Need help with stepper / driver selection (voltage-confused)

Other Hardware Motors, Mechanics, Power and CNC
1

Hi!

I have a question regarding stepper motors and drivers for my project. I am considering to purchase the 125oz.in stepper motor from Sparkfun (Stepper Motor - 125 oz.in (200 steps/rev, 600mm Wire) - ROB-13656 - SparkFun Electronics) and power it using the Big Easy-driver.

It is rated 3V at 2A/phase and have a suitable holding torque for my application. The rated values correspond to 1.5ohm/phase. Based on what I have learned from other threads, it is the current rating that is key, and the voltage rating is not all that important. According to the data sheet, the BED can feed a maximum of 1.4amp/phase, which is well below the 2A/phase motor rating. If I power the BED with 12V and the stepper phase resistance is 1.5ohm, this should correspond to 2,4amp/phase - - well above the maximum output of the BED. Then, if I turn the current limiting pot of the BED to max (1.4amp), what wil happen to the torque of the motor? Will it still be able to provide 125oz.in? Would I need a heat sink to cool the BED? Any other implications? I am really unsure, and have tried to figure it out by searching.

I.e. - - will the "weaker" 12V / 0,33a/phase (rated) motor performed better in terms of torque if I power it with a BED at 12v and limited current supply to motor maximum?

Probably not the best question ever asked, but then again I am a beginner :cold_sweat:

Thanks in advance!

Best regards,
Aleksander Hansen

2

The Pololu DRV8825 would probably be a better choice as it can handle a little more current than the A4988 on the BigEasydriver.

The rated torque will only be available at the rated current. I don't know how to figure out the reduction in torque for lower currents. Magnetic force is directly proportional to current but there are so many other factors including the coil inductance and back emf.

Low resistance motors driven with high voltages work better at high speeds than higher resistance motors driven at the same voltage. The higher voltage won't give more power (in the simple watts = volts * amps sense) but will allow the motor to maintain torque to higher speeds.

It is an interesting question (and I don't know the answer) whether a down rated 2A motor would have more torque than a 0.33A motor running at full current. The 0.33A motor will have many more windings on its coils.

You can, of course, by more expensive stepper drivers that can easily supply the full 2-amps. Maybe an option would be to start with a DRV8825 and upgrade later if needed.

You haven't said how much torque you need.

...R

3

As Newbie questions go, it is a much better question than we are used to hearing.
I think all the talk about motor current is appropriate and important but I think a more important question is how can you measure the torque to see if it meets your 125 oz-in criteria ?

4

Robin2:
The Pololu DRV8825 would probably be a better choice as it can handle a little more current than the A4988 on the BigEasydriver.

The rated torque will only be available at the rated current. I don't know how to figure out the reduction in torque for lower currents. Magnetic force is directly proportional to current but there are so many other factors including the coil inductance and back emf.

Low resistance motors driven with high voltages work better at high speeds than higher resistance motors driven at the same voltage. The higher voltage won't give more power (in the simple watts = volts * amps sense) but will allow the motor to maintain torque to higher speeds.

It is an interesting question (and I don't know the answer) whether a down rated 2A motor would have more torque than a 0.33A motor running at full current. The 0.33A motor will have many more windings on its coils.

You can, of course, by more expensive stepper drivers that can easily supply the full 2-amps. Maybe an option would be to start with a DRV8825 and upgrade later if needed.

You haven't said how much torque you need.

...R

Again, correct me if I'm wrong here. With a 1 inch arm, this motor should theoretically be able to "hold" 125oz / 34 newton (from datasheet). If we decrease the arm to 1/2 inch (1 inch, 16T pulley), the motor should theoretically hold 69 newton, and I just need a holding torque beeing able to withstand 40 newton. A a friend of mine told me that if I have a stepper that is close to its torque limits, it may start to "skip" steps and loose control of its position, so I thought this motor would offer the nessecary margin. However, if I limit the current to the BED's max of 1,4a/phase, I am afraid of the torque dropping close to my exact requirement, and hence start to skip steps - - if that is true..

