Application of Standard Dry Type Transformers in an Electrical System

Most utilities will only provide a customer with one service or electrical system. This system may be either single-phase or three-phase. Single-phase installations will normally be 120/240V AC, 3 wire systems. Three-phase installations could be 240 volt, 3 wire , 480 volt 3 wire, 600 volt, 3 wire, 208Y/120 volt, 4 wire , or 280Y/277 volt,4 wire. These are the most popular installations and their selection can be either on customer preference or availability of the system from the serving utility. With this many choices available , you may wonder why anyone would need a transformer, so let us offer an example. A new industrial plant moves into town and requires an electrical service in their new building. They have a great many motors in use at their company, so they decide it would be more economical to use 480 volt three-phase motors. For this reason they request a 480Y/277 volts three-phase system. This takes care of their motor loads at 480 volts and their office and plant lighting loads at 277 volts. However, to operate their office machinery and incandescent lighting they require 120 volts. They also have some small horsepower motors they want to operate at 208 volts. Since the utility will only provide them with 480Y/277 volt three-phase system , they require a dry-type distribution transformer to provide the 208 and 120 volt loads. This is the most typical of applications for dry-type distribution transformers. Other applications could be matching the voltage of a motor which does not match your system, isolating a computer or solid state device from system voltage due to voltage drop in an extremely long run of wire. The more important thing is to recognize what transformers can and cannot do . Below is a table of some of those things.

Operation	Yes	No
Change Voltage	*
Change Frequency		*
Convert single -phase current to three-phase current		*
Protect equipment (isolate line voltage from load voltage)	*
Stabilize fluctuating line voltage (constant voltage)		* Note
Compensate for voltage drop	*
Improve “power factor”		*

*Note: There are special purpose constant voltage transformers that can do this.

Selection of a Transformer

When a customer calls you for help in the selection of a transformer these are things you need to know:
1 What is the voltage of this load? The transformer you select must have an output voltage which matches his load voltage. (don’t get confused between system and utilization voltage - See Section V, Paragraph D.)
2 Is the load single-phase or three-phase? Remember the transformer cannot change phases. Three-phase loads must be fed from the three-phase transformer/banks.
3 What is the power requirement for this load? We ultimately need to arrive at a KVA value. If only amps are known, use the full load chart or the following formulas.

Single -phase “KVA”= Volts (loads) x Amps (load)
1000
Three=phase “KVA” = Volts (load) x Amps (loads) x 3
1000
Where 3 = 1.732

4 What is the frequency(hertz or Hz) of the load and line (source)? Remember, transformers cannot change frequency. Generally, all U.S. power companies generate power at 60 Hz. Therefore, the load must also be rated 60 Hz.
5 What is the supply or source voltage? Are primary taps required?
6 Is there a special temperature rise or insulation system requirement ? If not, quote our standard general purpose transformers.
7 Is the transformer to be installed indoors or outdoors? Some transformers, particularly small encapsulated units are rated for indoor or outdoor applications. Others sizes will require the addition of a weather shield for outdoor use.
With the above information you should be able to quickly select a transformer from the catalog.
Note: Other considerations which may require special units may include, but are not limited to: copper windings; low temperature rise units; units for applications in ambient temperatures higher than 40(C;units to be used at a high altitude above 3300 feet; special impedances; and many others. If requirements arise that do not fit the description of our standard units, be sure to contact your Federal Pacific representatives for assistance.

Problem: What is the proper transformer for a customer to supply an electric heater rated 100 amps, @240 volts, three-phase, 60 Hz? His available supply voltage is 480 volts, three-phase, 60 Hz. The transformer is installed indoors. 150 C with standard taps is required.
Solution: We have all of the information required with the exception of the load KVA. We know that: Three-phase KVA = Volts (load) x Amps (load) x 3 = 240 x 100 x 1.732 = 41.6
1000 1000
A 480 volt to 240 volt (Delta-Delta), three-phase, 45 KVA (which is the next standard KVA rating) general purpose transformer is required
Problem: A single line shows a 25 KVA, single-phase, 60 Hz, 150(C rise, 240X 480V to 120/240 volt transformer fed from a three-phase volt system. Is it correct?
Solution: Yes, the transformer has series-multiple primary windings so connection to 480 volt is acceptable. Remember, that if the transformer is single-phase, the source can be single-phase or three-phase. When the supplies three-phase, any two (2) lines or one (1) line and neutral will be used as shown below . Figure 1 applies for the 480 volt primary voltage in the above problem. Problem: An industrial plant requests a transformer that ca step down 480 volt three-phase to 240 volt three-phase and supply 200 KVA of load at 240 volt three-phase and 5 KVA of load at 120 volt single-phase. What would you quote?
Solution: A general purpose 300KVA, 480volt to 240 volt three-phase transformer with 120 volt lighting tap will work. Remember that the use of the 120 volt lighting tap requires a 30 % derating of the nameplate KVA and that the 120 volt loads cannot exceed 5% of the nameplate KVA. (See below)

Derating Calculations:
300 KVA x .30 KVA
300 KVA
-90KVA
210KVA (Available capacity for 30 (loads)

120 Volt Lighting Tap Calculations:
300 KVA x .05 = 15 KVA (Available capacity for 120 volt loads)

The following must be known before a transformer can be selected:

KVA the rating or capacity of the transformer
Phase Load requirements (single-phase or three-phase)
If the load is three-phase, both the supply and the transformer must be three-phase.
If the load is single-phase, the supply can be either single or three-phase, but the transformer must be single-phase.
Frequency usually 60 Hz (Hertz).
Primary Voltage Designates the load voltage for wich the primary winding is designated.
Secondary Voltage Designates the load voltage for which the secondary winding is designed.
Taps Adjustment capability for voltage variations.
Location of Installation Indoor or Outdoor
Other Considerations Mounting Requirements, Sound Levels, Impedance, Special Applications, K-Rating, Copper Windings, Electrostatic Shields, Temperature Rise, Insulation Class

Cost Comparison
Secondary Unit Substation Transformers

Mineral Oil Field	100%
High Fire Point Liquid
R- Temp	120%
Silicone	140%
Nonflamable Liquid	125%
Perclene TG	125%
Wescosol	125%
Wemco NF
VPI	120%
VPI-Epaxy Shieded	130%
Cast Soil	160%
Sealed Dry-Type	200%