VFD vs Softstart 1

[Jnox]

I'm trying to find what the advantage would be of using a VFD vs a soft start on a motor

Motor: 460V
350HP

What would we save if on average I ran three pumps at 75% using VFD vs running them at 100%.

I understand that VFD has some losses associated with it, and the losses increase as you decrease the speed.

But what is the relationship?

Thank you!

 

[thewellguy]

what type of pumps are they? Does your load on the pumps vary during the day? Do you run them at 70% now by chocking them back or would this be a new feture? More info please.

 

[Jnox]

The pumps run at 100% loaded 24/7. Water is recirculated, and amount used is either diverted back into system or used in process.

 

[LionelHutz]

This is an application where VFD's would definitely save energy.

The work done by a pump is proportional to the amount of liquid pumped. You pay for the energy used when the pump does work. So, you are paying to recirculate that water, possibly a fair percentage of the energy being used by the pumps at times.

To do this properly, you will need to install the VFD's and one or more pressure tanks to maintain pressure and then get rid of the recirculating valves. The VFD's will be controlled to change speed to keep up with demand while maintaining a constant pressure.

To actually decide how much energy will be saved you will need to evaluate the energy savings of the new system vs the expense of changing the process.

 

[jraef]

Just FYI, soft starters will not save you any energy, so the comparison is just to running at full speed.

Don't be overly concerned with VFD losses that "increase as you decrease speed". Those losses are as a percentage of power consumed, but in a centrifugal load your consumed power decreases at the cube of the speed. So although your PERCENTAGE of power losses increases, the AMOUNT of losses barely changes. For example let's say you reduce your speed by 20% in order to reduce your flow to 75% (I'm making those values up). At full speed your VFD throughput efficiency was 98%, but at 98% speed it may be 97%. BUT, at full speed the losses were 2% of 350HP, so 5.2kW then at 80% speed your HP consumed is only 179.2HP, so the now 3% losses is only 4.01kW. So although your percentage of losses went up, your actual losses still went down.

There are plenty of "VFD Energy Savings" calculators available for free from VFD manufacturers and on the web which can take your installation and operation data input to calculate your expected savings. They will factor in the VFD losses etc. But to reiterate what has already been said here, your's is the type of situation usually used as a prime example of the energy savings potential of VFDs. One thing you may need to know however will be the pump speed that you will operate at to achieve 75% flow in your installation. You pump supplier should be able to supply you with a speed / flow / head curve that you can use to determine that.

 

[itsmoked]

Keep in mind that changing to a VFD will probably cool your water somewhat as all that recirculating is heating it.

25% = 60kW of heating.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Jnox]

Thanks guys. I found a calculator on ABB's website if I can get a hold of pump data.

 

[charland]

Something to consider about the pump. You will need to control the VFD using data from the plumbing. If it suits your system, you can simply use a pressure transducer and a pressure accumulator. A PLC will speed or slow your pump to maintain the pressure in the accumulator. If you don't want to use an accumulator or if its a positive displacement pump (which means you will likely need a recirculation line to protect against over pressure with a pressure regulator valve), you can use a flow switch on the recirc line in conjunction with the pressure transducer on the output line. Use the flow switch to slow the pump down and the pressure transducer to speed the pump up

Charlie Harland