Size a Transformer
Transformers are used to scale the voltage available from the utility mains up or down to meet the needs of a load to be driven. While some loads may be suitable to be driven directly from the 120 volt alternating current (AC) mains, most loads will require a different voltage to drive the load. The requirement to scale the mains AC voltage up or down does not address the need for the transformer to support the full load current of the load. In order to support the needs of the load under all load conditions, the transformer must be sized to be capable of supplying the power (voltage times current) needed by the load. Use these tips to learn how to size a transformer.
Steps
- Characterize the load.
- Obtain the voltage required by the load. With this information, the scaling required in the transformer can be determined as the ratio of the AC mains voltage to the needed load voltage. Keep in mind that the utility company will only guarantee the voltage accuracy of the AC mains to plus or minus 10 percent.
- Determine the current required by the load under normal operating circumstances.
- Define the short circuit current that the load will present. Many loads, such as motors, require a large burst of current (referred to as inrush current) to break the load free of a standing start and transition the load to normal operating circumstances.
- Determine the transformer size needed to support normal load operations. The answer will be different for single phase transformers versus 3 phase transformers. Utilities provide 3 phase mains, using 3 wires. Often at the point of entry into a building only 1 of the 3 phases is supplied, using 2 wires.
- Figure the transformer size needed for a single phase transformer. This size is determined as voltage required by the load under normal operating circumstances (Vl) times current required by the load under normal operating circumstances (Il) divided by 1,000. The units of the answer are thousand volt amps (kVA). Transformers are sized in the industry by their KVA ratings.
- Calculate the transformer size needed for a 3 phase transformer. The 3 phase power requirements are determined as (Vl times Il times 1.732) divided by 1,000. The 1.732 factor used to size 3 phase versus single phase transformers is introduced by the need to calculate 3 phase power algebraically, rather than arithmetically.
- Make transformer size allowance for inrush current. Find the rated full load output capability of the transformer (Irfl) in amperes. The rated full output capability of a single phase transformer is determined as transformer kVA divided by Vl. The rated full load output capability of a 3 phase transformer is determined as kVA divided by (Vl time 1.732).
- Characterize the impedance of the transformer. The impedance of a transformer (Z) is Vl divided by Irfl. The answer for transformer impedance is expressed in units of ohms.
- Calculate the short circuit capacity (Isc) of the transformer. The short circuit capacity of the transformer will be Vl divided by Z. The short circuit capacity is expressed in amperes.
- Recalculate the required transformer size. The transformer Isc must support the inrush requirement of the load. If the load short circuit current found when the load was characterized is more than the calculated Isc of the transformer kVA chosen for normal operating circumstances, the next larger kVA size transformer must be chosen and the calculations redone. Continue iterating to higher kVA transformers until the Isc of the transformer can support the load short circuit current.
