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Thread: Questions from a VFD newby
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19th Jun 2013, 09:30 PM #1
Questions from a VFD newby
Hi Guys
With the recent post about a VFD, I realize I know very little about VFD technology and seek some basic information of what the can do,with one, to improve my lathe . Sure I have googled the subject but I get a mish mash of info and little is specific to my needs as I see them.
My lathe is a basic Chinese generic 12 x 36 with a 2 HP 240 15amp motor which gives my lathe 1800 rpm at max revs. It has a 9 spindle speeds and another nine if I change to another pulley. I have a 5” hydraulic disc brake from a scooter that I had intended to fit but other advice pointed me at a VFD..
I sort of understand that the lathe motor can be run with variable speed and can be made to brake quickly.
Beyond the above I understand little else. If my understanding is slightly or completely wrong please set me straight.
Can I fit this whiz bang doover work on my AC motor or does that need changing out?.
What about power consumption ? Does fitting a vfd improve that?
Are there is any other advantages that may be gained by doing the conversion?
If things look promising ,my intention would be to do the fit up work and get a sparky to make the connections.
Thanks in anticipation.
Grahame
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19th Jun 2013, 09:39 PM #2
Unfortunately Grahame, to use the variable speed offered by a VFD you would need to change the motor to a 3 phase unit. If you happened to have a 2HP 3 phase motor that could be connected in delta then you are only up for the cost of a VFD, which is quite reasonable now days.
I'm not sure if the combined cost of a VFD, motor and electrician to hook it all up would be offset by the advantages, given that you have a lathe the works perfectly for no cost at all. I know nothing about turning so experienced turners may say that the benefits obtained are worth it.
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19th Jun 2013, 09:41 PM #3
Hi Grahame,
First thing, you need a 3 phase motor. You cannot use a single phase motor. You will also need to wire the motor for 220/240v, for most motors that means in Delta (triangle) rather than star (Y). This can get complicated but is generally pretty simple.
The cheap VFD's don't really have braking (although i do believe that may depend on pot luck) but they have a ramp down function that can be set to slow the machine down quickly. This is limited by either your spindle nose (you can unthread you chuck on a threaded nose machine) or overloading the VFD. I have 2 ramp down speeds on the mars, one for high speeds (slower ramp down) and one for slower speeds (faster ramp down).
Not sure about power consumption.
Pluses are 3 phase motors are generally smoother (although mine has developed vibrations at certain speed ranges.....old motor though) and quieter, and there is no start or run cap to blow. I really think you need a tacho and a slightly larger motor to to make best use of the variable speed.
Cheers,
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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19th Jun 2013, 09:43 PM #4Golden Member
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mainly but not always used to convert 240v single phase to run a 3 phase motor for example i have a lathe with a 3 phase 1hp 3 phase motor( input wiring converted to delta by changing links) using a vfd or vsd ( same thing i think) i can run my lathe with variable speed / forward reverse and a whole heap of other parameters from normal 240 volt mains
john
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19th Jun 2013, 09:52 PM #5Philomath in training
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VFD's do have losses. If you compare a motor plugged directly into a source and one with a VFD between the source and the motor to get the same motor output you will need more power.
Michael
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19th Jun 2013, 10:41 PM #6Most Valued Member
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Hi Grahame,
As most here would know already I'm not all that excited about using VSD ramps for stopping.
Why not have a VSD and a brake? The brake will stop the spindle faster than a VSD ever will* and you have the option not to use it.(and in the unlikely event that its the VSD that is the issue the brake will still stop the spindle)
Stuart
*Using ramps. Likely faster than a VSD can stop with full DC braking also.
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19th Jun 2013, 11:11 PM #7
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19th Jun 2013, 11:17 PM #8Member: Blue and white apron brigade
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None of my VSD like to be slowed manually. On my wood work lathe, if I partially slow it manually using the chromed doo-hickey attached to the pulley at the back of the headstock and then take my hand off, the spindle speeds up again to the speed it is supposed to be doing at that point in the ramp down. Of course I haven't turned the ramp down parameters off.
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19th Jun 2013, 11:39 PM #9Most Valued Member
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This is exactly the problem I have with ramps. Not to put to fine a point on it but,,,,, if you have the vsd on say a 4 second ramp and it* grabs hold of you... you're going to have the longest 4 second wait in your life.
Now for normal stop with larger machines I may think differently, but I'd still want a brake.
Stuart
* "it" being the the machine of your choiceLast edited by Stustoys; 19th Jun 2013 at 11:47 PM. Reason: *
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19th Jun 2013, 11:46 PM #10Most Valued Member
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20th Jun 2013, 11:27 AM #11Cba
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> Can I fit this whiz bang doover work on my AC motor or does that need changing out?.
No, you would need a new 3 phase motor
> What about power consumption ? Does fitting a vfd improve that?
I do not think that should influence your decision, unless your lathe was running something like three 8 hours shifts a day, every day.
But to answer your question: fitting a VFD may improve power consumption.
> Are there is any other advantages that may be gained by doing the conversion?
I personally regard variable speed as the most useful single modification that one can do to a lathe, the smaller the lathe the more useful you will find it. This is what immediately springs to (my) mind:
- you can fine tune rpm to every situation on the fly, avoiding the regions of resonance where your lathe chatters. The benefit is much improved surface finishes, and faster material removal.
- when facing, you can vary the rpm during the cut, thus keeping surface speed constant. The benefit is better finish and faster metal removal rate.
- when parting off, you can vary the rpm during the cut, benefits as above, plus less tool breakage
- you will need to change gear or belt position MUCH less frequently (the larger you choose the VFD, the less gear changes you will need)
- You will double your maximum spindle speed (just make sure the bearings can keep up), and you will have infinitesimally slow speeds available for things such as threading to a shoulder, or winding coils.....
- soft starts will prolong the life of the drive train
- dynamic braking lets you shorten the time to stop the spindle - especially useful on a manual lathe, where many control measurements are taken
- there will be less noise, since the motor will not always have to run full spee
- there will be MUCH less motor vibration than with a single phase motor, surface finish WILL improve
I am sure there are more advantages....
Chris
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20th Jun 2013, 11:49 AM #12Cba
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Stuart, VFD's always use ramps for accellerating and stopping. Dynamic braking is using the motor as a generator, transforming motion energy into heat across a resistor. The effect is similar to electric eddy-current brakes in a truck. Just set the stopping ramp to a long value if you prefer it the behave like it was coasting to a halt. DC injection can be activated additionally to dynamic breaking. DC injection begins when the motor is almost stopped, it locks the lathe spindle for as long as DC injection is activated - usually just a few seconds. It can be very useful, for example think of using your lathe to wind a coil, DC braking can be used to keep the spindle from turning backward and slackening the wire thus unwinding the coil you just made.
You do have the option to set different ramp times depending on workpiece inertia, if you make sure the VFD control panel is easily accessible. Sure, a mechanical brake is much better at braking, but only larger industrial class lathes do come with mechanical brakes. A VFD gets some limited braking capability to a low cost home shop lathe - and it does so as a free byproduct of VFD technology. Free benefits are always appreciated but rarely perfect.... Chris
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20th Jun 2013, 12:02 PM #13Most Valued Member
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20th Jun 2013, 12:38 PM #14Cba
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For the question of reducing the need for gear changes, what matters is the drive size (size of both motor and VFD). You can say that for every time you double drive power, you halve the need for gear changes.
That said, you will already need much less frequent gear changes replacing a 0.55kW single phase lathe motor by a 0.55kW motor with VFD. Upsizing the VFD to 0.75kW was a good choice, but for other reasons than reducing the need for belt changes.
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20th Jun 2013, 12:59 PM #15Most Valued Member
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Questions from a VFD newby
Thanks Chris. The reason I up sized was to allow for a 30% derate at the maximum switching frequency of 16kHz. I wanted to be sure I could find a frequency that wouldn't drive me crazy. I'm pretty sure I can no longer hear 16kHz!
Chris