Changing belt drive to inverter control

Hi, like many others I have a round column mill with belt drive giving, in my case, 12 speeds. I don't have a problem with belt drives in general or this one in particular, other than it seems to take ages to change from one speed to another due to having to slacken the belts off and change pulleys and then retighten, usual story.

I had thought that it might be a whizzy idea to change to a 3ph motor with inverter control to give really rapid, simple easy and more precise speed changes. The cost of the changeover is another issue!

Now, rather that reinvent the wheel, has anyone actually done this and how did they achieve it in terms of pulley selection to achieve the speed range without sending the motor into too fast or too slow a speed. Thinking of motor over-or-under-speed and lack of cooling type issues here.

All comments on this gratefully received!

Chris

I changed my Myford VME mill to inverter control some years ago with excellent results, I do find if the pulleys are set for the highest speed I lose power if the speed control is set to run at a lot lower speed, I suppose that is inevitable?

Very convenient mod but not cheap.

Tony

Both of mine are all inverter drive (elliot omnimills) but I also retained the original pulleys so I can change ratio to suit power required. Drilling big holes needs slow high torque, you need a ratio change for that or the inverter will drop out (if it is configured correctly! ).

Mark

Chris,

i have a Chester Eagle 30 mill....it currently runs a 1.1 kw 3 ph 1400 rpm frame 90 motor with a vfd...

Im using the middle set of pulleys, but have the inverter set to 75 hz giving me a motor speed of 2000rpm....

Tony, Mark, John, am I right in assuming you have kept the original set of pulley and just changed the motor for a motor/inverter package? That sound like a logical solution, allowing fine tuning.

Also, have you added a read-out of (quill) speed, if so, where did that come from, how was it built/set-up?

Chris

Mine uses the existing standard (Old Age Pension) motors, three of which have been rewired delta for 240 Volt use. They all run on inverters but they also have a load reactor installed to smooth out the HF output. Your inverters will have a parameter to allow standard non-inverter motors to be used.

As a point of interest, three of my inverters are the Chinese "cheap as chips" variety and they seem to work very well

Mark

Chris,

Yes, thats right....I just changed the motor, retained the pulleys.....

I use a 1.5 kw rated inverter, wall mounted in an enclosure to protect from flying chips...the speed and direction controls are mounted in a small plastic enclosure on the mill just below the main E stop...

I had a Tom Senior mill with 3 phase motor. I fitted an inverter and programmed the speed to run between 50% and 100%. Then I used the belts to change the main speed but most of the time I ran the mill at about 500 rpm. This did most of what I needed.

Edited By David Clark 1 on 15/09/2015 08:41:25

I have a Myford VMB mill with 3 phase motor and inverter. This is the model with poly-V belt drive and it is a real pain to change the belt positions. As a result I keep the same belt setting for over 90% of the time and it works just fine. I could live with just one belt setting if I had to but if the machine already has the ability to have different belt settings it would be wise to keep this feature.

Doug

I have a VMC mill (1.5hp) with an inverter drive. The belts are set to 1180 rpm and this suffices for most jobs, including tapping under power using the jog and fwd / rev functions. I've probably only needed to change belt positions 4 or 5 times in the last 5 years or so, and that was driving a large shell mill. No cooling issues.

So impressed with the mill that I've also equipped my Myford S7 with an inverter package (1hp motor) - belts set more or less permanently to 1480 rpm with only the very occasional need to just change ranges. Never use back-gear. Again, taps and dies used under power (jog) and no cooling issues.

Costly, yes, but it moves the machines into a different class and is a great time saver (no laborious belt changing - a real pain), as well as giving you the ability to set the 'correct' speed for the operation in hand. If you go for it then I'm sure you won't regret it.

Chris,

Keep the pulley as is, it's not worth the hassle trying to change that set-up.

I just installed 3-phase motors with VFDs on my machines, and left the pulleys well alone.

As already mentioned, if I need extra torque/speed, I just move the belt or belts as in the case with the Marlow, and stand back and marvel at latest technology that allows me to adjust speed for cut or vice versa.

Geoff - Life's just too short to make changes not required.

Many thanks to all who have replied to my query - I now know the way I would like to go, just a question of finding the readies!

Chris

Unless you fit a larger motor you will always need more than one belt selected speed range to avoid running out of torque when using larger than usual cutters such as face mills or revs with smaller than usual ones. Its obvious that going too fast with face mills and flycutters creates major problems. Maybe not so obvious that under speeding small cutters can be as problematical due to chip clearance difficulties and high risk of cutter breakage due to the very slow feeds needed to avoid choking when running a small cutter too slowly.

Pre Bridgeport I had a one off Chester Lux variant fitted with a built in inverter controlled 3 hp 3 phase motor and two speed belt drive. Approaching double the usual motor power. The speed ranges were 125 to 1400 rpm in low and 250 to 2800 in high which was fairly satisfactory. The large overlap meant that it was almost never necessary to change belt setting, a right faff, during the job and power at the cutter was adequate for anything I cared to do. As the inverter had no speed display I fitted the control potentiometer with a knob having12 numbered divisions on the skirt and stuck a reference table on the front of the machine so setting suitable speeds was easy. In a perfect world a lower bottom speed (50?) and maybe higher top (3,500?) would have been nice despite the reduced overlap and resulting need to change belt settings more. Given a bit of cunning and creativity the change process could have been made much easier. Changing the B section Vee belt to multi groove would have been a good start.

