VFD supply by diesel gen set - low voltage application

[fab1961]

Dear all,

I have a similar case to that discussed in thread237-286989
but the generator is definitely oversized in comparison with the inverter (Customer says 200 kVA ?, while our machine is 58 kW total where VFD inside is 18.5 kW) and we are running at low voltage (415 V 50 Hz).
Randomly we get an alarm from inverter due to DCbus voltage changing too much.
My idea is to install a DC reactor on the VFD connected to one side of the DCbus (the VFd is a Omron E7 model, it has terminal blocks for that and the DC reactor is suggested in the operator manual in order to improve power factor).

In your opinion is the right thing to do?

Thank you.

 

[Skogsgurra]

A reactor is probably not going to help. It improves power factor and reduces short-term variations in DC link voltage when measured over 20 ms (or 16.667 if 60 Hz) but most likely it is about either the genset delivering unstable voltage or ramping down too fast.

What is the application? What other loads are connected to the generator?

Gunnar Englund

http://www.gke.org

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

 

[fab1961]

Thank you for the quick answer.

The application is turning a cutting disc saw for metal through a gearbox and it is a constant-power type of load. The motor driven is a normal 4 pole async motor. With normal cutting cycles the load on the VFD-motor is always under 80% of nominal current.

The other loads are
- a small (3 kW) async motor driving an oil pump started DOL
- 24V DC power supplies
- two air conditiong units for the lectric cabinet
- eight drives for brushless motors (two large units 48 A,
one 12 A and the others 3 or 6 A) with their EMC filters built-in
- other

I am not sure if the Customer has connected loads other than our machine, we are asking these days.

The VFD has already an Omron EMC filter connected at the mains - passive type.

I will let you know the end of the story asap.

 

[ScottyUK]

If the drive has a ramp down function then it will regenerate into the DC link as it brakes; the DC link voltage will rise as a result. A high inertia load like a saw disc contains a lot of stored energy which may exceed the drive's ability to dump the regenerated energy unless you add an external braking resistor to the drive, if it can accommodate one. Alternatively check the drive configuration and consider a DC injection brake instead of a ramp down if ramping is enabled?


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