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Servo motors vs the other motors

john lucas

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I'm really unfamiliar with servo motors. What are the advantages and disadvantages. To they require a special power supply. Can you retrofit one to other lathes, specifically mini or midi lathes.
 
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I have only worked with large servo motors. They require a controller like a VFD. You set an rpm and it will stay there no matter how light or heavy the load until you reach over current. They are very quiet. They cost quite a bit more to replace than a regular motor.
 
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I read thru the specs for the lathe in question;
The propaganda portion said the motor can be loaded up to 3 HP.
The specs say 1Hp Imperial or 750 watts Metric.
The NEMA (US) standard rates motors in continuous mechanical output so due to inefficiency about 1000 watts input power would be required per HP.
The EEC code is unclear to me if the watt rating is mechanical output or electrical power input and if it is the input then the mechanical power is less than 1 HP or 750 watts.
The standard 3 ph induction motors as used on most lathes have just as much if not more overload torque plus the VFD controllers have long ago reached commodity status.
The Servo controller and the servo motor will likely only be available from Harvey at inflated prices and are probably low quality.
 
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I read thru the specs for the lathe in question;
The propaganda portion said the motor can be loaded up to 3 HP.
The Servo controller and the servo motor will likely only be available from Harvey at inflated prices and are probably low quality.
I believe John posed a general question about servo motors and was not soliciting criticism of a particular product or of anyone's lathe choice.
 

john lucas

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I'm interested in all.discussions concerning these motors. I have not had time to actually look them up a d try to understand tech speak.
 
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I believe John posed a general question about servo motors and was not soliciting criticism of a particular product or of anyone's lathe choice.
I was only criticizing the deceptive marketing and trying to find a reason for a servo motor verses an ordinary induction motor.
 
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Basics of a servo motor, as I understand it, the motor can vary speed by means of changing supplied voltage.. a servo motor would have a speed sensing device so that when you set it to a specific speed it can detect when it starts to slow down (loading) or speed up (unloading) and within milliseconds, vary the voltage going to the motor in order to maintain a constant speed .. So, it seems to me, if the motor has enough power/torque in reserve, it should be able to maintain a very consistent and constant speed regardless of the amount of loading being applied to the work... Much like the Anti-Lock brakes system on a car, (and/or traction control system) which can spot changes in wheel speeds relative to other wheels and apply braking logic to select corner of the car to maintain traction or prevent lockup... by means of speed sensors... Seems to me, a servo motor would typically be able to maintain a smooth steady speed in varying conditions, as opposed to a belt drive which can slip or be limited to its torque to within a specific speed range (Torque curve)... But then, I am no engineer.. it is just my understanding of how a servo motor (as far as maintaining speed) would work.. The servo motors I am more familiar with are more like robotic controls (electronic governors, electronic throttles, electronic climate controls, and the like.. which are more of a linear control as opposed to rotational control)
 
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Don, I think you make a good point that we shouldn’t use the HP rating in sales brochure when comparing the output of one motor type (servo) to another motor type (3ph VFD motor). Sure there are other things to discuss regarding servo motors, but it’s nice to be reminded when HP ratings could mislead some people into making an unfair comparison between motor outputs.

Setting aside hp rating, do you think this servo is a reasonable size for this type of lathe based on the wattage?
 
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Basics of a servo motor, as I understand it, the motor can vary speed by means of changing supplied voltage.. a servo motor would have a speed sensing device so that when you set it to a specific speed it can detect when it starts to slow down (loading) or speed up (unloading) and within milliseconds, vary the voltage going to the motor in order to maintain a constant speed .. So, it seems to me, if the motor has enough power/torque in reserve, it should be able to maintain a very consistent and constant speed regardless of the amount of loading being applied to the work... Much like the Anti-Lock brakes system on a car, (and/or traction control system) which can spot changes in wheel speeds relative to other wheels and apply braking logic to select corner of the car to maintain traction or prevent lockup... by means of speed sensors... Seems to me, a servo motor would typically be able to maintain a smooth steady speed in varying conditions, as opposed to a belt drive which can slip or be limited to its torque to within a specific speed range (Torque curve)... But then, I am no engineer.. it is just my understanding of how a servo motor (as far as maintaining speed) would work.. The servo motors I am more familiar with are more like robotic controls (electronic governors, electronic throttles, electronic climate controls, and the like.. which are more of a linear control as opposed to rotational control)

Assuming that Brian is correct, and maintaining a constant speed is the difference between it and a DC motor with VFD, it leads one to contemplation as applied to woodturning. I'm theorizing how that can be applied to what we know concerning how vibrations emanating from the wood relates to the load applied to it, as it varies for light vs heavy cuts, and the overall quality of the cut.

