Oil-immersed transformers for motor loads require separate purchasing checks for steady running demand and motor-starting duty. Define the actual motor, starting method, driven load, supply conditions and loads already running before accepting a proposed transformer for the application. The project engineer needs that information to assess voltage performance and acceleration. An oil-immersed construction or a sufficient running-load kVA rating does not, by itself, demonstrate that a particular motor will start satisfactorily.
AI-generated motor-load illustration, not HEAKOR equipment or a project record; starting current, acceleration and transformer suitability are not established by the image.
This guide is for UK industrial EPC, consultant and procurement teams buying a three-phase oil-immersed MV/LV substation transformer for low-voltage induction-motor loads. It helps define the enquiry and study handover; it does not select a transformer rating, set a starter or approve a network connection.
Specify the starting event, not only the motor's running power
The purchasing record should describe what happens during the start, including the other equipment that must remain in operation. Starting current flowing through supply impedance can reduce the available voltage. Published motor-supply transformer selection guidance explains why insufficient voltage can prevent acceleration or affect the starter. That source concerns a particular low-voltage product scope; its typical values are not project limits or evidence for a HEAKOR transformer.
Request the actual motor and starter data instead of inserting a generic starting-current multiplier. State whether the driven machine starts loaded or unloaded, whether starts may overlap, and which loads are already connected.
As a concrete UK example, the NGED competitive-connection application asks for motor starting current, starts per hour, phase and starting arrangement. The form also allows for further detailed information. It is an example of a real connection-data requirement, not a universal form or acceptance rule for every UK network. Confirm the requirements of the project's serving network.
Keep motor starting separate from transformer energisation
Motor starting concerns a downstream load accelerating. Transformer energisation concerns the magnetic transient when the transformer is switched on. CIGRE's transformer-energisation overview describes the latter phenomenon, including the influence of core saturation and switching conditions.
A restoration sequence may involve both events, so identify them separately in the study scope. Do not use a transformer energisation-current record as a substitute for motor-start data. HEAKOR's separate dry-type transformer inrush guide addresses an energisation-data handover for that different transformer category; it is not a motor-start sizing method or an oil-immersed product specification.
Agree where voltage performance must be checked
Record the acceptance locations before comparing transformer offers. A statement such as “acceptable voltage dip” is incomplete without identifying where it applies, how long the event lasts and who accepts the result.
Use three separate review locations where relevant:
| Review location | Requirement to obtain | Responsible source |
|---|---|---|
| Network connection point | Applicable disturbance criteria and the supply conditions used for assessment | Serving network and project connection engineer |
| Shared distribution bus | Operating requirements of other loads that must remain connected during the start | Project designer and affected equipment suppliers |
| Motor terminals | Motor and starter requirements, acceleration objective and supporting load data | Motor, starter and driven-equipment suppliers, coordinated by the project engineer |
For each location, record the criterion, voltage reference, duration or repetition basis and reviewing party. Mark missing criteria as TBC rather than filling the schedule with a convenient percentage. This is a proposed purchasing record, not a standard-prescribed acceptance table.
Put the actual motor-start duty in the RFQ
A useful motor-start schedule brings electrical and mechanical information together. Motor torque, driven-load torque and inertia affect the acceleration process, as explained in this published softstarter application handbook. Its illustrative curves and settings should not replace the selected equipment's data.
| Information | What the buyer should obtain | Why it matters to the review |
|---|---|---|
| Motor identity and running duty | Manufacturer and model, phase, voltage, frequency, rated current, output power and available efficiency/power-factor information | Prevents shaft-output kW from becoming an unexplained electrical demand figure |
| Starting-current information | Manufacturer data with its stated voltage and operating basis; current/time or current/speed information if required | Defines the event instead of assuming a universal multiple of rated current |
| Starting arrangement | Actual starting method, starter or drive identity, and proposed controlled-start information | Makes the assessed start match the intended equipment and settings |
| Driven-load conditions | Machine duty, loaded/unloaded start, and torque-speed or inertia data where needed by the engineer | Supports the acceleration assessment rather than only an instantaneous current check |
| Repetition and sequence | Starts per hour, restart conditions, permitted simultaneous starts and loads already running | Identifies which operational cases the purchase must support |
If a motor or starter has not been selected, name the uncertainty and the party that will close it. A budget quotation can proceed on identified assumptions without pretending that the starting duty has already been accepted.
Request transformer data for the complete supply path
Ask the project engineer which transformer information is needed for the agreed assessment. Relevant requests may include the proposed rating, voltage ratio, connection and tap basis, impedance with its reference conditions and tolerance, and resistance/reactance information where the study needs it.
Keep that information connected to the single-line diagram, available source data, feeder details and relevant operating arrangements. A study for the normal utility supply should not silently become approval for an unassessed standby supply or different bus configuration. Record which operating cases are included and which are excluded.
Do not approve an offer merely because it has a larger kVA value or a lower quoted impedance. Ask the engineer to review the proposed transformer in the complete system and reconcile the consequences for the project. This article recommends no impedance target, tap adjustment or protection setting.
Compare bids against one agreed starting case
Make the same motor schedule and operating cases part of every enquiry. Then require each offer to identify the transformer data it provides, its assumptions, any deviations and the information still needed.
For example, two offers may address the same running demand while only one identifies the starting case used in its review. The next procurement step is to resolve the missing assumption, not to declare the larger transformer suitable without the study. Keep the motor-schedule revision, source assumptions, transformer-data revision and engineering review reference together.
Also distinguish equipment test documentation from the installation's motor-start acceptance. Agree whether the project requires a calculation, measurement or commissioning verification, who performs it and what is included in the purchase. Do not assume that an unspecified factory-test package demonstrates the performance of the complete site.
Frequently asked questions
Does a softstarter automatically allow a smaller transformer?
No automatic reduction should be assumed. The actual starting method must provide the required motor acceleration while meeting the project's supply and equipment criteria. Request the intended duty from the starter and motor suppliers rather than applying a generic saving factor.
Is one locked-rotor-current figure enough?
It can support an initial review, but it may not describe the acceleration sequence or a controlled start. Ask the project engineer which additional current, torque, time or speed information is needed for the selected motor and load.
Should I send data for only the largest motor?
Provide the relevant motor and load schedule, including permitted start combinations and already-running loads. Let the responsible engineer identify the governing cases. The largest motor's nameplate alone does not describe every operating condition the purchase may need to support.
Who should provide missing upstream source information?
Ask the connection engineer to obtain or confirm the applicable network information with the serving network. Keep it separate from the transformer supplier's own equipment impedance data. If preliminary source assumptions are used for budgeting, identify them and the party responsible for replacing or accepting them.
Submit the motor-start duty with your transformer enquiry
Use HEAKOR's oil-immersed substation transformer page for the equipment enquiry and the transformer RFQ checklist for the wider specification. The technical document centre helps organise requests for configuration-specific information; the availability and scope of each document require confirmation.
Submit Your Project Specification with the single-line diagram, running-load and motor-start schedules, source information, required voltage criteria, transformer requirements and UK project destination. Ask the offer to state the available transformer data, assumptions and unresolved items. This enquiry does not imply that HEAKOR includes a system study, motor-start test or performance guarantee; any such scope and the final equipment suitability require written agreement.