Simple rules for a steppermotor

(c) copyright 2004-2007

These rules are rather simple, but they are correct to a certain extent.

Number of steps of a ( freerunning ) steppermotor

Most steppermotors are electromotors of the hybride type. These are the strongest steppermotors with a very good price/performance ratio. The number of steps a steppermotor can execute with a fullstepdriver is usual 200. At every fullstep the shaft of the steppermotor rotates 1.8 degree and holds there. With a microstep driver the steps of a steppermotor are divided in so called microsteps. If one use's a microstepdriver with 40 microsteps, the shaft of the same steppermotor has 200 * 40 = 8000 posible position's. With every micro step the shaft of the steppermotor rotates 1.8 / 40 = 0.045 degree's. So a steppermotor doesn't need a feedback ( open system ) unlike a servomotor.

Torque of a steppermotor

The torque of the steppermotor is usually given in NewtonMeter ( Nm ). The torque is unfortunally by some given in other units, making the steppermotor looking stronger. For example a steppermotor with a torque of 1 Nm has a torque of 10 Kgcm and also of 100 Ncm. The latter apears to be stronger, but in fact there is no differens whatsoever. The torque given for a steppermotor is almost always the holdingtorque. This is the maximum torque the steppermotor can hold at a certain position. Keep in mind thoug, there is always a certain ( small ) part rotation of the shaft. Usual this is of no importance, but it is there. With a motor of 200 fullsteps this dislocation of the shaft is 0.9 degree maximum. If the force gets bigger, the steppermotor loses a step, and if will try to take the next fixed location. The needed torque can sometimes be calculated. Needed is the force wich will rotate a shaft and the radius ( of diameter ) of this shaft. Lets say we have a diameter of 1 cm ( radius is then 0.005 m ) and we need a force of 1 kg ( is 10 Newton ) to make it rotate. This means the required torque is then MINIMAL 0,005 * 10 = 0,05 Nm. Of cource there has to be some margin, so 0,1 Nm is a resonable MINIMUM torque. As the torque of most steppermotors decays with higher rotational speed of the shaft there has to be more margin. Also the needed force to let a shaft rotate is usual increased with higher speeds.

Shape of the steppermotor

Provided there is enough space, use a long thin steppermotor, instead of a short big one ( with the same torque ). A long thin steppermotor has a lower rotorinertia, and so accelerates faster. A steppermotor with a higher current is better than one with a low current. The higher the current, the lower the selfinduction of the steppermotor, so the less E.M.K. the steppermotor produces at higher rotation. So the steppermotor has a better torque at high speeds. All our stepermotors can be specified with a high current on request. The higher the voltage of the steppermotor-driver, the better the torque at higher speeds will be. If a steppermotor at high speeds misses steps, there is little point in increasing the current. This only increases the torque at low speeds, not at high speeds. Increase ( if posible ) the voltage of the steppermotor-driver. A steppermotordriver with microstep is better than one with full or half step.With microstepping, the steppermotor makes smaller steps, and so doesn't fibrate as much as a full step driven steppermotor. The complete system doesn't get that much fibration from the steppermotor. The only advantage of a full steppermotor-driver is the torque. At full step the torque is higher, as the steppermotor gets driven by all the coils. A steppermotor with 8 wires can be used with both unipolair and bipolair systems. Also there is the posibility to connect the coils in series ( low current ) and parallel ( high current, but also better torque at high speeds ). Use the coils parallel. The current is than higher, but also the torque at high speeds. When you use a unipolaire steppermotor-driver, a part of the coils isn't used, so there is always a loss in torque, both at low and at high speeds. This can be compensated for by increasing the current somewhat, but it can never completely be compensated for.


Maximum speed of a steppermotor ( RPM )

The ( maximum ) speed of a steppermotor depends on serveral things. First there is the start-stop speed of the steppermotor. This is the maximum number of steps a non-rotatingl standing steppermotor can follow from one moment to the other. This is influenced by the rotor inertia and of cource by the system. Also the voltage the steppermotor-driver can supply to the steppermotor is of influence. Wenn the steppermotor turns it can accelerate more, up to the point where the steppermotor generates a voltage of its own that counteract with the voltage supplyed by the steppermotor-driver. At that point there is no more acceleration posible, and the torque is zero. So the maximum speed is dependent of the voltage suplied to the steppermotor-driver, as the steppingmotor needs to turn faster to supply a countervoltage as large as the powersupply.

Start-stop frequency ( pull-in ) of a steppermotor

The start-stop frequentie is that frequentie which the steppermotor can follow without the need for a acceleration or deceleration. This is also called the pull-in frequentie.

Pull-out frequency of a steppermotor

The pull-out frequentie is that frequentie that the steppermotor can follow, but only with a acceleration or deceleration.

Maximum speed of a steppermotor

The maximum speed is for a large part dependent on the voltage the steppermotordriver can supply. As the steppermotor turns, it produces the counter-voltage and this is responsible for the drop in torque as the steppermotor starts to turn.

This is a rather simple way of looking at steppermotors, but for a beginner this is a good way to start.

(c) copyright 2004-2007

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