Can two VFD's run in parallel 5

[edison123]

For no-load open shaft testing various motors ranging from 50 HP to 1500 HP with maximum no-load current of 500 Amps at 415 V, 50 HZ, I am planning for to go for VFD.

If I buy two identical 250 Amps drives, can I parallel them on the output side for 500 Amps? This gives me an option of using them singly also up to 250 Amps as well as one to one standby, if one buggers off.

What do you drive gurus think?



Muthu

http://www.edison.co.in

 

[3DDave]

If the makers of the drives have certified they can be paralleled. Otherwise how would each detect the phase of the other?

 

[LittleInch]

Never ever heard of it before.

Sounds like an easy way to write off two VFD units to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

Not heard of it with VFDs. It's possible with certain types of AC inverters provided that they are interconnected as prescribed by the manufacturer so that their output phases are synchronised. Phase mismatch will cause something to go bang.

 

[FacEngrPE]

It is done with inverter type generators, which require a control cable connection.

https://generatorinsights.com/how-many-inverter-generators-can-you-parallel/.

Variable Speed Drives are not as far as I am aware built with that capability (at least as a standard product).

There are a few drive manufacturers that will provide drives with modified programs, as a custom product, so that possibly your requirements (or at least most of them) can be met.

 

[iop95]

There are VFDs that allow to add power modules to increase output power, but use same DCbus and sure, all module receive same control signals.

Separate VFDs may work in parallel if was designed for that and use a dedicated communication link for synch, load sharing and response to commands/faults.
I think may be problems (circulating currents) if VFDs use separate input source that are not close in impedance.

 

[edison123]

Thank you, all. Our supplier is going to ask the drive OEM about this possibility.

Muthu

http://www.edison.co.in

 

[jraef]

Parallel inverters are frequently done now for large drive applications, but not in the field. The inverters must be designed for that from the outset, typically using fiber optics for the firing controls to minimize latency in synchronizing the outputs. Most stand-alone single inverter drives do not have that capability.

There are drives DESIGNED for this kind of application, often referred to as having an "N-1" or "N-2" redundancy capability, meaning if you have two or three parallel inverter modules, the loss of one module will reduce the capacity of the drive. You can do it on purpose as well (if it has that feature).

As a general rule however, if you had, for example, a 1500HP drive with two parallel inverters, you can run a 750HP motor from one side, and most drives can effectively operate a motor down to 1/2 of the rating. So now you can get down to 350HP as the smallest motor. But since you are only going to be using this for testing, that rule may be less relevant, so long as you limit your use to V/Hz control


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Thanks, Jeff.

The vendor did confirm this Fiji drive does not have that capability of 2 in 1 power output.

Muthu

http://www.edison.co.in

 

[edison123]

Jeff

With 250 A drive, what is the lowest current it can be used for ? Vendor says 10% i.e. 25 Amps.

Muthu

http://www.edison.co.in

 

[jraef]

Muthu,
As a gross general rule, VFD mfrs typically tell you not to go below 50% of the VFD rated amps. That's because the current sensing technologies used in most drives, which are crucial for any form of vector control capability, run into error rates that effect performance at low speeds and low loads. There are however some drives on the market with newer current sensing capabilities that allow a much wider tolerance band, and 10:1 is what those are capable of. So it's possible that yours is one of those. But also, if you are NOT using any sort of vector control capability, i.e. open loop scalar control (a.k.a. V/Hz), then the current sensing is only used for overload protection, and that 10:1 size range may not be as much of an issue.

 

[edison123]

Thanks, Jeff.

Yes, I plan to run at open loop V/Hz mode in open shaft condition with our external optical speed sensor.

It's Fuji FRN0240E2S-4GB-210A HD / 240A ND drive.

 

[jraef]

Should be fine then.