Star-Delta Vs VFD in a pump motor ?!

[DNA990]

well guys, here is the deal , I'm a fresh graduate mechanical engineer and just started working 3 months ago,
and i'm facing some difficulties making a decisiion at work.
we need a set of booster pumps ( 1 duty + 1 stand by )to supply our plant with water, and some of the water supplied goes to our machines that require a constant pressure of 4 bar, anywayz, the pump has the following specifications : 5.5 KW , capacity : 27 Cu m / h and the motor is 3 PH 220/380 V , my question is for this motor do i actually need a Variable Frequency Drive or a star-delta would just do fine to protect the motor from the current load at the start and the system in general from the voltage drop due to this current load ? I'm confused because the price for a VFD is so high compared with star delta. so any advice ?
i have another question which is : is there any relation between the VFD and maintaining the constant required pressure for our machines ?? or is it only the pressure switch that takes care of this ?
thanks for your time ,

 

[itsmoked]

Never use an open transition star-delta starter for anything. They are abysmal P.O.S.es.

Instead use a solid state soft starter. They are vastly more flexible. They provide much better control. They greatly reduce the shock of starting to the mechanicals and the electrical network. They now cost the same as, or less than, a SD starter.

As for the VFD. You don't use a VFD in lue of a soft starter to just soft-start a motor because they cost about 3 or 4 times what an equivalent size soft starter costs.

You would use a VFD if you needed a soft start so gentle that the motor starting current never exceeds the full load amperage. This would be a very rare case. Perhaps a fragile system, or way-out rural, or a very undersized generator.

More typical reasons for using a VFD on a pump are to save energy if the pump doesn't run at or near 100% all the time, as a VFD will lose a few percent always. Or, you need to do pressure or flow control. A VFD can help you do pressure control and save a little energy at the same time.

Basically if you are going to be needing the system to vary flow and pressure often look at a VFD. It would always be better to just size the pump and the motor to run full out at the required operating point but if the system isn't going to fit that situation then look at a VFD.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DNA990]

Thanks Keith for your valuable input. really appreciate it.

I understand that I should not go for a VFD only because I need a " smooth" starting for the motor. but again, pressure has to be always 4 BAR as water goes directly to our machines that require this constant pressure. so in this case I should probably consider the VFD.

Thanks again,

Bara'a A. Habash
Maintenance Engineer

 

[itsmoked]

If the flow changes around I probably would look at using a VFD too. Yes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BigInch]

Or a fixed speed with a pressure control valve. It depends on how much flow varies. Look at controlling pressure and determine the flow variation that happens when you do that. If flow only varies between 50-85% of BEP speed when you are controlling pressure, a VFD might work out best. If you don't spend much time in that flow range and need to go to 100% BEP, a pressure control valve may be better. If you use a VFD, then be sure to size the pump for around 1.05 to 1.1 times the BEP you think you need now.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[DNA990]

Thanks guyz for your input , it did help. In fact , I did some more research and decided to go for a VFD. They will install the set within 2 weeks, hopefully.
so thanks again.

Bara'a A. Habash
Maint. Engr.

 

[itsmoked]

Let us know how it goes.

Keith Cress
kcress -

http://www.flaminsystems.com