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  1. #11
    Muzzer's Avatar
    Lives in Lytham St. Annes, United Kingdom. Last Activity: 12 Hours Ago Has been a member for 6-7 years. Has a total post count of 412. Received thanks 59 times, giving thanks to others 10 times.
    There's no black and white definition of AC and DC servo but generally a DC brushless servo uses trapezoidal ("6 step") drive current waveforms and an AC (brushless) servo uses a sinusoidal current. If you drive the motor and look at the phase voltages, so you find an AC servo produces a fairly sinusoidal voltage and a DC servo produces more of a wobbly trapezoidal (rectangular) voltage. A DC brushed motor has just 2 terminals and is driven by a DC current.

    Must admit, that machine of yours sounds more like a closed loop stepper. There are 2 quick tests you can do to check. A stepper will feel lumpy when you turn it by hand with the motor disconnected, whereas a true servo will run freely. You could also remove the belt to show that the noise is indeed due to the belt. Having said that, the belt shouldn't be making a racket....

  2. #12
    Quote Originally Posted by Muzzer View Post
    There's no black and white definition of AC and DC servo but generally a DC brushless servo uses trapezoidal ("6 step") drive current waveforms and an AC (brushless) servo uses a sinusoidal current. If you drive the motor and look at the phase voltages, so you find an AC servo produces a fairly sinusoidal voltage and a DC servo produces more of a wobbly trapezoidal (rectangular) voltage. A DC brushed motor has just 2 terminals and is driven by a DC current.
    The manufacturer calls it AC servo with integrated driver and that's good enough for me. I think they know what they are talking about, even if they are Chinese. JMC is selling all kinds of motors, so they have no reason to give us fake information about this motor.

    Quote Originally Posted by Muzzer View Post
    Must admit, that machine of yours sounds more like a closed loop stepper. There are 2 quick tests you can do to check. A stepper will feel lumpy when you turn it by hand with the motor disconnected, whereas a true servo will run freely. You could also remove the belt to show that the noise is indeed due to the belt. Having said that, the belt shouldn't be making a racket....
    Yes, I know the differences between motors, I have many different types and am an engineer, even if I am not designing motor controllers. Of course, I did the obvious tests, the motor when rotated with hands and is switched off, rotates freely with very little resistance, unlike a stepper. It feels totally different and is definitely brushless. I have also opened the driver case, and the motor is definitely a 3 phase motor, since there are three wires going into the motor case and there are of course no brushes. I did not hook up any oscilloscope since I did not feel I had the need to do it. Never the less, I think how sinusoidal the signal would look like depends on the quality of the driver also.

    Anyway, I have made a video even without any load, but have had no time to edit and publish it yet. Will do it as soon as I have time. Yes, the motor is quiet on the table without load, so I am perfectly comfortable with believing that it is a real AC servo, just like what JMC is saying. They do actually sell closed loop steppers of the same size and with similar looking integrated driver, but then they call it closed loop stepper, and not a 180W AC servo.

  3. #13
    ZASto's Avatar
    Lives in Belgrade, Serbia. Last Activity: 18 Hours Ago Has been a member for 3-4 years. Has a total post count of 48. Received thanks 1 times, giving thanks to others 1 times.
    BTW, what's an AC or DC servo? There is only one servo, but I think you call AC servo a servo which has a drive connected to AC current and DC servo one which has a driver connected to DC current. The only actual difference is that in one case you have the rectifier built into the driver and in the other case you have it built into the PSU. Mine is called AC servo with integrated driver, which requires 20-50VDC, but it is still an AC servo and not a stepper.
    DC servo is breushed DC motor with attached encoder for feedback.
    AC servo is kind of 3 phase BLDC motor with encoder for feedback.

    Drives for both types of servo motors could, by design, ve connected to AC or DC power source (at the end it is ALWAYS DC source).

  4. #14
    Quote Originally Posted by A_Camera View Post
    Actually, the sound is normal. The noise comes from the timing belt, not the motor. It is not fair to compare your servo on a test bench with one attached to this spindle through a timing belt.
    Chill your beans man, I was working on what you said "One thing I am disappointed is the noise. I find the servo is quite noisy." So I took it that most of that noise was from the motor and I was just showing how quiet a servo is compared to a stepper in case you didn't know.

    Also, if you are saying most of that noise is from the belts then you need to look at the setup/pulleys/belts because they shouldn't make that much noise. I connect most of my motors to ball-screws with timing belts so have fit 1000's of belts and they make a fraction of the noise you have there.


