Acme Electric Training (General Purpose Transformers)

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Acme ElectricTransformer Seminar

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Transformer Seminar

Basic Theory

Sizing and Selection

General Purpose Transformers

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Basic Transformer

• A transformer is an electrical device that has no moving parts and is designed to convert alternating current (AC) from one voltage to another voltage.

• It works on a magnetic induction principle, and can be designed to “Step-Up” or “Step-Down” voltage.

• A transformer consists of two or more coils of insulated wire wound around an iron core.

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When voltage is applied to one coil, usually called the primary, it magnetizes the iron core, which induces a voltage in the other, usually called the secondary.

Voltage Transformation

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• The turns ratio of the primary and secondary windings determines the amount of voltage transformation.

In this example we show 100 turns on the primary and only 50 turns on the secondary, this is a ratio of 2 to 1.

Note: Single phase units below 1 kVA should not be reverse connected because the secondary windings are compensated (additional turns) to overcome the voltage drop when the load is applied.

Turns Ratio

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• The voltage ratio between the primary and secondary will be the same as the Turns Ratio, except for single phase smaller than 1 kVA which have compensated secondary's.

• A practical application of this 2 to 1 ratio would be a 480 to 240 voltage change.

Note: Transformers should never be operated at voltages higher than nameplate rating, but may be operated at voltages lower than rated.

Voltage Ratio

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Core…Coil…Insulation…• The transformer’s core and coil are the primary components for voltage transformation

• The insulation; typically comprised of Mylar, Nomex, Kraft Paper, Varnish or Other Materials, is place between the turns of wire to prevent shorting to on another or to ground.

• Since a transformer has no moving parts, it will typically have a life expectancy of 20 to 25 years.

• Acme transformers carry a 10 Year Warranty against defects in material and workmanship.

Three Basic Components

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Isolation

• An isolation transformer is one in which the primary and secondary windings are physically separated from each other.

• Sometimes referred to as Insulated, windings are Insulated from each other.

• In an isolation transformer the output winding will be isolated or floating from earth ground unless bonded at time of installation.

Electrical Types

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Shielded Isolation

• A shielded isolation transformer has all the features of an isolation transformer plus it incorporates al full metallic shield (usually copper or aluminum) between the primary and secondary windings.

• This electrostatic shield (Faraday Shield) is connected to the earth ground and performs two functions:

1. Attenuates (filters) voltage transients (voltage spikes) (100 to 1)

2. Filters common mode noise (~30 decibels)

Electrical Types

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Shielded Isolation (cont.)

• The shielded unit is preferred over a standard isolation unit because it provides protection for sensitive and critical equipment.

• When more than one “shielded” unit is installed between the source and the load, the effect is referred to as “cascading” and greatly improves power quality.

• All Acme® General Purpose units 250 VA and larger are shielded at no added cost as a standard feature.

Electrical Types

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Auto-Transformer

Electrical Types

• An auto-transformer is one in which the primary and secondary windings are physically connected.

• Auto’s should only be used where isolation of the load is not required, single and three phase AC Motors are the most common application.

Note: Units manufactured as auto transformers must have the word AUTO on the nameplate.

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• Taps are a feature found on some transformers which allow you to correct for constantly high or low voltage conditions while still delivering full rated output voltages to the load.

• The NEMA designation for taps is:

– ANFC - Above Normal Full Capacity

– BNFC - Below Normal Full Capacity

• Spacing of taps is usually 2.5% or 5% of rated voltage.

Transformer Taps

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Diagram Showing Taps

Example:

Primary: 208V / Secondary: 120V

Condition: Supply voltage is low @187V

Untapped voltage: 108V (Fixed ratio)

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CATALOG NUMBER SUFFIX:

• No suffix = NO TAPS

• -S = SHIELDED BUT NO TAPS

• -1S = 2 X 5% BNFC

• -2S = 1 X 5%ANFC & 1 X 5% BNFC

• -3S = 2 X 2.5% ANFC & 4 X 2.5% BNFC

• -4S = 2 X 2.5% ANFC & 2 X 2.5% BNFC

• -5S = 2 X 5% ANFC & 2 X 5% BNFC

(This tap chart applies to all catalog numbers in Section I of the catalog. Exceptions may apply in other sections of the catalog.)

Acme Standard Tap Chart

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• UL – Underwriters Laboratories

• CE – Conformite Europeene

• CSA – Canadian Standards Association

• ANSI – American National Standards Institute

• NEMA – National Electrical Manufacturers Association

• IEEE – Institute of Electrical & Electronic Engineers

• ISO 9001 – Highest Quality Rating

Agency listing & Certification

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Sizing and Selection

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Enclosure Ratings:

• .050 - .150 kVA Totally Enclosed (3R)

• .250 - 25 kVA Epoxy Encapsulated (3R)

• 37.5 - 250 kVA Ventilated (NEMA 2)

• 37.5 - 100 kVA Non-Ventilated (3R)– (NEMA 2 becomes 3R with addition of weather shield)

Single Phase Transformers

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Enclosure Ratings:

• 3 - 15 kva Epoxy Encapsulated (3R)

• 30 - 1000 kva Ventilated (NEMA 2)

• 30, 45, & 75 kva Epoxy Encapsulated (3R)

• 30 - 112.5 kva Non-Ventilated (3R)– (NEMA 2 becomes 3R with addition of weather shield)

Three Phase Transformers

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Determine load data:

• Voltage required by load

• Amperes, horsepower, or kVA required by load

• Frequency

• Verify load is designed for single phase

Selection Guide: Single Phase

Determine Supply Data:

• Voltage of supply

• Frequency

• (Supply can be single or three phase)

• If the load nameplate expresses a rating in kva, a transformer can be selected directly from the charts in the ATD-01 catalog.

