can any one help me?

i have stepper motor and it works correctly but when i put the load on it to turn the gears it vibrate and not turn correctly.

 

Hi...

I also faced this problem till I unearthed the mystery about these motors...

Steppers are basically delicate motors that are good for precision movement but are unable to move heavy loads... their torque is less as compared to a Servo Motor of the same size...

You may try to reduce the stepper speed (by changing and slowing down the stepper swicthing logic) so that at lower speed it has a higher torque. If this works its fine otherwise go for a torque amplification of the motor by adding a gear wheel. More details are available for these motors at the robotics site.

 

A step motor does not turn smoothly, but it coggs. IOW, it turns in discrete steps, not a smooth continuous turn. What you are describing sounds like correct motor operation, but you probably don't notice the cogging until the motor is under load.

 

If I'm not mistaken, your motor was running in overload condition. try to increase one size of your motor.

 

If your stepper motor works correctly at no load and has problems with a load, then you may be driving it too fast, with not enough current, or without a slow enough ramp-up when you accelerate.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

That could be due to a wrong stepper motor dimensioning or, most likely to the kind of driver you're using.

Sophisticated stepper motor drivers usually send a BIG voltage/current initial peak (several times above plate specs) during some milliseconds to overcome the load resistance and motor winding inductance and force the shaft to advance one step.

The current is then reduced to a far lower value to mantain the required holding torque.

What really matters is that the average current does not exceed the nominal plate specs.

HTH. Eugenio.

 

The resonance over the entire frequency range is one of great disadvantages of stepper motors. This phenomena comes from basic construction and dependent upon load conditions.

The solutions maybe by both mechanical (higher damping or moment of inertia) or control (half step or micristepping) ways.

best wishes,
Leonid

 

Robert,

you never heard about midrange resonance of steppers?

Leonid

 

Leonid,

what's making you think that his problem is related to midrange resonance when you dont know what is the step rate he is atempting to run the motor at? He had to have very bad luck to have choosen a speed that will fall within that notch in the speed-torque curve that we know as midrange resonance.

Actually Mr Scott's advise is logical since there's some common mistakes almost everyone's doing when starting to work with a stepper and one of them is not providing an acceleration ramp (the motor will work fine with no load); expecting from the motor more than what the motor could give in terms of torque and speed being the others (many motors data sheets lack of enough info). Anyways, finding out if his problem has to do with midrange resonance is easy..all he had to do is accelerate the motor (loaded) from zero (max torque) to the intended step rate and look for loss of synchronism at some point but then if you keep accelerating the motor it should get back in synch (a helping hand would do ) ..if it doesnt it's just that you either have reached the max step rate or that that specific motor's torque is not high enough for the intended working conditions AT that step rate...Notice that by implementing an acceleration ramp he could have his problem solved without further testing

I' ve also seen not only on this list but on some other lists as well some conceptual mistakes regarding driving steppers..one of them being the use of a voltage higher than the rated voltage...some esoteric explanations have been given when in fact the answer and the reasons to do that can be explained by analyzing a simple LR circuit.

When a square wave is applied to the winding the current waveform will be smoothed and at high stepping rates the current could just not reach the nominal motor current. Under these circumstances, as the torque of a motor is approx. proportional to current there would be a loss of torque additional to the torque loss due the high speed. As the current rise time depends on V, L and R, and with L and R depending on the motor characteristics, its easier to play with V to lower the current rise time. Professional drives will recommend working with 4 to 5 times the nominal voltage. Notice that increasing the current wont take you anywhere cos you could at best overheat the motor and in the worst case saturate the magnetics to further degrade the motor torque. For sake of brevity..a higher voltage is used to achieve the highest possible torque at high stepping rates.and nothing else..and that techinque is always used with current limiting drives...either chopping drives or the brute force limiting method of adding a resistance in serie with the windings. Also if you need the highest possible perfomance from a given motor use bipolar driving (unipolar motors can usually be wired to be used with bipolar drives).

Regards, HEV

 

Hi. what is the highest holding torque a stepper motor can achieve? thanks!

 

> what is the highest holding torque a stepper motor can achieve? <

look for nema stepper motors....