Matrix Converter (AC to AC VFD)

[elec_eng]

I have a project where active front end (AFE) VFDs were specified due to the harmonic concerns, and the contractor submitted the Matrix Converter VFD (AC to AC). It is Yaskawa Z1000U model. I am not familiar with this Matrix converter at all and I have done some research but it seems very complicated and way over my heads. Have you ever had any experience with the Matrix converters? What are the typical application for this? Any drawbacks or limitations?

My application is HVAC fan control, fan wall system to be exact. Is the matrix converter is right for the HVAC application?

Any help would be appreciated.

 

[Ingenieur]

They say it is specifically designed for hvac fan applications
and has low thd

did your spec contain any performance specs?
thd, etc???
IEEE stds?
efficiency?

 

[elec_eng]

They say it is specifically designed for hvac fan applications
and has low thd

did your spec contain any performance specs?
thd, etc???
IEEE stds?
efficiency?

Per their spec it is specifically designed for the HVAC applications. You can google the Yaskawa Z1000U VFD. They are supposed to meet IEEE 519 and seems like they meet or exceed in some aspects, the every requirements.

I guess I am not just familiar with this new technology??

 

[Jraef]

In a nutshell:
A Matrix Converter is not all that new, there have been working versions since around 2001. The major difference is that instead of doing the AC-to-DC-back-to-AC that a conventional PWM drive does, the Matrix Converter just directly fires the AC power devices in a pattern that changes the relative output frequency and voltage (within it's limits) to the motor as AC. So yes, it's more complex, but at the same time, less complex, depending on your point of view.

Advantages:

• No diode bridge rectifier, so no harmonics created, so no harmonic mitigation is required.
• No DC bus, so no capacitors, so no capacitor failures, no issues of shelf life.
• Inherently line regenerative, which is good because without a DC bus, there is no possibility of Dynamic Braking.

Disadvantages:

• Even though there are no capacitors, there are a lot more power semiconductor components (IGBTs). The technology requires the use of 9 "bi-directional switches", but there are no bi-directional switches yet, so to make it work, they use 2 mono-directional switches, IGBTs, in inverse parallel pairs for each one. That means a Matrix Converter has 18 IGBTs in it, with 18 sensing and 18 firing circuits, all of which represent points of failure. So in general, they have about a 10% lower MTBF (Mean Time Between Failure) rating than a 6 pulse inverter drive, but about equal to an 18 pulse drive (because of the higher component count for an 18 pulse drive).
• They are more subject to damage and failure if there is an input voltage disturbance, which now is unfortunately becoming more and more common.
• The output voltage is limited to something like 87% of the input voltage, so to make them work for full speed / torque, you must have a boost transformer ahead of them. The proponents tend to avoid discussing this, but in their topology you will see an "AC Filter" box. That box includes the transformer to boost the line to compensate for the loss. It's basically the same as adding a line reactor ahead of a conventional drive, but in their sales literature, they go on saying how you don't need a reactor. OK, semantics... but it does still represent added cost, size and heat. Again, compared to an 18 pulse drive, it's better, but compared to a 6 pulse drive, it's about the same (from a size standpoint).
• They generally only make sense at 100HP and larger. They make them in smaller sizes of course, but in small sizes, the harmonic mitigation benefit is not as important in many cases and/or can be done less expensively (Matrix Converters are not cheap).
• Only two (related) Japanese manufacturers have pursued the technology (Yaskawa and Fuji), even though EVERY other mfr looked at the technology when it came out of Germany almost 2 decades ago. Why just them? I think because the Japanese control the world's pricing of IGBTs, so for them, they can make the economics look like whatever they need to. But the technology just didn't add up for most mfrs. So with only two suppliers, down time can be a real problem. If you use them, buy spares up front as part of the initial purchase. Remember, no shelf life.
• With no DC Bus there is no DC Injection Braking (DC Hold) either, so "anti-windmilling" of fans (braking before ramping up) is something they have more trouble with.

Bottom line: IF you are concerned for harmonics on a project where you have multiple large drives, it's something to consider compared to other available technologies*. I personally have had better success using smaller less expensive standard 6 pulse drives and adding an Active Harmonic Filter. The reason is, correcting harmonics at every drive works to mitigate the harmonics caused BY those drives, but does nothing for harmonics caused by OTHER equipment with power supplies, which is almost everything electronic now. An Active Harmonic Filter doesn't care where it came from, it just measures the harmonics and counteracts it.

