Tips/Techniques 3 Phase motor

[mbond]

I seem to have sparked a firestorm - which was not my intent.

I guess some theory / background for my comments seems necessary: All AC induction motors work on the principal of magnetic induction - hence the name . What that means, is that a changing electric current, generates a changing magnetic field in a sine / cosine relationship. Positioning 3 (or multiples of 3) circuits on a wheel allows the linear attraction of one magnet to another, to produce a rotation force.

When a rotor is under 'no load', then it moves 'immediately' in response to the next magnetic attraction caused by a new current in the next coil.

When a rotor is under some load, then it's movement towards the next position is delayed in proportion to how much load it is under. This time component is how the motor develops power. And also how the increased current is caused. The more load the motor is under, the longer the time lag between the 'ideal' movement of the rotor and its actual movement. This lag dives current, power and heat.

For anyone familiar with clutch plates, it's like a backwards version. With a clutch, the engine makes all the power that it can, and then as you push in the clutch, some and then more of that power is transferred to the wheels. An AC induction motor works the other way. It makes no power at all until some load is applied.

For many motors, the largest load that they will ever see is when you flip the on switch. Overcoming the inertia of the rotor and anything attached to it. This phase is called inrush. And is made much worse by a lack of capacitance within the motor itself. Many motors use things like fly clips to add capacitance at low RPM and switch them out as RMP increases. VFD is a much better choice


This message already feels too much like a lecture, so I'll stop here. On the other thread I burned a 100$ bit because I engaged it too slowly

If any of these 'bad' motors are anywhere in the GTA, I'd be glad to take it and wire it up

 

[Susquatch]

Yes, but you seem to be saying that the motor will draw the same current at no load, as with its maximum load: "If it takes a quarter hp to turn at motor at its 60hz speed, it takes a 1/4 hp period."

No, that's not what I'm saying, or at least not what I meant.

I'm saying that if it takes 1/4 hp to turn a motor at 60hz, it takes a 1/4hp period. Let's say you have a 2hp motor. And let's say that turning that motor with no load at 60hz takes 1/4 hp. Basically, this means that it takes 1/4 hp to run with no load because the motor friction, the belts, and the gears takes 1/4hp to turn. So if the system is running, 1/4 hp is happening. Any less than that and it won't turn.

If the system is turning, it is using the hp it takes to turn it. No more, no less. To make chips adds more load and requires more HP.

If that isn't clearer, send me a PM with your phone number and I'll call you.

Edit - I see that @mbond replied while I was drafting my response. I like what he said. It's prolly easier to understand.

My offer to talk on the phone is still there.

 

[Tomc938]

OK. So I don't want to make this painful, but...

The 3ph motor runs 2x the speed of the original motor, and loses torque at the lower end of the speed range.

The lathe has belts that can be arranged to change the speed of the lathe with the 1750 RPM motor. Using the belts (and back gears) the speed range is from 50 to 1500 RPM.

What if I would set the belts to a speed setting in the low/mid-range of the possible speeds. I THINK running the 3400 at full speed would then give the 1500 RPM that is the top end of the lathe, and slowing the speed down to half speed (1740) would give the same lower end that the lathe - has without engaging the back gears - as the 1750 would.

If I was turning something really big (I don't think I'll be doing a lot of very large diameter turning) I can always engage the back gear and run the motor at the 1750 and still get the lowest speeds.

If I am correct, I could get a similar speed range that I have with the 3400 RPM motor in this manner?

Am I understanding things correctly?

 

[Susquatch]

OK. So I don't want to make this painful, but...

No worries Tom. We have your back and we will get through this one way or another.

The 3ph motor runs 2x the speed of the original motor,

Yes, the motor plate you posted is 3400rpm. Your original is 1780.

and loses torque at the lower end of the speed range.

No. With modern VFDs, (not the old ones), it will hold torque below 3400, and lose torque above 3400.

It will also lose HP below 3400, and be relatively constant above 3400.

The lathe has belts that can be arranged to change the speed of the lathe with the 1750 RPM motor. Using the belts (and back gears) the speed range is from 50 to 1500 RPM.

We have to take your word that this is the case. It sounds quite reasonable.

What if I would set the belts to a speed setting in the low/mid-range of the possible speeds. I THINK running the 3400 at full speed would then give the 1500 RPM that is the top end of the lathe, and slowing the speed down to half speed (1740) would give the same lower end that the lathe - has without engaging the back gears - as the 1750 would.

I think there are some incorrect assumptions there.

If you do not change pullies or vfd frequency, using a 3400 rpm motor will mean that the range of your lathe will be from 100rpm to 6000 rpm. THIS IS NOT A GOOD THING.

If you set the VFD to 30 Hz, then the motor will run at half speed (1750) and your lathe will have its old speed range. However, 30Hz is prolly too low for a non-vfd rated motor. So your lathe will still run too fast.

You are WAY better off finding a 1750 rpm motor. It's either that or find a smaller pulley for the motor and a bigger pulley for the lathe input so that the lathe sees the same speed as it did with the old motor. I'm guessing that it's too much of a pulley size change to work well. So I'd focus on finding another motor.