    Quote Originally Posted by A_Camera View Post
    BTW, what's an AC or DC servo? There is only one servo, but I think you call AC servo a servo which has a drive connected to AC current and DC servo one which has a driver connected to DC current. The only actual difference is that in one case you have the rectifier built into the driver and in the other case you have it built into the PSU. Mine is called AC servo with integrated driver, which requires 20-50VDC, but it is still an AC servo and not a stepper.
    It's not that simple and the drives work differently regards the motor types, but in both cases, they are virtually silent compared to a stepper which was my point.
    I didn't think for a minute or was saying they were steppers but the noise in the video certainly sounds like a badly resonating stepper.
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

  5. #15
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    I would say that holding torque is better than what I'd expect to get from a 180W servo.
    You could probably improve it through tuning, but 180W is never going to have lots of holding torque.

    A quick search tells me a JMC 180W 3000RPM servo has 0.6Nm and 1.5Nm peak torque, so even through a 3:1 reduction, that's only 4.5Nm peak.
    You can put that kind of torque through a decent screwdriver with a bit effort, so on a 4" chuck, it's not going to take much effort to move things.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  6. #16
    Quote Originally Posted by ZASto View Post
    DC servo is breushed DC motor with attached encoder for feedback.
    AC servo is kind of 3 phase BLDC motor with encoder for feedback.

    Drives for both types of servo motors could, by design, ve connected to AC or DC power source (at the end it is ALWAYS DC source).
    That's the way I see it as well. But perhaps others see it differently, and that's OK as long as we understand what we mean. Mine is the brushless, 3-phase delta wired motor which I think all AC servos are. As for the drivers, yes, in the end, ALL driven by DC but installation wise it looks easier if you just have to plug one into the mains directly, compared to having to have a large power supply first. On the other hand, if a driver needs like 80VAC then it needs a large transformer and a box anyway, so while building a PSU out of that to provide DC output is a tiny bit more work, but that's really no big deal and in my opinion the benefits are basically gone. But sure, if directly mains driven then it is really nice and neat solution. Unfortunately, those motors are still pretty expensive.

  7. #17
    Quote Originally Posted by JAZZCNC View Post
    Chill your beans man, I was working on what you said "One thing I am disappointed is the noise. I find the servo is quite noisy." So I took it that most of that noise was from the motor and I was just showing how quiet a servo is compared to a stepper in case you didn't know.
    Actually, I guess that my comment was a bit too emotional. Here is the sound of the stepper at 380 chuck rpm (2280 stepper rpm).



    Somehow I expected (hoping for) some sort of miracle regarding noise, which didn't happen, but that's OK, I am fine with that for this experiment. In the end, if the whole lathe thing is going to work I may decide for a better solution and a more powerful motor. I think that a larger motor would also be more quiet, but that must wait for now.

    Quote Originally Posted by JAZZCNC View Post
    Also, if you are saying most of that noise is from the belts then you need to look at the setup/pulleys/belts because they shouldn't make that much noise. I connect most of my motors to ball-screws with timing belts so have fit 1000's of belts and they make a fraction of the noise you have there.
    I don't know the sort of installation you are doing, but I think that you don't build your motor holder the way I did here, which is the original 8 mm aluminium plate plus a 3 mm steel plate.

    Click image for larger version. 

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    The reason for this solution is that I could not for my life come up with a way of removing the original motor holder plate, so I could not replace it with a new, single 10 or 12 mm thick plate, which I intended to do from the beginning, and also because I only had 3 mm steel plate at home, and I have lots of that, and don't want to get other. Of course, I could visit a local workshop and they would most probably free of charge make such a plate for me, or could give me a piece of scrap plate which is thicker, but for now I am happy to be able to do it this way. I also needed longer belt, the 350 mm is been changed to 420 mm long.

    Click image for larger version. 

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    I also had to add some spacers otherwise the shaft would have ended up too far back. So I think the reason why you don't hear that noise is, apart from the fact that you probably don't run the motor for such long time, is that you also don't have such weird installation, which probably creates a lot of the noise also. With that in mind, plus the fact that my servo is also quiet on the table without load and with a dampening mat underneath, the comment I mage in the video is not fair, the motor is indeed quiet, but my own installation is generating a lot of noise.

