Hammond Buck Boost Transformers

Why Use Buck-Boost Transformers?
The advantages of using a buck-boost transformer over an equivalent standard isolation transformer are as follows:
Advantages
1) Used in a variety of applications
2) Inexpensive
3) Smaller and lighter
4) More efficient
5) 5-10 times increase in kVA
Disadvantages
1) No circuit isolation
2) Cannot create a neutral
3) KVA and voltages do not match what’s
on the nameplate kVA and voltages.
Buck-Boost Application
Buck-boost transformers offer an economical solution to the adjustment of line voltages that are slightly above or below normal. When a buck-boost transformer is connected as an autotransformer, only a portion of the load kVA is actually transformed. The majority of the load kVA is passed directly through to the source. For this
reason a buck-boost transformer may be used to supply a much larger load kVA than is indicated on the nameplate. Buck-boost transformers can be used to adjust stable voltages only. Fluctuating line voltages should
 be regulated with a Line Voltage Conditioner.

Buck-boost transformers represent an economical way to both raise supply voltage caused by line drop or equipment demand on the distribution system, or lower voltage caused by increased system voltages due to
supply line adjustments. Some loads including lighting and resistive loads require a stable supply to maintain
performance. The detrimental effects of incorrect supply line voltage can cause equipment failure. Buck-boost
transformers can correct line voltage within 5 to 25% of nominal.
Steps for Selecting Buck-Boost Transformers
The following information is required before selecting a buck-boost transformer:
(1) Line Voltage - The voltage that you want to buck (decrease) or boost (increase). This can be
determined by measuring the supply line voltage with a voltmeter.
(2) Load Voltage - The voltage at which your equipment is designed to operate. This is listed on the
nameplate of the load equipment.
(3) Load kVA or Load Amps - You do not need to know both - one or the other is sufficient. This
information usually can be found on the nameplate of the equipment that you want to operate. It is the
sum of all the equipment that represents the load.
(4) Frequency - The supply line frequency must be the same as the equipment to be operated - either
50 or 60 Hertz.
(5) Phase - The supply line should be the same as the load - either single or three phase.

