VFD and vari speed pulleys

[bw]

I am getting ready to install a VFD on my old supermax vari speed mill running Mach and would like a little advice and or suggestions about getting rid of the vari speed drive and going with direct pulleys.  The motor is a good 3 hp 3 phase and I have read I can get more RPM’s out of it with the VFD, 99.9% of my machining is aluminum so besides the occasional tapping head low speeds are not that much of an issue

What I don’t know is what kind of ratio as in 1 to 1 and what diameters I should be looking at for the pulleys, also a good source to get the pulleys.  I figure there has to be a few others out here who have done this and wouldn’t mind sharing the good, the bad and the ugly about doing it.

I am sure I will have a few more questions when I get to installing the VFD but I would like to do the whole job at once so any help will be appreciated.

Thanks.

[HimyKabibble]

I have a step-pulley BP clone with a VFD.   The vast majority of my work can be done with the motor in 4P mode, and the fastest pulley, which gives 2720 RPM at the spindle at 60Hz, and 5440 RPM at 120Hz.  This setting gives me plenty of torque for all but the largest drills, which require stepping down one pulley.  And my VFD is a V/Hz drive, so a sensorless vector drive will give better low-speed torque.  The only time I have to use the highest speed (switching the motor to 2P), is for finish cuts with small endmills, which lets me get up to 8200RPM.

Regards,
Ray L.

[Overloaded]

Hi bw,
  Here is a topic for reference.

http://www.machsupport.com/forum/index.php/topic,17770.0.html

..... and a link to where I got my pulleys and bushings.

http://stores.ebay.com/ElectricMotorSite/Tapered-Bore-2-Pc-/_i.html?_fsub=250396016&_sid=18379426&_trksid=p4634.c0.m322

They had a 35mm taperlock bushing that fit the stationary portion of the variable pulley at the spindle perfectly, keeping the brake as original. Made installation a breeze.

Regards,
Russ

[bw]

I like the idea of the v belts over timing as one of the things I hope to get out of doing this is to silence the beast.  My step pulley Bridgeport runs smooth and quite and I am hoping for the same on this when I’m done.

What I am having trouble wrapping my head around is the motor is 1725 rpm’s, and max rpm of vari speed is 4000 rpm, what diameter pulleys will I need to get that same ratio?  I know there must be some equation to figure it all out but I don’t know what it is.

Thanks

[Hood]

4000/1750= 2.29 so a ratio of 2.29:1 would give you 4000rpm. However as you are going to be using a VFD you will be varying the frequency of the motor to get the different speeds so you may be better actually going down the way in the ratio so that you get a slower rpm at base frequency and thus higher torque but you will still be able to get the higher speeds.
Hood

[HimyKabibble]

I like the idea of the v belts over timing as one of the things I hope to get out of doing this is to silence the beast.  My step pulley Bridgeport runs smooth and quite and I am hoping for the same on this when I’m done.

What I am having trouble wrapping my head around is the motor is 1725 rpm’s, and max rpm of vari speed is 4000 rpm, what diameter pulleys will I need to get that same ratio?  I know there must be some equation to figure it all out but I don’t know what it is.

Thanks

Speed ratio is determined by pulley diameters.  If you have a 4" motor pulley driving a 2" spindle pulley, the spindle will be running 2X as fast as the motor (4/2).  If you turn that around, and put the 2" pulley on the motor, and the 4" pulley on the spindle, then the spindle will be turning 1/2 the motor speed (2/4).

You need to decide what you want your max spindle RPM to be, and what maximum VFD frequency you will run, and select your pulleys from there.  You will no doubt be using a larger pulley on the motor than on the spindle, so the spindle is spinning faster than the motor.  If using the stock motor (not VFD/inverter-rated), it seems usually safe to run 2X the base frequency.  If using an inverter-rated motor, you can often go even higher.  USe the largest pulleys you can fit into the available space, to get the greatest belt contact area.

So, if your motor is rated 1725 RPM, and you run the VFD up to 120Hz, that will give you 3450 RPM at the motor.  If you want 5000RPM for your max spindle speed, then you need a pulley ratio of 5000/3450 or 1.45:1.  So if your motor pulley is 5" diameter, the spindle pulley must be 5/1.45 or roughly 3.5" diameter.

Finally, make sure your low-end RPM is reasonable.  A good sensorless vector VFD should provide usable torque down to perhaps 10Hz.  For the above example, this would give you 5000 * 10 / 120 = 416 RPM.  Note that, depending on the motor, running extended periods at low RPMs may cause the motor to run hot.

Regards,
Ray L.

[bw]

Ray, that is just what I was looking for.  I figure I am going to keep the back gear so low speeds shouldn’t be an issue and I of course would like the most rpm I can get on the top end without burning up the bearings. 

Thanks!

[HimyKabibble]

Ray, that is just what I was looking for.  I figure I am going to keep the back gear so low speeds shouldn’t be an issue and I of course would like the most rpm I can get on the top end without burning up the bearings. 

Thanks!

Like I said, I've been running mine up to 8200 RPM for years with no problems.  The bearings get warm (maybe 120F) after several hours of running.  That will be your first indication of trouble - if the spindle itself starts getting really hot.  Warm is OK, hot is not.  And it's neither difficult, nor terribly expensive, to replace the spindle bearings.

Regards,
Ray L.

[bw]

I will look and see if I can tell what bearings are in there when I get it apart to see what they are rated for.  I have replaced the bearings in the j head before so I know it's not that big of a deal, I have a set of good j head bearings on the shelf, if they are the same as in the supermax I may just swap them while I have it apart.

[Overloaded]

I used a Marathon Black Max inverter duty motor, max safe speed 5400 but I never needed to run it over 4500.
It runs at full rated HP down to 30 rpm, the lowest I have needed to run it so far.