Most variable frequency drives create harmonics because their standard six-pulse rectifier front end draws current in choppy pulses. One way to attack that at the source, rather than filtering it afterward, is to build the rectifier out of multiple bridges fed by phase-shifted supplies so that the worst harmonics cancel each other before they ever leave the drive. Twelve-pulse and eighteen-pulse drives do exactly this. This guide explains how a phase-shifting transformer and multiple rectifier bridges cancel the dominant harmonics, what each pulse number achieves, and where a multipulse drive fits against filters and reactors.
12-Pulse Rectifier Drive in one line: A 12-pulse or 18-pulse rectifier drive is a variable frequency drive whose front end uses two or three rectifier bridges fed by phase-shifted windings of a special transformer, so that the harmonics each bridge produces partly cancel at the input. A standard six-pulse drive produces strong fifth and seventh harmonics; a 12-pulse arrangement largely cancels those, and an 18-pulse arrangement cancels more, reducing harmonic distortion at the source rather than removing it with a downstream filter.
A conventional drive uses a six-pulse rectifier, a single bridge of six devices, which draws current in a distorted pattern rich in the fifth, seventh, eleventh, and thirteenth harmonics, with the fifth and seventh being the largest and most troublesome. The whole idea of a multipulse drive is to split the rectification across more than one bridge and feed those bridges from supplies that are shifted in phase relative to one another, so that certain harmonics produced by one bridge arrive in opposition to the same harmonics from another and sum to nearly zero.
A 12-pulse drive uses two six-pulse bridges fed from two sets of transformer windings shifted thirty electrical degrees apart, typically a delta and a wye secondary. With that thirty-degree shift, the fifth and seventh harmonics from the two bridges are in opposition and largely cancel, so the lowest surviving harmonics of significance become the eleventh and thirteenth. Because the eleventh and thirteenth are both higher in order and smaller in magnitude, the overall current distortion drops substantially compared with a six-pulse drive.
An 18-pulse drive extends the same principle to three six-pulse bridges fed from windings shifted twenty degrees apart, which cancels the harmonics through the eleventh and thirteenth and leaves the seventeenth and nineteenth as the lowest significant ones. Each step up in pulse number pushes the surviving harmonics higher in order and lower in magnitude, so an 18-pulse drive achieves lower distortion than a 12-pulse drive, at the cost of a larger and more complex phase-shifting transformer and more rectifier hardware.
The distinguishing virtue of a multipulse drive is that it prevents the dominant harmonics from being produced in the first place, rather than generating them and then trying to remove them. A line reactor or a passive harmonic filter, by contrast, sits in front of an ordinary six-pulse drive and attenuates the harmonics after the fact. Both approaches are legitimate, but they solve the problem in different places, and the choice depends on the size of the drive, the strictness of the harmonic limit, and how much space and capital are available.
Multipulse front ends shine on large, continuously running drives where the harmonic burden is significant and a clean input is worth the added transformer cost, which is common in oil and gas on big pumps, compressors, and electric submersible pump surface drives. The phase-shifting transformer they require is a real piece of equipment with cost, weight, and footprint, so the economics favor multipulse where the drive is large and the alternative would be an equally substantial filter. For smaller drives, a line reactor or a modest passive filter is usually the more economical route.
It is worth noting that multipulse drives are one of several source-side approaches and are not always the lowest-distortion option available. Active front-end drives, which use switching devices on the input to actively shape the current nearly sinusoidal, can achieve even lower distortion than an 18-pulse arrangement and can also correct displacement power factor, though at higher cost and complexity. The engineering decision balances the required harmonic limit against capital cost, efficiency, footprint, and the strength of the supply the drive connects to.
Specifying a 12-pulse or 18-pulse drive is a promise about harmonic performance, and that promise is only kept if the phase-shifting transformer, the bridges, and their balance stay healthy. A blown fuse on one bridge, an unbalanced transformer, or a device failure degrades the cancellation and lets the fifth and seventh harmonics reappear, quietly undoing the very benefit the drive was bought for. Because the drive keeps running, that degradation is invisible without measurement.
This is where continuous power-quality monitoring closes the loop. A meter at the drive input or at the facility bus reports current distortion and the individual harmonic magnitudes, and on a properly functioning multipulse drive the fifth and seventh harmonics stay suppressed. A rise in those specific low-order harmonics is a direct symptom that the cancellation has broken down, pointing an engineer straight at the rectifier or its transformer rather than at a vague harmonic complaint.
A cloud SCADA platform such as Merobix can trend the input harmonic content and drive status from power-quality meters and drive interfaces across a fleet of remote sites, so an operator can confirm that a low-harmonic drive is actually delivering low harmonics. Catching a degraded 12-pulse or 18-pulse front end early keeps a facility inside its harmonic limits and protects the investment made in the multipulse hardware, rather than discovering the problem when the utility flags rising distortion.
It uses two six-pulse rectifier bridges fed from two transformer windings shifted thirty electrical degrees apart, typically a delta and a wye secondary. With that phase shift, the fifth and seventh harmonics produced by the two bridges arrive in opposition and largely cancel, leaving the eleventh and thirteenth as the lowest significant harmonics. Because those surviving harmonics are higher in order and smaller in magnitude, the overall current distortion is much lower than a standard six-pulse drive.
A 12-pulse drive uses two rectifier bridges and cancels the fifth and seventh harmonics, leaving the eleventh and thirteenth. An 18-pulse drive uses three bridges fed from windings shifted twenty degrees apart, cancelling harmonics through the thirteenth and leaving the seventeenth and nineteenth as the lowest significant ones. The 18-pulse arrangement achieves lower harmonic distortion because the surviving harmonics are higher in order and smaller, at the cost of a larger phase-shifting transformer and more hardware.
Multipulse drives make the most sense on large, continuously running drives where the harmonic burden is significant and a clean input justifies the added transformer cost, since they prevent the dominant harmonics rather than filtering them after the fact. For smaller drives a line reactor or a passive filter is usually more economical. Where the strictest distortion limits apply, an active front-end drive can achieve even lower distortion than an 18-pulse arrangement, so the choice balances the limit against cost and footprint.
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