How does an 18-pulse drive work?

From an individual drive standpoint, or even thinking about it as a system approach for larger drives, makes a lot of sense.

18-pulse drives are what we call clean power drives. An 18-pulse drive has in it a front-end rectifier that has 18 diodes for switching from AC to DC. In this case, we also have a large line reactor that acts as a choke to limit the harmonic currents coming out of the drive. What we ultimately end up with is a significant reduction in harmonic current.

When you have an 18-pulse drive, you will see that the hertz and amperes on your system will increase, causing the distortion to be high. For example, you can go from 30 Hz at 8.2 amps up to 60 Hz at 10.8 amps. In the THD category, at 30 Hz, you’re running at 26.y% distortion. The distortion is significantly higher because the actual 60 Hz is low at that point. But, the actual amps of harmonic current that is doing the damage increases with load. It just doesn’t increase at the same rate as the 60 Hz current.

To describe in more detail, say we have an 18-pulse drive and we run it up to 60 Hz at a full load. When we do this and look at the waveform, we’re going to notice that the waveform is pretty clean. For 18-pulse, we’re going to have 18 plus or minus 1 harmonics, NP plus or minus 1. We’re going to expect 17th and 19th (36). So, two times 18 (36) plus minus 1, gives us 35th and 37th. Right away if we look at the current, we see the 17th harmonic is the most predominant. If we look at the actual amps of 17th, we’ll see that it’s right around 2.6 amps of 17th harmonic.

For this same example, if the actual load on the drive is reduced and we look at the THD, it shows roughly 6.3% distortion. Then, as we decrease the load more substantially, the THD starts to really go up. That’s where panic can set in based on the total harmonic current percentages. But if we go back to the 17th harmonic and look at how many actual amps of 17th harmonic we have, it’s hardly anything—1.0 amps of 17th.  When the speed is increased all the way up again, the actual value is going down but the actual amps are going up. This is important to realize with 18-pulse drives and total demand distortion vs. total harmonic distortion.

As we focus on how these drives work and where they’re important, say you have a drive, for example, that is 150 hp or larger, this is really the way to go because what you have is all that filtering and all the harmonic reduction benefit built into the drive. From a cost standpoint, this is more cost-effective than adding on additional filters and other types of solutions on top of the drive at that size. That’s why it makes sense to use 18-pulse drives—clean power drives.