VFD/Potentiometer/Motor Questions

I'm hooking up a VFD to my 1961 vintage 9" South Bend lathe (with .5 hp, 208 vac, 3 phase motor) and I have a few questions. I can hook up an external pot to control the speed of the lathe. The specs says the pot has to be between 3K - 5K ohms. What would be the difference in operation if I were to use a 3K ohm pot vs a 5K (or vice versa)?
What would be a reasonable maximum speed for the motor? The nomenclature plate says 1725 operating rpm, but I understand with a VFD you can go much higher. I don't want to hurt the motor, but the extra speed would be nice when using carbide. I've also got the ramp up and deceleration speed currently set at two seconds; is this reasonable?
I knew nothing about VFD's when I started the installation (I'm still not quite finished), but I have to say that it sure seems slick. Not sure of the advantages/disadvantages of using a VFD compared to a static phase converter (which is what I was using before). I understand with a static phase converter you lose 1/3 of your horsepower, not sure if the same thing happens with a VFD. I'm hoping not...
Thank you!
Dave Young

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no difference. VFD measures potential at the wiper.
i

Thanks Igor!
Ignoramus7204 wrote:
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You might want to add a resistor at each end of the pot to limit the max and min rpm. Total resistance to be between 3K and 5 K. Maybe a 1K resistor on each end of a 3 K pot.
Dan

Hi Dan,
Thank you for your reply. On this VFD (Automation Direct GS1 series) I can limit the top end parameter (but not the low end). I'm actually looking to buy a 5K pot, but I was curious to see if there was an advantage to using a 3K (or 4K, etc) pot.
Thank you again!
Dave Young
snipped-for-privacy@krl.org wrote:
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wrote:
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You do that with programming.
The larger value pot will simply give you a finer adjustment range.
Gunner
"If thy pride is sorely vexed when others disparage your offering, be as lamb's wool is to cold rain and the Gore-tex of Odin's raiment is to gullshit in the gale, for thy angst shall vex them not at all. Yea, they shall scorn thee all the more. Rejoice in sharing what you have to share without expectation of adoration, knowing that sharing your treasure does not diminish your treasure but enriches it."
- Onni 1:33

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without arguing with gunner, every VFD I've looked at excites the pot with a fixed DC voltage (usually 10VDC) and uses the voltage at the wiper for it's control voltage - any value will work so long as the current through the pot is less than the max it can source, and the impedance of the pot is about 10X less than the input impedance of the control voltage circuit - typically the input impedance is around 100K, so typically 3-10 K is fine. precision is unaffected by value of the pot. If you "float" the pot with resistors, you will get fewer volts per degree of rotation, and that will be greater precision of setting - but if you wanted precise setting, you could use a multi-turn pot or set the speed with the keypad.

They make 3 turn and 10 turn pots all of the time. Often used in Electronic gear.
Martin
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William Noble wrote:
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    Assuming a 10V control voltage, the 5K pot would dissipate 0.020 Watts, the 3K pot would dissipate 0.033 Watts. Essentially negligible. Operationally -- *no* difference. The range given is selected so:
1)    The resistance is not so low that it is drawing too much     current out of the reference output on the VFD.
2)    The resistance is not so high that the wires pick up too     much electrical noise -- which would result in constant     variation of the speed.
    Anything within the range which they specify will work without problems, and can be considered equivalent to each other.
    If you remote the pot some distance from the VFD, I would suggest that you run the wiring through a shielded twisted pair wire. The shield connects to ground and the low side of the pot, one side of the twisted pair connects to the pot's high side, and the other to the pot's wiper. The VFD's manual will tell you where to connect each wire at the VFD end. And put the pot in a metal (aluminum will do) box at the lathe end of things, with the shield connected to the box metal as well. This will minimize noise pickup.
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    This depends on the motor itself. Often the same rotor is used in several speeds of motors by the same manufacturer, so if you can find data from the manufacturer of the motor indicating that the same part number rotor is used in the 3450 RPM version of the motor, you should have no problems running the existing motor up to twice its speed. Anything beyond that would be a serious problem.
    If you can't find such information, pick up a 850 RPM motor with a bit more horsepower, change the pulleys to get the same spindle speed with that motor, and you should be able to run that one to twice its speed with no problems -- since you are unlikely to have a particularly large motor on a 9" South Bend anyway. The larger the motor, the more likely that overspeed will cause it to self-disassemble.
    If your VFD can go up to 400 Hz (some can), *don't*. Stick to no more than 120 Hz unless you have a motor *designed* for 400 Hz (such as aircraft motors.)
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    It should not lose any of the horsepower, as it is producing a full three phase full time. The static phase converter only makes up a pretend third phase for long enough to get the motor spinning, and then turns it off -- pretty much akin to the centrifugal switch and start winding on a single phase motor.
    Good luck,         DoN.