This is also the reason for my initial question. I was first planning on purchasing the previously mentioned "weaker" 12v - 0.33amp/phase motor which had close to exactly the momentum i needed , but I read that driving it on 12V (rated voltage) would make it loose its torque at higher rpm's (the torque was close to its limits at rated voltage and current), and that the main reason for increasing voltage is to make it stronger on higher RPM's. Since not going above 12V was an option, I was afarid the motor would start to skip steps and loose positional control when gaining speed (I am using accel stepper library with accelleration). This is the reason why I thought of the above mentioned motor 125oz.in as a better alternative. Lots of numbers here now.. :slight_smile:

Best regards,
Aleksander Hansen

5

A stepping motor has it's maximum torque when it is stationary. you get that torque at a current no matter what the voltage is.

As the motor speed increases from zero the torque drops off. The higher the voltage the slower it drops off with increasing speed.

As the speed increases the torque finally drops to a point where the motor stalls and skips steps.

Bottom line, with a higher voltage you can go faster before you stall.

6

Grumpy_Mike:
A stepping motor has it's maximum torque when it is stationary. you get that torque at a current no matter what the voltage is.

As the motor speed increases from zero the torque drops off. The higher the voltage the slower it drops off with increasing speed.

As the speed increases the torque finally drops to a point where the motor stalls and skips steps.

Bottom line, with a higher voltage you can go faster before you stall.

Thank you! :slight_smile: But as the motor is approaching a stall situation, will it start to "skip" steps, or will it just go slower without this skipping of steps? Im not even sure what my friend ment about it, but it sure did not sound like something i wanted to happen!

7

will it start to "skip" steps,

Yes

will it just go slower without this skipping of steps?

No.

8

Thank you.

Is it possible to set up a torque / rpm graph?

9

Yes this is normally in the motor's data sheet.

10

Motors and coils are dynamic devices they will do any work given to them. What usually happens is they are either denied the required power to do the work or there internal components can't handle the power being drawn and fail. Theoretically given infinite current and indestructible wires a 5V toy motor could drive a bus.

Stepper motors are not fundamentally designed for strong loads especially with high speed applications, they are for high precision position movements. What your doing it making quiet an advanced combination, I think theoretical calculations have gone out the window and the best you can do is what you suggested, take your min specs and then get the largest motor you can find in your budget.

In some applications you will hit a calculation wall and have no choice but to do real world testing to see what happens.

Using the motors rated voltage will give you its strongest output, however you should never make a system that operates at its rated current draw. Unless otherwise stated in spec sheet always assume its the absolute max rating , meaning you should have at least 10-20% lower current draw then the motors rating to avoid damage from current spikes.

11

harddrive123:
Using the motors rated voltage will give you its strongest output, however you should never make a system that operates at its rated current draw. Unless otherwise stated in spec sheet always assume its the absolute max rating , meaning you should have at least 10-20% lower current draw then the motors rating to avoid damage from current spikes.

This is not true for stepper motors.

Firstly, as discussed earlier, the rated voltage is irrelevant for all practical purposes.

Secondly, stepper motors are designed to operate at their rated current 24/7.

...R

12

Ratings can be given for intermittent or continuous duty cycle, you always
have to read what the datasheet says, not assume things.

Modern high performance steppers are not easily damaged by current spikes, since they
use NdFeB magnets, whereas older motors used permanant magnets that weren't
so permanent. The max current rating might be a thermal limit (in fact this is likely).
It may be for a temperature rise that is more than you can tolerate in your application
too, so watch out for that.

Lower spec / older steppers use alnico or ferrite magnets which can be demagnetised
or remagnetised by over current (in fact just disassembling such a motor will
demagnetise it somewhat). For such a motor the current limit may well be
magnetic.

When talking about other motor types, short overcurrent bursts are often characterised
in the datasheet, since the continuous current is a thermal limit.

