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View Full Version : Constant Torque vs. Varaible Torque VFD



robhrzic
08-16-2007, 08:39 AM
I have a vertical knee mill with a 3hp 3 phase motor that I would like to power with a VFD, using mach 3 for speed control. It is a 4 pole, 1725 RPM, 8.9 amp unit.

Would I need a constant torque VFD, or a variable torque VFD for this application? What is the difference?

Would the motor lose any power by using a VFD vs. using a rotary phace converter?

I went to www.driveswarehouse.com (http://www.driveswarehouse.com) and I am a little overwhelmed with the choices. Could someone reccomend a brand / model?

If I understand correctly, I can use the VFD to increase the maximum RPM of my spindle, by increasing the frequency. How much higher would I be able to increase the frequency without risking damage to the motor, bearings, etc...

The head is a variable speed model with a low (60-600 RPM) and a hi range (600-4250 RPM). Would I just leave the head in hi range at the maximum RPM setting, and adjust the actual RPM using the software and VFD, or would I still have to shift ranges to meet speed requirements?

protman16
08-16-2007, 02:04 PM
robhrzic,

Constant torque would be the way to go, especially if you want to do tapping. Torque changes with speed/frequency on a variable torque unit.

As for your motor you should be able to double the speed with out much concern. Normally a motor can run between 20 to 120Hz and still be reliable.

With so many brands out there I can understand why you would be overwhelmed. Personally I like the AC Tech or Lesson brand VFD's. I have had good results with them in the past.

Good luck,

Trevor

in2steam
08-17-2007, 06:20 AM
I have a vertical knee mill with a 3hp 3 phase motor that I would like to power with a VFD, using mach 3 for speed control. It is a 4 pole, 1725 RPM, 8.9 amp unit.

Would I need a constant torque VFD, or a variable torque VFD for this application? What is the difference?

Would the motor lose any power by using a VFD vs. using a rotary phace converter?

I went to www.driveswarehouse.com (http://www.driveswarehouse.com) and I am a little overwhelmed with the choices. Could someone reccomend a brand / model?

If I understand correctly, I can use the VFD to increase the maximum RPM of my spindle, by increasing the frequency. How much higher would I be able to increase the frequency without risking damage to the motor, bearings, etc...

The head is a variable speed model with a low (60-600 RPM) and a hi range (600-4250 RPM). Would I just leave the head in hi range at the maximum RPM setting, and adjust the actual RPM using the software and VFD, or would I still have to shift ranges to meet speed requirements?


They used to be called variable Horsepower and constant horsepower, I guess torque works too. You are wanting a constant, variable torque units are for fans and pumps were the faster you spin the more torque they require.
Please understand that when you decrease below the rating of a motor that you loose torque, and also above the rating for the motor it normally flatlines. To put it simply normal 3 phase motors run best at there name plate rating form there its a give and take. So you will not have a ton of torque at 20 Hz vs 60Hz the drive maybe able to compenstate, depending upon it and the motor, some motors react well to slow speed. There is a disadvantage to running at slow speed, you get alot more heat, and if there is a fan to cool the motor it doesnt work so some secondary cooling might be in order. Your gear box will still be used, I assume that its a variable speed belt since you list a range of speeds if not that then you may consider leaving it in a mid range speed. Needing to shift would fully be depandant upon what you are doing, I can see it not spinning at the 10HZ in high gear and high speed as you tend to need alot of torque to spin at those speeds.
As for how fast you can get the head to go depends upon how well the motor reacts to being overdriven, I have never had a problem with a motor reaching 80 Hz, after that it can be hit or miss, I have had some go past 120Hz but most don't make it past 90Hz. Since its a 4 pole motor you are normally spinning at 1750 rpm so more then likely you can take more, but you really should check your bearing number and see, sometimes they are listed on the motor nameplate. The spindle on the other hand may not like spinning in excess of 5000 rpm, thats a common rpm rating for bearings, so again you should check your spindle bearings and see if they can handle it. If you are planing on using electronincs in the near this machine you will need a filter/line reactor at that HP, otherwise you will get odd results in the computers.

robhrzic
08-17-2007, 12:49 PM
Thanks for the input protman16

I ordered this VFD today, as I think it will fit the bill.

http://www.driveswarehouse.com/Drives/AC+Drives/Sensorless+Vector+VFD/SV022iC5-1.html


Since its a 4 pole motor you are normally spinning at 1750 rpm so more then likely you can take more, but you really should check your bearing number and see, sometimes they are listed on the motor nameplate. The spindle on the other hand may not like spinning in excess of 5000 rpm, thats a common rpm rating for bearings, so again you should check your spindle bearings and see if they can handle it.

