Stepper motor amperage

I'm planning on building a camera slider for a DSLR camera. The slider would also work verticle so with a camera weight of 1.5kg I am looking at stepper motor with +2kg torque.

I found 2 motors with the same torque but different amperage. What impact does the amperage have on the motor/torque/speed?

NEMA 17 0.4A 2800g/cm

NEMA 17 1.3A 2800g/cm

The low-current motor will have a higher coil resistance and inductance and as a consequence will have a lower top speed for a given power supply voltage.

How many steps per second do you need for your application?

The weight of the camera on a slider that moves horizontally is not of great significance. The important factor is how much force is needed to move the slider - if it is well constructed with ball bearings or rollers the friction should be very low. Of course a heavy camera will be more difficult to accelerate even if the slider friction is low.

...R
Stepper Motor Basics
Simple Stepper Code

Thanks Robin for the reply.

I will be modifying this camera slider so there shouldn't be much friction.

I think that I understand now the current/coil relationship in regards to speed and I see where you're going with the steps/sec. The slider is 100 cm and the timing pulley would have a 25mm outer diameter which would be 78.5mm circumference. So from A to B the shaft would need to rotate 12.7 times. Other than the setup, i.e. to calibrate the position, the slider would be used mostly for timelapse so from a fraction of a step per second to several steps per second.

Based on your reply I found this website: daycounter.com.

The results after plugging in the numbers are max 500 steps/s for the 0.4A and 1924 steps/s for the 1.3A.

If I got that right then the 0.4A should be good enough for my job with the understanding that it may take a few seconds to calibrate the position from one end to the other.

What driver would you recommend for both motors?

*Edited because I can't multiply correctly...:slight_smile:

I have some low current stepper motors that I control with Pololu A4988 stepper drivers. That driver would probably also be OK for 1.3 amps, but the Pololu DRV8825 has slightly more current capacity and would probably be a more sensible choice. It should also work fine with the low current motor.

Be sure to adjust the current limit on the stepper driver to suit your motor.

...R

25mm diameter sprocket? That's pretty large, a 16 tooth MXL sprocket is about 10mm diameter for instance and
will be fine for your slider and you won't need crazy torque.

Don't use a 0.4A motor unless you want to go upto an 18 or 24V supply, from a more convenient 12V supply pick
a motor with 1 to 1.5A current and a DRV8825 driver. [ These motors tend to have about 0.4Nm of torque
at low speed ]

[ BTW you'll need some way to protect the camera should the power fail in vertical mode ]

Mark, Robin,

Thanks for the advice. DRV8825 it is.

Re pulley...I'm just working on the parts list now so all feedback is appreciated. I chose the 25mm based off of plans that I found on the internet...not really understanding the mechanics of it. 10mm it is.

As for the power supply, I was thinking of running it off a 20V 2.5 Ah power tool battery that I have so wouldn't a higher amperage (1.7A) drain the battery quicker? Speed is not a factor in my application but battery life is with potentially several hours of time lapse so it seems that the 0.4A motor is the better solution.

Thanks again.

Steppers are high power devices, you won't get good battery life because they draw pretty much
maximum power constantly, whether anythings moving or not.

Battery life depends only on power of the motor, choosing a low current motor isn't going to help at all,
in fact it will limit the top speed. Stepper drivers are power converters, they'll draw the same supply current for any motor with a given power rating, basically.

Thanks,

How long would a 2.5 Ah 20V battery last on the following stepper motors:

  1. 12V rated 0.4 A
  2. 12V rated 1.7 A

Speed in my case in not a decisive factor.

I read in Robin's Stepper Motor Basics that "For all practical purposes the nominal voltage of a stepper motor is irrelevant." Is there any danger in using this battery to drive the stepper motors?

You must ensure that the current limit on the stepper driver board is adjusted so that it protects the motor. Then you can use any voltage up to the maximum acceptable by the driver board.

A 12v 0.4 amp motor will require about 0.8 amps (two coils) and that gives 12 * 0.8 = 9.6 watts (say 10).

To get 10 watts from a 20v battery you will draw 0.5 amps. If you are drawing 0.5 amps then 2.5Ah will be sufficient for 5 hours.

