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Thread: VFD and 120Hz wood shapers...
03-08-2007, 11:26 PM #1
I've posted this question at owwm.com and it was suggested that I ask over here as well...
I'm thinking of purchasing an old Whitney No. 134 direct drive double shaper from a friend. It's a nice looking machine - compact, clean, etc. The only hitch is that it came out of a factory that was also running the original frequency converter with it. As it is, the converter was much larger than the shaper it powered...remember that I said I like the small size of the machine!
The motors are both 4 HP rated at 120 cycles, 7200 RPM. I was planning on getting two small, solid state VFDs and using those to get each motor up to speed. I don't fully understand (and I'm OK with being a little in the dark) the process, other than it essentially creates a synthetic sine wave. I understand that some people believe that the process producing the wave can damage the insulation in the windings. I will be using the machine in a one person shop as a hobby, so most people have told me they don't think I'll have any problems with motor damage, but I was hoping to get some feedback from the folks on this site as well.
Now for the questions:
1. Here is the VFD model I was planning on using: SAFTRONICS CV10 COMPACT AC VECTOR DRIVE. MODEL CV102005-9...will it work???
2. Will these VFDs hurt the motors in the shaper?
3. If the motors are already set up to run at 120Hz, what exactly is the VFD doing? I was under the impression that a VFD was used to speed up a motor that was made to run at a different RPM...like make one wired for 1725 rpms go faster/slower...where is my thinking skewed?
Thanks for your help.
03-09-2007, 12:37 AM #2
1) Yes that will work. By the way, you can buy that exact same drive from Teco, who makes it and sells it to Saftronics. Pricing is likely better from Teco, but if you have a favorite supplier who takes good care of you, by all means stick with them. If you are buying over the web and are willing to go it alone, go for the best price you can find.
2) In general, the motor problems associated with using VFDs are associated with the higher voltage ratings, i.e. 380 - 575V, and/or where there is a long distance from the VFD to the motor (+25ft). If you are a little worried you can always add a reactor to the output of the VFD, or if you are really paranoid you can buy an output filter. I personally wouldn't bother, but then again, I didn't pay for the motors either
3)For a motor to run at 120Hz, you need to give it 120Hz. Your utility power is at 60Hz. The output of a VFD is like a completely new power source, it uses your utility power only as a "raw material" so to speak. So you will need to program the VFD to provide 120Hz as the maximum frequency when it is at full voltage (assuming 230V). You don't probably need the VFDs ability to lower the speed, in fact that's probably not a great idea on a shaper because you could smoke the bit. So you will just be using it as a frequency converter only.
Without the VFD, you have no way of getting 120Hz power (other than re-using that big box you mentioned).
By the way, I hope you have 3 phase power. 4HP is too big to take advantage of a 1 phase input on that VFD. You would need to buy the 7.5HP version.
03-09-2007, 10:05 AM #3
Thanks for the reply.
I like the idea of extra protection for the motors...I checked on the Bay and found a fair number of reactors (under Business/Industrial), but they were 50/60Hz. I'm assuming I need one rated for 120Hz. Any ideas on where to find these guys?
05-24-2007, 12:08 AM #4
JRaef - Thanks for your answers to my questions.
A slight change of plans - the double shaper I'm getting is a different machine, although it's still direct drive, so I'll need to use the VFDs. The only difference is that the motors are 5hp each. My Saftronic VFDs are 20.5A input (3ph) and 17.5A output (3ph). I believe this is still OK to power the 5hp motors.
I was speaking with a friend and he told me to ask you if it's OK to run these VFDs off a 3ph RPC panel if it's well balanced (within 5%). He said that he thought it would be OK, but that you would really know the answer. I can put something in between the RPC panel and the VFDs to help "filter" if need be.
Also, since I'm going to be running wood through the machines these VFDs are powering, I was thinking it would probably be a good idea to put the units in some sort of enclosure so the innards don't get covered in sawdust. Thoughts?
05-24-2007, 11:22 PM #5
You never stated the voltage, but I'll assume 230V. 5HP @ 230V should be around 15.2A give or take, so you should be fine in that aspect, but...
If you only have 1 phase power, that VFD is too small to run directly. It needs to be 2x the rating of the motor to be safe, so a 10HP drive is necessary. Using an RPC on the front end will help that situation because it will sort of balance the loading on the input diodes, but not completely, and there will still be additional ripple on the DC bus that may be too much for the caps to take care of. The problem is, you will not find out if it was good enough until AFTER the magic smoke comes out. Sorry to be the party pooper, but I calls 'em like I sees 'em.
