More power?

Hi all,

I've just got ahold of two sparkfun stepper motors http://www.hobbytronics.co.uk/stepper-motor

And am using each with a SN754410, but they're not quite a torquey as I thought they might be. When they're spinning I can just pinch the shaft and it will stop turning quite easily - I figured they would put up a bit more resistance.

Is there a way I can get a bit more power out of these puppies?

I have those motors and I control them with Pololu A4988 stepper drivers and a 20v power supply (laptop power unit).

H-bridges are a poor choice for driving stepper motors.

...R Stepper Motor Basics

Thanks Robin, I'll be sure to check out you write up too.

Are you able to get a reasonable amount of resistance / load on that shaft before it starts skipping steps with your setup?

Once the motors are powered up I don't think I can turn the shaft with my fingers - can't test it right now. I reckon I am getting the specified torque.

...R

With high impedance motors like this you won't get many rpm before the torque gives out. 100 or maybe 200rpm.

You need current control and low-impedance steppers if you want torque at any speed. You will see in the datasheet the winding inductance is a whopping great 46mH (which they print as Mh because motor manufacturer's datasheets are usually awful). A good snappy stepper would have 1/20th as much inductance and so run 20 times faster before the torque drops.

If you want fast, get motors with an ohm or two of resistance and a few mH inductance. Use a chopper drive such as DRV8825. Higher supply voltage helps with speed too (remember these are current controlling drivers, the winding current doesn't increase with supply voltage, just the speed the current can be changed).

Thanks for all the info guys, I really appreciate it!

I've ordered a couple of A4988's which should be here tomorrow so I'll give them a blast :)

Robin, I'm curious about the current limiting. Can they safely go past 0.33A ? I'm sure I read somewhere that somebody was running 0.7A per motor in their config.

You shouldn't put more than 0.33A into your existing motors - there's no point using current control with them at all unless from a much higher supply voltage than 12V as the winding resistance already limits the current.

Low impedance motors are what you need for torque at speed.

What speed do you want?

lilsancho: Can they safely go past 0.33A ? I'm sure I read somewhere that somebody was running 0.7A per motor in their config.

I have not seen that so I am not going to encourage you to let the smoke out.

...R

0.7A will put 4.5 times more heat into the windings so you would have to limit the duty cycle to 20% or so and on-time to a fraction of the thermal time-constant of the motor.

More importantly you could risk demagnetising the rotor (although this is less likely with modern motors with NdFeB magnets)

Hi guys, I'm back again with a few more questions :) thanks again with all your input so far its been really helpful!

I think I have an EMF proplem now, I've sometimes noticed that when I plug my battery in I see a little spark when the connections touch. (This is for my board supply not the motors). So I connected the + and - together with a jumper (without the battery connected) and gave the wheels a spin, that's when I noticed my power indicator led light up for a brief moment!

I figured the A4988's would have protected me from this (newbie) but clearly I was wrong.

So now I'm worried about exploding batteries (lipo).

Would a diode between my common gnd and + dissipate this build up and keep my batteries safe?

gave the wheels a spin, that's when I noticed my power indicator led light up for a brief moment!

Most motors act as generators when the shafts are spun. This is expected and not a problem.

The spark when you connect the battery is also not surprising -- it is probably due to a capacitor charging.

The A4988 has internal protection diodes.

jremington: Most motors act as generators when the shafts are spun. This is expected and not a problem.

The spark when you connect the battery is also not surprising -- it is probably due to a capacitor charging.

The A4988 has internal protection diodes.

Thanks for the assurances :) but I've since noticed that the motors have stopped working :( I've checked all the connections and supply rails but notthing happens when commanded to move. Is it possible that in my excitement of spinning the wheels to make the led light up (with batteries disconnected) has fried something? I did take some reading when I spun the wheels and it looks like they generated about 100mA per wheel, the volts only seamed to be a few mV but I not sure if my meter was able to accurately measure them.

spinning the wheels to make the led light up (with batteries disconnected) has fried something?

That is possible, but I consider it unlikely. What WILL destroy a motor driver is to disconnect the motor leads while the circuit is powered.

Triple check all your wiring and when the system is powered, measure all the voltages.

jremington: What WILL destroy a motor driver is to disconnect the motor leads while the circuit is powered.

Ah, I can't say that I recall doing something like that, but I won't rule it out. I'll check the voltages when I get home. Is there any partular area that I should pay attention to?