Thanks for the feedback in2steam.

I really didn't plan on running the spindle faster than about 5000 rpm, I was just wondering what a safe limit would be. I don't see any bearing info on the nameplate, unless I'm not looking in the right place. See attached slighty out of focus pic.

I was really kind of hoping to find a "sweet spot" for the spindle (for example, 2500 rpm) and then adjust the frequency up or down using Mach 3 and the VFD to attain the necessary speed.

Is there a way to "feedback" (please excuse the incorrect terminology)the actual spindle speed to Mach 3? I don't see how you can have accurate spindle speed control through the software, if there is no way for it to verify the speed. Especially if there is going to be hi / lo range shifting involved, I would think that this would be difficult to keep track of.


If you are planing on using electronincs in the near this machine you will need a filter/line reactor at that HP, otherwise you will get odd results in the computers.

What is a filter/line reactor? Could you please elaborate, and suggest source/specifications? I am going to have a couple of computers within close proximity of the VFD, due to space constraints.

Al_The_Man
08-17-2007, 01:20 PM
I look at it this way when running a 4 pole motor on 120hz, most manuf. also make a 2 pole motor, It is very likely the motor manufacture uses the same materials (bearings etc) for both style of motor, I can't see them stocking two part numbers to save a couple of ¢.
One other issue is balance, again, I would think the same techniques apply to both.
I run my 4 pole motors up to 120hz and so far have not seen any problem.
If you search for 3 phase choke or line reactor, both motor and VFD re-sellers usually stock them, they are sized according to Max motor/VFD current.
Al.

Al_The_Man
08-17-2007, 06:00 PM
BTW if you have a 2 speed system (gear/belt) then you need a way of informing the control, usually by a switch if manual change, this should change the scaling of the output to the VFD accordingly.
Al.

robhrzic
08-19-2007, 09:09 AM
Al_The_Man, what type of switch? Where would I install it? Please elaborate.

Al_The_Man
08-19-2007, 10:13 AM
In a system that has auto gear change, there would be a switch back to the control, usually off of a gear shaft to indicate that the gear change had completed, the control would then make the necessary change in the scaling before issueing the S value.
In the case of a manual gear change, the control should stop the spindle and after the manual change was made a toggle switch or touch screen button etc would be operated manually indicating to the control of the range change and again alter the scaling accordingly.
The only thing with the manual change, the operator has to make sure it is in the appropriate position when the start button is pressed.
Al.

robhrzic
08-19-2007, 11:52 AM
I see, thanks again Al_The_Man for your input.

cjmerlincnc
08-19-2007, 12:15 PM
Hi, You can control the speed of a VFD with Mach. As most VFD'S have a speed input of 0-10v you will need a converter as Mach uses PWM to control a spindle speed. There is a board called Digispeed (if you do a search on the web) which does this and then you will need a sensor on the spindle to get the rpm fed to Mach.

robhrzic
08-19-2007, 12:28 PM
Thanks cjmerlincnc, I have purchased Bob Campbell's Combo Breakout board, which "also includes the spindle speed function that can control a variable frequency drive (VFD) through an optically isolated 0~5 VDC or 0~10 VDC."

http://www.campbelldesigns.com/Combo-board.php

In fact, I just received it Thursday. I just hope that it won't be too big of a pain to figure out how to get all of them to communicate. I mean, of course, Mach 3, the Combo Board, the VFD, and the spindle motor.

Thanks, again.

in2steam
08-22-2007, 03:45 AM
I see your motor is of Chi-com origin, bewarned I have never had alot of luck keeping those motors from going south, esp when in inverter use. Another thing on the several odd off times I have had a failure I took them apart they had sleeve bearings, which don't tolerate higher speeds. Just some words of warning.