    So for the future, if I will otherwise be happy with this, then I will remove the original plate through cutting it off and replace it with a thicker aluminium plate in one piece. I think I can do that, but right now I don't want to destroy any part of the machine. Of course, if anyone have an idea about how to remove the original plate without cutting, I'd be glad try and do it that way, but I could not find any video or description in explaining how it was installed the first place or how it can be removed without removing the whole spindle assembly and risking the pre load of the bearings, or causing damage to the spindle or the spindle housing. All I know is that removing the chuck and the large pulley plus the four screws is not enough, the plate can still not be removed.

    Quote Originally Posted by JAZZCNC View Post
    It's not that simple and the drives work differently regards the motor types, but in both cases, they are virtually silent compared to a stepper which was my point.
    I didn't think for a minute or was saying they were steppers but the noise in the video certainly sounds like a badly resonating stepper.
    I agree, it does sound like steppers, but again, that's only with the belt and everything attached. Regarding the driver, actually, my driver can start to buzz with some configuration. I did some experimenting and at one point it sounded like an old cheap Chinese built Toshiba 6560 chip based driver I had when I started my very first CNC build. That driver was also buzzing a lot when the motors were standing with the holding current applied.

  8. #18
    Quote Originally Posted by m_c View Post
    I would say that holding torque is better than what I'd expect to get from a 180W servo.
    You could probably improve it through tuning, but 180W is never going to have lots of holding torque.
    I did play with the parameters and did some tuning, but perhaps it can be improved even more. What I wish I could do is to prohibit the driver from going into low current mode, because I think that's what is the biggest problem. When the motor is on holding current, the driver reduces the current even more very fast after stop, which is OK if you are using the motor as a spindle motor, but not if you want to use it as a rotational axis, where the motor is mostly on holding. Of course, there are motors with dedicated electromagnetic brakes, but that might not be necessary if the current settings would allow full current all the time when the motor is stopped, as long as the driver is enabled. So maybe I missed one setting or maybe the PID regulator can be tuned even better.

    Anyway, I have seen some YT video where the same issue was fixed by tuning, but those motors all had V6 firmware, and this is V5. I was in contact with the manufacturer and they said sorry, you need a V6 motor, which is why I complained to the seller, who in the end gave me full refund because he could not ship the motor he advertised. I am happy this way, but feel sorry for those who wont get a reasonable compensation, if the problem is due to the PID in the motor and requires parameters which don't exist in the V5.

    Quote Originally Posted by m_c View Post
    A quick search tells me a JMC 180W 3000RPM servo has 0.6Nm and 1.5Nm peak torque, so even through a 3:1 reduction, that's only 4.5Nm peak.
    You can put that kind of torque through a decent screwdriver with a bit effort, so on a 4" chuck, it's not going to take much effort to move things.
    Yes, that was the way I was thinking also, I mean, 0.6Nm x 3 should work fine, but in the worst case, I can even change it back to 6:1, so the total holding torque should be fine, but what I noticed now that it seems like a sudden quick pressure can start the rocking, while a slow increase can not, so I think the problem with the PID must be fixed for this to work as I want it. Otherwise I can not use it. I don't know how other people manage with this servo, but it is my understanding that this is a very popular servo in the DIY segment of CNC builders.

    What I also find strange is that I can NEVER, not once, force maximum current consumption, not even for a very short time. Because I think that is not good, since in that case it can not be called 180W at all. The maximum I could ever measure was below 2A. In my opinion, the driver should increase the current until maximum is reached when I try to twist the shaft when on holding, but long before maximum current, the motor throws an error and just release the tension all together, instead of increasing the current and applying more holding. So perhaps, the motor is a totally different one, while the label says it's a 180W but in reality, maybe it's just a 100W motor.

    Anyway, I want to finish my "lathe" first, before spending too much time on the seemingly weak holding torque.

  9. #19
    In case somebody have done this before, please help with some advice.



    I don't understand this crazy design. Why did they do it that way...?

  10. #20
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    Quote Originally Posted by A_Camera View Post

    What I also find strange is that I can NEVER, not once, force maximum current consumption, not even for a very short time. Because I think that is not good, since in that case it can not be called 180W at all. The maximum I could ever measure was below 2A. In my opinion, the driver should increase the current until maximum is reached when I try to twist the shaft when on holding, but long before maximum current, the motor throws an error and just release the tension all together, instead of increasing the current and applying more holding. So perhaps, the motor is a totally different one, while the label says it's a 180W but in reality, maybe it's just a 100W motor.
    180W is the rated continuous power at rated speed.
    Power output is directly proportional to speed, so at 300RPM aka 10% of rated speed, continuous power output is only 18W.

    The only time the drive will draw full power from the power supply is when the motor is running at high speeds and loaded.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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