My initial thought on this, is a servo motor has no benefit applicable to woodturning. As the amount of wood being removed increases with a heavy cut, it has a direct relationship to the amount of vibration emanating from the source. This, instead of maintaining a constant rpm, is what directly effects the quality of the cut that is possible.

I spend a lot of effort finding a most perfect rpm, but that is only helpful for the finest of finishing cuts, where the changing dynamics of the cut is at it's least point. Once a heavy cut comes into play, the whole dynamics relating to the quality of the cut changes. Since those dynamics change, maintaining a specific rpm is no longer a determining factor in those things that promote the best cut.

My thoughts are not the result of actual experience with a servo motor, so I'm interested to hear further discussion from those who do have the experience.....or, from those who have additional theoretical thoughts.

-----odie-----
 
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Assuming that Brian is correct, and maintaining a constant speed is the difference between it and a DC motor with VFD, it leads one to contemplation as applied to woodturning. I'm theorizing how that can be applied to what we know concerning how vibrations emanating from the wood relates to the load applied to it, as it varies for light vs heavy cuts, and the overall quality of the cut.
A DC motor is not controlled by a Variable Frequency Drive, there is only 1 type of motor that operates on a VFD and that is an AC 3 phase induction motor. The VFD / 3ph motor combination will maintain a constant speed sufficient for wood turning. Servo motors are normally used to supply adjustable constant speed and positioning control on machine tool axis.
 

john lucas

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Part of why I asked this question was because I've gone through several.lathes and changed motors and drives. My first 2 lathes were single speed motors with Reeves drives. They werent bad but you had to keep the parts clean and could only vary the speed.with the lathe running which was interesting if you turned a pen at 3000 rpm and then your next project was a bowl and you forgot to crank the speed.down. my next lathe was single speed with a step.pulley. I converted it to a DC drive. The DC drive had a problem of losing power when you lowered the speed which of course was a.problem when roughing bowls. I learned about 3 phase motors and variable frequency drive so I installed one of those. They maintain the power at low rpms which was great. You also get reverse and electronic breaking. So im.wondering what the advantage are.of servo motors over 3 phase motors a d VFD s.
 
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A DC motor is not controlled by a Variable Frequency Drive, there is only 1 type of motor that operates on a VFD and that is an AC 3 phase induction motor. The VFD / 3ph motor combination will maintain a constant speed sufficient for wood turning. Servo motors are normally used to supply adjustable constant speed and positioning control on machine tool axis.

Hello Don......This is the motor I have on my Woodfast lathe:

https://www.woodturnerscatalog.com/p/110/5521/leeson-DC-Variable-Speed-Control?term=leeson+minarik&term=leeson minarik

Leeson DC Variable Speed Control 1-1/2 HP 220v

-----odie-----
 
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I think the answers lies in why all the big well known wood lathe companies, where cost is not a factor and have used the best parts they can, have never used a servo motor. But one company is now boosting a servo motor on their seemingly over priced and questionable marketing literature on their servo motor. I for one will never be the guy that buys the new car the first year off the line. I always let the other guy buy all the bugs and problems in a new build. If it’s around by the second and third year one can read about all the pros and cons of the new product in real life. Then make an informed decision. A company will never tell you the limitations of the new lathe, just give you all the marketing hype they print in there selling literature. Maybe in 10 years all lathes will have servo motors, or this one will be long gone. We shall see in time.
 
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Servo motors have been used on metal lathes for years. Nothing wrong with servo motors. They will hold a constant rpm. They are very quiet. I don’t have experience with small ones. I have installed 15 HP to 40 HP on plastic extruders, form presses, trim presses, grinders and other machines. They ran 24/7/365 for years with no problems. Larry I wouldn’t worry about. It’s not like it’s new technology.
 
