I m trying to understand better how a stepper motor works in order to make an order for some projects.

What I know is the max holding torque (which is related to my load). For example, I know that I need a motor with max torque of 4.2 kg.cm

The questions is :

I have found 2 different bipolar motors, both with 4.2 kg.cm holding torque, but in the first, the current is 0.5A and in the second the current is 2.4A. How or what is the relationship between current, voltage and torque in a specific speed ? Is it possible with different current/phase to produce the same torque ?

The motor with the lower current will have many more turns of wire in its coils. Magnetic force depends on the magnetic field and the current and the length of the wire.

The motor with more turns will have greater impedance to the rising current so it will take longer for the current to rise from 0 to max in a pulse. If you use the same voltage on both motors that means the one with more turns will have a lower max speed because the duration of the pulse will not be long enough to allow the current to reach its max for an appreciable proportion of the pulse.

I m trying to understand better how a stepper motor works in order to make an order for some projects.

What I know is the max holding torque (which is related to my load). For example, I know that I need a motor with max torque of 4.2 kg.cm

The questions is :

I have found 2 different bipolar motors, both with 4.2 kg.cm holding torque, but in the first, the current is 0.5A and in the second the current is 2.4A. How or what is the relationship between current, voltage and torque in a specific speed ? Is it possible with different current/phase to produce the same torque ?

thanks

Read the datasheet for the particular motor - different motors are different (should be obvious). Unlike
DC motors there is no single parameter you can use to determine performance, you look at the graphs
in the datasheet, and you also have to take into account resonance for your particular mechanical setup,
which puts a big notch in the torque v. speed graphs unless you have really effective damping.

The frequency of the resonance is dependent on mechnical details, so it won't necessarily be mentioned
in the datasheet, but its usually the limiting factor for performance at speed.

I need to move a wheel-car, with MAX speed of 0.15~0.20 m/sec. Becouse the wheel is 6mm diameter, that means a speed at wheel around 60 rpm (wheel speed).
I have also calculated that motors' torque should be around 3.5 kg.cm at max.

I will use a spur gear of 10mm diameter which will lead another gear of 50mm diameter. that means that the gears analoguw is 5:1

which i think that this means that the speed of the motor's gear should be 5*60=300rpm. Right ?
Also that means that the FINAL rated torque (Lets say "N") of the stepper motor will be : N= rated (ungeared) torque * 5

Please let me know if I m doing any mistake in my analysis.

I need a NEMA 14 motor (9~12V) , but how can I find which one is proper for me ? All stepper motor at ebay have no curves, cannot understand how to select one.

Can you please help/guide me (with the given data above) of how to find the proper motor for the example above ?

Maybe try something cheap and cheerful like this plastic gear motor to get you started. They seem to be widely available (not necessarily from Pololu).

You are unlikely to be able to control the position of a robot by measuring how many turns the wheels make - there is too much slippage between the wheels and the ground. So I can't see any value using either a stepper motor or an encoder on a DC motor if the purpose is to figure out the position.

The PWM method to control speed, becomes a trouble without an encoder, as at lower speeds (lower volt) the torque cannot move the vehicle (and a load of 4kg). Also soft-starting is a mesh. It would be perfect if I have an encoder, but it is really hard for me to find the proper encoder for the specific motor, how to implement it and then how to write the code for the encodr etc.

The stepper motor is a decent solution, with almost constant torque in low speed (don;t forget max speed is just 0.2 m/sec) and without the problems of soft starting of DC. As it concerns the power consumption, with easy driver there is a sleep and enable power pin, so there is no problem.

So, having the above data (of the example) I reall need a guide of how to select a stepper motor. I believe It will be usefull for many other users

I can see the advantage of the stepper motor for speed control.

In the link I gave you in Reply #1 there is a suggestion for a simple way to measure the required torque.

You are correct that the reduction gear will multiply the torque but it will also introduce a lot of friction. I would estimate the required torque and then double it for a margin of error.

If slow step rates (say 1000 steps per second or less) are acceptable a big advantage of the stepper motor that only requires 0.5 amps is that you can control it with a low cost driver like a Pololu A4988 or DRV8825. The 2.4 amp motor would be too much for those drivers - though a TB6560 may be suitable and would not break the bank.

I just read your excellent introduction article about stepper motors. Thanks. What is missing is how to calculate the max speed before torque falls to zero. But lets take it from the start with an example please.

As you said it your article V=IR => 10V=0.5A20ohm when motor is stationary. That also means (theoriticaly) that P=VI=100.5= 5W. right ? (general principle).

Now (just to understanding better), if my power supply is for example 7V, then in order to have the same behavior (torqe and speed) that means current would be I=P/V = 5/7= 0.71 A ???? right ?

2nd Question
In the specific motor of the example, if we don't have a curve. How can we calculate or to be more specific ESTIMATE the max speed (no load) before torque fall to zero ?

gspir:
I just read your excellent introduction article about stepper motors. Thanks. What is missing is how to calculate the max speed before torque falls to zero.
....
Now (just to understanding better), if my power supply is for example 7V, then in order to have the same behavior (torqe and speed) that means current would be I=P/V = 5/7= 0.71 A ???? right ?

2nd Question
In the specific motor of the example, if we don't have a curve. How can we calculate or to be more specific ESTIMATE the max speed (no load) before torque fall to zero ?

The big flaw in this thinking is the notion that the voltage is meaningful. It is normal to driver stepper motors at voltages above the nominal voltage, Also, once the motor starts moving things get far more complex than applying Ohm's law because of the impedance, inductance and back emf of the motor. The only way to know the max speed for a given supply voltage is either by experiment or by studying the manufacturer's performance curves (and those curves are probably based on the manufacturer's experiments).