chris

Big Daddy
08-22-2007, 11:16 PM
What type of mill is it specifically? Is it a Bridgeport? I did this conversion 2 years ago on a V2XT. I find a very nice difference with variable torg especially in heaver cuts. You can laterally hear it working. It sound like its shifting gears but you can really see the difference in the finishes compared to standard CNC or (Stupid control). Which have a tendency to just bog down when you least expect it. As far as variable frequency; the things that are the most important is your insulation rating, meaning the higher the letter (example -J-) the longer the motor life. But you probably know that. Second and probably just as important, make sure your frequency changes (or your settings) are in increments of 60. 60/120/180 est. You don't want to try something like 100 or 75 for instance! It has to do with voltage balance over a Y and true sinusoidal voltages. 120 isn’t bad but I find my mill likes it at 180. It runs pretty cool & quiet there. I still have all of my wiring diagrams too if you need any direction. The directions I got when I started were wrong so it took a bit of doing and several thousand dollars paid out in service calls before I got something that was working right. But dont let that discourage you, the end product you'll find is well worth it. good luck man.

Big Daddy
08-22-2007, 11:45 PM
Check everything I said if your running on single phase. I'm running on 3 phase.

in2steam
08-23-2007, 02:03 AM
big daddy,

Constant torque avoids the change in torque you are hearing, plus you get better low end preformance. The drives do the same thing they just go about it slightly different. The changing in "gears" can be done on just about any drive, that is the carrier freq, it changes based on several inputs and setttings, mostly speed-this can be bad sometimes. Any drive should give better preformance then in across the line setups, mainly because the drive can compensate for load some(torque boost) and this is esp on 3 phase inputs. As for your frequency changes motors are a fickle, some motors which are designed to run on 50Hz prefer those increments, most american made motors like the 60Hz incremtents and variations on 2,4,6,8 poled motors(3600,1800,etc). Other things like gearing, and bearings can plan into finish also. Since you more then likely have a 6 pole motor on your mill you can get higher freqs(180 would not spin most 2 pole motors unless designed to do so), 6 pole motors in my experience have been less the ideal(like a loud ringing similiar to a bad bearing), but I have had other people tell me they work fine in there applications. I worked at Leeson electric for awhile in there engineering lab I got to test all kinds of motors, the 6 pole was my least favorite.


chris

Big Daddy
08-23-2007, 10:19 PM
First I don’t doubt your credentials, you sound like you know a thing or two about the subject. But over the past couple of years I’ve been exposed to some of the best in the business with degrees & their own companies that do this for a living out here in the west. So I’m not exactly dumb to the concept either. Let’s just say I had to pay thru the nose for a crash coarse! What I’ve learned about 3 phase power vs. single phase power and true sinusoidal and what it can do for the lucking few that choice to incorporate it was astonishing! What I can’t figure out is why anybody would want to go convert a 3 phase system back to a single phase system? Run the phase converts! Put buck/ boosters in place if need be to condition the power. For the most part you have a better balanced 3 leg configuration then almost any grid system in the country! Then if you plug into the mix something like oooo, a senorless vector control with true sinusoidal capability, variable frequency plus variable torque control to boot? Man that’s over the top! All I can tell ya is my machine runs & sounds like no other machine I’ve ever seen! O’ya by the way,,, it’s set up 4 pole, I wouldn’t do it any other way.

Big Daddy
08-23-2007, 10:40 PM
I mean think about it for a minute...What we’re talking about here is plugging in true intelligence into your spindle! A whole separate computer that’s sole purpose in life is to think / control & monitor the power being applied to a machines spindle! That’s incredible to say the least. Don’t you think? For me, I'd love to take advantage of that kind of technology.

in2steam
08-24-2007, 12:55 AM
Amen brother,

But i don't know whom you are talking about converting from 3 to 1 thats foolish. If you only have a single phase line, and you only plan to run one or 2 pieces of equipment which are say 3 HP at the most you can either just buy a static converte if the equipment is not to touchy, or a VFD in some cases. Rotary converters are nice, but expensive and over kill if all you are doing is turning a 3 phase motor on a mechincal press brake for instance. Most the equipment my work purchases nowadays is VFD equiped, one in particular only runs at one speed, but instead of putting it across the line it uses a drive to bring it up to speed and compensate at a good load, this saves on the belts and motor and shaft bearings which are about 10x harder to replace then a few hundred dollar drive every 5-6 years which only takes 5 minutes.
I have to laugh, someone actaully refered to me as having creditials, I just look at myself as a boy with a dream, and a degree in industrial maintenance to boot. The funny thing is that I would have never thought I would end up working in an R&D lab with that degree, its ashame I could not have stayed but 9/11 changed just about everyone.

