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Mill motor

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Hukshawn

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#1
Since I thought my mill motor blew up last week (drum switch fried, fuzed contacts of the start winding, blew capacitor. Since replaced cap and drum switch, all's well)
I had been pondering changing to 3ph/VFD, as a replacement, and as to gain more motor control.

My friend has an old barn with LOTS of stuff... In my rummaging I found a 1hp 3ph motor, 220v
If I were to rig it up on the mill with a VFD, does anyone really think I'd miss the 2hp I have now? I can honestly say I've never bogged the mill down, or taken a cut deep enough to make it sweat... The biggest mill I have is a 2" 3 flute face mill and i really can't see me getting anything bigger. Not for my hobby use... Bigger tooling like that would wind up scaring my pants off...
If I went VFD for speed control, I'd leave the pulleys I have. Like I did in my lathe when I converted it to DC. I can keep the same speeds range I have but with varying torque by changing the belts. I could achieve similar with the mill.

Thoughts?
 

mksj

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#2
With a VFD as you decrease the speed below the base speed (60 Hz) your horsepower decreases in a linear fashion down to 0 RPM. You also have issues of constant torque below the base speed with older motors, so may have constant torque to 30Hz and then it decreases. Newer VFD rated motors have constant torque ratios of 10:1 so good down to around 6Hz but no Hp at that frequency. I would stick with a 2Hp VFD rated motor, I think you give up a lot of Hp and torque going to 1 Hp. Dpends on the mill, pulley ratios and what you do with it. Typically if you are using a VFD to control the motor speed, you would up size the motor and not decrease it. If it is a knee mill I would stick with a minimum of 2 Hp.
 

Hukshawn

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#3
Food for thought. Thank you very much. I'll keep looking. Somethings bound to turn up eventually. And in the mean time, my mill works again.

Over the speed control, I think what I want most is the braking ability with a VFD. I use the brake shoe a lot when I'm running the mill.
 

mksj

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#4
Specific to VFD braking, with a braking resistor you should be able to stop the mill spindle in 2-3 seconds with electronic braking. It will not hold the spindle when stopped, only used for deceleration. On my mill with electronic braking, I do not use the mechanical brake to stop the spindle, it stops within 3 seconds regardless of speed.

The term over speed is specific to the motor, not the spindle. So on my mill the motor's speed/frequency range is 20-200 Hz for a mill spindle speed of 450-4500 RPM, back gear is 45-450 RPM. Above the motor base speed of 60Hz the motor makes constant Hp up to 200Hz, but torque falls off in a somewhat non-linear fashion. If you were to look at the applied spindle power compared to a standard 1750 RPM 60 Hz motor runing a 1:2 belt ratio to achieve the same mill spindle speed of a VFD 1750 RPM motor running at 120Hz (3500 RPM) with a 1:1 belt ratio, the latter would have twice the Hp and about the same torque with a spindle speed of 3500 RPM in both cases. So over speeding the motor gives you a wider motor operating range while maintaining power to the spindle. Decreasing a motor below its base speed, you loose Hp and although motor torque is flat, spindle torque decreases in a linear fashion relative to a fixed speed motor because of the mechanical advantage (changing pulley ratio) of the latter. This is one reason for over sizing the VFD motor to compensate for the performance drop off below the motor's base speed.

A 2 Hp mill would be up sized to 3 Hp for VFD use, a 3 Hp would be up sized to 5 Hp for use with a VFD. Also VFDs can boost a standard motors operating parameters for short periods and can provide tighter moter spindle speed control when using sensorless vector or encoder feedback.
 
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Hukshawn

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#5
This braking resistor you speak of... can you walk me through it? Can you do it in any induction motor? I am complete;y fine with my mill/motor set up, its the braking is like more than anything.
 

mksj

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#6
A braking resistor is specific to use with VFDs, it is a means for the VFD to sink the motors generated voltage into heat, a VFD can also inject voltage into the motor coils to reverse/counter to the motor inertia. This is also done on DC motors.
 

strantor

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#10
With a VFD as you decrease the speed below the base speed (60 Hz) your horsepower decreases in a linear fashion down to 0 RPM. You also have issues of constant torque below the base speed with older motors, so may have constant torque to 30Hz and then it decreases. Newer VFD rated motors have constant torque ratios of 10:1 so good down to around 6Hz but no Hp at that frequency. I would stick with a 2Hp VFD rated motor, I think you give up a lot of Hp and torque going to 1 Hp. Dpends on the mill, pulley ratios and what you do with it. Typically if you are using a VFD to control the motor speed, you would up size the motor and not decrease it. If it is a knee mill I would stick with a minimum of 2 Hp.
You are correct of course, but the way you said it could be taken the wrong the way. The way I like to say it is that, when you decrease speed by pulleys or gears, you get a corresponding increase in the available torque. When you decrease speed with a VFD, the available torque remains the same. So since HP is a function of torque and speed, technically HP goes down as you said. But if the torque you used at high speed was sufficient, then you will still have that torque at low speed (not considering the torque curves and nonlinearity you already covered nicely).

It depends on your motives for lowering speed. For example, if your saw is meant for an 8" blade and you're putting a 12" blade on it, then use mechanical methods. If your 1" endmill is spinning too fast and you want to slow it down, VFD

My reason for posting is that I think people (future people reading this) who are noobs, might read this and zoom in on "HP Loss" and think a VFD is not a good option, when it almost always is. For this application specifically, I concur with at least a 2hp 3ph motor.
 

MikeInOr

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#11
I suppose I could try with the 1 hp motor I have. A 2 hp VFD would suffice either way.
That is what I would do. Try the 1hp motor but buy a 2hp VFD. If the 1hp motor falls short you aren't really out anything if you have to buy a 2hp motor. And you never know, a 2hp 3ph motor might show up on craigslist for cheap and fall into you lap! :)
 
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Bob Korves

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#12
This is what I would do. Try the 1hp motor but buy a 2hp VFD. If the 1hp motor falls short you aren't really out anything if you have to buy a 2hp motor. And you never know, a 2hp 3ph motor might show up on craigslist for cheap and fall into you lap! :)
When I was buying a VFD for my surface grinder with a 1.5 hp motor, the price of a 3 hp Chinese VFD was only $10 more than a 2hp version. Things change in the future. Be as adaptable as possible.
 

4gsr

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#13
I too have a 1 HP motor on a 645 Index mill. Been running it on a I think it's a 1.5kw Delta VFD. Been in operation for over ten years now. I don't have a braking resistor either, a hand brake works fine for me for stopping when needed. It decelerates to zero in about 2-3 seconds. Original factory settings I'm running with. Have never had a problem with HP or torque loss during any cutting operation. I do have the option to change speeds by pulleys and belts. I have it set where max speed is at 70 herz and spindle RPM is about 1200. If this is of any help. Ken
 

Hukshawn

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#14
I too have a 1 HP motor on a 645 Index mill. Been running it on a I think it's a 1.5kw Delta VFD. Been in operation for over ten years now. I don't have a braking resistor either, a hand brake works fine for me for stopping when needed. It decelerates to zero in about 2-3 seconds. Original factory settings I'm running with. Have never had a problem with HP or torque loss during any cutting operation. I do have the option to change speeds by pulleys and belts. I have it set where max speed is at 70 herz and spindle RPM is about 1200. If this is of any help. Ken
I use the hand brake because it'll take 20-30 seconds to fully spin down. The motor is a big ol' 2hp. The old style that looks like a 10 horse. Hence why it would be nice to have a electric brake and not wear out the brake pad. Which I've already had to shim.
 
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