18-Pulse Autotransformers
Phase-shifting autotransformers that feed multi-pulse drive rectifiers to cancel low-order harmonic currents at the source.
Cancelling Harmonics at the Rectifier Instead of Filtering Them Afterwards
An 18-pulse autotransformer produces three sets of three-phase voltages shifted by 20 degrees from one another. Each set feeds its own 6-pulse rectifier. When the three rectifier currents combine at the transformer primary, the 5th, 7th, 11th and 13th harmonic currents largely cancel, leaving the 17th and 19th as the lowest characteristic harmonics.
Because it is an autotransformer, it carries only part of the load power magnetically and is smaller than an isolation design of the same rating, but it gives no galvanic isolation. It needs a drive with three rectifier inputs or an external 18-pulse rectifier assembly. Compare options on our harmonic filters page or try the harmonic mitigation selector.
Where 18-Pulse Solutions Fit
- ■ Large VFDs: Pumps, fans and compressors where one large drive dominates the harmonic load.
- ■ IEEE 519 specifications: Projects that call for low current distortion at the drive or the PCC.
- ■ Generator-backed sites: No capacitors, so no leading power factor at light load.
- ■ Drive OEM packages: Integrated 18-pulse drive lineups and engineered drive systems.
- ■ Medium-voltage and multi-pulse retrofits: Phase-shifting transformers for multi-pulse rectifier systems.
18-Pulse Compared With Other Options
| Approach | How it works | Points to weigh |
|---|---|---|
| 18-pulse autotransformer | Phase-shifted rectifier inputs cancel low-order harmonics | No capacitors; sensitive to supply voltage imbalance and background distortion; needs a multi-pulse rectifier |
| 12-pulse (30 degree) transformer | Two rectifiers, cancels 5th and 7th | Simpler; leaves the 11th and 13th |
| Passive tuned filter | LC branches absorb specific harmonics | Lower cost; leading power factor at light load; resonance check needed |
| Active harmonic filter | Injects cancelling current | Holds performance across the load range; electronics to maintain |
| Active front end drive | IGBT rectifier draws near-sinusoidal current | Can regenerate; drive-integrated; needs line filtering for switching ripple |
What to send for a fast, accurate quote
- Drive make, model, horsepower and input current
- Whether the drive has three rectifier inputs or needs an external rectifier
- Supply voltage, frequency and measured voltage imbalance if known
- Isc at the point of common coupling and the harmonic target
- Enclosure, ambient and mounting
- Generator operation
Evaluating 18-Pulse for a Drive Project?
Send the drive data and the harmonic requirement. The harmonic filter spec sheet includes an 18-pulse option.
Build a harmonic filter spec sheet Contact us18-pulse autotransformers: common questions
How does an 18-pulse autotransformer reduce harmonics?
It creates three three-phase supplies shifted 20 degrees apart, each feeding a 6-pulse rectifier. Combined at the primary, the 5th, 7th, 11th and 13th harmonic currents largely cancel, so the lowest characteristic harmonics are the 17th and 19th.
What current distortion does an 18-pulse system achieve?
Published figures for well-designed 18-pulse systems are commonly in the single digits of percent THDi at full load. Actual results depend on supply voltage imbalance, background voltage distortion and load level, so ask for the expected performance under your site conditions.
What is the difference between an 18-pulse autotransformer and an isolation transformer?
An autotransformer shares windings between input and output, so only part of the power is transformed magnetically. That makes it smaller and more efficient, but it provides no galvanic isolation. A phase-shifting isolation transformer is larger and isolates the drive from the supply.
Can an 18-pulse autotransformer be added to any drive?
Not directly. It needs three separate rectifier inputs. Some drives are built with them; for others, an external 18-pulse rectifier feeds the drive DC bus. Check that the drive supports DC bus input and that the manufacturer approves the arrangement.