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D r y - T y p e D i s t r i b u t i o n T r a n s f o r m e r s

Transformer Selection Steps

Three Phase Loads

1. Determine electrical load

Voltage required by load.

Amperes or kVA required by load.

Frequency in Hz (cycles per second).

Verify load is designed to operate on three phase.

All the above information is standard data normally obtained from equipment nameplates or instruction manuals.

2. Determine supply voltage

Voltage of supply (source).

Frequency in Hz (cycles per second).

The frequency of the line supply and electrical load must be the same. A three phase transformer is selected which is

designed to operate at this frequency having a primary (input) equal to the supply voltage and a secondary (output) equal

to the voltage required by the load.

3. If the load nameplate expresses a rating in kVA, a transformer can be directly selected from the charts. Choose from

the group of transformers with primary and secondary voltages matching that which you have just determined.

Select a transformer with a standard kVA capacity

equal to or greater than

that needed to operate the load.

Primary taps are available on most models to compensate for line voltage variations.

(Refer to question #2 in the Transformer Questions and Answers Section on page 6.)

When load ratings are given only in amperes, tables 3 and 4 or the following formulasmay be used to determine proper kVA size for the

required transformer.

(1)

To determine three phase

kVA

when volts and amperes are known:

Three Phase kVA =

Volts x Amps x 1.73

1000

(2)

To determine

Amperes

when kVA and volts are known:

Amps =

3 Phase kVA x 1000

Volts x 1.73

Three Phase Example

Question:

Select a transformer to fulfill the following conditions. Load is a three phase induction motor, 25 horsepower @ 240 volts, 60 Hz

and a heater load of 4 kilowatts @ 240 volts single phase. The supply voltage is 480Y/277, three phase, 4 wire.

Answer:

Compute the kVA required.

Motor

—From table 4 the current is 68 amps.

240 volts x 68 Amps x 1.73

= 28.2 kVA

1000

(The kVA can also be obtained from Table 4)

Heater

— 4 kVA

A three phase transformer must be selected so that any one phase is not overloaded. Each phase should have the additional 4 kVA rating

required by the heater even though the heater will operate on one phase only. So, the transformer should have a minimum kVA rating of

28.2 + 4 + 4 + 4 or 40.2 kVA. Refer to the appropriate selection chart. A 480 delta primary—240 delta secondary transformer may be used

on a 4 wire, 480Y/277 volt supply. The fourth wire (neutral) is not connected to the transformer. To not overload the transformer, a 45 kVA

transformer should be selected.

Note:

Any two wires of the 240 volts, 3 phase developed by the secondary of the transformer may be used to supply the heater.

Any 2 wires of a 3 phase system is single phase.