Primary/Backup Motors on one VFD

[CtrlSystemsEng]

I'm putting together a system with two 40 hp pump motors. One pump will function as the primary, and the secondary pump will be used as a standby in the event that the primary fails. Both pumps will never operate at the same time.

I would like to use a single VFD for both pumps and install a relay on the output side of the VFD to switch control from the primary to the standby. The system is controlled by a PLC, so if a pump failure is detected on the primary, then the VFD's output would be returned to zero before starting the standby pump. This would be an automatic switchover operation as controlled by the PLC. I also intend to have hardwired and PLC programmed interlocks to prevent both pumps from being energized.

I would like some input on the feasibility of this strategy from a design and safety standpoint. If it is acceptable, then what would be the nature of the relay I might need as well as load reactors that also might be required? What other issues should be considered? The other option is to implement two separate VFD's, but that seems unnecessary. If anyone has a good schematic for this type of application, it would be greatly appreciated.

Thanks!

 

[itsmoked]

Generally when someone goes to the bother and expense to put in a standby/backup motor this signals an exceptional need for high reliability.

As such you should not consider using one VFD for both motors. A VFD has about the same chance of failure as the motor it runs.

The correct setup would be two VFDs and some logic to run both motors alternately. This prevents bearing failure from non use. It also returns most of your investment rather than having one unit essentially rotting away while the other does all the work.

IF you can't afford two VFDs you should consider whether or not you could start the backup DOL without system issues. If that's the case then set up the backup for that. It won't let you alternate motors for proper wear averaging. In this case it might possibly a better alternative to use a soft starter if you want less of a network hit on starting. Personally.. I'd go for a second VFD.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

I agree fully with Smoked. The VFD probably isn't the most expensive component. And special solutions usually don't make things more reliable than standard applications.

BTW, if availability is important, don't forget to take care of the bearing problems you are likely to meet. What voltage are the motors?

Gunnar Englund

http://www.gke.org

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

 

[CJCPE]

I agree with the others who have said that the motors should not be less reliable the VFDs. If you have had some experience to the contrary, you probably need to do the things that assure that the motors will be as reliable as they can be.

1. Purchase motors that are advertised by the manufacturers as being suitable for use with VFDs. The manufacturer’s description should include a statement that the motor insulation meets NEMA standard MG1 part 31. That means that the insulation has a certain level of voltage withstand capability vs. the ringing voltages that occur with IGBT VFDs and long motor cables. Information about continuous torque vs. speed should be provided, but “suitable for variable torque” would be a sufficient statement.

2. Consider using load reactors or dv/dt filters depending on the motor cable length. Look at the VFD manufacturer’s recommendations and/or the recommendations of the manufacturers of the reactors and dv/dt filters:

http://www.mtecorp.com

http://www.transcoil.com

3. Consider providing protection against bearing currents. Look at the VFD manufacturer’s recommendations. I believe that shaft grounding is the leading remedy.


CJC

http://www.vfdriveinfo.com

 

[ozmosis]

I cannot disagree with the above comments but a large number of VFD manufacturers include pump control (cascade, duty/standby etc) within their drive capabilities.
A number of drives already with this
example:
Danfoss VLT Aqua

http://mcliterature.danfoss.com/WebPublish/doc_MI38C302.pdf

ABB ACS800

http://library.abb.com/global/scot/...700505759/$File/Pumping_DEF (NXPowerLite).pdf

Vacon PFC

http://www.vacon.com/Default.aspx?id=462019

with most of the above, there is no need for additional PLC control or your own logic/relays.

 

[CtrlSystemsEng]

I wanted to thank all of you guys for your valuable inputs. I started to consider the one drive approach because we had some space limitations. I can see now that dedicated drives are the right thing to do. Cycling the pumps to ensure wear averaging was also great input.

Skogsgurra, this is a 480 VAC application. The drives are spec'd for 40 hp at 52 Amperes.

Once again, thanks for the great info and useful links. The design phase kicks off tomorrow.

 

[itsmoked]

Happy mis-no-details!

Keith Cress
kcress -

http://www.flaminsystems.com