VFD Operational Practices 1

[cub3bead]

Need some opinions on best operating practices for a pump station where two pumps are fitted with VFDs and the third is constant speed.

The operators normally operate the two VFD pumps at 100% speed rather than operating the fixed speed pump and one VFD.

Isn't the operator's preferred mode burning up a lot of more kWhs over 1 VFD + 1 constant speed operation?

 

[DickDV]

That depends on the flow rate. If all of the flow from two pumps is required, then nothing is being wasted. If less flow than two pumps is required, then considerable energy can usually be saved by slowing down one of the pumps.

 

[jraef]

If all of the flow from two pumps is required, then nothing is being wasted.

DickDV,
What about losses in the VFD? You will have more losses in 2 VFDs than in 1 + 1 at full voltage through a contactor, no? Even the best VFD designs are only around 97% efficient at full speed, and that is without reactors etc.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[itsmoked]

Good point jraef. I guess the hassle of adding bypass contacts would depend on just how big these pumps are.

 

[jraef]

Bypass contactors for VFDs are a tricky business. It's not like a soft starter where you can just close the contactor when at full speed, because in a VFD the transistors cannot come into contact with the line power without carefull coordination of phase synchronicity and some inductance, and most drives are not capable of that (it's called synchronous transfer). The safe way is to isolate the VFD output before connecting the motor in bypass, which means an open transition. On a pump, that means a surge will occurr when you reconnect it.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[DickDV]

jraef, it is true that modern drives have about 2-3% of motor load kw in losses which is only a factor when comparing systems at or near full speed. Below full speed, the pump or fan savings become so large that VFD overhead losses quickly become insignificant.

 

[jraef]

Absolutely, but his question was specifically in reference to running 2 pumps at 100% speed on VFDs vs 1 + 1.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[cub3bead]

One set of pumps have 300 hp drivers and the second set have 400 hp drivers.

No doubt that the part load savings justify the VFDs but this is a base load plant.

Wasn't involve in the selection of 2 VFDs and 1 constant speed mototr but suspect that I would have advocated 1 VFD + 2 constant speed for this installation and saved the cost of the extra drives.

 

[cub3bead]

Thanks for the insight jraef. A star to you.

 

[Marke]

I am beginning to see this a lot.
VFDs are being put onto fixed flow installations with the promise of energy savings. If the pump always has the same constant flow, it would be better to size the pump correctly and run it with a soft starter. This energy savings concept has conditional requirements. I advocate VFDs for variable flow requirements but question VFDs for fixed flow installations.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[mil3]

I agree with Marke.
If all you want is fixed speed operation, then plan for soft starters to remove most water hammer and that's it.
No reason to use VFD's for fixed speed pump applications except for some minor reasons.
Maybe the use of the VFD's here isn't fully utilized for the system. Maybe speed control still has to be implemented but no-one has realized the benefits yet.
Also agree with you cub3bead. I would have also just installed a single VFD and two fixed speed pumps. Once the VFD is at full speed, then bring on a fixed speed pump and back of the VFD and so for the 3rd pump aswell. Common line though.
This depends on your piping design too though and how the system is designed.

 

[aolalde]

I agree with Jraef, the more elements in the chain the more losses you will have to pay in electric energy consumption. Besides, motor’s efficiency is affected by the non-sinusoidal wave generated by the VFD.
A logic evaluation to get optimum initial cost, performance and operational efficiency seems necessary.