How do Stepper Motor Drivers Overcome the Speed Restrictions Due to a Charging Coil?

[mattnedgus]

Gekodrive have a useful intro to how stepper motors work with some details on speed v torque and the causes of resonance. The link below is to part 1 of 9. There's a drop-down menu on the right of the page to access the rest of the series.
https://www.geckodrive.com/support/s...or-theory.html

Kit

I had another read through that thanks Kit - it's a good resource but I wanted something more concrete.

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I think this is my answer, in the simplest terms:

UPDATE 05 June 2019: To anyone that reads this in the future I think I arrived at some wrong conclusions. I've' added a new post explaining why (Post #18). I'm very much an amateur!

All About Circuits: Maximum Stepper Motor Speed

The problem I was having was with the result of the last Max RPM calculation t=4LI/V here:

MassMinds: Estimating Stepper Motor Size

From the calculations I've now done comparing a few motors (from Zapp) in this table**:



I get a max theoretical speed for my present parallel-configured 8-wire motors of 759RPM, which agrees with the problems I've had trying to run them at 800RPM. A serial-configured version of this motor (or a standard 4-wire bipolar stepper) would run at a max 379RPM according to the same t = 2LI/V equation when plugging in the values on the datasheet. This makes sense because the specified current and inductance of this serial configuration is 0.5x and 4x respectively. These values increase the top line of the equation (and thus the value of t) by a factor of 2. And since t is on the bottom in the equation: MaxRPM = 60/(200*t) if t goes up by a factor of 2, the RPM comes down by a factor of 2.

I think the author of the article added this '2 factor' into the equations top-line artificially so that when you enter the real ("datasheet-specified") values for an 8-wire stepper motor in a serial configuration, you end up with an RPM half of what it should be.

This is what confused me so much - I struggled to believe that the 3.4Nm, 4.5Nm and 8.7Nm NEMA 34 motors could only reach maximum RPM's of 359, 265 and 195 (at voltages specified in the table). This seemed too much of a drop in RPM from the 759RPM the 3Nm NEMA 23 parallel-wired motors should reach.

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Do the values calculated seem like reasonable figures to be expected from stepper motors or could I expect higher from modern stepper drivers?

Maybe because they're using chopping and PWM as opposed to AC waveforms for example, or reducing the coil current at the higher speeds shown in datasheet graphs to achieve them perhaps?

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Now I know everyone says that 'bigger isn't always better' in stepper motors and I feel this goes some way to explaining (at least in simple terms) why.

I hope this information might helpful to someone else in the future.

I welcome any correction or any addition to my understanding!

Matthew

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(**Notes on Supply Voltages in the table: I used 68V for the NEMA 23 because that's what I'm using even though it's 20% above maximum recommended. The 77.78V NEMA 34 values are RMS values of the specified 110Vac shown in their datasheets. I figured this would be a reasonable DC value to use. I chose 48V for the 3.4Nm because 77.78V seemed way too high.)