VFD For 2 Speed Motor

[The Reaper]

I have a customer with 2 speed motors on their cooling towers. They are 30HP 3PH 460V. They would like to put VFD's on the fans and would like to reuse the existing motors. Can we just use the high speed windings and abandon the low speed? I would love to sell him some new motors, but they just don't have the money for both. Thanks in advance for any help.

[DDC_Dan]

It's best to use inverter-rated motors in VFD applications. They have beefed-up insulation to withstand the voltage spikes resulting from high-speed switching. Standard motors will certainly run on a VFD, but you run the risk of shortened motor life due to insulation breakdown because of the switching transients.

[22nomo]

Motor VFD tip sheet.pdf This might be of help for your question.

[The Reaper]

Thanks for the info guys.

[waregl82]

you also need to verify the bearings are rated for low rpm applications.

[Abrnth3]

you also need to verify the bearings are rated for low rpm applications.

Most bearings are not rated for these low rpms. grease does not get distributed and bearings will burn up in short time.

[DDC_Dan]

Most bearings are not rated for these low rpms. grease does not get distributed and bearings will burn up in short time.

You know, this is the second time I've heard this on this forum. I know that sleeve bearings do indeed have a minimum speed necessary to develop the hydrodynamic oil wedge that prevents metal-metal contact, but I have never heard of a minimum speed for ball bearings - and as far as I know all inverter-rated (and all motors inverter or not, above a HP or so) have ball bearings.

I looked up Timken's engineering guide for ball bearings and could find only one indirect reference to low speed. Here's a link:

http://www.timken.com/en-us/products...%20Section.pdf

Here's the reference (on page A165):

BearingSpeed.jpg


According to this graph ball bearings - no matter what the lubrication - have no issues down to zero RPM. If anyone has info on minimum speed for ball bearings I would like to see it - I would definitely like to learn something new on this.

Now, if your load has sleeve bearings then low speed operation is definitely a concern.

[Balkandroid]

My opinion, You can use it at hte high speed (I have use it few times), but the lowest freqency is must be at least 15-20Hz, because otherwise the motor can burn (not the bearings). I have see once a motor which has been burned in 5Hz - because the cooling of the motor was not enought.

[bearfromobx]

I've converted several VAV AHUs from inlet guide vane to VFD, reusing the original motors (all 1.15 SF but sized to keep current at or below FLA) and gotten more than 10 years on motors with at least 5-10 years or more of original service before the conversion. The biggest failure I've seen has been "washboarding" of the bearing races related to high voltage discharge through the bearings. I always suggest shaft grounding on a VFD driven motor to give the induced voltage somewhere to go instead of through the bearings. So far, I've not seen failures related to bearing lube failures, even at low speed, but have had two fail after conversion because a contractor turned down the minimum speed below 25% (15Hz input to motor), which cooked the winding insulation. I can't think of any problems created by abandoning the low speed windings right off, but I've attached a couple of links below for more reading which might help. Good luck.

http://www.evapco.eu/sites/evapco.eu...ncy-Drives.pdf
http://www.usmotors.com/Service-Supp...otorsVFDs.ashx
http://www.industry.usa.siemens.com/...tion-guide.pdf

[bearfromobx]

Makes me wonder about the split oil bushings operating as mid shaft bearings on the Evapco multiwheel towers. I've seen several with VFDs and their operating speed averages about 900 to 1000 RPMs at the shaft. How do they maintain enough oil wedge to keep clearance or are they just built that loose to begin with?

[DDC_Dan]

We just converted two old (late '70's) air handlers to VFD from inlet vanes, the first of several we will be converting at a hospital. We spec'd new inverter-duty motors and required the grounding kits as well.

[Abrnth3]

We have been converting a lot of our systems over to VFDs. Always setting min hz above 12, usually 15hz. Due tomotor burn out, especially on air over motors. No burn outs yet and several have been running for 3-4 years.

[DDC_Dan]

Always setting min hz above 12, usually 15hz.

Me too. The OP's application was a cooling tower - just run the speed down to 15 Hz and on further reduction of temp just cycle the fan off, done that many times with good results.

[sdt1104]

This may be an echo of whats been said. Frequency drive motors should have ball bearings. You need a motor designed for inverter use. We have over 300 VFD motors 25 to 100 HP and we set the minimum speed at 20Hz or off. You can expect some fluting on the motor bearings; that is a static build up that discharges at the motor bearings causing premature failure. We tried the "AGIES" grounding rings with little or no success. On the up side, we have gotten really good at changing motor bearings. Using the existing motors, you will most likely get to sell them the correct motors in a few months anyway.

[The Reaper]

Thanks for all the help everyone. The customer doesn't want to take any chances and will be buying new motors.

[EEAllen]

yes, you can just put the motors on their high speed, and use the VFD on the power supply

[wcsmith]

I have never heard of a low RPM bearing requirement on a cooling tower shaft; however, if the tower has a gear reducer low RPM's can affect bearing lubrication and an oil pump may be required to operate using a VFD at low RPM's.

[jbarron4]

The problem is not with the motor (except for the cooling issues where the motor fan is not running fast enough and an sxternal fan drives off the line will take care of this). The real problem is that the gearbox is not turning fast enough to properly lubricate the top end.
Usually the cooling tower manufacturer places the limit at 25% speed when VFDs are used. I would ask the manufacturer.
Also the problem with bearings can be eliminaated by installing insulated bearings on the ODE end and the Aegis bearing grounding on the shaft or now you can buy the CDR series INPRO seal that has the grounding brushes built in and replaces the INPRO seals in IEEE 481 motors.
To recap:
Insulate the rear bearing (SKF makes insulated bearings)
Install shaft grounding
Install Inverter duty motors (with the insulated bearings and CDR not all have them unless ordered special) Your motor shop can help with this.

[jbarron4]

VFD Bearing.JPG
I have attached a picture of a bearing removed from a 250 HP 480 VAC motor in service for 4 months without insulated rear bearing and shaft grounding. THis motor was repaired by replacing bearings (rear bearing insulated SKF) and added IMPRO CDR shaft grounding brush and seal.
Evident is the washboarding caused by current discharge through the bearing.
While this can happen with an accross the line starter in a system with poor power quality it is very common with VFD drives especially with long feed lines and inadequate line and load reactors.

JDB.