Opening DC Bus on VFD

Kyle Katzmeric

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Madison
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Application Sales
I have an application where the customer requires a backup spare 150hp drive on standby in the same enclosure, the customer wants to use the same external braking transistors and resistors but requires the drives to be electrically isolated from each other. My concern is that in order to do this we would need to install large DC contacts and chokes to protect the drive in the event of the contactor opening up inadvertantley. Should I deter them from doing this and just add additional DB units and resistors or can DC contactors and chokes be added successfully?
 

petersonra

Senior Member
Location
Northern illinois
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engineer
I suspect it will be as cost effective to just add a set of brake resistors and transistors for the backup unit as opposed to screwing around with what they are proposing.

How would you interlock them with the contactors that isolate the primary drive?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Plus the added components and the ways they're configured just adds more possible mechanisms and components that can lead to a failure. Redundant standby systems have to be very carefully designed so that they make things better and not worse.

The transistor switches are effectively contactors themselves, but they are more sensitive to how they are driven and connected. So I'd be very leery about trying to use one transistor unit for both drives.
 

Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
Are they 480V VFDs? If so, wait until they get a look at DC contactors rated for 850VDC (the voltage at which the Dynamic Brake is triggered) at the current you need for the drives! If they survive that shock, then tell them the price!

On some drives you can, however, tie the DC busses together and SHARE the same braking resistors all of the time, there is no need to isolate them from each other. In addition or if you have drives that are not capable of tying the DC busses together, you can buy external brake modules from people like Bonitron to do this.
 
Last edited:

paulengr

Senior Member
Are they 480V VFDs? If so, wait until they get a look at DC contactors rated for 850VDC (the voltage at which the Dynamic Brake is triggered) at the current you need for the drives! If they survive that shock, then tell them the price!

On some drives you can, however, tie the DC busses together and SHARE the same braking resistors all of the time, there is no need to isolate them from each other. In addition or if you have drives that are not capable of tying the DC busses together, you can buy external brake modules from people like Bonitron to do this.
I thought that was most drives do this. Only thing is adding the DC rated fuses between drives.

This again gets into reliability getting killed by overdoing it but you can also get modular/parallel inverter drives at least from Schneider and ABB. They are made so that you can say make an 800 HP drive with four 200 HP ones but you can also add a hot spare or two.
 

Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
I thought that was most drives do this. Only thing is adding the DC rated fuses between drives.

This again gets into reliability getting killed by overdoing it but you can also get modular/parallel inverter drives at least from Schneider and ABB. They are made so that you can say make an 800 HP drive with four 200 HP ones but you can also add a hot spare or two.
There are some cheap drives out there that don't give you access to the DC bus (without disassembling the drive).
 

garbo

Senior Member
With the price of drives becoming lower then years ago I can not see any advantage of combining four 200 HP drives to run a large motor. I PM'd hundreds of drives, troubleshoot, replaced boards, IGBT'S, caps & replaced them. Think it could become a nightmare attempting to troubleshoot 4 interconnected drives.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
I have an application where the customer requires a backup spare 150hp drive on standby in the same enclosure, the customer wants to use the same external braking transistors and resistors but requires the drives to be electrically isolated from each other. My concern is that in order to do this we would need to install large DC contacts and chokes to protect the drive in the event of the contactor opening up inadvertantley. Should I deter them from doing this and just add additional DB units and resistors or can DC contactors and chokes be added successfully?
If I may dig deeper into what could happen when the drive motor is running at "cruising" speed. . .and the VFD crapped out unexpectedly.

The VFD failure would set in motion-- braking sequence --whether it is regen or dynamic braking or other braking scheme.

Now, you want the standby VFD to take over and to have a seamless transition as if everything is normal.
During this "failure" event the electronics of the standby unit is forced from deep sleep to full functionality in a split second. Motor still coasting etc.

I haven't seen a SYNERGY system design that will provide seamless functionality especially when two sources of energy are working together as one. . .the mechanical and electrical energy.
Save for the UPS (Uninterruptible Power Supply)

Some newer VFD motors have feedback signals to inform the VFD about the status or condition of the drive motor itself.

