General Transformer Data

Load

It may seem obvious but telling the transformer designer about your specific needs in terms of the voltage, and current you require is vital in ensuring that you get the right product. It may be that there are other factors such as harmonic currents that will impact on the performance of the item, these should also be advised with your enquiry.

Another point to make is that you should state in your enquiry if the voltage you are advising is to be obtained at no-load or at rated load. These values are different. The no-load voltage of the transformer will be reduced by applying resistive or inductive loads, and may be increased with capacitive loads.

Supply Frequency

Due to the basic science of transformers the supply frequency that they are to operate on is fundamental to their design.

Transformers designed for 50Hz will operate on 50 or 60Hz. Generally 60 Hz transformer do not have sufficient flux handling capacity to operate on 50 Hz without becoming overheated, noisy, pulling excessive no-load current or just simply saturating resulting in operation of primary protective fuses.

Frequencies below 50Hz will result in a larger transformer, while frequencies greater than 50Hz may enable the size to be reduced.

Ambient Temperature

All electrical equipment is designed to operate within specific parameters. One of the principle parameters is the temperature of the air surrounding the item that is used to cool the item. All international and national standards for transformers and the like lay down standard operating temperatures. They are used as a corner stone for the design of the equipment.

If the item you are seeking is to be used under high ambient temperatures such as one would find in say a room with restricted ventilation, or one that is heated by other equipment within the room or surrounding process plant then by advising of this condition we can take it into account within our design ensuring that you obtain the right product for the job in hand.

Temperature Rise

Hand in hand with ambient temperature goes the winding temperature rise of an item. Wound components are designed to be within a specific winding temperature rise while being cooled by air at a known temperature. This way the product designer can ensure that the item when in use operates at a temperature which is not damaging to the insulation system of the product.

Any temperature rise limit that your specifically require should be stated within your enquiry. If none is provided the standard temperature rise limits will be applied as detailed in the applicable international or national standard for the insulation class used.

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 cost and 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.

A typical application would be to interface a piece of equipment from overseas with a local electricity supply of different voltage, say a 380 Volt piece of machinery to operate from a supply voltage of 415 Volts. If the total load is 100 kVA then the equivalent frame size of the auto transformer would be in the order of 8.5 kVA. What does this mean; well the auto transformer rating still needs to be stated as being for 100 kVA to ensure the designer allows for the right current ratings etc, but the materials needed to manufacture it as similar to that of a transformer with a normal double wound / isolating transformer rating of 8.5 kVA. This compares favourably with using a 100 kVA double wound isolating transformer certainly on price and physical size.

Multi Tapped Transformers

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:

Generally the use of this formula becomes limited by other factors such as voltage reduction on load, known as regulation, by the time an off to on ratio of 3 or 4 to 1 is reached.

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, no-load to full load voltage reduction.

Is determined given by the formula below:

Efficiency

Is determined given by the formula below:

Effects of Ambient Temperature on Ratings

As detailed above the ambient air temperature in which a piece of electrical equipment is designed to operate is an important part of the design. If however you have a piece of equipment designed for one ambient condition and you are operating it under different conditions a reduction in maximum load make capacity may allow it to operate satisfactorily without say producing alarm and trip signal etc.

Insulation Classification

Read about k-factor and harmonic mitigating transformer data.