Transformer

When you get a call for a transformer Here is what you need to know:

1 What is the voltage of this load?
2 Is the load single-phase or three-phase?
3 What is the power requirement for this load?

We have to find 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 1.732
1000

(Square root of 3 = 1.732)

4 What is the frequency?
5 What is the supply or source voltage? Are primary taps required?
6 Is there a special temp or insulation ? If not, use standard general purpose transformers
7 Is the transformer to be installed indoors or outdoors?

With the above information you should be able to select a transformer.

Note: Other things that may require special units may include, copper windings; low temperature rise units; units for applications in ambient temperatures higher than 40 deg 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 standard units, contact your dealer.

Calculation Examples:

What is the proper transformer to supply an 3 Phase electric heater rated 100 amps, at 240 volts, 60 Hz? The supply voltage is 480 volts, three-phase, 60 Hz. The transformer is installed indoors. 150 C with standard taps is required.

Solution: You have all the information 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 needed.

Example:

You have a motor load of 160 amps at 480 volts 3 phase and a supply voltage of 600 volts 3 phase.

Step 1- 160 Amps x 480 Volts x "square root of 3" = 133 KVA
Step 2- The next bigger size is 150KVA   therefore
Step 4- 150KVA divided by "root 3",then divided by 600 V= 144 A Primary
Step 5- 150KVA divided by "root 3", then divided by 480V= 180 A Secondary
Step 6- Secondary conductor size =180A*1.25=225A or 4/0
Step 7- Primary Breaker=144A*1.25=180A
Note- CEC Rule 26-256(3) says you need next size larger so: 200A

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

KVA --The rating or capacity of the transformer

Frequency-- Usually 60 Hz.

Primary Voltage-- Designates the voltage for which the primary winding is designated.

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.

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