Why is impedance important?

It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.  Calculate as follows:

Normal Full Load Current =

Maximum Short Circuit Amps =

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.


How do you select transformers?

  1. Determine primary voltage and frequency.
  2. Determine secondary voltage required.
  3. Determine the capacity required in volt-amperes.
    • This is done by multiplying the load current (amperes) by the load voltage (volts) for single phase.
    • For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.
    • ALWAYS SELECT  THE TRANSFORMER LARGER THAN THE ACTUAL LOAD. This is done for safety purposes and allows for expansion, in case more load is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000.
  4. Determine whether taps are required. Taps are usually specified on larger transformers.
  5. Use the selection charts found on the website.

What is a transformer and how does it work?

A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life.

It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

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