A crane or hoist transformer should be assessed against the motion cycle and the drive’s energy path. Lifting, lowering, braking, reversing and idle periods can impose different electrical states. Regenerative mechanical operation does not automatically mean energy returns through the transformer: the drive may export upstream, exchange energy on a common direct current (DC) link or dissipate it locally.
Trace the drive architecture before assigning reverse power
Identify the front-end converter, braking arrangement and any shared DC bus. Ask the drive supplier which operating states produce upstream active-power export and which do not. Keep motor-side behavior separate from transformer-side current.
Converter-transformer application guidance addresses specialized converter interfaces within its scope. General transformer application guidance covers the broader rating and operating context. [1] [2]
A single motor power rating cannot describe several drives sharing one transformer. Include travel, trolley, lifting and auxiliary loads where applicable, and identify which motions can occur simultaneously.
Preserve the cycle rather than averaging it away
Record current or P-Q demand for each motion state, its duration and repetition. Include the relevant load range and the intended operating schedule. An average energy figure can be small when export offsets import while winding current remains significant in both directions.
For an illustrative loss-free energy account, importing 100 kWh while lifting and returning 40 kWh during lowering gives 60 kWh net import. It does not mean that the transformer experienced only 60% of the associated current duty. The separate intervals and current magnitudes remain necessary for thermal assessment.
The appropriate loading framework should use the actual construction, ambient conditions and chronological sequence. Mineral-oil loading guidance is one defined framework, not a universal crane-duty multiplier. [3]
Include the states that change coincidence
| Motion or system state | Question to resolve |
|---|---|
| Normal lifting cycle | Which drives and auxiliaries operate together? |
| Regenerative lowering | Where does the recovered energy go? |
| Repeated reversals | What current sequence and recovery interval occur? |
| One drive unavailable | Does the remaining operating pattern change? |
| Idle | Which equipment remains energized or thermally active? |
Do not assume favorable cancellation between one lifting crane and another lowering crane unless the electrical architecture and operational control support that assumption. A shared transformer can see different aggregate states as crane positions and workloads change.
The drive input spectrum should also be supplied at relevant operating points. Harmonic guidance for nonlinear equipment supports using actual converter data rather than a generic intermittent-load description. [4]
Specify mechanical and electrical boundaries separately
A mobile or vibrating crane installation may impose environmental and mechanical duties on the transformer in addition to the electrical cycle. Those conditions need their own support, mounting and exposure data; the duty-cycle calculation does not qualify the physical installation.
The procurement package should include the motion sequence, converter topology, simultaneous-drive matrix, current spectra and site conditions. Ask the supplier to identify the cases governing continuous, temporary and repeated operation.
Acceptance should preserve the implemented drive settings and operating restrictions used in the assessment. Enabling upstream regeneration or changing the permitted simultaneous motions can alter transformer duty without changing the motor nameplates.
The correct specification follows both current and energy through the complete motion cycle. “Intermittent duty” and “regenerative” are useful descriptions only when their actual timing and electrical paths are defined.
Core procurement checkpoint: Do not use net regenerative energy to specify the core
Crane and hoist energy returned to a bus does not cancel the individual current duties of their transformer windings. The transformer manufacturer should approve the operating and magnetic design before core procurement. Retain the selected voltage-frequency basis, geometry and assembly interface in the order, while keeping regenerative controls, intermittent loading and complete-unit thermal assessment outside the core supply claim.
For the core enquiry: Approved duty reference; core drawing; voltage-frequency basis; steel specification; delivery state and inspection requirements.