1. Rated Power / Construction Volume:
A commercially available four pole 2.2kW motor (1500 min-1) with a connecting voltage of 400V has to be adapted to a 500V mains. The rated current of such a motor is approximately 5.4 A. The formula for the three-phase load yields the following necessary apparent power:
This example shows that the output active power of 2.2.kW differs substantially from the input apparent power of 3.74kVA. The transformer must always be dimensioned on the basis of the necessary apparent power and in the above example, trafomodern recommends the use of a 4.0kVA transformer.
2. A Lot Does Not Always Help a Lot!
Short-time Power and Continuous Power
When dimensioning control voltage transformers, attention should be paid to the indicated short-time power as well. Applying this selection assistance will lead to cost and space savings.
Determining the Correct Dimensions of a Control Transformers
Dimensioning Based on Short-time Power
If mainly large protections are to be connected, it is recommended to select the control transformer on the basis of the short-time power.In most cases, this will reduce the necessary transformer power.Make sure that the holding power does not exceed the continuous power.
Inexpensive Transformer = Hot Transformer
Due to the prevailing pressure on prices, everyone looks for possibilities to simplify products and make them smaller and cheaper. This trend unfortunately also applies to the transformer industry. The use of better insulation materials allows an increase of the operating temperature of the coiled products. Switching from the insulation class B (130°C) to insulation class F (155°C) results in a decrease in size of approx. 16% and correspondingly lower purchasing price. However, the load losses (copper loses) increase by about 30% and the efficiency is lower. For the user, this means in practical terms that more heat needs to be removed from the switch cabinet, i.e. there is a need either for a more complex cooling system or ventilation. These additional costs for a more powerful air conditioning of the switch cabinet should be compared to the supposedly “saved“ costs for the transformer.
trafomodern has opted for the following method for its products:
The indicated model power is based on the use according to insulation class B (130°C). For a transformer power of 6.3kVA and above, the lists also contains a value (approx. 16% higher) for a use according to insulation class F (155°C) (DTSP or DTFSP for example). This will allow you to select the product for your particular needs (loss optimised or price optimised).
3. Separate Winding Transformer vs. Autotransformer
In case of doubts, our sales force»Contact will be pleased to assist you in selecting the correct transformer type.
4. Types of Operation
The required transformer for the operation would have a model power of 32kVA. If however the load cycle is known (in our example: 3 minutes 20A, 2 minutes 45A and 5 minutes pause, i.e. off-load), the following formula will yield a significantly optimised transformer:
Consequently, the necessary apparent power is:
The optimised transformer now only has a model power of 12.5kVA, is significantly smaller and therefore less expensive than the 32kVA transformer.
The thermal heating times of the transformers need to be taken into account as well. The heating time is defined as the necessary time for a transformer to reach its operating temperature if connected to the rated current. The length of the load cycle should never be longer than this time. Typical heating times are
|Type power||Heating time|
|0,1 kVA||approx. 15 Minutes|
|1,0 kVA||approx. 60 Minutes|
|10 kVA||approx. 3 Hour|
Also, the ratio of the calculated continuous current to the maximum current should not significantly exceed the ratio of 1:2, in order to avoid unnecessary high voltage losses.