VFD vs Softstart
VFD vs Softstart
(OP)
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!
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!





RE: VFD vs Softstart
RE: VFD vs Softstart
RE: VFD vs Softstart
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.
RE: VFD vs Softstart
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.
RE: VFD vs Softstart
25% = 60kW of heating.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD vs Softstart
RE: VFD vs Softstart
Charlie Harland