Hi all,
I'm in the midst of restoring an old Walker Turner 900 series drill press. Although the 600rpm minimum speed is not terrible, I think it's still about 2x too fast for some types of work I might get into. Secondly, I have a nagging fear that I will crack the pot metal spindle/motor step-pulleys during some future (unsuspecting) belt swap.
So, logically, a 3-phase VFD conversion sounded about right. The next step was to find a motor to replace the original 1/2HP "The Driver" motor. The below outlines what I've learned over the last few days. Please fact-check me if I'm wrong about anything!
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A number of online posts allude to the need for higher HP motors when attempting a 1ph-to-3ph conversion. Initially, I thought this was because the motor lost some capability at lower-than-nameplate speeds. However, in my research, I found that torque is theoretically constant between 0 - 100% rated speed. (I suppose there must be some lower limit there, and that is torque is not truly constant down to 0%.)
At present, my understanding is that the implied "need" for higher HP motors for VFD purpose is really just related to the thermal capability of the motor. For TEFC motors, the motor drives its own cooling fan, so obviously the cooling performance is worse at reduced speeds. I found that "Inverter Duty/ VFD-Ready" motors are typically given with a constant torque ratio, given as the ratio of reduced speed to nameplate speed. Per my understanding, the meaning of this is simply "how slow can the motor spin without overheating". In the case of a 1725rpm motor with a 10:1 constant torque range, the motor can spin at 172RPM @ nameplate torque, while maintaining temperatures (at least) lower than the insulation class rating.
Does that sound about right?
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So, now I am less convinced that a higher HP motor is truly necessary, as long as a decent inverter duty ready motor with large constant torque ratio is used. I searched many catalogs (Baldor, US Motors/Nidec, WEG, Dayton), found some OK options, but most "only" had a 10:1 or 5:1 constant torque. Then I found the "W Series" from Book Crompton, which seems like a lot of bang-for-your-buck. 15:1 constant torque ratio, NEMA MG-1 P31 rating, 1.25x service factor and <$250 for most 1/2HP and 3/4HP models.http://www.brookcromptonna.com/Product Catalogs/Aluminum.pdf. Unlike most rolled steel motors, the Brook Crompton is cast aluminum with lots of cooling fins. Intuitively, this type of motor should be able to dissipate heat much more effectively, which probably explains its 15:1 torque ratio and 1.25x service factor.
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*Now my main question is: Am I likely to harm the tool (drill press) by over-sizing the motor to 3/4HP or 1HP (for instance)? I would imagine that the drivetrain has some limits. In a drill press, probably the splined spindle shaft is the highest stress point(?) If the full-load torque is increased by say 25 or 50%, then the drill press will undoubtedly "see" that extra stress under heavy load, right?
Given my lack of experience with machine tools in generals, I want to make sure I'm making sense. Anyone have any input or advice?
At this point, I'm leaning towards a Brook Crompton 1/2HP/1725RPM inverter duty motor (as mentioned above). They are about $220, which is not bad
Thanks!
Tim
I'm in the midst of restoring an old Walker Turner 900 series drill press. Although the 600rpm minimum speed is not terrible, I think it's still about 2x too fast for some types of work I might get into. Secondly, I have a nagging fear that I will crack the pot metal spindle/motor step-pulleys during some future (unsuspecting) belt swap.
So, logically, a 3-phase VFD conversion sounded about right. The next step was to find a motor to replace the original 1/2HP "The Driver" motor. The below outlines what I've learned over the last few days. Please fact-check me if I'm wrong about anything!
----
A number of online posts allude to the need for higher HP motors when attempting a 1ph-to-3ph conversion. Initially, I thought this was because the motor lost some capability at lower-than-nameplate speeds. However, in my research, I found that torque is theoretically constant between 0 - 100% rated speed. (I suppose there must be some lower limit there, and that is torque is not truly constant down to 0%.)
At present, my understanding is that the implied "need" for higher HP motors for VFD purpose is really just related to the thermal capability of the motor. For TEFC motors, the motor drives its own cooling fan, so obviously the cooling performance is worse at reduced speeds. I found that "Inverter Duty/ VFD-Ready" motors are typically given with a constant torque ratio, given as the ratio of reduced speed to nameplate speed. Per my understanding, the meaning of this is simply "how slow can the motor spin without overheating". In the case of a 1725rpm motor with a 10:1 constant torque range, the motor can spin at 172RPM @ nameplate torque, while maintaining temperatures (at least) lower than the insulation class rating.
Does that sound about right?
-----
So, now I am less convinced that a higher HP motor is truly necessary, as long as a decent inverter duty ready motor with large constant torque ratio is used. I searched many catalogs (Baldor, US Motors/Nidec, WEG, Dayton), found some OK options, but most "only" had a 10:1 or 5:1 constant torque. Then I found the "W Series" from Book Crompton, which seems like a lot of bang-for-your-buck. 15:1 constant torque ratio, NEMA MG-1 P31 rating, 1.25x service factor and <$250 for most 1/2HP and 3/4HP models.http://www.brookcromptonna.com/Product Catalogs/Aluminum.pdf. Unlike most rolled steel motors, the Brook Crompton is cast aluminum with lots of cooling fins. Intuitively, this type of motor should be able to dissipate heat much more effectively, which probably explains its 15:1 torque ratio and 1.25x service factor.
-----
*Now my main question is: Am I likely to harm the tool (drill press) by over-sizing the motor to 3/4HP or 1HP (for instance)? I would imagine that the drivetrain has some limits. In a drill press, probably the splined spindle shaft is the highest stress point(?) If the full-load torque is increased by say 25 or 50%, then the drill press will undoubtedly "see" that extra stress under heavy load, right?
Given my lack of experience with machine tools in generals, I want to make sure I'm making sense. Anyone have any input or advice?
At this point, I'm leaning towards a Brook Crompton 1/2HP/1725RPM inverter duty motor (as mentioned above). They are about $220, which is not bad
Thanks!
Tim