Can you help me with Stepper Motor Selection? NEMA 23

I'm using this driver [files.panucatt.com]

which says it supports up to 2 amps and up to 35 volts max (with additional cooling). Its typical to use NEMA 17 motors with these drivers but I would like to run one axis of a 3d printer using a NEMA 23 motor so I can get higher torque. The problem is that I don't know enough about steppers to know what is good and what is bad. I need it to run fairly cool as they are mounted using ABS brackets.

Here are the stepper options I'm looking at [store.kysanelectronics.com]

What factors are important to consider? I'm really interested in the 57BYGH301
2 phase
5.88 volts
1.4 amps
4.2(O) resistance
15mH Inductance Phase
15 Kg./cm Holding Torque

That seems like a good option, but all of this almost seems to simple, that is nearly 3 times the torque of my current NEMA 17 steppers. Am I overlooking something?

Please let me know

Thanks,
Clay

... it's really that simple - a NEMA23 has much higher torque with the same energy consumption because of the bigger diameter.

The other values are OK ...

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Viktor
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The coil resistance and inductance both seem a bit high to me. I ran the math and, compensating for backemf, you should expect a maximum full-step rate of 152 Hz at 12V, not counting any stepper driver oddities. 24V gives you 384Hz and 30V gives 497Hz.

On a 2cm pulley, 152Hz is 47mm/s, 384Hz is 120mm/s and 497Hz is 156mm/s.

[Edit: This is only true at full torque. If you derate the torque (it's proportional to current), you can speed it up dramatically. E.g. at half-torque & 12V, you get 235Hz and at 24V, you get 515 Hz]

To find the step rate, I solved the following equation for t, which is the full-step time:

I = (V-K/t)/R * (1-exp(-t*R/L))

where I is the maximum coil current
V is the driver voltage
R is the winding resistance
L is the winding inductance
K is the step-wise torque constant

K is derived from the holding torque and coil current:
K = (Holding torque in N-m / coil current) * (2 * pi radians/ 200 steps) = 0.0329867 volt seconds/step

Edited 1 time(s). Last edit at 10/17/2012 05:46PM by Annirak.

I've done some more research, and it seems that I have an error in my math. It stems from how stepper torque is rated vs. how it is used in microstepping drivers. Microstepping drivers have a maximum torque with one winding energized at maximum current, while stepper motors are rated with both coils energized at maximum current.

The adjusted value for K is:
K = (Holding torque in N-m / (2 * max coil current)) * (2 * pi radians/ 200 steps) = 0.0329867 volt seconds/step

With that change, the picture is not so bleak.

+---------------+------------------+------------------+------------+
| Drive Voltage | Full Torque Freq | Half-Torque Freq | Stall Freq |
+---------------+------------------+------------------+------------+
|           12V |            370Hz |            546Hz |      727Hz |
+---------------+------------------+------------------+------------+
|           19V |            776Hz |            956Hz |     1151Hz |
+---------------+------------------+------------------+------------+
|           24V |           1055Hz |           1245Hz |     1455Hz |
+---------------+------------------+------------------+------------+
|           30V |           1381Hz |           1587Hz |     1818Hz |
+---------------+------------------+------------------+------------+

Thanks for sharing, Annirak. What does that frequency relate to? Step rate of full steps? Half steps? Microsteps?

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Quote

Microstepping drivers have a maximum torque with one winding energized at maximum current, while stepper motors are rated with both coils energized at maximum current.

Microstepping drives have constant torque in each position because the vector sum of the currents lies on a circle.

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[www.hydraraptor.blogspot.com]

Traumflug Wrote:
-------------------------------------------------------
> Thanks for sharing, Annirak. What does that
> frequency relate to? Step rate of full steps? Half
> steps? Microsteps?

It refers to the full-step rate, regardless of micro stepping level. E.g. if you're using 1/16 drivers then multiply that frequency by 16 for the micro step rate.

@Nophead, that's what I meant. Is my K value correct? Or do you get .707 times the full-step torque when micro stepping?

With these drivers you get exactly the same torque with full step or micro step, and in any position.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> With these drivers you get exactly the same torque
> with full step or micro step, and in any position.

Perhaps I'm not being clear. The majority of stepper motors are spec'd with torque measured when both coils have Imax flowing through them (a vector sum of Imax * 1.414). Meanwhile, microstepping drivers use a vector sum Imax in all positions. This should mean that microstep torque is 0.707 * rated torque. Does that sound right?

Say the motors are speced at 1A, two coils on.

An allegro puts both coils on at 70%, even in full step mode, so to to get the rated torque you set VREF so that the current is 1A when two coils are on, i.e. 1.4A when a single coil is on. Power dissipation is constant all the way round as well, so the motor have the temperature rise stated on the data sheet for two coils on.

When people say microstepping reduces the torque they are just not comparing like with like.

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[www.hydraraptor.blogspot.com]

this is certainly interesting for me .... i like to know more ... (hungry chinchilla looking for pellets of data)

so if say my sanyo denkis ...

9VDC, 0.85A, 59oz/in ...

theoretically energized @ 2coils x0.7 factor of the rated power in a A4988, it should get the 59oz/in holding power? is that the way to put it?

That is my best guess, provided that they're not too old. It seems that older stepper motors were rated assuming full-step only and rated current in both coils. I can't seem to find any information in any datasheets about what current the torque rating applies to.

hmmm well then i guess the only other way is to attach a weighted device to the stepper and make it move then ...

How would I know if stepper motor is bad or going bad. When I ease off brakes a little bit, get the buzzing sound that lasts for 3to 5seconds. Too it to the dealer, they don't know what the cause of the sound is either. A friend said inquire about the stepper motor. Help please.

Also Posted in Stepper Motor Forum-stepper,torque, volts, pps
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Hi all

Trying to make some sense of this
Objective : maximize torque and rpm and validate my (mis)conceptions
Application is non reprap

Motor :wantai nema 23, type 57byg621 torque curve attached

[www.wantmotor.com]

Max torque, 12.5 kg cm at 24 V, half step
Halfstep gives 40% more torque than full step, but some say 4988 enegizes both coils at full step too, but can't find it in the spec sheets-confirmation would be great

Typical power supply is 12V so torque=50%?
If a gearbox is used, for torque multplication, this often needs an increase in rps, to keep speed, this increases pps, torque is reduced -high pps drops torque ?-see attached torque curve(also efficiency of gearbox costs up to 50%)

Possible solution:increase volts to max 25 times (according to Gecko) the rated voltage, say max 45/48 volt? (using DRV driver instead of pololu) Driver amps set to motor amps in spec sheet . Motor will run hot. This doubles torque?? (Almost) but assume that BEMF becomes an issue-considering to use L6470 driver for compensation

If microstepping is used, torque as total remains the same, just divided per microstep?
But microstepping increases pps, so lower torque again-as per curve?


Appreciate your help in this

Coolerooney
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Edited 1 time(s). Last edit at 07/08/2013 04:25PM by Coolerooney.