CNC ignoramus seeking stepper help
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Hi,
believe it or not, there is no such thing as Nema24, Nema23 is a known and standardized size, but not so Nema24.
Nema 23 means that the motor is 2.3 inches square or 57mm. What is commonly refered to as Nema24 is in fact 60mm square.
So Nema23 is the American (inch units) motor whereas the Nema24 is the metric equivalent. In terms of motor performance there is little
or no difference.
All stepper motors lose torque the faster they go, thats just the physics of steppers. The inductance is a resonable measure as to how bad that torque
degradation will be and therefore limit how fast your machine will be. So inductance is VERY important. Most first time buyers don't know about inductance
an so buy on the basis of highest torque...makes sense right? Well no, high torque often means high inductance and consequent loss of high speed performance.
The manufacturers know that newbies buy on torque so they make high torque motors as cheap as possible to capture them but such motors are as slow as a wet week.
In fact inductance is only part of the picture, the true measure of how fast a motor might be is the 'Inductive Time Constant' which is the product
of the winding resistance and the winding inductance.
1st Nema23:
0.83 x 2.2 =1.826msec
2nd Nema23:
1.1 x 3.6 =3.96msec
1st Nema24:
0.5 x 2.2 =1.1msec
2nd Nema24:
0.35 x 1.3 = 0.455msec
So clearly the 2nd Nema24 is by far and away the fastest motor you've listed.
The classical way to make a motor run fast without losing steps is to use the highest voltage you can, I would not limit your machine with a paltry 36V
drivers. The highest voltage driver commonly available are 80V....get them and run them at 80V....don't mess around...if you want your machine to perform
then use the highest voltage you can.
As to the size of the power supply.....that's not quite as straight forward as you might believe.
Lets imagine you have a 5A stepper motor with a 1 Ohm winding resistance and a 50V power supply. If you hooked up the 50V supply to the stepper
the current would be 50/1 =50A!!! and it would blow up almost instantly.
The driver applies the 50V available UNTIL the current reaches 5A AND THEN uses PWM to limit the current to the rated and safe 5A.
This is where it gets a bit tricky, when the driver goes into PWM limiting the current drawn from the power supply reduces also.
Output Voltage =5V, Output Current= 5A
input Voltage= 50V Input current =0.5A
Yes....you read that correctly....the driver is effectively behaving like a transformer for DC. If you want to read more about this then read the theory
of how buck regulators work....its fascinating.
The point is that only for very brief periods of time, micro seconds only, is the power supply required to deliver 5A, for the rest of the time it can loaf along
at lower currents.
Most people recommend that you size you power supply to meet the rated current of two (of three) steppers. Thus if you had three 5A steppers then a 10A supply
is needed. The very heavy current burst required of a power supply are best delivered by a linear supply, that is a mains transformer/rectifier/capacitor combination
rather than switch modes supplies. Linear supplies are very much more tolerant than switch mode.
Craig
Originally Posted by Mickyas
