Mine Hoist Transformers: Use RMS Cycle Screening Without Hiding Hot Spots

1. Define the duty

Use the hoist’s winding-current history to screen average load losses, then use a transformer thermal model to check winding hot spots and operating limits. A signed average of motoring and regenerative current is unsuitable: losses depend on current squared. Conversely, a maximum motor shaft power can overstate average heating while failing to describe converter winding currents. The transformer is one part of the hoist drive package; ABB’s Kiruna system shows separate main and excitation transformers and a four-quadrant converter. That architecture is an example, not a universal hoist design.

Primary references — ABB Kiruna hoist system

2. Required design inputs

Ask for current versus time at each transformer winding, voltage, speed, torque, payload, travel distance, acceleration, braking and dwell durations. Identify the busiest sustained production sequence, not merely an average annual cycle. Include commissioning, balancing, inspection, abnormal recovery and repeated restart cases. State harmonic spectra at representative speeds, ambient temperature, cooling stage, altitude and the initial thermal condition. Provide the sequence order because identical RMS values can produce different short-term temperature peaks.

3. Illustrative engineering example

Illustrative balanced 120 s cycle at one winding: 400 A for 20 s acceleration, 250 A for 50 s travel, 300 A for 20 s regenerative braking and 40 A for 30 s dwell. Use magnitudes even during braking. The calculation below gives 261.0 A equivalent current. Compared with a 300 A rated reference, the simple I²R load-loss ratio is 0.757. This is a screening result at assumed constant resistance, not permission to select a 261 A winding or overload a 300 A design.

4. Compare failure cases

Compare the repeating production cycle with consecutive high-payload trips and an interrupted cooling system. RMS aggregation erases the chronological heating and cooling that determines a hot spot. The long-term oil temperature and the faster winding response do not share one time constant. IEC 60076-7 addresses mineral-oil loading, ambient conditions and thermal ageing; select the correct model for the actual insulation and cooling technology instead of transplanting mineral-oil parameters into a dry transformer.

5. Procurement evidence

Require the supplier to provide the evaluated duty trace, winding loss assumptions, harmonic allowances, cooling controls, thermal constants or justified model inputs, peak predicted temperatures and margins against the agreed operating limits. Obtain a separate statement for peak current and short-circuit mechanical withstand. A low equivalent current does not demonstrate adequate converter commutation performance or voltage regulation during acceleration. Agree the allowable sequence when a fan or pump is unavailable.

Primary references — IEC 60076-7:2018

6. Commissioning and operation

At commissioning, time-align transformer current, temperature indications, cooling status and hoist cycle records. Validate that the actual trace falls inside the approved envelope; discrepancies should return to the supplier model. Routine winding sensors do not necessarily measure the hottest conductor location, so compare their expected behavior rather than assuming equality. Preserve a representative production record for later payload or cycle-rate changes. Thermal protection must coordinate with the hoist stopping strategy and cannot be tuned from the RMS example alone.

7. Frequently asked questions

Is RMS a hot-spot model? No. It screens average current-squared loss only.

Does braking cancel heating? No. Regenerative winding current still creates losses.

8. References and related guidance

• Primary references — ABB Kiruna hoist system
• Primary references — IEC 60076-7:2018
• Primary references — ABB power regeneration

Related engineering guidance:

• Winding hot-spot modeling
• Regenerative conveyor duty