I understand the priority of the customer in terms of economic advantage. . . which of course could save him money.
But in the real world. . .what seems workable on paper doesn't work what we're made to believe.

My solution would be:
Set up two independent systems completely isolated from one another.

The resultant savings that the customer expect is certainly a benefit that cannot be denied.. .more dollars in our pocket is better.

This philosophy has been examined at length by Aristotle in his treatise that:

"The whole is greater than the sum of its parts."

The "WHOLE" portion in the treatise is the ROI. (the money)
Advantageous in terms of economic reason but not good when it comes to technology. Maybe even money-loser.

This is the basis of Synergy Design System in Electrical Engineering. Are you still awake ? lol

Capturing Aristotle's logic is not an easy reading.
Philosophy is not an easy subject if your aim is to reconcile it with Technology.
 

paulengr

Senior Member
With the price of drives becoming lower then years ago I can not see any advantage of combining four 200 HP drives to run a large motor. I PM'd hundreds of drives, troubleshoot, replaced boards, IGBT'S, caps & replaced them. Think it could become a nightmare attempting to troubleshoot 4 interconnected drives.
Big reason. How many 800 HP drives are stocked and ready to go? Who is going to do this considering market size?

ABB and Schneider are two that offer a modular system. You link together multiple VFDs that work as one large system. You can even add an extra redundant module.

So it makes all the sense in the world. Now they can “stock” any size VFD up to maybe 2000 HP. You can even have an inline “hot” spare. A “monolithic” 800 HP VFD isn’t going to have 6 gigantic IGBTs anyways. It will have several individual ones wired in parallel. This just puts them in separate enclosures and spreads out the cooling.
 

paulengr

Senior Member
If I may dig deeper into what could happen when the drive motor is running at "cruising" speed. . .and the VFD crapped out unexpectedly.

The VFD failure would set in motion-- braking sequence --whether it is regen or dynamic braking or other braking scheme.

Now, you want the standby VFD to take over and to have a seamless transition as if everything is normal.
During this "failure" event the electronics of the standby unit is forced from deep sleep to full functionality in a split second. Motor still coasting etc.

I haven't seen a SYNERGY system design that will provide seamless functionality especially when two sources of energy are working together as one. . .the mechanical and electrical energy.
Save for the UPS (Uninterruptible Power Supply)

Some newer VFD motors have feedback signals to inform the VFD about the status or condition of the drive motor itself.

I understand the priority of the customer in terms of economic advantage. . . which of course could save him money.
But in the real world. . .what seems workable on paper doesn't work what we're made to believe.

My solution would be:
Set up two independent systems completely isolated from one another.

The resultant savings that the customer expect is certainly a benefit that cannot be denied.. .more dollars in our pocket is better.

This philosophy has been examined at length by Aristotle in his treatise that:

"The whole is greater than the sum of its parts."

The "WHOLE" portion in the treatise is the ROI. (the money)
Advantageous in terms of economic reason but not good when it comes to technology. Maybe even money-loser.

This is the basis of Synergy Design System in Electrical Engineering. Are you still awake ? lol

Capturing Aristotle's logic is not an easy reading.
Philosophy is not an easy subject if your aim is to reconcile it with Technology.
Redundancy is a tricky thing. As you add complexity it decreases reliability, precisely the thing you were trying to fix. Not saying you want to catch a falling knife which is what you are talking about doing. But let’s look at a vastly simpler design. Put in semiconductor fuses. That stops a shorted IGBT or diode in its tracks. Now if you need say an “800 HP” drive and you out in a “1000 HP” version and simply run it if a component fails the fuses take it out of service in under 4 milliseconds. Since we were running all IGBTs before (cooler) now we have fewer working transistors but since we haven’t exceeded capacity it just works. A 1/4 cycle “hiccup” won’t affect motor operation.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
. . . Not saying you want to catch a falling knife which is what you are talking about doing. But let’s look at a vastly simpler design. Put in semiconductor fuses. That stops a shorted IGBT or diode in its tracks. Now if you need say an “800 HP” drive and you out in a “1000 HP” version and simply run it if a component fails the fuses take it out of service in under 4 milliseconds. Since we were running all IGBTs before (cooler) now we have fewer working transistors but since we haven’t exceeded capacity it just works. A 1/4 cycle “hiccup” won’t affect motor operation.
Wishful thinking does have a place in every endeavor that may experience difficulty ...that in essence will result in downtime. I don't see your argument on how you can work-around and achieve a seamless operation as OP has intended to do.