*Other available technologies are growing rapidly too. You will see a LOT more Active Front End (AFE) PWM drives released in the next year or two, virtually every mfr will have one available.

 

[junkhound]

re: Disadvantages:


+1, amen.
Plus, if you try to build a matrix converter for 400 Hz or 800Hz, IGBTs cannot switch fast enough and one needs MOSFETS.
For an 800 Hz (or 3600 Hz soon to be in aircraft) matrix converter, the switching noise and timing get to be pretty troublesome.
Noise and timing manageable for 60 Hz switching at say just 20 kHz.

Biggest advantage is bidirectional operation.

 

[elec_eng]

In a nutshell:
A Matrix Converter is not all that new, there have been working versions since around 2001. The major difference is that instead of doing the AC-to-DC-back-to-AC that a conventional PWM drive does, the Matrix Converter just directly fires the AC power devices in a pattern that changes the relative output frequency and voltage (within it's limits) to the motor as AC. So yes, it's more complex, but at the same time, less complex, depending on your point of view.

Advantages:

• No diode bridge rectifier, so no harmonics created, so no harmonic mitigation is required.
• No DC bus, so no capacitors, so no capacitor failures, no issues of shelf life.
• Inherently line regenerative, which is good because without a DC bus, there is no possibility of Dynamic Braking.

Disadvantages:

• Even though there are no capacitors, there are a lot more power semiconductor components (IGBTs). The technology requires the use of 9 "bi-directional switches", but there are no bi-directional switches yet, so to make it work, they use 2 mono-directional switches, IGBTs, in inverse parallel pairs for each one. That means a Matrix Converter has 18 IGBTs in it, with 18 sensing and 18 firing circuits, all of which represent points of failure. So in general, they have about a 10% lower MTBF (Mean Time Between Failure) rating than a 6 pulse inverter drive, but about equal to an 18 pulse drive (because of the higher component count for an 18 pulse drive).
• They are more subject to damage and failure if there is an input voltage disturbance, which now is unfortunately becoming more and more common.
• The output voltage is limited to something like 87% of the input voltage, so to make them work for full speed / torque, you must have a boost transformer ahead of them. The proponents tend to avoid discussing this, but in their topology you will see an "AC Filter" box. That box includes the transformer to boost the line to compensate for the loss. It's basically the same as adding a line reactor ahead of a conventional drive, but in their sales literature, they go on saying how you don't need a reactor. OK, semantics... but it does still represent added cost, size and heat. Again, compared to an 18 pulse drive, it's better, but compared to a 6 pulse drive, it's about the same (from a size standpoint).
• They generally only make sense at 100HP and larger. They make them in smaller sizes of course, but in small sizes, the harmonic mitigation benefit is not as important in many cases and/or can be done less expensively (Matrix Converters are not cheap).
• Only two (related) Japanese manufacturers have pursued the technology (Yaskawa and Fuji), even though EVERY other mfr looked at the technology when it came out of Germany almost 2 decades ago. Why just them? I think because the Japanese control the world's pricing of IGBTs, so for them, they can make the economics look like whatever they need to. But the technology just didn't add up for most mfrs. So with only two suppliers, down time can be a real problem. If you use them, buy spares up front as part of the initial purchase. Remember, no shelf life.
• With no DC Bus there is no DC Injection Braking (DC Hold) either, so "anti-windmilling" of fans (braking before ramping up) is something they have more trouble with.

Bottom line: IF you are concerned for harmonics on a project where you have multiple large drives, it's something to consider compared to other available technologies*. I personally have had better success using smaller less expensive standard 6 pulse drives and adding an Active Harmonic Filter. The reason is, correcting harmonics at every drive works to mitigate the harmonics caused BY those drives, but does nothing for harmonics caused by OTHER equipment with power supplies, which is almost everything electronic now. An Active Harmonic Filter doesn't care where it came from, it just measures the harmonics and counteracts it.

*Other available technologies are growing rapidly too. You will see a LOT more Active Front End (AFE) PWM drives released in the next year or two, virtually every mfr will have one available.

Jraef,

Thanks for the summary. This is very helpful.

As with any other technologies, it offers no advantages without drawbacks. That was very much expected. However, I still have so many other questions and I would appreciate your input on them.

1. So it seems Matrix converter (MC) is very comparable to AFE. You mentioned the cost of IGBT was the blame for the less popularity of MC (although Yaskawa managed to beat their comparator’s price in my case). If the cost is not an issue, would you choose the MC over AFE? I wouldn’t think Yaskawa decides to go with MC over AFE only because of their price advantage on IGBTs??

