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

[Kitwn]

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

[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.)

[Voicecoil]

I
(**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.)

Are you sure the 110V AC figure isn't a RMS value to start with? It's the normal was to quote an AC voltage unless you specifically say "110V AC pk" - anyway it does say also 5.9A constant current, so the voltage is kind of irrelevant.

[mattnedgus]

Are you sure the 110V AC figure isn't a RMS value to start with? It's the normal was to quote an AC voltgae unless you specifically say "110V AC pk" - anyway it does say also 5.9A constant current, so the voltage is kind of irrelevant.

To be honest I wasn't entirely sure, but it felt right.

For the three NEMA 34 motors respectively I used the maximum voltages (32*sqrt(L)) of 42, 64 and 82Vdc. The specs from the same manufacturer declared Vdc for the NEMA 23 and Vac for the NEMA 34 speed-torque graphs. My logic was that the drivers for these motors can be attached directly to a 110Vac supply like in the US and thus would have a numerically lower DC equivalent.

If I use 110 as the value for the supply voltage in the equations I also get 1645, 750 and 552RPM respectively for the max speeds for these same motors. It takes the power for each upto 649, 605 and 506W respectively. These values just felt way too high.

The supply voltage is one of the factors that determines the time it takes to reach the 5.9A maximum current. A higher voltage fills the coil to 5.9A faster. For example on my motors I get a current rise-time of 0.198ms and a max 759RPM on a 68V supply but if I were to swap this to a 36V supply the rise time goes up to 0.373ms and I get a max theoretical RPM of 402.

I'd really like to know what effect chopping stepper drivers have on the maximum speed of a motor and how half/microstepping the motor might influence these values though!

[Robin Hewitt]

History is against you. The problem is that some bod in America decided that all stepper motors should be 2 phase, 200 full steps/rev regardless of the frame diameter, and that holding torque was actually something worth printing on the spec. sheet. The Chinese shrugged collectively and said, "Whatever" and set about making them.

Unfortunately as motors get bigger the inverse square law is not on their side so they lose power. Holding torque can be enormous but it is the pull in torque that moves you to the next next step. Pointing this out to people who have already spent money on enormous motors and unsuitable toroidal transformers, is not going to win you any friends on places like this.

You may think you can overcome this by putting in loads of microsteps but then things become springy and your tolerances quickly become slack.

You obviously appreciate the problem or you would not have asked. If you have not already blown your budget, go to the Oriental Motor Company web page and look up some speed/torque graphs for their 5 phase motors.

[mattnedgus]

History is against you. The problem is that some bod in America decided that all stepper motors should be 2 phase, 200 full steps/rev regardless of the frame diameter, and that holding torque was actually something worth printing on the spec. sheet. The Chinese shrugged collectively and said, "Whatever" and set about making them.

Unfortunately as motors get bigger the inverse square law is not on their side so they lose power. Holding torque can be enormous but it is the pull in torque that moves you to the next next step. Pointing this out to people who have already spent money on enormous motors and unsuitable toroidal transformers, is not going to win you any friends on places like this.

You may think you can overcome this by putting in loads of microsteps but then things become springy and your tolerances quickly become slack.

You obviously appreciate the problem or you would not have asked. If you have not already blown your budget, go to the Oriental Motor Company web page and look up some speed/torque graphs for their 5 phase motors.

Thanks Robin, I'll take a look them - my options are still open.

Ah, it wasn't my intention to offend anyone - I couldn't find the information in a way that I could understand and hoped that maybe by sharing what I'd found it might help someone else later down the line.

To be honest I think I fell lucky on my current machine. It was my 2nd build (after trying V rollers on L alu-extrusion with ACME and T6-something drivers - it was hopeless!) and I had some excellent advice from Gary at Zapp regarding the motor and driver combination. I'd just hoped to increase my performance on this new build and didn't want to risk the outlay before I had a slightly better understanding, especially when I've read more than once about NEMA34's not necesarily providing the hoped increase.

[Robin Hewitt]

Ah, it wasn't my intention to offend anyone

I am sure you will not offend anyone. Sadly I am an incorrigible know it all and my foot is permanently in it