VFD vs Soft Start vs Fluid Coupler

[Dobber1978]

Trying to put together a list of pros and cons for each of the mentioned. We would need 2 drives each one running a 500HP mine dewatering pump, one motor would be on all time and the other would be to pick up the slack, we would like to keep both the same to reduce spare parts so the option of one VFD and a softstart is out of the question,

Thanks
Jeff

 

[aolalde]

Hi Dobber1978

These are what I think.

VFD:
PROS 1- Low inrush current very torque effective.
2- Smooth acceleration for the motor even fully loaded.
3-Allows adjust for optimum operational speed.
4- Total release for the line and substation while the motor starts.
CONS 1- High initial investment
2- Could impose surges to the winding insulation and bearing induced currents.
3- Could be a source of harmonic distortion for the line.

SOFT STARTER
PROS 1- Reduced Investment
2- Partial release for the line and substation while the motor starts.

CONS 1- The current reduction follows a big torque reduction.
2- A surge of current could be developed while being tranfereed to the line.
3- Could fail to accelerate a fully loaded motor.

FLUID COUPLER
PROS 1- Easy motor start no loaded.
2- Smooth engagement of the load.

CONS 1- Initial investment.
2- additional element in the mechanical coupling

 

[dpc]

If all you are concerned about is starting, then the solid-state reduced voltage starter is the clear choice. It will be much less expensive.

If you need to control the speed in order to control the flow, then it has to be a VFD or fluid coupler.

The VFD and fluid coupler will give you a soft start, but that is not really why they used in most cases.

 

[Dobber1978]

Any thoughts on running costs of the three?

Jeff

 

[macmil]

There is also the option of a MagnaDrive adjustable speed drive. Disclaimer; I sell them. But of course depending on your circunstances, it could be a good application.
mac

 

[Barry1961]

Pumps and fans as a rule are very easy to start so any of the starting methods would work. The VFD and Soft Starter will reduce the big current surge when starting. I would think with a 500hp motor you would use at least a soft start with your fluid coupling.

The VFD will save you money if one of the pumps is going to be running at less than full speed for any significant amount of time. There are bunches of companies that make VFD’s for fan and pump applications. They are very popular in water and sewage where you have peak demands in morning and evening then reduced flow at other times. There are several online calculators to figure cost savings with VFD’s.

There are companies that sell pre-package and programmed drives for multiple pump operation. These come with manual or automatic emergency bypass in event of VFD failure and other standard things to make it easy. If you do a google for “pony pump” you will probably get a bunch of hits. The pony pump is the small pump that runs during low flow hours like after midnight.

Some of the more popular VFD’s for fan/pump/HVAC are ABB, Hitachi and Omron, but there are many more.

Barry1961

 

[DickDV]

At 460V 500hp, a VFD will cost you at least 8 times what a soft starter will cost you. You really need to have a reason for variable speed to justify a VFD unless---------- your object is to drive starting inrush current down as far as possible without regard to cost.

Otherwise, an electronic soft starter is the better choice. You will be able to reduce your starting inrush from the usual 6-8 times FLA to about 3 times FLA and reduce starting torque to whatever level is necessary to just break the load into motion. On fans and pumps, that is typically very low torque.

A hydraulic coupling will limit torque ok but will not reduce inrush current. The duration of the inrush current will be shorter but the magnitude will be just as high as starting across the AC line.

Use of an electronic soft starter will normally translate into much longer motor life since the bearings, shaft, and windings are not being subject to the huge shock loads seen with across the line starting.

Bottom line: very hard to justify a VFD. Think softstarter.

 

[jraef]

I had a different take on your original meaning. Before I was able to post this you have shown that I was wrong, but you should still consider this.

Since a soft starter cannot change speed, and you mention speed trimming as a necessity, I originally took your post to mean that you were considering 2 prospects: 1) VFD and 2) Soft Starter combined with a fluid coupling, because with a fluid coupling you still need to start the motor, and on large motors that usually means reduced voltage starting.

My additions / comments then to aolalde's list:

VFDs, nothing to add or change

Soft Starter / Fluid Coupling combo
Pros Add:
Smooth acceleration without transition
Soft starter will prevent voltage drop from starting the motor.

Cons Add:
Slightly higher system losses (through the fluid coupling) compared to VFD (I may be wrong about this though, it seems to me we have had this discussion in here before).
Risk of fluid leak or contamination, hazmat containment issues to deal with.

I would also beg to differ on aolalde's assessment that soft starters surge when transferred to the line and can fail to accelerate a loaded motor. The only time either of those things are an issue is when they have been misapplied. The same would hold true of a VFD if you use one that is not rated for the job.

As to cost issues, at 500HP (assuming 460V) the VFD is going to be 6-8 times more expensive than a soft starter, but again, if speed control is a necessity then you must add the Fluid Coupling cost. I'm no expert on those, but likely the combo is more than a VFD.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[cbarn24050]

An energy optimising VFD will pay for itself over and over again in saved electricity costs. In some countries there are climate change tax breaks on these items as well.

 

[ozmosis]

Dobber1978
Just another point. You mention that the application is for a mine. Just make sure there are no environmental safety issues concerning the electrical installation required. If there are, this will bump up the cost of any electrical device considerably. You haven't mentioned your voltage supply either, this will also have an impact on the cost of electrical devices. Mines are often on higher voltages than the 'norm' (400~460).

 

[thewellguy]

You never mentioned what type of dewatering pump you are running. keep in mind that many types of pumps require that the reach a minimum speed in a short period of time to prevent damage to the motor or pump. This may not effect your decision as to which pump, but it many effect programing.