chris

s76tools
10-13-2007, 08:51 AM
Sorry guys. I don't mean to hijack the thread from robhrzic. I am on the same situation with him regarding vfd's but I am lost and not sure which one to buy. (there is constant, variable, sensorless). I have a 3 axis cnc knee mill I just bought. It is has a 3 hp 3 ph motor and the cnc portion is centroid. On the data plate on the side it says 3 hp 25 amps. I opened the big electrical box and traced the ac input voltage and found (3ea) 25 amp fuse on each line.
I connected the 3 ph ac input line of my newly acquired cnc to my old toshiba vfd which I use to power up my other non cnc mill 3 hp bridgeport mill and the new cnc will not run. It says on the vfd's digital window (current overload).
I know why because my old vfd is only rated 2.2KW and is not enough power to drive the 3 hp motor and the cnc control. I need a higher rated vfd. but not sure which one to buy.
I checked the hitachi SJ100 and SJ200 7.5 hp running around $650 but not sure about what some of the specification exactly meant. (constant, variable, sensorless).

Question is:
1. Which one should I buy?

2. There is only one ac input connector to power up this cnc machine. The machine is in my garage. I have no choice. I have to buy a rotary converter or a vfd.
If I buy a rotary converter there will be no problem but if I buy a vfd and in case I want to change the speed in the future with frequency, I guess I am going to have a problem.
because one connector with 3 phase coming in. I am sure it is split some where to power up the centroid cnc control side with a 1 ph 220 or 110. If I am changing frequency on the output 3 ph of course the output 1 phase will also change and I am sure will do something crazy with the control.
How should I do it?
Right now the speed of the motor is controlled manually. I guess I have to find the input for the control and run a separate 1 ph line?
Any help or comment is appreciated!
Looks like Al the man and In2steam are the most experienced in this area.
Al?
In2?

Thanks!

Al_The_Man
10-13-2007, 10:10 AM
Personally I would not supply the whole machine from a VFD, if that is what I interpret you are doing, you need to separate the 1ph from the 3ph components, and preferably just feed the spindle from the VFD. The control, and possibly the servo's are single phase, if the servo's are 3ph SCR, you may again have a problem.
It depends what you have on the machine.
Al.

Big Daddy
10-13-2007, 03:06 PM
I set mine up with the SJ 100, when I bought it they (the experts) said this would be big enough. But after I had got it installed and ran it for a while I realized that I might have preferred the bigger SJ200. Because it will easily handle the demands of a heavier load while in ball busting cuts.
You set it up the Hitachi to run your spindle motor “ONLY”! Very important.
Your also going to need some good Schematics to help wire it up. I got mine from EMI, maybe they'll help you out. Lastly,, if everything goes badly for you make sure you have a back-up plain. Namely a good machine tech that can step in and finish if the job if it gets to much. I still have my diagrams, they arent the greatest but I'm willing to lend you a hand anyway I can, just use my private mail box. GOOD LUCK!!!

Big Daddy
10-13-2007, 08:04 PM
It doesn't matter you probably already have that set right, my other question is do you have a auto turning feature on your VDF? If so use it and that should solve your problems.

Sorry I posted this on the wrong thread! It suppose to be on the other VDF thread!

bkamali
03-31-2011, 12:09 PM
Dear All,
I have an application to replace DC variable speed motor with an AC VFD controlled.
The application is a traightener and Withdrawal in a continious casting machine. I think I need a variable toque system but I am not sure.
Does anybody help me to decide about that.
Thanks in advance.

perrybd
02-16-2012, 05:09 PM
The difference between CT and VT are their overloads. A VT drive has an overload of 120% for a minute a CT drive will have a 150% overload for at least a minute.

I'd recommend you take a look at VFDs.com buying guide. It goes over some of the VFD basics you should look into when buying a VFD. Here's a Link to it: Variable Frequency Drive Buying Guide (http://www.vfds.com/vfd-buying-guide).

Good Luck with your VFD

mike_Kilroy
02-16-2012, 08:46 PM
actually the difference is the amount of torque ur motor will provide BELOW BASE SPEED.

ct will give same v/hz line from 0 to base speed so u get nameplae rated torque the whole way.

vt reduces the v/hz curve below base speed along a semi exponential curve so the torque available goes down significantly with lower commanded speed. NOT for a machine tool spindle - for a fan or pump