Selecting a dry type transformer comes down to a short list of inputs. Most specification problems in service trace back to one of them being assumed rather than stated, and the ones that cause the most trouble are the load profile and the installation environment.
Determine Your Power Requirements
Start from the connected load and then decide how much of it will run at once. Adding up nameplate ratings of every motor and then buying a transformer for the total produces an oversized unit that runs at low load, where it is inefficient and where core loss is paid for continuously whether it does any work or not.
What the design actually needs is the expected demand, the peak demand, the duration of the peak, and the power factor. A short peak can often be handled by the thermal inertia of the winding rather than by a larger rating, but only if the overload and its duration are known at the design stage.
Include the load growth you expect. Adding 25 to 30 percent is a normal allowance. Adding 100 percent because the plant might expand produces a transformer that runs inefficiently for its whole life, and the money would usually have been better spent on the connections.
Voltage Requirements
Record both primary and secondary voltage, and check whether the system uses the wye or delta relationship. In a wye-connected system the line voltage is √3 times the phase voltage; in delta the line current is √3 times the phase current. Getting this wrong produces a transformer that is electrically correct but cannot be connected as intended.
Decide whether a standard ratio will do or a custom one is required. Custom ratios are routine in industrial work, and they avoid the losses and space taken up by an extra conversion stage.
Check the tap requirement. Off-circuit taps on the primary allow the ratio to be adjusted for the actual supply voltage at commissioning, which is useful when the supply is at the edge of its declared range.
Finally, consider voltage regulation. As load increases, secondary voltage falls by an amount set by the impedance. Where the load is sensitive to that, either the impedance has to be chosen with regulation in mind or the tap has to be adjustable.
Installation Environment
This is where the specification most often goes wrong. Record ambient temperature, and note that a transformer rated for 40 °C ambient will not deliver its full rating at 50 °C. Record altitude, because air density falls with height and air is the only coolant a dry type unit has.
Record humidity, dust, and any corrosive atmosphere. Cast resin construction is the answer to most of those, and it is worth specifying from the start rather than adding after a unit has already failed.
Record the enclosure requirement. Indoor units may be open or in a simple sheet steel enclosure; washdown areas need an appropriate IP rating; outdoor and coastal installations need coating and sealing to match.
Core, Winding, and Insulation Choices
- Grain-oriented silicon steel is the standard core material. Core loss figures vary between grades and show up directly in running cost.
- Copper windings conduct better and stand up to fault forces better; aluminium reduces cost and weight. The choice is driven by both price and duty.
- Insulation class sets the temperature the design works against. Class F and Class H are common in industrial dry type units because they allow a smaller unit for the same losses.
- Forced air cooling raises the rating available in a given footprint, at the cost of fans that have to be maintained and a rating that depends on them running.
Impedance and Fault Level
Impedance controls two things at once. Higher impedance limits fault current, which reduces the duty on switchgear downstream. It also increases voltage drop under load and worsens regulation. The value chosen is a compromise between the two, and it has to be set against the actual system fault level rather than assumed.
On systems with large motor loads, the impedance also affects how far the voltage dips when a motor starts.
Calculating the Rating
The working check is straightforward. Take the expected demand in kVA, add the growth allowance, then correct for ambient and altitude using the manufacturer’s derating factors. If the load is non-linear, apply the harmonic factor as well rather than treating the kVA figure as final.
If the result sits between two standard ratings, the decision is about what happens on the worst day rather than about the average. Load a unit at 95 percent continuously and the insulation ages at a rate the design margin was meant to prevent.
Commercial and Life-Cycle Factors
- No-load loss runs 24 hours a day for the life of the unit, so it is worth more attention than its share of the nameplate suggests.
- Load loss varies with the square of the load, so a lightly loaded unit has low loss even with an inefficient design.
- Delivery time and the availability of a replacement matter more than a small price difference when the transformer is on a critical production feed.
- Whether the site has the resource to maintain forced air cooling, because a fan-cooled rating is only available if the fans are working.
Information to Send Us
- Primary and secondary voltage, with the vector group if one is required.
- Rated capacity and the load profile, including peak and duty.
- Ambient temperature, altitude, humidity, dust, and corrosives.
- Enclosure or IP rating, and the intended cooling method.
- Any impedance, tap, or temperature rise requirement.
- Access constraints and the required delivery date.
Given those, the design follows. See the dry type transformer range, the cast resin transformer range, or contact us.
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