VFD revisited

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Thirteen years ago we put in an Altivar drive on a submersible well. We hadn’t been back until two weeks ago when the guys correctly diagnosed bad splices in the drop cable. Thrust bearings were at limits of wear, too.

I stopped today by when a new pump and cable was installed and uploaded the parameters to new software I have.

One parameter that stood out was that I have the Max HZ set at 50 vs 60.
IDR why I did that. I would have had tech support, I’m sure.

What would be the advantage of doing that? Any mind readers?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If the pump has more than adequate flow and pressure capability, you might save on power consumption depending on various factors (e.g., if something else is limiting the flow rate). Or it might have been done to reduce the amount of on-off cycling with somewhat oversized pump.

Or maybe you were out of the country a lot back then and had 50 Hz on your mind. ;)

One thing to consider is that the ramp up and down times should be kept relatively short to minimize the time of metal-to-metal contact on hydrodynamic thrust bearings.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The default setting on Altivar drives used to be 50Hz (I think they finally changed that on drives sold in North America). Maybe you just didn’t realize it and never changed it.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
The pump installer had not used a VFD at that time so we studied pretty hard trying to make it right.
The submersible is more than adequate IIRC.
The bearing wear was a thought so I did look at the ramp times. Up is .5 seconds to 30 HZ with 2 second deceleration. I would typically use Coast to stop now. I believe it runs almost continuously at 36 HZ throughout the day and hits sleep mode only during late night, early morning. There is a flow meter and water is being used.

Is 13 years a short bearing life?
 

Besoeker3

Senior Member
Location
UK
The pump installer had not used a VFD at that time so we studied pretty hard trying to make it right.
The submersible is more than adequate IIRC.
The bearing wear was a thought so I did look at the ramp times. Up is .5 seconds to 30 HZ with 2 second deceleration. I would typically use Coast to stop now. I believe it runs almost continuously at 36 HZ throughout the day and hits sleep mode only during late night, early morning. There is a flow meter and water is being used.

Is 13 years a short bearing life?
I think, from what I have read, that's quite good.
 

drktmplr12

Senior Member
Location
South Florida
Is 13 years a short bearing life?
bearings are rated in hours. 13 years operating daily, assuming 18 hours a day is 85410 hours. depending on their construction, i would expect about 100,000 plus/minus 10% hours from bearings. you can get lucky and get 150,000 out of them. a VFD may shorten that depending on the quality of output power.
 

Besoeker3

Senior Member
Location
UK
bearings are rated in hours. 13 years operating daily, assuming 18 hours a day is 85410 hours. depending on their construction, i would expect about 100,000 plus/minus 10% hours from bearings. you can get lucky and get 150,000 out of them. a VFD may shorten that depending on the quality of output power.
Yes, VFDs can cause motor bearing currents if the proper precautions are not taken. That shortens their like, sometimes considerably.
 

Besoeker3

Senior Member
Location
UK
VFDs are built with an expected life of 100,000 hours.


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That's about as long as you might expect the DC link capacitors on a VFD to last but sometimes it is much less depending on ambient conditions:

Aluminum Electrolytic Capacitors, Power High Ripple Current, Screw Terminals Fig. 1 FEATURES • Long useful life: 10 000 h to 15 000 h at +85 °C
From Vishay.
 

paulengr

Senior Member
If you have lots of money and space metaluzed film caps last almost forever. But it doesn’t matter. The next limitation is that RoHS silver solder joints fail at about 15 years. Beyond that you go to sintered joints. Semikron offers this on for instance modules made for Siemens traction drives but it’s not something you find in 99% of the VFDs out there.
 

Dzboyce

Senior Member
Location
Royal City, WA
The thrust bearing in a submersible motor is a Kingsbury style “frictionless” bearing, At 30 hz or higher there is a film of water between the two bearing surfaces. I like to see the motor turning 45 hz or more during normal conditions. Most submersible pumps won’t develop any flow at the target pressure until they are above 45 hz anyways. This is why a submersible motor needs to accelerate to 30 hz in 1/2 second or less.

I did lose the thrust bearing on a 5 hp 100 gpm pump couple of years ago. The check valve stuck open and the pump had to run at 36 hz to maintain 50 psi, and never went to sleep. It would speed up when there was any water flow.

When the thrust bearing wears out, the impellers in the pump end drop into the bowls and wear out. When we assemble or disassemble a pump, we have a go/no go gauge to check the motor shaft stickup. Which is actually measuring the height of the thrust bearing in the bottom of the motor.
 

paulengr

Senior Member
Aka “cut less” bearing or “gutless”. Ok so what is the lubricity of water? Yep with almost no viscosity, nothing. Oh and what does any amount of sand or grit do? 45 Hz is not a hard number but not far off usually. It depends on surface speeds between the two bushing surfaces. Submersibles are fairly hardy though unlike vertical hollow shafts. I’ve seen one lift station with 60 foot line shafts. They had wye delta but disabled it for delta only when the equipment wore out. Hard to described hearing and watching a 60 foot shaft wrap itself up and unwrap. Sounds roughly like a dead short at high power when the conduit and wiring vibrate except with a metallic bang.

Full speed in 0.5 seconds is reasonable on small under 5 HP motors but on big 50+ HP sewage and storm water pumps you won’t get that even ATL. All centrifugal pumps have an operating range of roughly +150/-50% of their design speed. If you fall off the curve you can quickly get in trouble. Plus some have such a steep performance curve that VFD operation is worthless. You really need to pay attention to and know how to read a pump curve if you are doing any pump work to stay out of getting called bad names because you didn’t keep the valve and pump people out of trouble.

It’s like the day a pump company sold a mine a 24” butterfly control valve on a 24” pipeline (hint, hint!!) to go from 300 PSI to atmospheric on a 15,000 GPM system firing into an elbow. Let’s just say I’ve never heard that sound like that before. Sounded like multiple full auto 22 rifles hitting the elbow. Also watched the 150 kV flange gaskets blow right out at me as they brought it online.


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