appears in the system, it is discharged through surge protecting device before the transformer gets damaged. BIL rating specifies the minimum voltage that transformer can withstand under this condition.
BIL graph  
The method of testing of the transformer for BIL has been defined and set by IEEE and ANSI standards. The wave shape has been also defined which is commonly known as 1.2/50 μs voltage wave. The impulse wave shape shows the magnitude of the voltage in KV (Kilo volts), Rise time (tf, time that takes the voltage rise from zero to its peak value in μs (Micro seconds)),and duration of the surge (T) sometime referred as Tail time (time that takes the voltage drop to 50% of its peak value in μs (Micro seconds).
This test is done with the initial transformer design to validate the integrity of the insulation and its high frequency surge withstand capability. It is considered one of the design tests for any transformer and needs not to be repeated with every transformer manufactured. However, a quality control impulse test (QC impulse test or production impulse test) is offered as an optional test whenever required. Design impulse test consists of a reduced voltage, 2 chopped wave, and a full voltage impulse applied to the transformer. Voltage and current wave shapes are captured during the above tests for comparison. Any deviation from the reduced wave to full voltage wave shape should be studied. In general, they should be very close to each other. Any new bump in the full wave can be considered as a failure point. Based on the location of the bump, an educated guess can be made as where the failure has occurred. After subjecting the transformer to above voltage surge tests, transformer should pass hi pot test at 60Hz. and double induced voltage test 400 Hz.
During quality control, impulse-only full voltage surge is applied to all of the bushings or the terminals of the transformer before hi pot and double induced test is performed.

What is polarity, when associated with a transformer?

Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

What is exciting current?

Exciting current is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and less to approximately 2% on larger sizes of 750 KVA.

How do I determine the required amp rating of breaker to be used on primaries?

To determine the required amp rating breaker to use, follow these simple steps:

1. Click calculator here, or on the main navigation bar on the left.

2. Input rated KVA and primary voltage of your transformer and click calculate.

3. Take the resulting amperage rating and multiply it by a minimum of 1.2 (recommended 1.4) do determine the amperage required.

4. Breakers are made in standard ratings, you should purchase a breaker rated equal to or higher than your requirements.

When reverse feeding a transformer the breaker required will be larger due to reduced primary voltage. Follow the same procedure to determine your breaker requirements.

NOTE: In case of discrepancy, provisions of NEC 450-3 and/or other local/national codes shall prevail over the above general guidelines.

Can air cooled transformers be applied to motor loads?

This is an excellent application for air cooled transformers. Even though the inrush or starting current is about 5 to 7 times normal running current, the resultant lower voltage caused by this momentary overloading is actually beneficial in that a cushioning effect on motor starting is the result.

Can a three phase transformer be loaded as a single phase transformer?

Yes, but the load can not exceed the rating per phase and the load must be balanced. (KVA/3 per phase)

For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.

How many BTU’s of heat does a transformer generate?

The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the room by 3.41 to obtain the BTUs/hour.
For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.

Do you have to ground a transformer?

In most cases the enclosure of the transformer is grounded for safety reasons. However, a transformer will function properly without being grounded. Be sure to research all grounding requirements for your specific application against the NEC (National Electrical Code) as well as any local electrical codes.

Do your transformers contain PCB?

All Liquid filled distribution transformers manufactured by MGM Transformer Company are PCB (polychlorinated biphenyl) free at the time of shipment.