Regardless of the type fuses you have-- in a critical electronic circuitry-- it will have an impact on the overall operation.
Disabling an IGBT in its track will not guarantee a continued operation. Perhaps you need a better explanation in this regard.
A 1/4 cycle hiccup is more than enough for precision design machine to cause a major throw-up. I can probably figure out some of Aristotle's logic-- but a classic shibboleth coming from you is unbelievable.

You have a white paper to support that?
 

paulengr

Senior Member


Two examples that do exactly what I described using exactly the scheme I described. Not new technology. I don’t understand why you can’t understand it. These have existed for years. It’s not for reliability but 12, 18...36 pulse drives naturally load share over a lot more switching elements. ABB has done tons of papers on surviving energized failures and touting DTC as a simple solution to the challenge.

Trying to spin up a “hot spare” should be simple but I gave you an alternative where the drive can survive failures in service.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE


Two examples that do exactly what I described using exactly the scheme I described. Not new technology. I don’t understand why you can’t understand it. These have existed for years. It’s not for reliability but 12, 18...36 pulse drives naturally load share over a lot more switching elements. ABB has done tons of papers on surviving energized failures and touting DTC as a simple solution to the challenge.

Trying to spin up a “hot spare” should be simple but I gave you an alternative where the drive can survive failures in service.
During the late seventies when I got envolved in variable speed drives, no US manufacturers were heavily involved (in earnest) -- the design and manufacture of VFDs.
It wasn't even called VFD . . . It was called PWM.
At the time- I was a rookie engineer working with an engineer from Frankfurt, Germany on modernizing an old paper mill. Those on your catalog were latest entry in the band wagon.

PWM or VFD was invented in Finland by Martti Harmoinen in the early seventies.
At the time I was working with this German engineer in the paper mill-- no VFD distributors were at hand that we had to put together components on our own using the old hardy SCRs.
Finland was already using PWM on their street cars while US was still experimenting on them.
Reliance Motors was making bare bone mother boards to mount SCRs for a "put together" shop-made unit for primitive VFD.

I was involved on PWM/VFD much earlier than thirty years that you are talking about.
That's a brief history of VFD.

What I'm concerned mostly is your cavalier approach in proclaiming edicts that goes against the grain of sound engineering practice.
And you try to avoid the subject about "where is the source of info" that you can burn up a fuse and still keep the VFD running.
That's one touchy issue. And then you alluded to REDUNDANCY is a bunch of HOOEY.

Redundancy is subsumed in the design of avionics control system. Also applies in military equipment where survival takes the front seat.

As an example-- the attack warplane --the Warthog A10 also known as Thunderbolt is a survival piece of machinery. It is designed to destroy enemy tanks.

Because of its design for deployment in close - to- ground enemy fire--it can continue to fly for a safe landing even if the fuselage is almost blown apart.

The control system components is housed in a titanium enclosure and can take enemy fire from below.

An ugly looking airplane but if you're the pilot --you will feel assured of better chance of survival and get home alive because of its extensive redundant features.
In fact the aircraft's redundant features include its ability to return to airbase with half its wings missing.

I was at a sub-contractor's repair/assembly plant in Arizona where the fuselage was assembled at a maintenance plant.
I serviced their CNC Machines there years ago.

This is an example of the value of redundant design. . .it is not a decrease in reliability.
It is designed keep people alive.

......
 
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