Stepper speed with current limiting driver


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    Default Stepper speed with current limiting driver

    So I know there are other influences on max speed but this just a theoretical.

    There are various online calculators for max RPM for steppers with mH, Amp and volts. My question is if the current is limited, is the max RPM higher? Then if it is and is a higher rapid is required, will a driver further limit current to further increase speed.

    As far as I understand, the speed limit is all about the time taken to charge/discharge the coils in the motor. So I'm guessing the driver will continue to charge until the set current is reached and then move onto the next step. If this is the case then setting a lower current limit will mean the time taken to reach this level and move onto the next step will be less meaning a faster RPM.

    School me please.

    Happy Days.

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    Default Re: Stepper speed with current limiting driver

    quote "So I'm guessing the driver will continue to charge until the set current is reached and then move onto the next step"

    you have confused two functions of the modern stepper driver

    the stepper driver uses a combination of two control systems

    the first part
    could be an up/down counter controlled by the step (clock) pulse and the direction ( count up / down ) control
    the output of the counter is the address input to a read only memory used as a lookup table (code converter)
    the output of the lookup table then controls the two H bridges that switch the currents that passes through the coils

    these days the up/down counter and look up table is emulated by a micro controller

    if the stepper supply voltage was equal to the motor voltage this would work for full and half step patterns


    the second part
    the "chopper current control" circuit has an internal clock oscillator running at a higher frequency than your maximum step frequency

    a flip flop is first set by an internal clock pulse , switching on the supply to the motor coils

    when the current measured as the volt drop across a fractional ohm resistor (that connects the H bridge to the negative supply)
    is equal to the reference voltage that's used to set the current , the flip flop is reset , switching off the current to the coil

    the duration on the on-time depends on the reference voltage, the drivers motor supply voltage & coil inductance

    the current is only turned on again by the next internal clock pulse

    by adjusting the reference voltage that controls the current according to the current step
    you can generate the magnetic fields for micro stepping

    John

    PS
    Stepper speed with current limiting driver-step-pmm-current-waveform-jpg

    Last edited by john-100; 06-26-2017 at 04:25 AM. Reason: add PS - quote & diagram


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    Default Re: Stepper speed with current limiting driver

    Quote Originally Posted by joel0407 View Post
    So I know there are other influences on max speed but this just a theoretical.

    There are various online calculators for max RPM for steppers with mH, Amp and volts. My question is if the current is limited, is the max RPM higher? Then if it is and is a higher rapid is required, will a driver further limit current to further increase speed.

    As far as I understand, the speed limit is all about the time taken to charge/discharge the coils in the motor. So I'm guessing the driver will continue to charge until the set current is reached and then move onto the next step. If this is the case then setting a lower current limit will mean the time taken to reach this level and move onto the next step will be less meaning a faster RPM.

    School me please.

    Happy Days.
    I think I see what has happened here.

    You were looking at this:

    Stepper Motor Calculator

    And you noticed that the speed increased as the input current decreased in their calculator. And you read this quote from the calculator website

    "This calculator computes the maximum speed of a stepper motor, which is limited by the time it takes for the coil to energize to it's maximum holding current, and then de-engergize as polarity flips."

    Well, that's a pretty cruddy calculator. It doesn't even give you a graph.

    I think a simplistic explanation might be that the current is limited by the driver, but the motor doesn't take the maximum current from the driver unless it needs to produce more torque. So at faster speeds, the motor can only use less current before it starts to loose steps, and there is less torque available.

    Therefore, in your example of limiting the current, this would result in less torque being available at lower speeds, and exactly the same characteristics as before at higher speeds. Not what you were looking for, I know.

    So, I took those formulas, from the link above, and put them in a spreadsheet by incrementally decreasing the amps. They did have a few differences in their formulas and their calculator, there was an extra 2x in the power equation, and the speed equation didn't convert from milliseconds to seconds. So this was my result:

    Stepper speed with current limiting driver-stepper-calcs-jpg

    You can see I put an actual graph of a motor, 7A, 2mH, at 48V for comparison to what the calculator says. Also, I did one for 80V, and I put in my best guess for what the actual might be at 80V.

    Now, if you were to increase the voltage, and limit the max current to a lower value via the driver, I think this could result in less torque at lower speeds, but better characteristics at higher speeds, while not needing to supply as big of a power requirement increase due to the higher voltage power supply.

    The typical answer that people usually give to the "what maximum voltage should I use" question comes from the "Gecko Step Motor Basics Guide" which states that

    Max Voltage = 32 x sqrt(Inductance)

    So for the example of the motor I used, 32 x sqrt(2) = 45 Volts

    However, I personally don't put much faith in that equation any more. Yes, Geckos are great products, and the guy who made them is way smarter than me, and they're an excellent product, etc, etc. It's that one equation I have taken issue with.

    This equation is to limit overheating of the motor at higher voltages. I am now wondering if the resistance of the motor should be factored in. I have read in a few places that the heat increase is directly proportional to I^2 x R. The motor from CNC Router Parts I used as an example has a low resistance, and there is a similar one from oriental motor that also has a low resistance and inductance that is driven at 80V in their graphs, which flies in the face of the Gecko equation.