• If motor horsepower is known, you must first select kva from the charts in the front of Section I or from the KVA Card.

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Note: Always size based on load volts and load amps!

• To determine kVA when volts and amps are known:

kVA = Volts x Amps

1000

• To determine Amperes when kVA and volts are known:

Amps = kVA x 1000

Volts

SIZING: SINGLE PHASE

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Load: 1-phase, 60Hz, 240v, @ 10A

Supply: 1-phase, 60Hz, 480v

Single Phase Selection

Problem #1

Problem #2

Load: 1-phase, 60Hz, 120v, @ 50A


(See next slide for answers)

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kVA = (240V x 10A) / 1000 = 2.4kVA

Single Phase Selection

Answer #1

Answer #2

kVA= (120V X 50A) / 1000 = 6 kVA

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Problem #3

Load: 1-phase, 60Hz, 3 HP motor,

240v, starts 2 times/hour


Single Phase Selection (Cont’d)

Problem #4

Load: (1) 3/4 HP, 1-phase, 240v motor

(2) 1/4 HP, 1-phase, 120v motor

(4) 50 watt lamps, 120v

Supply: 1-phase, 60 Hz, 480v

(See next slide for answers)

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Answer #3

• From Table 2 in Section I, a 3 HP motor requires 4.1kVA of transformer

• Since motor starts 2x per hour, kVA increases 20%

• 4.1 x 1.2 = 4.92kVA or a 5kVA unit

Single Phase Selection (Cont’d)

Answer #4

• ¾ HP = 1.66kVA

• ¼ HP = 0.7kVA

• 4 x 50W = 200W or 0.2kVA

• Total kVA = 1.66 + 0.7 + 0.7 + 0.2 = 3.26kVA

• Use a 5kVA unit

Note: When dealing with incandescent lighting and resistive heating, wattage will equal VA.

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Determine Load Data:

• Voltage required by the load

• Amperes, horsepower, or kVA required by load

• Frequency

• Verify load is designed for three phase

Selection Guide: Three Phase

Determine Supply Data:• Voltage of supply

• Frequency

• (Supply must be three phase)

If the load nameplate expresses a rating in KVA, a transformer can be selected directly from the charts in the catalog.

If motor horsepower is known, you must first select KVA from the charts in the front of Section I or from the KVA card.

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Note: Always size based on load volts and load amps!

• To determine kva when volts and amps are known:

– KVA = Volts x Amps x 1.73 1000

• Note:

– 1.73 due to phase shift of 120 degrees between phases

– 1.73 = square root of 3

• To determine amperes when kva and volts are known:

– Amps = kVA x 1000

Volts x 1.73

• Note:

– A three phase transformer can be seen as (3) single phase transformers.

Sizing: Three Phase

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• Delta: A three phase, three wire system, that may be grounded or ungrounded and is represented by a triangle.

Delta-Wye Connection

• Wye: A three phase, four wire system, where the fourth wire (neutral) has the same voltage relationship to each of the phases. It is represented as the letter “Y”, with the neutral being the center.

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• The delta primary shown here is the same as on the previous slide

Note: The secondary here is also a delta configuration but, that it has a center tap on one winding (middle coil). This tap can be used for single phase power, but is limited to 5% of nameplate kVA.

Delta-Delta Connection

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Problem #1

Load: 3-phase, 60 Hz, 208v @ 30A


Three Phase Selection

Problem #2

Load: 3-phase, 60 Hz, 240v @ 50A


Problem # 3

Load: (1) 3-phase, 240v, 30 HP motor

(1) 1-phase, 240v, 5 HP motor

(1) 1-phase, 120v, 2 KW heater

Supply: 480Y/277, 3-phase, 60 Hz(See next slide for answers)

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Three Phase (Cont’d)

Problem #1

kVA = (208V x 30A x 1.73) / 1000 = 10.8kVA

Next larger std kVA is 15 kVA

Problem #2

kVA = (240V x 50A x 1.73) / 1000 = 20.76 kVA


Problem # 3

30 HP = 32.4 kVA

5 HP = 6.72 kVA

Transformer kVA = (32.4 + 6.72 + 6.72 + 6.72) = 52.56 kVA(Note: 1-Phase load was added 3 times to give equivalent 3-Phase rating.)


Education

Acme Electric Training (General Purpose Transformers)