Factors which affect Transformer Choice
Load VA
The major factor affecting the transformer physical size is the product of Load current and volts.
Supply Frequency
Transformers designed for 50Hz will operate on 50 or 60Hz, frequencies below 50Hz will result in a larger transformer.
Frequencies greater than 50Hz may enable the size to be reduced.
Temperature Rise
Any temperature rise limit that may be required should be stated on enquiry. If not given then the full load temperature rises is limited by values in the standard for insulation class.
Effect of Ambient Temperature Rise
High Ambient Temperatures may affect the size and rating of the transformer.
Auto Wound Transformers
If no isolation is required between primary and secondary windings, the use of an auto wound transformer can have the
benefit of reducing the physical size.
As a simple guide to sizing an auto transformer the following formula can be used:
The closer the value of high and low voltage values the smaller the transformer physical size.
Multi Tapped Transformers
| A large spread of voltages will mean an increase in the size of the transformer | |
| 0-380-550v | Increased by 18% |
| 0-110-240v | Increased by 25% |
| 0-240-415v | Increased by 25% |
| 0-110-240-415v | Increased by 40% |
| Secondary taps may also increase the transformer size. | |
Duty Cycle
If the transformer is not to be used continuously, allowances can be made with the design giving a reduction in the size. The
size of the transformer may be calculated using the following formula:
D.C. Rectification
To calculate the transformer size (VA) which is to supply D.C. from a bridge rectifier the following calculation can be used.
Single phase un-smoothed.
VA=1.3 x IDC x VDC
Single phase with capacitor smoothing
VA= 1.5 x IDC x VDC
Three phase with un-smoothed
VA= 1.1 x IDC x VDC
Regulation
Is determined given by the formula below:
Efficiency
Is determined given by the formula below:
Effects of Ambient Temperature on Ratings:
| Average | Peak | Suggested Rating Reduction |
| 30°C | 35°C | Nil |
| 35°C | 45°C | 6% |
| 40°C | 50°C | 12% |
| 45°C | 55°C | 20% |
Insulation Classification:
| Class | Continuous Temperature Rating | Temperature Rise |
| A | 105°C | 60°C |
| E | 120°C | 75°C |
| B | 130°C | 80°C |
| F | 155°C | 100°C |
| H | 180°C | 125°C |
| C | 220°C | 150°C |