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

[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

[m_c]

You are correct in that nema34 motors usually don't give you the required performance increase above nema24 motors.

In all honesty, given the speed you're looking for, I'd look at servos.

[mattnedgus]

You are correct in that nema34 motors usually don't give you the required performance increase above nema24 motors.

In all honesty, given the speed you're looking for, I'd look at servos.

I'd love to but aren't they about £500 (motor+driver) per axis?

[routercnc]

Hi Matthew

Just to clarify Robin’s earlier comment. You are not offending anyone here. Far from it, you are asking interesting questions. Robin was referring to new posts from builders who come on this site having bought some big motors and are sometimes offended when told they have bought the wrong thing. It has happened a lot.

[m_c]

I'd love to but aren't they about £500 (motor+driver) per axis?

Aliexpress.
I've not personally used them (I opted for Kinco from Zapp), however at least a couple people on here have used the cheap Chinese servos without any problems.

Drive/Motor set can be had for under $300.
The main drawback is the drives are only powered via the 240VAC, so if they lose power, you lose position. Whereas the logic in the Kinco drives is powered via 24VDC, so you can cut the 240VAC (i.e. an estop) and all the encoder circuitry remains active.

[mattnedgus]

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

Hi Matthew

Just to clarify Robin’s earlier comment. You are not offending anyone here. Far from it, you are asking interesting questions. Robin was referring to new posts from builders who come on this site having bought some big motors and are sometimes offended when told they have bought the wrong thing. It has happened a lot.

Thank you, I understand, I think I must have misunderstood originally - I definitely didn't want to be putting anyones nose out of joint, especially after having had such good advice on here in the past.

Aliexpress.
I've not personally used them (I opted for Kinco from Zapp), however at least a couple people on here have used the cheap Chinese servos without any problems.

Drive/Motor set can be had for under $300.
The main drawback is the drives are only powered via the 240VAC, so if they lose power, you lose position. Whereas the logic in the Kinco drives is powered via 24VDC, so you can cut the 240VAC (i.e. an estop) and all the encoder circuitry remains active.

Thank you, it's definitely worth thinking about - I started learning a little more about servo motors after I saw your reply. All this time I was under the impression they all had a built in gearbox like those servos you get in model-aircraft and RC-cars, but that doesn't seem to be the case!

[mattnedgus]

Hey Guys,

My apologies, I think I've made some real errors of judgement above! I'm still very much an amateur but here's what I think is happening for anyone that reads this and gets as confused as I did, in the future:

A coil in one phase will have current go from all in the +ve direction, through 0, to all in the -ve direction, and then back through 0 to +ve again in one revolution of the motor. In the case of a 4.2A motor it has a total of 16.8A to move (+4.2A --> -4.2A --> +4.2A) - so massminds worse-case calculation I linked to in an earlier post, I think, is correct after all (although in contradiction to the calculation on All About Circuits website - how?).

For my motor (I=4.2A, L=3.2mH, V=68V) I thus get a max speed of 380RPM. The first time I calculated this it didn't feel right so I did more reading and convinced myself that my gut was right (that 759RPM should be my max). Reading the info on the All About Circuits website seemed to persuade me of this - but it didn't expain why the graphs on some spec sheets for motors were shown with usable torque at even higher speeds.

BUT, it just dawned on me that this (380RPM) is the CORNER SPEED, NOT the MAX SPEED!! This is the speed under-which the coil should be able to fully saturate (build the maximum field strength) in both directions and thus maintain full torque (or very near to) upto this speed. This is what Geckodrives graphs show with the flat portion: https://www.geckodrive.com/support/s...or-basics.html

At speeds faster than 380RPM there is fall-off in torque because there is no longer time to fully saturate the coil with the maximum current. Torque drops off after this speed because its proportional to current and reaches its max speed when insufficient current can be pushed into the motor to make a step.

[hanermo2]

Tech:
Yes, no.

Steppers are not much different from servos.
The servos step up the "next" step magnetically before the rotor is there.
They anticipate the position of the rotor and tightly couple it to the angle of the rotor via the encoder.

And using the skewed rotors (afaik) +magnets they can keep this up indefinitely.

And can use very heavy currents temporarily, since the duration is so short.
Servos used to run 12 kHz, 0.1 ms each.

But servos have better magic steel and magic wire.
Very complex thing with magnetic flux rise time, decay time, heat losses, efficiency, mag. flux strength, inductance, reluctance, etc.
The servos have probably 1% difference in materials but might have 100x difference in terms of magnetic inductance/reluctance effects at very high speeds == 500 kHz - 4 Mhz of modern ac servos.