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Topic: Moving a closed loop stepper motor VERY slowly (Read 470 times) previous topic - next topic

zwieblum

"Warning: For NEMA17 closed loop motor, need to change the value of motor peak current from 80 to 30, if not, it will burning the motor as too high current output."
What does the motor formfactor has to do with the motor current rating?

johndg

What does the motor formfactor has to do with the motor current rating?
Generally speaking, the smaller motors have a lower maximum current rating. For example, the HS range of motors sold by one well-known supplier quote the following maximum current ratings per phase (in amps):
HS17 0.26  - 1.0
HS23 1.5 - 4.7
HS34 3.1 - 4.7
So, while there is obviously some overlap between the ranges, NEMA 17 motors generally have a significantly lower maximum current rating. But you are correct in that whatever the frame size, the datasheet for the specific motor should be consulted.

zwieblum

NEMA17 only defines the front facing side of the motor, not the volume nor it's thermal capacity. The max. amp rating has nothing to do with the size of the motor, it's a property of the winding wire diameter. The max. const. current amp rating is a property resulting from the prev. properties and the available cooling. ... don't be misguided by marketing texts, read the datasheets.

johndg

read the datasheets.
The values I quoted were from the datasheets. I also said "whatever the frame size, the datasheet for the specific motor should be consulted"

zwieblum


johndg


MarkT

Microstepping divides steps and momentum. e.g. fullstep ~ 10Nm --> halfstep 5Nm --> 1/4 step 2.5Nm ... so each fullstep position you get 10Nm, inbetween that you get for 1/2 step positions 5Nm, inbetween that you get for 1/4 step position 2.5Nm ... better get a geared stepper with a 1:36 ratio - or any ratio that translates 200steps/rev to 360steps/rev
Not sure what you are trying to say, as you say "momentum" and use "Nm" which is the unit of torque,
not momentum.   Microstepping does not reduce torque, this is a myth.  It actually tends to _increase_ usable
torque by reducing the sensitivity of the motor to resonance effects.

[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

MarkT

Generally speaking, the smaller motors have a lower maximum current rating.
Size is not related to current rating - you can rewind a motor with thinner wire and decrease
the current rating without affecting its performance as a motor at all, it just increases the
impedance, requiring higher voltage and lower current to drive it.

What size is correlated to is maximum torque, which is a result straight from the
physical equations governing motors.  (construction type and material choice
also affects torque, but all else being equal the volume of the motor's rotor determines
the torque).

For instance I've a little BLDC about 25mm in diameter that can easily pull 20A as
its windings are a few milliohms, and a similar sized BLDC (a "gimbal motor") which is
about 10 ohms and takes 0.5A max.  Mechanically they are similar, so the power, speed
and torque ratings are comparable.  The difference is the number of turns and thickness
of the wire in the windings.  The total amp-turns will be similar at full power and that's the
only thing the mechanical response of the motor cares about.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

MarkT

Hi all,

I have a project in which a 450 mm long arm will rotate in steps of 0.5 degrees from 0 to 90 degrees. Each step will take 3 seconds and the motor has to stand still for 2 seconds before going to the next 0.5 degrees.

The arm will be actuated by a stepper motor with a shaft thats attached to the arm. The shaft is supported by two bearings, so the load of the arm won't have to be carried fully by the stepper motor shaft.
Typical stepper has 1.8 degrees per step, so will have an accuracy of that order, whether using
microstepping or not.  Microstepping will increase the precision, but not so much the accuracy.

I'd suggest using the maximum microstepping ratio you can get so that there are many steps per
0.5 degrees.  

Don't expect that level of accuracy, but you ought to be able to get smoother motion.

I worry that you have a large torque given the length of the arm - bearings won't affect this,
but a counter-balance can, so if you can counter balance the arm you will decrease the torque
the stepper sees - do you have any idea of the level of torque you are dealing with?  If its more
than the motor can handle you're going to be disappointed.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

johndg

Size is not related to current rating -
I stand by my generalisation (for stepper motors, which are specified in the title of this topic).
Source of my data: McLennan Servo Supplies HS series motors.
Summary:
NEMA17 0.26  - 1.0A
NEMA23 1.5 - 4.7A
NEMA34 3.1 - 4.7A


More from McLennan, Ametek MAE series.
Summary:
NEMA23 1.0 - 3.0A
NEMA34 4.3 - 6.4A


And yet more,  Applied Motion HT Series.
Summary:
NEMA17 0.14 - 1.0A
NEMA23 0.71 - 3.3A
NEMA34 2.05 - 4.45A


Some from Micromech: Stogra motors
Summary:
NEMA23 1.4 - 6.5A
NEMA34 2.3 - 10A
NEMA42 4.0 - 12.5A

I could go on (I have examples from Parker SY range and Kollmorgen T series), but I think this has wasted enough bandwidth already. I'm having difficulty finding a range of steppers which is a counterexample to my generalisation. Can you?






Robin2

I stand by my generalisation ...
I agree with @MarkT. I have a few of these Nema 17 stepper motors. They only draw 0.33 amps.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

johndg

I agree with @MarkT. I have a few of these Nema 17 stepper motors. They only draw 0.33 amps.

...R
But surely that's supporting my thesis (small stepper, small current)? Mark is stating that size and current are unrelated.

Robin2

But surely that's supporting my thesis (small stepper, small current)? Mark is stating that size and current are unrelated.
Not at all. The Nema 17 motors you have referred to draw 3 times the current of the ones I have and you can get Nema 17 motors that draw 2 amps or more.

In other words, for any physical motor size there can be a wide range of coil currents. The only practical approach is to refer to the datasheet for the specific motor that is being used.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

johndg

The Nema 17 motors you have referred to draw 3 times the current of the ones I have and you can get Nema 17 motors that draw 2 amps or more.

My first reference to a NEMA 17 current rating was 0.26A. Also, I did not find a NEMA 34 with a rating of less than 2A. Yes, there can be a wide range of current ratings for a given frame size, but in general, the trend of larger steppers having higher current ratings  pervades. I have found no manufacturers' range of motors for which this is not true.
As we both agree, the datasheet for the specific motor should be consulted.

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