Don,
You've answered some of my questions in the past, and I want to thank you once more for taking the trouble again.
I learned a lot from you regarding the shielded wiring. I had already ordered twenty feet of 18/4 shielded wire from Ebay ($3.99 for ten feet!) for both the pot and for the drum switch (the VFD does in fact have 10 volt control voltage). On page 2-10 of the users manual (http://web2.automationdirect.com/static/manuals/gs1m/ch2.pdf) it shows the shielded wire connected to the CM (common terminal I assume). So with this, would I still ground the shield to the box the pot is in?
The motor was made by Westinghouse. I'll set the max speed to 150% of rated RPM (to about 2500 RPM) and keep an eye on it. My VFD will go to 400 Hz, and I'll heed your advice to not go there. I really don't think I need that kind of speed anyway.
Thank you again!
Dave Young
DoN. Nichols wrote:
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Generally speaking, a good quality 4-pole(1800 RPM) motor will handle 120 Hz without damage. 2-poles are another story and you should probably limit top speed to 90 Hz. The worst thing that can happen is to throw a rotor bar into the air gap, mechanically jamming the motor or cutting an winding end turn. In either case, the VFD's trip circuit will shut down the motor power in a fraction of a second with no damage to the VFD.
Randy

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    You're welcome.
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    Hmm ... I've not bought wire *new* for a long time, but that seems rather high by hamfest prices. (If you don't know hamfests, consider them to be electronics focused flea markets. :-) Of course, when I see a wire which I am likely to use, I tend to buy the entire spool, not a short length.
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    Hmm ... 18 gauge wire strikes me as overkill for the signals involved. I would be happy with 28 gauge wire for those -- though if you found an oil-resistant wire, that might be good to deal with the hazards of the shop. The 28 gauge would be better off inside some kind of protective jacket -- possibly conduit -- if the VFD is remoted from the machine. However, you *will* need heavier gauge wires for the actual power to the motor from the VFD. And you *might* benefit from shielding there as well, to reduce RFI (Radio Frequency Interference) making a hash of AM radios or TV images nearby.
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    I would -- unless the entire box and the knob on the pot were well insulated from contact with you or the machine. You could get away with a plastic box if the shield stops fairly close to the pot, and if you aren't gripping the metal shaft of the pot to possibly act as an antenna.
    But it would not hurt to first check for voltage difference between the shield and the lathe's frame. If you have that, it is probably from leakage in the motor's windings. In that case, you start with a large gauge wire (say 16 ga or 10 ga) between the ground terminal on the VFD and the frame of the machine tool. Then, if there is significant leakage in the motor, it will trip the protective circuits in the VFD instead of making your machine a source of shocks.
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    As someone else suggested, you are probably alright up to 120 Hz on a four-pole motor (such as your base RPM suggests). Anything over 1800 RPM (typically 1750 or 1725 in reality with slip taken into account) would be 2-pole, and be unlikely to have ever had the rotor tested for higher speeds than the standard 60 Hz line would provide. Again, the larger the motor, the more likely self-disassembly would be when overspeeded.
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    You're welcome.
    Good Luck,         DoN.

I personally think that he would be just fine using any twisted wire, no need for shielded wire.
i

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    Twisted pair is good when two-wire driving a balanced load, such as the differential drives to differential SCSI, or balanced audio through phone lines.
    But when one side is tied to ground or a common (as in this case), and when you have three wires instead of two (two connected to hard points, ground or +10V, and the other variable and sensed as a voltage level, you get cleaner behavior with the shield tossed into things.
    You can likely get away without it, but it is little more trouble to add shieded to the twisted pair, and he needs three conductors anyway to go to the pot.
    Enjoy,         DoN.

Dave Young wrote:
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IF the pots all have the same taper they will all work the same. The pot is hooked across a reference voltage and the wiper is used as a voltage divider for the input to the speed ckt. 5 k will use a little less power.
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Depending on the bearings in the motor and how well it was originally balanced. 150 percent of the speed will work in most cases.
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A static phase converter is similar to a start capacitor on a single phase motor. Since you are using only some of the windings to run the motor, you only get some of the rated power.
A vfd will generate full hp at the rated rpm of the motor. when you reduce the speed. you reduce the HP output even though the torque will be maintained. If you go above the freq./ rpm rated speed of the motor, the hp output will be more than the rating on the motor.
John
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On Fri, 11 May 2007 01:50:01 GMT, Dave Young
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No difference. The pot is just a voltage devider. Be sure you use the right "taper" pot - likely Linear, not logarithmic. A higher resistance pot loads the control circuitry a little less, so I'd likely use an industry standard 4.7L linear pot, or a 5K
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Generally not much more than 20% overspeed.
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The VFD is significantly more efficient and has the extra plus of variable speed.
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Again, my many thanks for all the input. I'm waiting for the shielded wire to come in I ordered last week. I still need to wire the motor and run it to the VFD and mount the box for the pot somewhere comvenient. But things are coming along nicely....
:-)
Dave Young
clare at snyder.on.ca wrote:
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I've got the VFD installed (minus the remote pot) and running. I'm going to take the motor back off and see if it is actually wired for 220 (verses 440). I just don't have the torque that I think I should have. Admittedly, .5 horse isn't that large, but I can easily stall the motor with not so deep cuts. Torque-wise, it's not any better than a static phase converter, although the adjustable speed is wonderful. Is it possible to run a 440 motor on 220 and just not get the torque?
Dave Young
Dave Young wrote:
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Yes, it is exactly what you should expect to happen if the motor is wired for 440.
Randy

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Torque will not be improved or lessened by using a VFD (except on the extremely low ends). Its a matter of pully ratios. Its better to run the motor faster because you have a smaller pulley on the motor, than anything else, in my experience.
and you are right.. .5hp is suckage. Lots of machines use them..but they have geared down the spindles usually via pulley ratios.
Personally..I dont think anything under 3/4hp is worth much on a real lathe.
Shrug
Gunner
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