If I was turning something really big (I don't think I'll be doing a lot of very large diameter turning) I can always engage the back gear and run the motor at the 1750 and still get the lowest speeds.

If I am correct, I could get a similar speed range that I have with the 3400 RPM motor in this manner?

Am I understanding things correctly?

Yes, but again that assumes you can really run at 30hz. That may or may not be the case and you won't know till you try it. 40Hz with a fan is prolly very realistic. 40Hz would get you to a minimum speed of 133rpm.

If I could, let me talk a bit about my own lathe. My current motor is a 2hp 1750rpm single phase 220V motor. My minimum speed right now is 70rpm. I have found 70 to be a bit too fast for my old brain and hand/eye coordination levels.

So I would like to be able to run at 50rpm or even lower on occasion. I am not worried about top speed. 50rpm is totally doable with a regular 1750 rpm motor and a regular VFD.

I have already acquired a 3 phase 2.4HP 1750 motor. I would have been happy with anything up to 3hp. But I found a 2.4 so that is what I plan to use. I also have a 3Hp VFD for it.

With this combination, I can run my lathe as low as 50 rpm without any concerns and I don't need to change the drive pulley ratio.

I will also set my VFD for smoother SLV mode and slower accel/decell times so that the 2.4hp motor does not overstress the drive train on my lathe, and I will install a braking resistor to give me better faster e-stops.

The other thing this motor/vfd gives me is the ability to fine tune the lathes speed within reasonable limits on the fly as my cuts change so I can get better parting operations and better finishes.

This conversion will happen over the winter.

You can certainly make a 3400 rpm motor work Tom, but I think you need to ask yourself what your objectives are first and then figure out how to achieve them. I would think a 1750 to motor is a better start on that journey.

 

[DavidR8]

Tom you and I have basically the same lathe. Mine has a 1725 rpm motor now that when I convert to 3-phase it will be with another 1725 rpm motor.

 

[Dabbler]

@Tomc938 I have the same lathe also. Still with the original motor...

 

[Tomc938]

OK. I REALLY appreciate al the input. I think I will pass on the 3400 motor. Besides the speed issue, there are also issues with mounting it as the new motor has mounts very different from the original.

It was just so tempting because it is local and for sale for a years so likely to go cheap when it finally does.

I'm having DM conversations with a forum member who might have something I can make work, perhaps easier.

In the meantime, there is nothing wrong with the current set up, just that this other motor came along.

 

[YotaBota]

@Tomc938 - would you like us to keep an eye down this way for a 3ph motor? Going to need a good pic of the data plate and shaft dia/length.

 

[Tomc938]

@Tomc938 - would you like us to keep an eye down this way for a 3ph motor? Going to need a good pic of the data plate and shaft dia/length.

That would be awesome. Probably not as many as out in Ontario, but more than up here, that's for sure.

I'll get the data plate picture later this evening.

 

[DavidR8]

<peeperspeeled/>

 

[Tomc938]

This is the best I can get:

(the only thing missing is the wiring for CW/CCW.

 

[Susquatch]

Wise decision Tom. It's a bitch getting good photos of a data plate sometimes. The best I can do with my eyes says it has a YL90S? Frame. I can't quite make out the last letter - or if there even is one.

 

[Dabbler]

@Tomc938 You will still likely to be using the back gear, even if you VFD this lathe. The belts lose quite a bit of power, and using a VFD, it feels like you lose some more at low frequencies. So at the very bottom end, you might have some trouble making even shallow cuts on larger pieces. But you will always be able to engage the back gear, and runt the VFD in reverse.

 

[DavidR8]

Here's an edited version that's a bit easier to read.

 

[justin1]

 

[DavidR8]

View attachment 41334View attachment 41335View attachment 41336

Is that your motor?

 

[Tomc938]

Is that your motor?

@justin1 has kindly offered for me to see if this one that he has will work.

Sure looks like it to me. What say you all? Mounting might need a wee bit of work, and the pulley perhaps. I'll check these tomorrow.

 

[DavidR8]

@justin1 has kindly offered for me to see if this one that he has will work.

Sure looks like it to me. What say you all? Mounting might need a wee bit of work, and the pulley perhaps. I'll check these tomorrow.

Right rpm, right HP.
Give it a go.

 

[justin1]

@justin1 has kindly offered for me to see if this one that he has will work.

Sure looks like it to me. What say you all? Mounting might need a wee bit of work, and the pulley perhaps. I'll check these tomorrow.

Making a pulley isn't that hard to make. Made one for the big lathe before I bought a keyway brooch I just used a endmill to make the keyway in pulley and added 2 set screws just incase lol

O ye make sure you add enough extra length on pulley to be able to slide into or away from motor to help line up the v belt if there isn't enough adjustment on motor mounts or room to adjust

 

[Dabbler]

@Tomc938 @justin1

Welcome to the wonderful world of NEMA motor sizing... Especially with offshore equipment!!!

I use this chart, and it hasn't let me down, yet. This way you can be sure the motor diameter, the mounting base and bolt holes, and shaft diameter will work, and tells you what to measure..e Saves a lot of trouble.