DC LINK 1

[Lc85]

Can anyone please explain exactly what the function of the DC link in an inverter drive is?

Thanks

 

[VEBill]

Presumably it's the DC power rail between the power supply section and the inverter section.

Since this is outside my area, standby for someone else to confirm or correct.

 

[Noway2]

VE1BLL is correct. The "DC Link" is the common name for the DC portion of a UPS or Variable Frequency Drive. It is the section after the rectifier and before the inverter. It can be a very dangerous part of the circuit as it is often high voltage at DC and may have a lot of storage potential due to the capacitors that are placed on it to filter out ripple in the DC voltage. If the application is a UPS, it may also have batteries attached to it, in which case, it must be treated with utmost care because a bank of batteries can deliver staggering amounts current in a very short period of time.

 

[DRWeig]

Here's the very basic, consistent with VE1BLL and Noway2 comments:

AC power at fixed frequency (50 or 60 Hz) ==> rectify to DC ==> DC Link ==> invert to AC at variable frequency



Best to you,

Goober Dave

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[Lc85]

Thanks for the replies but I'm still not sure how this smoothing works? Also can items be powered from the dc link?

Thanks

 

[DRWeig]

Imagine the front end (line side) of your VFD as a huge DC power supply. That's all it is. There's a full-wave rectifier that produces what would normally be horribly rippled. The DC level charges the capacitor, which acts as a filter.

I don't think you need to be accessing the DC link to power other things. I don't know your supply voltage, but on a 480 VAC drive the DC link is in the 700 VDC range...

Best to you,

Goober Dave

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[Skogsgurra]

The DC link is used to supply auxiliary power to the inverter (via a DC/DC power supply unit) and there's usually some spare capacity to supply small loads. But if you need more than a few hundred milliamps at 24 V or so, you need to use a separate PSU.

I actually had a customer come up with the same question once. He needed around half an ampere at 24 V and thought that it would be cheap and easy to drop the 560 V DC via a resistor down to 24 V. There are three problems with that approach:
1. there will be around 250 W dissipated
2. there will not be any voltage regulation
3. there will be no insulation between grid and load

On top of that, you need very powerful fuses or brakers to protect your system in case of a short or a ground fault.

So, don't even think of that.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ozmosis]

The technical term for a traditional VFD (variable frequency drive)is a converter, meaning; an AC supply is rectified to a DC and then the inverter section provides the switching function (typically a PWM- Pulse width modulation) to recreate an AC waveform that can be controlled. As described above. The DC link is simply that, a link between a rectifier and an inverter.
So, in essence, an inverter hangs on the DC link. In drive systems where you have one large rectifier (controlled or uncontrolled) you can feed multiple inverters from this DC supply. This is called a common DC bus drive. In applications such as paper or steel, you can have driven loads or regenerating loads on the same bus. As long as you do the correct engineering to size it correctly. This is when the drives business gets fun.
On a traditional converter (VFD), you will have capacitors hanging onto the DC bus required to smooth out the supply and sometimes DC link reactors to provide additional smoothing and minimising harmonic distortion.
For a VFD, measuring the voltage on the DC bus is the means to understand if the unit is functioning correctly; both for rectification and also in the event of any regeneration (if on an uncontrolled rectifier). So, typically on all vfd's, you will have terminals that can be used to measure the dc voltage.
Likewise, on some but not all drives, you will have some means to dissipate regenerated typically via an additional transistor connected on the DC bus that monitors the DC bus voltage and if it reaches a level that would usually cause the drive to trip, it 'dumps' the excess into a series of resistors that can be connected onto the DC bus.

So, there are plenty of things that can be connected onto onto the DC link, as long as it is not fingers. That would hurt.
I use the term "traditional" throughout because there are many different topologies these days for a 'drive'.

 

[waross]

Some of the longest DC links are used for rail transit drives. DC bus bars run adjacent to the rails to supply DC to the pickups on the rail cars. The DC is inverted onboard to drive either induction motors or a Linear Induction Motor. The DC bus bars may be fed from multiple substations. Regeneration is used to slow the cars. Extremely little friction braking is needed. Normally the energy from regeneration is used to power accelerating and running cars, reducing the draw on the grid. When the regeneration exceeds the draw, the DC bus voltage increases. This triggers the substations to switch in load bank resistors to dissipate the excess energy.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

There are also what are called "common DC bus" drive systems using the same concept Bill just described, often used in coordinated drives for things like paper machines and rolling mills. So if your converter section is sized properly, there are plenty of possibilities for using the DC link, but as Gunnar said, it may not be as simple as it appears.

Rather than playing a game of 20 questions, why don't you state the reason for your question? It sounds as if you have an idea of something you want to try doing with the DC connections in the drive.

"Will work for (the memory of) salami"

 

[Lc85]

We were planning to use the dc link of a Mitsubishi A740 to power a power supply so we can power some instrumentation. Just to see if this has been done before?

 

[DRWeig]

My company sells those Mitsubishi drives. The product engineer votes don't do it. There is some capacity there, but you'd void your warranty if it's a relatively new drive. Plus, you'd have a strange connection in there if you got qualified service one day.

Boy, the cost of a DC power supply sure seems small compared to the cost of that drive and the cost of the energy you'll expend and the other disadvantages of the voltage divider or even a DC-DC converter. See Skog's note four posts above this one, read it again. You have 480 VAC (or possibly 200 - 240 VAC) right there for power, too.

480:120 150 VA control transformer: USD$50.00
120 VAC to 24 VDC power supply 100W: USD$100.00
Avoiding a risky solution: priceless

That's an Eaton transformer and an Idec switcher. You could get the whole deal for USD$50 if you do it on the cheap.

How much current do you need at 24V?

Best to you,

Goober Dave

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[DRWeig]

As usual, I forgot my most relevant comment: The A740 comes with a 100 mA 24 VDC power supply connection for external use... If you don't need more than 100 mA, it's pretty simple to use.

Best to you,

Goober Dave

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