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.
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.
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:
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.
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.
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.
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?
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.
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.
Last edited by Abrnth3; 03-17-2013 at 12:22 AM.
Reason: add info