24-Pulse & Multi-Pulse Phase-Shifting Transformers
Phase-shifting transformers that feed several rectifiers so low-order harmonic currents cancel before they reach the plant supply.
More Pulses, Fewer Harmonics
A multi-pulse transformer has several secondaries, each phase-shifted from the others, and each feeding its own 6-pulse rectifier. A 12-pulse design uses two secondaries 30 degrees apart, 18-pulse uses three 20 degrees apart, and 24-pulse uses four 15 degrees apart. The more pulses, the more of the low-order harmonics cancel at the primary.
24-pulse is used on large drives, DC power supplies, electrolysis and furnace rectifiers, and medium-voltage drives, where the load is big enough that the extra transformer cost pays back in lower distortion. It is often built as an isolation transformer, or as two 12-pulse transformers with a 15 degree offset. For most low-voltage drives, an 18-pulse autotransformer is the smaller choice. Compare options with the harmonic mitigation selector.
Where Multi-Pulse Transformers Fit
- ■ Large VFDs and MV drives: Where one drive dominates the plant harmonic load.
- ■ DC power supplies and rectifiers: Electrolysis, plating, and DC bus supplies.
- ■ Induction and furnace rectifiers: High-power melt and heating supplies.
- ■ Mining and pumping: Large pumps, conveyors and hoists.
- ■ Data center and BESS: Large rectifier and converter loads.
12-, 18- and 24-Pulse Compared
| Pulse number | Phase shift between secondaries | Lowest characteristic harmonics | Points to weigh |
|---|---|---|---|
| 6-pulse (no phase shift) | n/a | 5th, 7th | Simplest; highest distortion |
| 12-pulse | 30 degrees (2 secondaries) | 11th, 13th | Common on large drives and rectifiers |
| 18-pulse | 20 degrees (3 secondaries) | 17th, 19th | Often an autotransformer for LV drives |
| 24-pulse | 15 degrees (4 secondaries, or two 12-pulse units) | 23rd, 25th | Lowest distortion; largest and most costly |
What to send for a fast, accurate quote
- Load type: drive, rectifier, furnace, or DC supply
- kVA or kW, input voltage and frequency
- Pulse number required or the harmonic target at the PCC
- Isolation needed, or autotransformer acceptable
- Rectifier configuration and secondary voltages
- Enclosure, cooling, ambient and altitude
Sizing a Multi-Pulse Transformer?
Send the load data and harmonic target. We route it to a manufacturer that builds phase-shifting transformers and come back with a quotation.
Open the harmonic spec sheet Contact usMulti-pulse transformers: common questions
What is the difference between 18-pulse and 24-pulse?
18-pulse uses three rectifiers fed 20 degrees apart, so the lowest characteristic harmonics are the 17th and 19th. 24-pulse uses four rectifiers fed 15 degrees apart, pushing them to the 23rd and 25th. 24-pulse gives lower distortion but needs a larger transformer and more rectifier hardware.
Is a 24-pulse transformer an autotransformer?
It can be, but large 24-pulse systems are usually isolation transformers, or a pair of 12-pulse transformers with a 15 degree offset between them, which also gives galvanic isolation.
Do multi-pulse transformers work with an unbalanced supply?
Harmonic cancellation depends on balanced voltages. Supply imbalance and background distortion reduce performance, so ask for expected results under your measured site conditions.
When is a harmonic filter better than a multi-pulse transformer?
Where there are many small drives instead of one large one, where the drive has no multi-rectifier input, or on retrofits with no room for a large transformer, an active or passive filter is often more practical.
Does AMP Magnetics build these transformers?
No. AMP Magnetics is an independent manufacturer's representative. We help you specify the requirement and route it to a manufacturer suited to build it, who confirms ratings and delivery on the quotation.