My friend John kept the standard motor when converting his J head Bridgeport to inverter drive and finds it best to use the two middle belt drive speeds and back gear in the usual way. He says its rarely necessary to alter the belt settings mid job as almost all work can be treated as either high speed i.e. belt on the upper middle or low speed ones. There is normally sufficient torque and speed overlap in backgear to cope without belt shifting.

Clive

If you fit the speed control pot (remote) of the VFD with a 10:1 reduction knob with dial, you will have 100 divisions.

Control is, for all practical purposes linear, so top Speed/100 gives an easy way to set speed (if you really need to be that accurate).

I'm glad the belts and they torque they can give cropped up. It seems that there is a retailer around that might say buy a 2,800 rpm motor to get a 2,800 rpm spindle speed, throw other pulleys and any gearing away and set what ever speed is needed via the inverter. If only.

I haven't done my mill but have changed my lathe. I did what a lot of people do upped the motor power. This means that I can reduce speed and still have the same torque at the cutter as I did have. It can give a reasonable speed range. This subject often crops up elsewhere and some one said it's wonderful and posted a video where the inverter was used to bring the speed right down. It's an excellent example of when extremes may work out. He drilled a sizeable hole with the inverter at 50Hz, this took some time. He then changed the cutter to a sort of fly cutter to face the area around the hole and then brought the speed right down with the inverter and used it which took seconds so there was no chance of burning out the motor due to the cooling being reduced. I've always wondered what happens if a separate fan is fitted as the inverter will control the motor current. Mitsubishi inverters will model the thermal behaviour of the motor which should help but for their own motors. They also have a random switching setting which does a lot to remove the noise problems this type of drive has.

It is possible to get motors that are intended to be driven over a wide speed range. They usually have a fixed speed fan and are balanced for higher speed running and are costly. I asked Brooks about running at high speed and got the answer that he doubted if they made an 1400 rpm armature that would have problems running at 2,800. Wont burst in other words. TEC show how they think their motors should be run on their inverters in one of their catalogues if it can be found. This should give people an idea what ordinary motors can do. One of the catalogues doesn't provide any data at all.

I went from 1/2 to 1hp on my lathe. In part because I had let some of the smoke out of the 1/2 hp a few but there was still enough smoke left in it to allow it carry on working after it had cooled down. As I was making the change I also fitted the inverter. I generally run it at 40 to 80Hz but mostly up to 60Hz. I'm in no rush when I work so didn't change belts that often anyway.

John

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I have a Chester Champion round column and changed it to inverter drive. I machined an extension shaft for a single pulley as I couldn't retain the original cone pulley due to no 3ph motor having enough meat or length of drive shaft to fit. The extension shaft is grub screwed at the top to the motor output shaft and sits in a roller bearing at the bottom which is stud locked to the base plate. This keeps things nice and true and reduces vibration. So I don't change pulley ratios with just having the one I just crank up the rpm!!

John

I've put 3ph + VFDs on my Centec mill, Chester lathe, and Naerok drill. All straight replacements with no change to the pulleys or gears. Most of the time I use the mid range of the pulleys/gears and change the motor speed to suit the job.

Because power = torque x RPM, the general rule of thumb with VFD controlled motors is: below rated speed (typically 50Hz) the motor gives constant torque and reduced power, and above rated speed it gives constant power with reduced torque. If you need more torque for a particular job, then you have to reduce the physical gearing.

Yes I used the original pulleys, I haven't got an rpm indicator, after 40 years as a Toolroom machinist I have a pretty good idea what speeds work with various cutters & different materials.

I would guess Ebay or a Google search should turn up something?

Tony

I too dont have a spindle speed readout........

I listen to the machine and the cutter, letting them tell me if I'm pushing too hard....

that said, it shouldnt be too dificult to jury rig a readout...In fact if you stick with the fixed pulley method, then the spindle speed is a ratio of motor speed, and if your vfd has the means of accepting the number of poles your motor has hen its easy to do.....with a bit of maths...

eg my motor is 4 pole, ergo its speed is 1400 at 50hz, raise the frequency motor goes faster....if your pulley speed at the spinle was 800 for the same speed in at the 1st pulley, then go figure.....easy...

I've just donethis on my Aciera F3.

http://www.model-engineer.co.uk/forums/postings.asp?th=110426

You can work out the spindle speed at 50HZ from the pulley ratio, mine is 837 (pulleys of 2.3/8" & 4" on a 1410rpm motor). You can then 837 %50 x<frequency of VFD>

Although I used a RPM meter on the spindle which was £8 from ebay.

Minimum speed is 88rpm although it was struggling a bit with a M6 tap in mild steel.

Maximum speed is 1783 which is a bit slow I think.