How to Get More Torque from a Stepper Motor?

More energy in, the more energy out (sometimes).

Whether your sequences are more efficient or practical (or both) will require some instrumentation. I'd be wary of overheating issues. Interesting concept though.

Hi frankd20,

There is no reason this shouldn't work, but try running your motor under load for awhile and note the temperature rise (with your hand, at a minimum). You are running your motor at 50% greater total power, after all. Microstepping your motor using this inverse scheme should work as well. Just keep an eye on the temp. rise and your motor should be fine.

Can you vary your step sequence/rate under program control, or is it hardwired? I'm asking because you can exercise your motor, say, make it turn one way then another, under load, and see how it performs. Give it a good workout whilst noting the temp. rise.

Take care,

-e

In this particular application temperature isn't much of a concern of mine, so far nothing gets all that hot, but I do realize how this would increase the temperature. The motor driver is running on a PIC chip of my own coding, so I can make the thing do whatever I want with in the limits of my programing skills (the real limit in this case)and the hardware. I am just surprised that you can't find any information on this method of driving a stepper.

On another note I have a new stepper with a much higher torque rating on the way so this trick will not be necessary for this application in the near future.

One place to look is the US Patent and Trademark Office, or USPTO. Another search tool is the Google Patent Search. If it has been done and found to be patent-worthy, you'll find it here.

For your motor exercise program, try using one of the PIC's timers to toggle the wave sequence back and forth under program control and drive it, say, 5-10 seconds one way, then the other. If you can ramp the sequence rate up & down, all the better. Otherwise you may "hop" steps during the acceleration/deceleration phases.

you can make simple power measuring equipment using shafts, pulleys, ropes and weights..

either as a brake, or as a drum hoist. by varying the weight, on the same drum, you can at least get an idea of the difference between the two methods, as to the difference in power.

I have read about an easier way to measure torque, that uses a ruler and some weights. Same concept just easier to do. For my purpose I can go even simpler since I don't need a number, I just need to compare the two different driving methods. I have no idea what the model # or spec'd torque output is anyhow so I can't compare to that.

If the OP needs dynamic torque, this will give comparative values; the comment mentioning a ruler & weights is only good for static torque [which may be fine]. I had need of a way to measure dynamic torque of a small motor once, and put a spool (miniature drum) onto its shaft, with a light string attached. At the other end of the string was a length of chain in a heap on the floor directly beneath the drum. As the motor turned, it began to lift the chain; when it stalled, I measured the amount above the floor, and had the radius of the drum and weight of that length of chain as basis for my torque value. In that case, something like bead chain was suitable. I've used sash chain in a similar way since then, and it is obviously scalable to much larger systems. The Christopher Becker "Chain-O-Matic" beam balance was my inspiration.

You have more torque because you are dumping more power into the motor. However, the locking position of the rotor will be slightly different because you will be pulling and one of the poles from two different magnets. Also, if you are driving three coils with the rated current per coil you will be dissipating more heat than was intended by the design spec which may not be a problem if you only do it for intermittent moves and are able to lower the power when stopped. You can actually micro-step the motor by ramping the current with chopper drivers and you also reduce the cogging effect. The additional torque from higher voltage with limited current is only during the period when you incur the 1/LR time constant of the motor and so only becomes significant when the windings are only on for very short times during high speed operation. You aren't really getting more than rated torque you are just getting it for closer to 100% of the step period instead of waiting for it to ramp up with the LR time constant.

That is one possibility I have thought of, and someone who I work with also suggested to me. It makes sense that this could be the situation.

I guess if I really want to find out I would need to measure the torque output and compare against the motor spec. I don't know the motor spec for this particular motor so I would have to use another one.

you can increase the torque with a gearbox. It will reduce the step length and increase the torque with the same rate.

It will also reduce the speed which is what I need. If I reduce the speed of the motor the torque goes way up. Right now I am driving the motor about as fast as it can go before it starts to skip to do what I need, I don't need accuracy. I just need a motor that will turn at a very constant yet adjustable rate that won't vary when the load does.

if you hope to increase torque, as other recommand. increase input power, the larger current, the stronger torque .(of cause the higher voltage, we can ge t more current). power = torque x turns/m

I know that increasing the voltage gives you more torque as I stated in my original question. I am already driving the motor with about twice the voltage it is rated at, but it wasn't enough.

If you use a proper chopper driver (duty cycle modulation) you can run the motor at a much higher voltage. I have operated very large stepper motors at 2000 sps (600rpm) by using ten times the rated motor voltage. You should select a motor with a lower rated voltage but higher current. That in effect makes the winding have less inductance which is what you are fighting. Twice the voltage is hardly worth the effort especially if you have a relatively high voltage motor.

frankd20,

Kudos for creative thinking out of the box, but as you might guess there are some complications that may or may not concern you. By turning on three phases at once the larger maximum B field position in the stator is less clearly defined. In other words going from position one to position two the magnetic field is larger but position one and position two may not necessarily be where you think they should be. Your external load (gravity, inertia, and friction) may pre-load the motor so that this becomes a moot point. To test this you may find that when you are holding a load in one position in your three coil activation approach, you'll find a larger region that motor can drift from a supposedly fixed position compared with the full or half step norm. Since most motor manufacturers expect that the at rest motor position is a critical criteria (why else use a stepping motor) they probably don't recommend this approach. Therefore, no web based mentions of this approach. As I mentioned before though this may not be critical in your application.

Also since you have all three coils ON at once, the parasitic IR power heating the motor chassis will be at least 50% higher than anticipated. Again, your operating modes may not care about this. But since it can overheat the motor in some applications, no manufacturers will recommend this.