Problems Powering a VFD with Open Delta feed ? 1

[HVmazz]

Powering a VFD with Open Delta

Should I run a 10hp VFD on Open Delta 3 phase or use a higher power rated VFD and feed it single phase 240V? I need to run a 10 hp 240 volt three phase pump for a water fall.

What are the potential problems?"

 

[Skogsgurra]

The problem with open delta and VFDs is that the impedance is different in the three phases. Which causes voltage unbalance when loaded. And that many (most) VFDs are sensitive to voltage unbalance.

Technically, a voltage unbalance makes four of the diodes in the mains rectifier do the job while two of them don't. So the four have to work harder and get overloaded. And, to make things worse, out of these four diodes, two work the hardest. That's why there is a tolerance limit around 2 % on mains unbalance. And that is difficult to stay within if an open delta gets heavily loaded.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar;
I know that 10 hp is a little large for single phase, but would a single phase rated VFD be safe on open delta?
Respectfully

 

[jraef]

Waross,
I don't know of anyone who makes a 1 phase input VFD that goes quite that high without derating a larger unit. 5HP is the highest I've seen, from Siemens and AC Tech. Above that the usual trick is to double the size of the VFD to provide for the larger diodes and increased bus capacitance.

Some VFDs however are not capable of that, especially those which use SCR front-ends instead of diode bridges, because they need to have relatively clean power on the front-end in oder to gate the SCRs properly.

I had already recommended to the OP in another forum that he can most likely use a 15HP Constant Torque rated VFD for his pump application since it will be effectively double the necessary size for his 10HP VT pump load.

 

[LionelHutz]

I'd also say use the 15hp VFD connected on the open delta.

 

[HVmazz]

Thanks again...
Great discussion and advice.
You have helped me a great deal.

I think we will go with a 15 hp VFD as jraef suggests...
Thanks.

 

[itsmoked]

Hey jraef why does 15HP = 2 x 10HP? I didn't follow that. What's "VT" got to do with anything?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com