    It also depends on the insulation class of the stepper motor wires, and the maximum temp that the stepper can get to without damage. Also the size of the motor and how well it's design can dissipate heat.

    So in summary, this is my humble opinion on the matter, please keep in mind that I am not an expert on this subject.

    1. You can increase the speed characteristics by increasing the voltage. This results in about double the torque (for double the voltage) at faster speeds and around the same torque at the lowest speed. How much you can increase the voltage by, I don't know, but don't arbitrarily trust the equation from Gecko as the best answer.

    2. If you reduce the max current via the driver, this will result in reduced torque at lower speeds.

    3. The calculator I posted a link to, and probably the one you saw, isn't very good. If anyone knows of a better one, please post a link .



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    Default Re: Stepper speed with current limiting driver

    Thanks to both of you for your responses.

    John, your response is pretty technical and I will need some time to study to understand it better.

    NIC 77, You're exactly right that I used one of calculators. I haven't purchased my motors of controllers yet as I'm still researching. The first motors I chose had ridiculous high inductance which therefore meant they were quite slow. I've evolved from 3.5amp motors at 5.0mH to 5.66amp motors at 1.8mH. I've also gone from 36v to 60v power supplies which meant a change of drivers to handle the extra voltage. What started at costing me $425AUD is now at $618 for 4 motors, 4 Drivers, and 4 power supplies.

    Part of my problem is I have already purchases 1605 ball screws. I am aware that I was advised to at least go 1610 but I couldn't justify the cost for my hobby machine. If it's not fast enough then I'll drive it with a belt and a ratio of pullies. Then my only limit will be whip.

    At 48v where I was with a $603AUD package would have given me 139 IPM according to the calculator and 60v will give me 173 IPM. Now I only suspect this. I figure that speed is maximum before I'd likely start missing steps as the coils aren't energizing and then DE-engergizing fast enough. I expect my cutting speed would be lower again. I figure that would be my rapid speed. I'm not too worried as I should be able to travel my 700mm X axis in about 10 seconds plus allowing for acceleration time.

    My current package is 4 x AM882 drivers with 4 x 23HS33-4008D NEMA23 motors. At 60v (still have a few choices of PS). That voltage might be too much for those motors though. With the inductance of 1.8, square root is 1.341640786499874. 32 x 1.341640786499874 = 42v.

    So what I understand from your post. I'll still be able to rapid faster than the RPM of the calculator but torque will drop off. I think I'll have ample torque anyway so that shouldn't matter too much.

    I will have a look but I found another motor with near exact specs to the 23HS33-4008D but a lot better data sheet. I'm guessing the 23HS33-4008D is a copy. I think the other motor was leadshine. I'll look for it soon. It had a graph for the motor and I was surprised at the performance in wiring it half coil. Half coil kept the inductance the same but reduced the current to 4 amps (vs 5.66 in Bi-polar parallel). That increased the speed in the calculator as well.

    Happy Days



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    Default Re: Stepper speed with current limiting driver

    Hi Joel

    another diagram to go with my 23:52 post from yesterday

    Stepper speed with current limiting driver-inside-a4988-jpg

    it should make it easier to see how the stepper driver works
    than just my description that nearly turned into "war and peice" !!!!

    John



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    Default Re: Stepper speed with current limiting driver



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    Default Re: Stepper speed with current limiting driver

    Quote Originally Posted by joel0407 View Post
    They look very similar indeed.

    http://www.leadshine.com/UploadFile/Down/57HSxxd.pdf

    57HS22 has an inductance of 1.8, amps of 5.7, resistance is 0.4.

    A couple more options to consider:

    Something like this

    IMS MD0F3424, MDOF3424 Servo Motor MDrive 34 NEW NO BOX. | eBay

    the driver is built in, and that goes up to 2000 RPM.

    Also check out this thread:

    http://www.cnczone.com/forums/steppe...pservos-3.html

    Posts #30 to #35

    I actually don't know how much torque you might need at higher speeds, or how big your machine is, how heavy, lengths of screws, dual driven gantry, etc, so I'm going to refrain from saying that a particular motor is a good choice, because I don't know anything about your build other than you have 5mm lead ballscrews.



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    Default Re: Stepper speed with current limiting driver

    Quote Originally Posted by NIC 77 View Post
    Also check out this thread:

    http://www.cnczone.com/forums/steppe...pservos-3.html

    Posts #30 to #35

    I actually don't know how much torque you might need at higher speeds, or how big your machine is, how heavy, lengths of screws, dual driven gantry, etc, so I'm going to refrain from saying that a particular motor is a good choice, because I don't know anything about your build other than you have 5mm lead ballscrews.
    I've checked out that other thread. Seems I should be fine with speed. From other threads, I think I'm going to be fine with torque as well.

    This is a thread about my design.
    http://www.cnczone.com/forums/diy-cn...ml#post2063142

    Happy Days.



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Stepper speed with current limiting driver

Stepper speed with current limiting driver