Problem with lack of torque from stepper motors

Hey so i am driving a device using two stepper motors and the motors are slipping to the point where the wheels dont even rotate. The stepper motors im using have a 48 N.cm holding torque rating and 1.7A/phase and i am using a big easy driver ramped up to the full 2A/phase to drive them. I am however only using two 9V in series (18V) to power them. Is there not enough current being supplied or is the only option to decrease the radius of my wheels to reduce the torque on the motors?

Don't be offended but you using two PP3 9V smoke alarm batteries in series and being surprised you're not getting 1.7A is completely ludicrous. You don't have a chance in hell of getting 1.7A out of a smoke alarm battery if your life depended on it. You can throw those in the trash right now and start looking for a replacement because you never had a chance to begin with using those. The short answer as to why is the internal resistance is too high. You need to get some LIPO batteries A 4S Lipo would give the maximum torque possible The one linked below is one of the cheapest because it is only 2200mAh which means 2 x 1.7 = 3.4A and 2.2Ahr/3.4A = 0.64 hours = 38minutes of use on a full charge.

Be sure to use a genuine Lipo battery charger and NEVER charge a LIPO battery UNATTENDED because they can burst into flames if overcharged , which is why you need a lipo charger. Never charge a lipo battery on any surface that can burn .(meaning it must be in a ceramic pot or metal box or a concrete or brick barbaque pit but NEVER on wood).

http://hobbyking.com/hobbyking/store/index.asp

Thanks for confirming that the batteries are the issue :slight_smile:

Would the equivalent voltage using AA's get me sufficient current? I was thinking 8 AA's per driver?

Fresh alkaline AA batteries will supply 2 amperes for a few minutes, but in full step mode your motors each require 3.4 amperes, considering both windings.

No. I'm sorry but I guess you must be new at this. None of the household batteries would work unless you ganged up two battery packs of 12 "D" size batteries. 12 "D" size in one 18V pack in parallel with another 12 "D" batteries in another 18V pack for a total of 24 "D" size batteries at 18V. That would have a chance of delivering 1.7A at 18V for each motor. "D" size batteries can have a mAh rating as low as 2000 or as high as 10000, meaning it could deliver between 2 and 10A for one hour. I think 10000 is a bit on the optimistic side . You might get by with only 12 "D" batteries in series but they probably wouldn't last very long which is why I would say at the very least you need two packs of 12 batteries each. Anything smaller than a "D' battery would have no chance of delivering 1.7A at 18V for each of the two motors. Since 12 "D" batteries is a lot of weight for a robot, you should consider the Lipo battery option. A single 4000mAh 4S
battery wouldn't be too much weight for the motors. If you try to use Nicad it would be even heavier.

A single half way decent 4000mAh 4S LIPO battery
[ Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking](http:// Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking)

like this cheap one rated for 20C could deliver:
80A for 3 minutes
or
40A for 6 minutes
or
20A for 12 minutes
or
10A for 24 minutes
or
4A for 60 minutes .

(20*4=80A. .)

Your load is 2 x 1.7A = 3.4A so you could expect just over an hour on a full charge from the above battery.

i am driving a device using two stepper motors and the motors are slipping to the point where the wheels dont even rotate.

How about providing a little more detail about the mechanical design. Is this something mobile or is it like an x/y table or what ?

Sorry yes i am new to all this :slight_smile:

It is something mobile. The 2 rear wheels are driven by a stepper motor each.

Total weight of vehicle ?

2.6 kg with diameter 10cm wheels at the moment

5 lbs is a significant load to push around for a battery powered vehicle. That's probably double what an RC car weighs. Your going to need every bit of that 1.7A per motor at 18V. I think your only option is the lipo battery.

Stomper90:
2.6 kg with diameter 10cm wheels at the moment

Sounds plausible, 0.48Nm with 0.05m radius wheel = 9.6N or about 1kg force per wheel.
(which gives a healthy margin of safety to allow for various losses and the drop off of
torque with speed in a stepper)

The BigEasyDriver is a chopper so that 3.4A at the motors does not mean 3.4A from the 18V supply,
probably more like 1A at kow speed, ramping up to 2A or more at higher speeds.