Page 1 of 2 12 LastLast
Results 1 to 20 of 25
  1. #1
    SND
    SND is offline Diamond
    Join Date
    Jan 2003
    Location
    Canada
    Posts
    9,221

    Default Servo Motors and Spindle Motors

    I've started looking at various components for cnc machines. Slowly piecing things together...

    Reading the GE-Fanuc website. They list Spindle Motors, and Servo Motors. But if you click on each link, they appear to be the exact same thing, same name. Alpha i or Beta i.

    So My questions is. Can a normal servo motor indeed be used to drive a spindle for a cnc machine say up to 4000-6000rpm? is that how it normally works? as they have position encoders built right in, seems which surely would be useful. Or is there more to this?


    http://www.gefanuc.com/cnc_en/cnc_pr...es_motors.html

  2. #2
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    My Chiron uses a servo motor as the spindle drive.Runs of the axis drive so I don`t see a problem.It`s a Siemens 1FT motor and Siemens 610 drive.

  3. #3
    SND
    SND is offline Diamond
    Join Date
    Jan 2003
    Location
    Canada
    Posts
    9,221

    Default

    Ahh ok. I'm starting to look into the siemens cnc controls now. So if its run off the axis drive itself, then theres no need for a further VFD to control that type of spindle motor?

    "Edit" From what I read it sounds like a the digital servo drive(Like Siemens Simodrive611?) is essentially a super fancy vfd that tracks everything and takes the orders from the control?

    I'm thinking of building a gang-tool lathe. Where I'm getting stuck is on the cnc/control/drives side of things as I know very little about that stuff. Theres the "hobby" stuff talked about on *******, gecko drives, mach, etc, gotta piece it together from many suppliers and hope it works all together. So I'd kind of prefer a more industrial system. As this isn't meant to be a hobby gang-tool lathe(if I ever do build it) I'll need .0001" resolution, easy enough programing and all that. Fagor appears popular on the smaller gang-tool lathes also.

    There'll be many more threads/questions coming soon, steep learning curve ahead

  4. #4
    CarbideBob is offline Titanium
    Join Date
    Jan 2007
    Location
    Flushing/Flint, Michigan
    Posts
    2,797

    Default

    At the risk of oversimplify it spindle motors are just bigger than servo motors.
    DC spindles need a field winding supply.

    Avis servos drives have more control/response requirements than spindles so their drives are more complex inside, A true servo drive big enough for a spindle is more expensive than a dedicated spindle drive.

    Spindle motors often run 10,000 RPM. Not many people spinning ballscrews at this kind of speed.
    Axis motors need to produce full torque at zero rpm, most spindle drives won't do this.

    Yes you can drive a spindle with a big axis servo motor. Most important is that the drive matches the motors capabilities.

    As you've seen on the other place you can mix and match all kinds of things. It just becomes a performance tradeoff. Many hobbyist builders consider 300 IPM rapids as fast and most are happy if their machine can make parts within +/-.001.

    Bob

  5. #5
    SND
    SND is offline Diamond
    Join Date
    Jan 2003
    Location
    Canada
    Posts
    9,221

    Default

    Thanks for the info,

    Would a spindle drive running one of those spindle/servo motors like listed on the fanuc website, require anything additional to keep track of spindle orientation when it comes to threading?

    I ask because it seems many cnc's have a separate encoder on the spindle for that purpose, so I assume its because they use a "normal" motor?



    I'm confusing myself in my own questions

  6. #6
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    The Siemens 1FT motors are ac servo`s,have encoders,position sensors and three phase tachos built in,all leads to very accurate positional control.
    The 611 drive is a higher dc link voltage than the 610,runs at 660 vdc and uses the higher voltage 1FT motors.Pdf`s can be downloaded from Siemens Automation website or from www.doconweb.com (might not be exactly the right address).
    FWIW I recently got quotes for a Siemens 802D and a Fagor 8055 controls for a retrofit,here in the UK the Fagor was $2000 more expensive.If you can run to it Siemens will do you a complete kit of control and integrated 611 drive,according to retrofitters I have spoke to,it`s the easiest one they have ever fitted.

    Mark.

  7. #7
    Perry Harrington is offline Titanium
    Join Date
    Oct 2006
    Location
    Boulder Creek, CA
    Posts
    2,351

    Default

    The trend is towards servos as spindles, because the C axis is builtin. The thing you should know is that AC servos are permanent magnet brushess DC motors, otherwise known as synchronous AC motors. This is because the rotor field is provided by the magnets instead of an induced current. Induction motors are used for spindle motors because they operate at high loads and speeds, and an induction/asynchronous motor is cheaper to make in those power levels than a synchronous motor.

    The drive logic for asynchronous motors is different than synchronous motors. You need a drive that is designed to run the type of motor you are using.