Selection Chart

Single Phase - Group A

Single Phase - Group B Selection Chart

Single Phase - Group C Selection Chart
 

Three Phase - Group A Selection Chart
 

Three Phase - Group B Selection Chart
 

Three Phase - Group C Selection Chart
 

Group A
Primary Voltage 120/240 Secondary Voltage 12/24 -50/60 Hertz
 

Group B
Primary Voltage 120/240 Secondary Voltage 16/32 -50/60 Hertz
 

Group C
Primary Voltage 240/480 Secondary Voltage 24/48 -50/60 Hertz

Buck-Boost Transformers - Questions & Answers

1. What is a buck-boost transformer?
Buck-boost transformers are small single phase transformers designed to lower (buck) or raise (boost) line
voltage from 5-20%. The most common applications for buck-boost transformers include boosting 208 volts to
230 or 240 volts for air conditioning systems, boosting 110 to 120 volts and 240 to 277 volts for lighting
applications, heating systems and induction motors of all types. Many applications exist where supply voltages
are frequently above or below nominal.
Buck-boost transformers are conventional low voltage, single phase distribution transformers, with standard
primary voltages of 120, 240 or 480 volts, and secondary voltages of 12, 16, 24, 32 or 48 volts. They are
available in sizes ranging from 50 VA to 10,000 VA. The primary and secondary are wired together to form a singlewinding autotransformer. Utilizing the additive and subtractive polarity, small amounts of voltage are
 either added or subtracted from a distribution circuit.
2. How does a buck-boost transformer differ from an isolating transformer?
A buck-boost transformer is manufactured as an isolating transformer, with separable primary and secondary,
and is shipped from the factory in that configuration. When the end user at site connects it, the primary is
connected to the secondary changing the transformer’s electrical characteristics to those of an autotransformer.
This provides the smaller voltage correction that is typical of buck-boost. The primary and secondary windings
are no longer isolated as they are connected together.
3. What is the difference between a buck-boost transformer and an autotransformer?
As noted above, when the primary and secondary are connected together to buck or boost voltage, the
transformer becomes an autotransformer. If the connection between the primary and secondary winding is not
made, then the unit remains as an isolation transformer.
Applications
4. Why are they used?
A buck-boost transformer is a simple and effective way of correcting off-standard voltages. Electrical and
electronic equipment is designed to operate within a standard tolerance of nominal supply voltages. When the
supply voltage is consistently too high or low - typically more than 10%, the equipment will operate below peak
efficiency.
5. Can buck-boost transformers be used to power low voltage circuits?
Installed as two-winding, isolation transformers, these units can be used to power low voltage circuits
including control, lighting circuits, or other low voltage applications that require 12, 16, 24, 32 or 48 volts output,
consistent with the secondary of these designs. The unit is connected as an isolating transformer and the
nameplate kVA rating is the transformer’s capacity.
Operation and Construction
6. Why do buck-boost transformers have 4 windings?
A four winding buck-boost transformer with 2 primary and 2 secondary windings can be connected eight different
ways to provide a multitude of voltages and KVA’s. This provides the flexibility necessary for the broad variety of
applications. A two-winding transformer can only be connected in two different ways.
7. Will a Buck-Boost transformer stabilize voltage?
Autotransformers will not stabilize supply line voltage. The output voltage of an autotransformer is a function
of the input voltage. If the input voltage varies, then the output voltage will also vary by the same percentage.
Load Data
8. Are there any restrictions on the type of load that can be operated from a Buck-Boost transformer?
There are no restrictions as to application for Buck-Boost, including single or three-phase motor loads.
9. As an Autotransformer, how can a Buck-Boost transformer supply kVA power?
This is a function of adding voltage - a small amount of voltage is added and a small amount of corresponding
power capacity is added as well. For example, if the transformer is connected in such a way that 22 volts is
added to a 208 volt primary, a 230 volt output will result.
Using this example, the calculation for autotransformer kVA is as follows:
kVA = Output Volts x Secondary Amps
                            1000
kVA = 230V x 41.67 Amps = 9.58 KVA
                           1000