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Hello Don......This is the motor I have on my Woodfast lathe:

https://www.woodturnerscatalog.com/p/110/5521/leeson-DC-Variable-Speed-Control?term=leeson+minarik&term=leeson minarik

Leeson DC Variable Speed Control 1-1/2 HP 220v

-----odie-----
Odie your dc motor is controlled by a vsd. The variable speed drive changes the voltage to control the speed. Hence they can be used on dc and ac motors. A vfd changes the frequency of the wave and the voltage and hence can only be used on a ac motor, Don is right. Unless I’m reading your reply wrong Odie.
 
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Guys,

I have been using my Harvey with a 1hp servo motor for over a year. After running it several times a week during this time, I have not had a single problem. It runs smoothly and quietly. Because I am not a production turner who has to maximize wood removal, I have never bogged it down.

One thing nobody has mentioned is size and weight of those ac motors compared to my servo motor. It is small in comparison. I bought the lathe to fit into my tiny shop. I needed a compact, powerful lathe for my style of turning. The Harvey T-40 fit the bill.

By the way, it was cheaper than the PM 2014.

Jon
 
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Odie your dc motor is controlled by a vsd. The variable speed drive changes the voltage to control the speed. Hence they can be used on dc and ac motors. A vfd changes the frequency of the wave and the voltage and hence can only be used on a ac motor, Don is right. Unless I’m reading your reply wrong Odie.
OK, got it, Glenn! I wasn't aware of the difference.

In my original sentence using VFD incorrectly, the substance remains the same. I'm not seeing any benefit to a constant speed, for the reasons I mentioned. Now, if there is a real benefit to the act of woodturning, and not about the size of the motor (as in @Jon Minerich post above this one), I'd like to know more about it.

Assuming that Brian is correct, and maintaining a constant speed is the difference between it and a DC motor with VFD, it leads one to contemplation as applied to woodturning. My initial thought on this, is a servo motor has no benefit applicable to woodturning.
 
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OK, got it, Glenn! I wasn't aware of the difference.

In my original sentence using VFD incorrectly, the substance remains the same. I'm not seeing any benefit to a constant speed, for the reasons I mentioned. Now, if there is a real benefit to the act of woodturning, and not about the size of the motor (as in @Jon Minerich post above this one), I'd like to know more about it.
I’m not seeing any benefit either. I assume my Oneway has enough torque to keep a constant speed. And I doubt the Harvey lathe cannot be stalled like any other lathe. Now if it had a 20 hp servo motor that would make a difference, but it would also mske a similar difference if my oneway had a 20 hp motor.
 
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One thing for sure is if the servo motor or its drive needs replacement it's going to be more expensive than an equivalent AC motor or VFD.

Example: on one of my machine tools a Baldor servo motor was not performing as it should. Baldor referred me to a factory authorized repair outfit to have the motor repaired. After a $200 diagnosis they decided it couldn't be repaired. $1100 later I ended up with a new Baldor servo motor. That was a lot of money for a motor of that size. $1100 will go quite a ways toward an AC motor and a VFD.

If maintaining constant speed (rpm) under load is the issue you can achieve that with a flux vector AC motor and VFD (more expensive than conventional AC motor and VFD). The motor rpm is monitored by the drive and corrects as needed.
 
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Servo motor is a broad category. What is the type of servo, DC, AC Stepper motor we are talking about? They are all different and require different type of driver and have different responses.

As for the power (HP) of the motor. Some types have large thermal mass and can take quite a bit of over-current for a brief period of time. The power must come thru the power supply (driver) and can be momentary. I assume this is what the advertisement is about. Is this good engineering, or marketing. As an engineer I did not like to tell marketing everything and this is why
 
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One thing for sure is if the servo motor or its drive needs replacement it's going to be more expensive than an equivalent AC motor or VFD.

Example: on one of my machine tools a Baldor servo motor was not performing as it should. Baldor referred me to a factory authorized repair outfit to have the motor repaired. After a $200 diagnosis they decided it couldn't be repaired. $1100 later I ended up with a new Baldor servo motor. That was a lot of money for a motor of that size. $1100 will go quite a ways toward an AC motor and a VFD.

If maintaining constant speed (rpm) under load is the issue you can achieve that with a flux vector AC motor and VFD (more expensive than conventional AC motor and VFD). The motor rpm is monitored by the drive and corrects as needed.
We used to have our 15 HP servo motors rebuilt for $5,000 because a new one was around $11,000.
 
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