13

Hi,

I tried finding information in the datasheet regarding the current ratings, but it is closer to a technical drawing, just containing the basic information. Torque / rpm graphs, conditions for current ratings etc, are completely left out. However, is holding torque is only present at rated power, and torque is proportional to coil current, it should be possible to determine at least holding torque given other values? I mean - - if I change the applied voltage from 3V to 12V and limit the current by 50% of rated value, should'nt the holding torque be 50% of the specified? But because we increase voltage, it should keep the (reduced) torque longer before approaching stall?

Best regards
Aleksander Hansen

14

if I change the applied voltage from 3V to 12V and limit the current by 50% of rated value

I am not sure you understand the voltage / current relationship or how a driver limits current.

Current is limited by applying the voltage and monitoring the current, when the current rises to the set value the voltage is removed. There is now no voltage applied for the rest of the switching time.
The reasion why a higher voltage allows you faster speed is that it allows the current to get into the coils quicker. With a lower voltage when the stepping time approaches the time it takes the current to rise to the set value that is when the torque drops off.

15

And you also need higher voltage to overcome the back-EMF from the windings due
to the speed of the motor.

16

Firstly, as discussed earlier, the rated voltage is irrelevant for all practical purposes.

Secondly, stepper motors are designed to operate at their rated current 24/7.

That is so fundamentally wrong. When the manufacture gives you the max current it is derived from when using the max voltage, that whole ohms law thing. Increasing the voltage past that will put more current through the coils, a current they might not be rated to handle and fail. Less voltage will get you less current, but then again that will weaken its strength/torque.

As I said, unless otherwise noted in the spec sheet you have to assume the given current is its absolute maximum rating. Not doing so is just poor design and you have a fairly high risk of shortened life span of the device or outright failure.

17

harddrive123:
Increasing the voltage past that will put more current through the coils, a current they might not be rated to handle and fail.

That is only true if you are not using a proper stepper motor driver board which can power the motor at high voltages without allowing excess current to flow in the motor coils. I still stand behind my statements.

Stepper motors powered at their rated voltage will not perform as well as those powered at high voltage.

...R

18

Stepper motors powered at their rated voltage will not perform as well as those powered at high voltage.

Yes 100% spot on.

When the manufacture gives you the max current it is derived from when using the max voltage, that whole ohms law thing.

No that is not right. The voltage given is not the maximum voltage, it is the voltage you need to use to generate the rated current in the coil when that current has built up to it's maximum.

A coil is an inductor, therefore it takes time to build up current, in the same way that it takes time to charge up a capacitor. So using a higher voltage is a way of reaching that rated current more quickly. Once that current is reached then the driver shuts off the voltage or behaves like a constant current switching regulator to keep the current at that level. So the faster you can get the current into the coil the faster you can step the motor without loosing torque.

19

What your saying and I am saying are two different things.

I'm not saying it won't work at higher voltages, I'm saying you shouldn't if you want to take into consideration the life span of the motor and prevent un-necessary risks of damage to it.

Stepper motors powered at their rated voltage will not perform as well as those powered at high voltage.
Yes 100% spot on.

LOL, well ya as is true with every motor and coil, that is until it starts smoking and/or catches on fire.

No that is not right. The voltage given is not the maximum voltage, it is the voltage you need to use to generate the rated current in the coil when that current has built up to it's maximum.

Think about that for a second, they just gave you a voltage and current rating. Why would they pick that number and not a higher value, they will sell more units if it has higher specs. Because it has a much higher risk of self damage if you go beyond those values.

Stop giving bad design advice. Saying it will work isn't the point, one usually wants it to keep working with very low risk for a long time.

20

IMO sensibly spec'd bipolar steppers give a resistance, current and insulation voltage rating.
No operating voltage at all.

I think a lot of people don't realise and simply multiply I by R to give a bogus V.
Although arguably its perfectly reasonable to drive a bipolar from dual H-bridge at low speeds, just not taking full advantage of the motor's capabilities.