10. How are single phase and three phase load amps and load kVA calculated?
Single phase Amps = kVA x 1000              Three phase Amps = kVA x 1000
              Volts                                                       Volts x 1.73
Single phase kVA = Volts x Amps              Three phase kVA = Volts x Amps x 1.73
                 1000                                                         1000
Three Phase
11. Can Buck-Boost transformers be used on three phase systems?
Interconnecting two or three single phase units will readily accommodate three phase systems . The number of units to be used in a three phase installation depends on the number of wires in the supply line. If the three
 phase supply is 4-wire wye, then three buck-boost transformers are required. If the three phase supply is 3-wire wye (neutral not available), two buck-boost transformers are needed.
12. Should Buck-Boost transformers be used to develop three phase 4 wire wye circuits from three phase 3 wire delta circuits?
No - a three-phase “wye” buck-boost transformer connection should be used only on a 4-wire source of
supply. A delta to wye connection does not provide adequate current capacity to accommodate unbalanced
currents flowing in the neutral wire of the 4-wire circuit.
13. Why isn’t a ‘closed delta’ Buck-Boost connection recommended?
This connection requires more kVA power than a “wye” or open delta connection and phase shifting occurs
on the output. The closed delta connection is more expensive and electrically inferior to other three phase connections.
Connection and Frequency
14. How do you know how to connect a Buck-Boost transformer?
A connection chart is provided with each unit that shows how to make the corresponding connections.
These same charts are also shown in this section.
15. Can 60 Hertz Buck-Boost transformers be operated on 50 Hertz?
Due to ‘saturation’ of the core, 60 Hertz buck-boost transformers should only be operated at 60 Hertz, and
not 50 Hertz. Units manufactured as 50 Hertz units will however, operate at 60 Hertz.
Nameplate Data
17. Why are buck-boost transformers shipped from the factory connected as isolating transformers, and not pre-connected autotransformers?
The same 4-winding buck-boost transformer can be connected eight different ways to provide a multitude
of voltage combinations. The correct connection can best be determined by the user when assessing the supply
voltage at site.
18. Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?
Shipped as an isolating transformer, the nameplate is required to show the performance characteristics
accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kVA’s would be
impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.
Safety
19. Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?
Buck-boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small
increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding
transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is
grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For
this reason, qualified personnel only should maintain conventional autotransformers.
Sound Levels
20. How does the sound level differ between Buck-Boost and isolation transformers?
Buck-boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer
capable of handling the same load. The isolation transformer would have to be physically larger than the
buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10 kVA is 35 dba
and a 75 kVA is 50 dba.
Cost and Life Expectancy
21. How does the costs compare between a Buck-Boost transformer and an Isolation transformer handling the same load?
For most buck-boost applications, the savings are about 75% compared to the use of an isolation transformer
for the same application.
22. What is the life expectancy of a Buck-Boost transformer?
Buck-boost transformers have exactly the same life expectancy as other dry-type transformers.
23. Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?
Autotransformers are very common and recognized by all the safety and standard authorities. You can
refer to N.E.C. Article 450-4, “Autotransformers 600 Volts, Nominal, or Less”, as a reference publication.
Item (a) details overcurrent protection for an autotransformer and item (b) covers an isolation transformer being field connected as an autotransformer for a buck-boost application.
24. When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the overcurrent protective device, such as the fuse or circuit breaker?
The NEC Article 450-4 outlines overcurrent protection for autotransformers. It is reproduced as follows:
“NEC 450-4 - Autotransformers 600 Volts, Nominal, or Less
(a) Overcurrent Protection. Each autotransformer 600 volts, nominal, or less shall be protected by an
individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device
shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An
overcurrent device shall not be installed in series with the shunt winding.
Exception: Where the rated input current of an autotransformer is 9 amperes or more and 125 percent of
this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher
standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes,
an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.
(b) Transformer Field-Connected as an autotransformer. A transformer field-connected as autotransformers
shall be identified for use at “elevated voltage”.
Example: A 1 kVA transformer, Catalog No. Q1C0ERCB, is rated 120 x 240 to 12 x 24 volts. It is to be connected
as an autotransformer to raise 208 to 230 volts single phase. When connected as an autotransformer in
this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining
the full load input amps and the corresponding size of the overcurrent protection device, either a fuse or breaker.
Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts
When the full load current is greater than 9 amps, the overcurrent protection device - usually a fuse or nonadjustable breaker, the current rating can be up to 125 percent of the full load rating of the autotransformer
input current. Max. current rating of the overcurrent device = 46 amps x 125% = 57.5 amps
The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere
rating of the overcurrent device. This is shown in Article 240-6 of the N.E.C.
Max. size of the fuse or circuit breaker = 60 amps

o     HAMMOND INDUSTRIAL CONTROL TRANSFORMERS

o     HAMMOND  BUCK BOOST TRANSFORMERS

o     HAMMOND   LINE REACTOR TRANSFORMERS

o     HAMMOND DRIVE ISOLATION TRANSFORMERS

o     HAMMOND MOTOR STARTING AUTO TRANSFORMERS

o     HAMMOND LOW VOLTAGE LIGHTING TRANSFORMERS

o     HAMMOND GENERAL PURPOSE DISTRIBUTION TRANSFORMERS

o     HAMMOND K-FACTOR TRANSFORMERS

o     HAMMOND ENCAPSULATED TRANSFORMERS

o     HAMMOND  AUTOTRANSFORMERS

o     HAMMOND GENERAL PURPOSE MEDIUM VOLTAGE TRANSFORMERS

 Power Transformer Information:

 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 Transformers -  Federal 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 Transformers -  Hammond 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.

 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.