    The largest servo I've seen is 5HP, but I'm certain they are available in larger. BLDC motors make high brake torque because of the permanent magnets, that's why they work for axis drives. The torque and power curves for BLDC and induction motors is very similar. They both apply voltage scaling, variable frequency/variable current drive with field weakening above base speed.

    The motors are rated in Ke or Volts per 1k rpm and base speed rpm, and current. With these 3 pieces of data you know how to run the motor.

    The drive provides constant current, variable voltage up to base speed, this is why the torque output is constant. When the motor hits base speed, the current stays constant, but the voltage is reduced as the frequency increases. This is field weakening. This is how you make sense of the torque and power curves of spindle and servo motors.

  8. #8
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    "
    The trend is towards servos as spindles, because the C axis is builtin. The thing you should know is that AC servos are permanent magnet brushess DC motors, otherwise known as synchronous AC motors. This is because the rotor field is provided by the magnets instead of an induced current. Induction motors are used for spindle motors because they operate at high loads and speeds, and an induction/asynchronous motor is cheaper to make in those power levels than a synchronous motor.

    The drive logic for asynchronous motors is different than synchronous motors. You need a drive that is designed to run the type of motor you are using.

    The largest servo I've seen is 5HP, but I'm certain they are available in larger. BLDC motors make high brake torque because of the permanent magnets, that's why they work for axis drives. The torque and power curves for BLDC and induction motors is very similar. They both apply voltage scaling, variable frequency/variable current drive with field weakening above base speed.

    The motors are rated in Ke or Volts per 1k rpm and base speed rpm, and current. With these 3 pieces of data you know how to run the motor.

    The drive provides constant current, variable voltage up to base speed, this is why the torque output is constant. When the motor hits base speed, the current stays constant, but the voltage is reduced as the frequency increases. This is field weakening. This is how you make sense of the torque and power curves of spindle and servo motors."

    You`ve really lost me here Perry.How do you weaken the field in a PM motor?
    I have never seen a base speed quoted for a bldc motor,have you got an example?
    A base speed is usually quoted for a brushed dc motor in conjunction with a max speed which is attained by field weakening.
    Mark.

  9. #9
    Perry Harrington is offline Titanium
    Join Date
    Oct 2006
    Location
    Boulder Creek, CA
    Posts
    2,351

    Default

    Brushless DC motor = Permanent Magnet AC motor = synchronous AC motor.

    The ratings of the motor are based on the magnetic flux. So you try to keep the flux constant. Most motors are constant torque up to base speed, which would mean constant current. After base speed the rpms increase, but the torque decreases and the power decreases. So, if the torque decreases and we know that torque is related to current input, the current must drop as the rpms increase. You add more voltage to increase the rpms, reduce the current, increase the frequency. So, field weakening must include current weakening as voltage rises and frequency rises, keeping the magnetic flux constant.

    So my previous statement was incorrect (it was based on what I was told by someone else). Bear with me, because of all the resource material I've read, no-one wants to spell out all the details in one place.

    Base speed = speed at which motor is rated. Eg 100v 3000rpm 8 amps.

    My Sanyo Denki servo has all that data right on the nameplate. Induction motors are rated the same way: My spindle motor on my mill is 230v, 1725rpm, 10.2A, 60Hz.

    The servo on my Hardinge is a Sanyo Denki P50B08040-2000. It's rated 1.372nm cont, 3.92nm peak torque, .171nm per amp, 6000rpm max, 9.2a stall current, 29a max, 17.88v per krpm.

  10. #10
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    Perry ,your statements on their own are all true.Collectively applied to servos as in your previous post are all wrong.
    An ac servo motor is a three phase motor,supplied with dc on three phases.It has a permanently excited field,i.e. magnets.Voltage is not applied to the magnets therefore you can not have field weakening.

    Base speed refers to the crossover point in a wound field dc motor.It is the speed the motor will be at when the armature is at maximum volts and the field is at maximum volts.This is the point at which field weakening begins to raise the armature speed.Additionally some dc drives will also start to taper off the armature current.

    From your own figures the motor on your mill and the motor on your hardinge are not rated the same way

  11. #11
    macona's Avatar
    macona is offline Diamond
    Join Date
    Jun 2006
    Location
    Beaverton, OR
    Posts
    5,442

    Default

    Usually the difference between brushless AC and brushless DC s the method of commutation. DC usually uses hall effect or resolvers and AC usually uses Encoders.

    The difference between servos and spindle motors is a little different. Often they have multiple sets of windings for different speed ranges. The drive switches between these windings electronically as it changes speeds.

    Here is the thread where I installed a 2KW servo in my Monarch 10EE. I am very happy with it.

    Retrofit of 10EE Drive to AC Brushless Servo Motor

  12. #12
    Perry Harrington is offline Titanium
    Join Date
    Oct 2006
    Location
    Boulder Creek, CA
    Posts
    2,351

    Default

    Mark, makes sense. So with a BLDC motor it is simply Ke and current, no flux feedback needed, just feed it voltage to max rpm.