Also, protection from dust and dirt is always good. Ventilate with fans and filters if you can (and you are good about maintenance), but if not, use a box that is 6 x the volume of the drive itself. So if the drive is 10 x 6 x 5, that is 300 cu. in., so use a box that is 1800 cu. in. To figure the size, start backing out maximum dimensions you can live with, i.e. if you don't want anything over 10" deep, 1800 / 10 = 180 sq. in., and you don't want anything over 20" high, 180 / 20 = 9" wide. So a 20 x 10 x 10 sealed box should work.
Side note: some of you who have seen me post this in the past may notice that I have adjusted my previous formula from 4x volume to 6x volume. That is because I have noticed that in the past 5 years, newer drives are smaller than they were when I learned that formula. I recently had one get a little too hot on me using the 4x value.
05-25-2007, 10:57 AM #6
No...these drives were going to be run off the balanced 3ph from the RPC. So there's no way to know whether or not it will work until after the VFD is fried?
Are there any other options for me to explore? Each VFD is rated good for a 5hp motor and I will be using them on 5hp motors.
Thanks for your help on this
05-25-2007, 09:14 PM #7
The problem is, the RPC output may be balanced as far as a meter or a motor sees it, but the rectifier in the VFD front end might react differently to the capacitor supported 3rd phase. I didn't think this was an issue in the past, but it was brought up in here once before and I have since asked around. (previous thread) Nobody is quite sure. My fear is that since there is no inductance in the rectifier it will instantly pull down the cap manufactured leg and your input will end up being just as imbalanced (as far as the VFD diodes are concerned) as if you fed it 1 phase.
But I am not an expert in RPCs, that is just my understanding of them from reading previous posts in here. Maybe I am wrong. Other regulars in this forum are more knowledgeable on RPCs, maybe they will chime in. peter5322 would be a good resource for that. link to peter5322
05-27-2007, 01:24 AM #8
I asked peter5322 and these were his responses:
(question)...Since these VFDs are only rated for 5hp at 3ph input, can I run them off my RPC panel if it's well balanced (within 5%)?
+/- 10 percent should be good enough, but +/- 5 percent is better.
(question)...Is there something I can put between the RPC panel and the VFDs to
>protect them? Like a "filter" or something?
Possibly a "line reactor", which is a length of steel pipe several inches
long, and with several turns of the motor feeder's conductor passed
Practical Machinist once had an example of making a line reactor out of
pipe and wire, but I couldn't find it.
There are commercial line reactors, of course, but these are usually very
expensive, and are very often over-kill.
05-27-2007, 01:39 AM #9
Jraef - I found this line reactor - I'm still waiting to hear back from peter5322, but I assume this would be a good "extra protector" to put between the RPC and the VFDs to make sure that no surges run back to the RPC.
05-27-2007, 06:16 AM #10
This whole discussion hinges on the motor's name plate electrical ratings. Please state them in full: Volts, Amps, Hz, dual voltage, etc. Without these figures the people best qualified to help you are flying blind.
A modern VFD will solve a wide variety of motor drive problems but you still need hard number to determine the suitability of application.
Don't get lost on minutia like line reactors. They are critical in some applications particularly when running older 460 volt motors at full load from a VFD because of the energy of the voltage spikes characteristic of PWM electronics running inductive load. Reduced loads and dual voltage motors connected for the low voltage will impose fewer problems arising from voltage spikes. In any case a suitable reactor can be extemporized from a short piece of pipe with a suitable troidal winding. High priced pro gear like line reactors are low tech. They are nice but in their absense a handy home shop owner can whip up an expedient from the scrap box.
Single phase input is solved by oversizing the drive by an approtiate factor. Not to worry. 5 honest HP at a woodworking spindle is about all a strong man can muscle against. You don't have to take full HP cuts and it's likely you never will. The VFD cannot be hurt by overloading. It will sense out of parameter operation and self limit or trip into "fault."
05-28-2007, 12:04 PM #11
Oh...sorry! Here's the info from the motor plates:
Does that info help? Thanks.
05-28-2007, 02:44 PM #12
Yup. As it happens that is a rather light electicl load for a motor rated 5 HP. You could probably run it from a 3 HP VFD. VFD's are rated for the largest HP motors likely to be connected and a 3 HP which are those having 1.15 service factors. Since the load on a shaper is intermittant the spindle motor and VFD can be rated accordingly.
Further, most VFD's can be set so their overload default mode is to slow down the motor giving the user an audible cue to lighten up.
So while you can use a 3 HP drive and save a few bucks while sacrificing little productivity, the 5 HP option is still opn to you.
Given the modest ratings of the motor and the robust ratings of the VFD I would guess without further analysis you could run the three phase only 5 HP VFD from single phase without a worry for the input diodes so long as you set the max motor current at 12.5 A.
05-28-2007, 05:13 PM #13
You're suggesting that I run my 5hp VFDs (and therefore the shaper) off single phase rather than off my converted 3ph? The input on the VFD says 3ph 220v...
Can you further explain? Sorry for my ignorance.
05-29-2007, 06:49 PM #14
I see you have recieved some excellent responses to you question about the shaper.
Here is an easy an practical test you can do with no danger of damaging any equipment.
Find out if the VFD will work on single phase by wiring the VFD to a 3 phase motor and connecting the VFD to single phase. Motor connects to T1 (U), T2 (V) and T3 (W). Try L1 (R) and L2 (S) for the single phase input first. if the VFD powers up then it will run on single phase. BE AWARE YOU MUST HAVE a MOTOR CONNECTED before you power up the VFD.
If the VFD works then connect it to the shaper when you have the shaper and give it a go. The VFD is protected so you can not hurt it unless you try to operate it without the motor connected.
If the VFD will not power up on single phase then post here and we will give you another suggestion on how to get it going. I am not familiar with the model you have but I think I remember some comments about it running on single phase.
05-29-2007, 09:14 PM #15
Hmmm... I don't know about that. Running a VFD not designed for 1 phase input without derating it is playing with fire, almost literally. I understand your point Forrest; his motor is not drawing typical 5HP current. But the N3-205-C Teco drive (which is what that Saftronics drive really is) is rated for 17.5A maximum. So even that 12.5A motor is going to pull almost 22A from the 1 phase line if that motor is ever fully loaded. That is too much for that VFD.
If the VFD does not say it is rated for 1 phase input, and you connect it to single phase, and you do not derate it, then you run the risk of frying the input rectifiers. ALL of the protection circuits for the VFD will be derived from the DC bus. The rectifier is unfortunately BEFORE the DC bus, so if you have a 5HP VFD and connect it to a 5HP 3 phase motor and it pulls the 3 phase current of a 5HP motor, the current through the rectifier will be 1.732 times the motor current. So the protection systems may think that everything is hunky dory, while at the same time the silicon in your input rectifiers is becoming a full time conductor (shorting in other words).
You will also have trouble with ripple current in that the difference between the peaks and valleys of the DC voltage after the rectifier will be significantly higher when fed with 1 phase power. If the VFD was not rated for 1 phase input, the smoothing capacitors on the DC bus will be unable to supply enough boost voltage, leaving a lot of ripple in the DC line as it feeds the output transistors. When that happens, the output transistors may be firing at a peak one time and a valley the next. But at the same time this transistor is firing at a valley, the opposing transistor in the other phase may be firing at a peak, so now the power actually flows the wrong way! That sets up negative sequence currents flowing back through the transistors, which overheats them quickly and the magic smoke escapes.
05-29-2007, 09:34 PM #16
So should I run the VFDs off the converted 3ph power (which will be balanced to 5%...unloaded) and put a line reactor to protect stuff?
05-30-2007, 04:05 AM #17
Yes, although I'm not sure the line reactor is all that necessary. Still it never hurts to have one, kind of like cheap insurance.
05-30-2007, 08:37 AM #18
05-30-2007, 05:43 PM #19
I recently put a Mitsubishi Freqrol FR-A200- 5.5K (7 1/2 hp) vfd on my SB Nordic lathe with a 7 1/2 hp motor. I was origionally going to power it with my 15 hp RPC (even ran the 3ph plug outlet). I also ran a 1 ph line with this outlet plug next to the 3 ph plug. Thought I would give it a try and if it didn't work on 1 ph, had the 3 ph outlet avaiable.
Have been using it for a couple months on single phase with no problems. I routinely take 0.100" deep cuts on 3" crs bars at a feed rate of 0.010 ipr at 550 rpm. I was surprised it would take that cut, but it does - just throws hot chips everywhere.
I had to modify the lathe because the Nordic has (had) FWD, REV, and STOP clutches. Removed the clutches and made it direct drive - and let the VFD stop it.
Can't speak for other VFD's, but the Fr-200's seem to function well on 1 ph (at least up to 7 1/2 hp). I have 4 other machines (1 to 3 hp) powered by Fr-200's - all on 1 ph. I rarely need my RPC now (just for my surface grinders and hyd press).
05-30-2007, 09:40 PM #20
As I alluded to earlier, the problems will be seen by components IN the system, but not the VFD system as a whole. So you could be overloading your diode bridge during those heavy cuts, but there is nothing in the VFD that is going to tell you that until the bridge fails one day. The same is true of the transistors being fed with too high of a ripple current. there is nothing in the VFD that monitors this condition because it is not supposed to happen in the first place, so you won't know about the incremental damage being done to the transistor silicon substrate until the moment it fails. overloading / ripple current creates component heat. Heat x time = failure in electronic components. More heat, less time to failure.
Still, it also depends upon the load you are putting on the motor. If even at these deep cuts you aren't using all of the available power from that motor, then it's a moot point. Have you observed the output current when doing those tougher jobs? If it's still less than 1/2 of the VFD's capability, you're golden. It's just a risk factor you are taking.