2. Is the MC better technology than AFE? (It seems MC can offer everything AFE does and more?)

3. Does MC offer better efficiency?

4. I assume MC has the same UL listing and the requirement for the CB and conductor sizing wouldn’t be different from any other VFDs?

5. You mentioned MC is more subject to damage and failure if there is an input voltage disturbance. What do you have to do to mitigate that?

6. What kind of spares would you need for MC?

 

[Ingenieur]

What HP range are we discussing?
what percentage of the total bldg load are the vfd's?
private or public money?
were acceptable mfgs listed in the spec? Approved equivilents acceptable?

these look like a good fit for your application
they were voted Consulting-Specifying Engineering product of the year
https://www.yaskawa.com/pycprd/newsdetails/item&2016-z1000u-product-of-year&1
granted, cost is a factor, but a technically problematic product would not be selected

imho I would not get too bogged down in the technicalities/architecture
it looks well suited for hvac applications in the 60 Hz range

 

[Jraef]

Jraef,

Thanks for the summary. This is very helpful.

As with any other technologies, it offers no advantages without drawbacks. That was very much expected. However, I still have so many other questions and I would appreciate your input on them.

1. So it seems Matrix converter (MC) is very comparable to AFE. You mentioned the cost of IGBT was the blame for the less popularity of MC (although Yaskawa managed to beat their comparator’s price in my case). If the cost is not an issue, would you choose the MC over AFE? I wouldn’t think Yaskawa decides to go with MC over AFE only because of their price advantage on IGBTs??

2. Is the MC better technology than AFE? (It seems MC can offer everything AFE does and more?)

3. Does MC offer better efficiency?

4. I assume MC has the same UL listing and the requirement for the CB and conductor sizing wouldn’t be different from any other VFDs?

5. You mentioned MC is more subject to damage and failure if there is an input voltage disturbance. What do you have to do to mitigate that?

6. What kind of spares would you need for MC?

1. Most companies that have already released an AFE drive are price competitive at 200kW and up, but don't have a good solution yet for smaller loads if you want something like an AFE or Matrix Converter. That gives the MC the edge because although I have seen a couple of quotes for small AFE drives from ABB, they are really very expensive still. Right now, I think ABB is the only one available under 200kW so far (as far as I know); Schneider has one down to 55kW, not UL listed yet; Vacon/Emotron/Danfoss/Graham (all merged now) says they have one down to 55kW too, but again, no UL listing. Siemens has one for HVAC, but it is an FFE drive, not really AFE, and those come with other issues. A-B is releasing an AFE drive next month at Automation Fair but starts at 250HP. So for smaller HP sizes, MC may have the pricing edge for the time being. BUT, in most cases if you compare a group of smaller MC drives to a group of standard off-the-shelf 6 pulse drives + an AHF unit, the 6-pulse + AHF will probably come out less and in my opinion is a more flexible / reliable solution.

2. Is it "better"? I don't know. Judgement call really. I'm not a big fan of technology for technology's sake. They are a solution to a problem that already had solutions. Is it a better solution? Who am I to say?

3. Better efficiency? Again, I don't know. To paraphrase Mark Twain / Bejamin Disraeli, "There are lies, damned lies, and efficiency ratings". They claim higher efficiency that "other low harmonic solutions", without further explanation.

4. I can't see how or why the SCPD issues would be any different for an MC drive vs a conventional VFD. they are still "power conversion equipment" per the NEC.

5. I don't know how one would mitigate the risk of damage from line disturbances to be honest. I'm very familiar with how conventional VFDs are affected and why line reactors mitigate that, but I'm not clear on the risks to MCs, just that it is substantially higher by report from some industry white papers I read. It makes sense, being that the transistors are on the AC line and that is always problematic, but it might be that the "AC filter" assembly they need to boost voltage serves a similar function in protecting them as a line reactor does in a rectifier. I'm just not that fluent in the deep details, my exposure to them was in evaluating them for consideration a few years ago.

6. My point on spares was that because there are so few suppliers and from my experience, even Yaskawa and Fuji themselves don't stock these drives, you should carry one complete drive of every frame size that you use in spares inventory. It could take weeks to get a replacement. That too is a judgement call, but I've done something like that before and when equipment is down and YOU are the guy responsible for choosing an oddball product that takes weeks to get a replacement for, you had better have a spare unit to drop in or a resume ready to go...

 

[elec_eng]

Jraef, Thank you for your insight. much appreciated.