General Transformer Data

Factors which affect Transformer Choice

Load VA

Load current and volts is important transformer data as it affects the physical size of the transformer.

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:
Auto Wound Transformer Data
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:

Duty Cycle Transformer Data

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:

Transformer Regulation Data

Efficiency

Is determined given by the formula below:

Transformer Efficiency Data

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

Read about k-factor and harmonic mitigation transformer data.

Site Designed and Developed by First Internet Manchester web design