Then you need to allow a good margin of error in the calculations - maybe you will really be drawing 0.7 amps for a theoretical run time of 3.5 hours

Personally I would not expect to get 2.5Ah from an alleged 2.5Ah battery - manufacturers are very optimistic. Think of 1.5 hours rather than 3.5 hours. AND you must also be VERY CAREFUL not to over-discharge Lithium cells.

You can do the sums for the other motor.

...R

PS... Check my maths carefully. I often make mistakes.

Robin2:
You must ensure that the current limit on the stepper driver board is adjusted so that it protects the motor. Then you can use any voltage up to the maximum acceptable by the driver board.

A 12v 0.4 amp motor will require about 0.8 amps (two coils) and that gives 12 * 0.8 = 9.6 watts (say 10).

No, it requires 0.4A per winding, but when one winding is at 0.4A the other is at 0V, the maximum
total current is actually sqrt(2) x 0.4A, but this isn't useful information, you just set the driver to 0.4A setting
and it does all the work.

A 30 ohm 0.4A stepper takes a constant 4.8W from the driver since the sum of the squares of the winding
currents is constant (quadrature drive)

Thanks Robin and Mark,

I was taken off by Mark's comment: "Battery life depends only on power of the motor, choosing a low current motor isn't going to help at all".

If I understand correctly though...a lower amp motor will draw less current, therefore the battery charge will last longer on a low amp motor than for a higher amp motor so the 0.4A motor will allow the slider to work longer than the 1.7A motor with the same battery. Correct? Granted, I understand that the 0.4A motor will require a longer time to charge the coils and therefore will be slower but that is not a concern for me as the slider will be moving at several steps per second.

When reading through the comments under the Stepper Motor Basics, there is a discussion on proper voltage for the motor. I'm a bit lost in the discussion so just to confirm...the 20V LION battery that I specified can be used with the 12V motor and DRV8825 driver and the battery won't be harmed?

As to the correct settings on the driver...still not sure what is the correct setting 0.4A or 0.5A.

As to protecting the LI-ON cells. That is a good point. I haven't considered that. Perhaps I could do some type of voltage volt controller like here. I could probably get the low volt value from the battery when the power tool turns off and use that in the code to turn off the slider at that time. Any ideas as to if this idea would work and what type of resistors to use to split the 20V voltage to something acceptable by the Arduino?

As to securing the camera from falling. Yes I haven't thought of that as well. Maybe I'll just use a pillow but I am open to more sophisticated solutions.

Once again...thanks to both of you for your help so far.

Just a follow up to my previous post re voltage divider.

I found this page that calculates the voltage output based on different resistor values and voltage input.

My battery is 20V but I tested it right now and fully charged the voltage is actually 20.6V so using 21V as the base, I could use a 5.1kΩ and 1.5kΩ, which should give an output voltage of 4.8V (4.4 ratio). Would this work?

Current from the battery will be Vbat/(R1+R2), 20V/6600 = 3mA.

If that's a problem long term, use higher value resistors.

Thanks Crossroads.

roli001:
Thanks Robin and Mark,

I was taken off by Mark's comment: "Battery life depends only on power of the motor, choosing a low current motor isn't going to help at all".

If I understand correctly though...a lower amp motor will draw less current, therefore the battery charge will last longer on a low amp motor than for a higher amp motor so the 0.4A motor will allow the slider to work longer than the 1.7A motor with the same battery. Correct?

No, that's not how things work with steppers - the stepper driver does power conversion so only the
power of the motor and losses in the driver contribute to battery consumption. Steppers tend to draw
fairly constant power all the time.

roli001:
If I understand correctly though...a lower amp motor will draw less current, therefore the battery charge will last longer on a low amp motor than for a higher amp motor so the 0.4A motor will allow the slider to work longer than the 1.7A motor with the same battery.

I seem to have missed this.

You need to consider how many watts the motor draws, rather than just how many amps.

If you have two motors that have the same holding torque but one draws 0.4A and the other draws 1.7A you will probably find that they both draw a similar number of watts.

...R