PDA

View Full Version : How to get 60 to 100 RPM



madman
12-31-2009, 11:46 AM
From My Bridgeport Mill. I need to really slow down the mill for some work im attempting to do. What is the proper way to get it down to these speeds without sacrificing Torque? i am using my Mill for a Pile of Cylinder Head Projects for some new Drag bike engines Im building. thanx guys and Im wishing you all a Happy New Year. Take Care

Doozer
12-31-2009, 11:48 AM
Sensorless vector VFD.

--Doozer

Dawai
12-31-2009, 11:50 AM
YOURS still three phase Mike? if so.. hook that 3ph inverter I sent with you to it via twist lock plug..

Mine has a back gear.. seems you had a older one?? dang thing turns in reverse in the back gear thou.. It takes a drill bit hours to drill through something turning the wrong way..

madman
12-31-2009, 12:18 PM
Mine is a 3 phase motor converted to single phase. It was rewound and a small box with a capacitor inside it is used to run it. My electrical xperience is poor at best.

Richard-TX
12-31-2009, 12:34 PM
Mine is a 3 phase motor converted to single phase. It was rewound and a small box with a capacitor inside it is used to run it. My electrical xperience is poor at best.

That sounds like someone installed a static converter and not rewound it for 1 ph. Converting a motor to true 1 ph requires a lot more than just the adding of a capacitor.

If you need lots of torque, then a belt or gear reduction is what you want.

Carld
12-31-2009, 12:40 PM
That's what I was thinking. The cap may be a balance cap that just lets the 3 ph motor start on single phase and still be wired as a 3ph.

I am thinking to be single phase the motor would you would have to replace all the windings if it can be done at all.

Forrest Addy
12-31-2009, 12:55 PM
The motor was re-woumd? That is the original three phaase windings entirely removed and rewound for single phase? Or did you take the motor to a shop explained the problem and they equpped you with a static phase converter and charged you for a re-wind?

I think the bet way to check this is to look in the motor's junction box and see if there are 9 wires numbered 1 through 9. If you were cheated and th motor i still three phase that's good news because you can still connect a VFD to it.

Here's an object leon for everyone. Replace a motor with a single pase but keep the old motor. You will never know when you wih you had the cnvenience and superior operability that a VFD gives to a maching whose spindle peed changes in a serie of belted or geared teps.

80 RPM to 60? No sweat. Use a pony motor. Shift to 80 RPM. Make a bracket so you can mount a small motor (1/4 HP is plenty, the R8 taper will slip long before you transmit 1 HP at 80 RPM.) with a suitable V-belt pully in line with the large pulley on the existing motor. You will have to relieve the BP belt case to clear the belt. This is brief to describe but a PITA to do but it's not that hard in concept. Start by calculating pulley ratios and holding the pulley selected looped in a sample belt to see just where to mount the motor.

Alternatively, find a three phase motor and use the VFD.

But the very first step is to determine if the existing motor is still wound three phase.

madman
12-31-2009, 03:03 PM
I will do it. That is check the number of wires. However i do think the entire motor was rewouind and a cap in a box and a resistor was also wired in there . It has worked for me flawlessly for over 15 years or so. It originially cost 350 bucks for everyehing and a replacement one phase at the time was a staggering 1200 dollars ,Thanx Oh does it (the motor keep its torque at even 60 rpm??

aboard_epsilon
12-31-2009, 03:33 PM
you could try putting some insulation tape around the pulleys ..
theres not that much difference between 80 and 60 ..perhaps not much tape may be required .perhaps the amount required wont even effect the slip on the belt ..lot of maybes ..but maybe worth a try

all the best.markj

Paul Alciatore
12-31-2009, 04:00 PM
That sounds like someone installed a static converter and not rewound it for 1 ph. Converting a motor to true 1 ph requires a lot more than just the adding of a capacitor.

If you need lots of torque, then a belt or gear reduction is what you want.

Motors are designed to work best at their design speed. So mechanical reduction is always going to provide the best torque. Any electronic control will always be somewhat lacking by comparison. But it is difficult to add mechanical reduction to an existing machine, hence all the VFDs as a second best solution.

My Grizzly gear head mill does 60 RPM with gears and hence provides great torque. But the high speed end is somewhat lacking as it only goes to 1500 or so.

EVguru
12-31-2009, 05:13 PM
From My Bridgeport Mill. I need to really slow down the mill for some work im attempting to do. What is the proper way to get it down to these speeds without sacrificing Torque? i am using my Mill for a Pile of Cylinder Head Projects for some new Drag bike engines Im building. thanx guys and Im wishing you all a Happy New Year. Take Care

What you haven't told us is which milling head you have.

The variable speed 2J2 head goes down to 50rpm and the J belt head down to 80rpm (with ~1500rpm motors).

Older heads were the M, C and R.

If you're not sure which head you have check on; http://www.lathes.co.uk/bridgeport/

whitis
12-31-2009, 09:03 PM
Motors are designed to work best at their design speed. So mechanical reduction is always going to provide the best torque. Any electronic control will always be somewhat lacking by comparison. But it is difficult to add mechanical reduction to an existing machine, hence all the VFDs as a second best solution.


What you say is correct but could be misinterpreted to mean that torque decreases substantially when you crank down the speed, especially since the OP asked about sacrificing torque rather than HP.

A good motor control can provide relatively constant torque from the design speed on down (from design speed up, constant HP instead of torque). The problem isn't that torque decreases at low speeds but that it doesn't increase. A gear or belt drive reduction amplifies torque. With a VFD, since available torque remains the same and speed is reduces, available HP goes down while gearing keeps available HP about the same (minus losses of gears).

An otherwise equivalent motor designed for a slower speed (more poles) will also have higher torque at its design speed. You can think of it as the distance the magnetic field needs to pull between poles or think of it as "gearing" down the number of electrical revolutions to a smaller number of mechanical ones.

Slap a 6" diameter face mill in your mill and you need lower RPM, vs. an end mill with same number of cutting edges, to keep the same surface speed and higher torque at the spindle to maintain the same cutting force at the cutting edge at the same metal removal rate. A cutter diameter that is too big can thus be a poor mechanical impedance match for a small mill that can't be geared down low, as can a too small diameter cutter that needs to run faster than the spindle allows.

However, you will often be limited by other factors to using less than the full horsepower of your motor. Cooling, taper slipping, deflection of the machine due to higher forces, human response time, chatter, incorrectly sized tooling, etc. With a VFD or continuosly variable transmission, it is very easy to change the speeds when you need to; with simple gears or belts, you may get lazy and also have a smaller number of available speeds, none of which is quite right. If your spindle taper is close to slipping at medium speed, its gripping power isn't going to increase when you gear down the motor and the HP you lose by cranking down the VFD instead of using gears is HP you couldn't use anyway.

Here is a graph of torque and horsepower vs speed for a VFD:
http://www.kilowattclassroom.com/Archive/VFDarticle.pdf
It does confirm the statement that motors are best at their design speed, slow it down you lose HP and speed it up and you lose torque. There are also other reasons why the motor may work best at its intended speed. On the other hand, if you are only concerned about one of those two parameters at a given time you can have a range of speeds.

Someone else suggested adding tape to the pulleys to change the speed from 80 to 60. You would need enough tape to increase the diameter of the pulley by 8/6 = 4/3 or a 1/3 increase in size. Too much tape and not enough metal.

doctor demo
01-01-2010, 02:41 AM
It takes a drill bit hours to drill through something turning the wrong way..
Hey Dave, You ever put a bandsaw blade in bassakwards? It takes forever to cut:) . Don't ask Me how I know:o .

Steve