    It wasn't explained to me that field weakening was in reference to the rotor. I assumed it was in reference to the stator.

  13. #13
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    Perry,in a brushed dc motor with wound field,the field is the stator.

  14. #14
    macona's Avatar
    macona is offline Diamond
    Join Date
    Jun 2006
    Location
    Beaverton, OR
    Posts
    5,442

    Default

    The servo drives are constant current drives. But max speed is based on the pole configuration just like a regular three phase motor. Also the control may have parameters that limit max speed. If the control sees the motor is lagging in position it will allow more current into the motor to try and get it back into position. What limits the time you can run past peak continuous power is cooling. Servos are usually not directly cooled and a lot of the heat is sinked through the face of the motor. Spindle motors are forced air cooled and have air channels through the laminations to help keep the motor cool.

  15. #15
    Perry Harrington is offline Titanium
    Join Date
    Oct 2006
    Location
    Boulder Creek, CA
    Posts
    2,351

    Default

    Mark, I'm not talking about brushed DC motors. I've been speaking of Brushless DC motors, otherwise known as synchronous AC motors.

  16. #16
    CarbideBob is offline Titanium
    Join Date
    Jan 2007
    Location
    Flushing/Flint, Michigan
    Posts
    2,797

    Default

    Perry,
    In a servo motor basically voltage = speed, current = torque.

    The difference in a DCBL and AC servo is the drive. A DCBL is driven by a stepped waveform (known as a trapezoidal drive) and a AC servo is driven by a sine wave (know as a UV drive).

    An AC servo drive needs higher resolution feedback to maintain alignment of the sine wave to the motor so they use resolvers or encoders.

    In a torque mode system (tachless) the current output of the drive is proportional to the input voltage.
    In a velocity mode system (with tachs) the motor speed is proportional to the input voltage. In this case there is a speed control loop in the drive itself. Most large CNCs use velocity mode drives as they allow better multi-axis synchronization during contouring albeit at some reduction in responsiveness .

    Check out the engineering notes on AMC's website.
    Bob

  17. #17
    Mark McGrath is offline Diamond
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    4,241

    Default

    "Mark, I'm not talking about brushed DC motors."

    When you talk about field weakening,you are,whether you mean to or not.

    Mark.

  18. #18
    Perry Harrington is offline Titanium
    Join Date
    Oct 2006
    Location
    Boulder Creek, CA
    Posts
    2,351

    Default

    The two references I've seen to field weakening were for induction AC motors. One was a paper on rewiring the windings to give a higher speed and the other was from a guy who's had a lot of hands on experience with motor controllers. If you search google, you'll see that field weakening is applied to PM motors, BLDC motors, and induction motors.

  19. #19
    Limy Sami is online now Diamond
    Join Date
    Jan 2007
    Location
    Norfolk, UK
    Posts
    11,417

    Default

    Now before this gets out of hand, I'll be the first to admit electrical engineering is NOT my subject, but I'm trying to keep up, and at the moment TRYING is the operative word.

    From where I'm sitting it looks like either somebody doesn't actually KNOW what they're talking about, or trying to be deliberately provocative by talking at cross purposes.

    Please can we establish what type of motor applies to which type of control.

  20. #20
    Clive603 is offline Titanium
    Join Date
    Aug 2008
    Location
    Sussex, England
    Posts
    2,228

    Default

    Limy Sami

    As so often happens on a forum correspondents are tying to combine detail with accuracy and brevity. This always fails in greater or lesser degree and becomes highly confusing to the uninitiated.
    Getting down to basics:-
    1) an electric motor is an electric motor, its job is to convert volt-amps (i.e watts) to torque and speed.
    2) There are myriads of design parameters associated with electric motors. The designer chooses a configuration and set which, in his view, best meets the load to be driven, the way it is controlled and what the customer will afford.
    3) A spindle (also traction) motor is controlled to speed.
    4) A servo motor is controlled to position either rotational (normal motor) or linear.
    5) In principle any design or configuration of motor can do any job. Linear motors are just normal motors unrolled then parameter optimised.
    6) In practice see 2 above and take the various performance characteristics into consideration before buying or obtaining a motor.
    7) Detail things like field weakening and so on are only important when you are trying to match a controller to a motor.
    8) Its much easier to get someone else to match controller and motor.
    9) Designing a controller from scratch is a right pain (bin there, dunnit, never again OK).
    10) If you want to know more Electric Motors and Drives by Austin Hughes, published by Newnes IBSN 0-7506-4718-3 is pretty good. Light enough on the maths for a general reader but enough sums to let you work out WTHIGO if you need to. Third edition is much better than first two. Ask your library.

    Clive

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •