Page 45 - Kollmorgen PMX Series Stepper Motor Selection Guide
P. 45

Effects of Available Current
                                                                                     SPEED (RPM)
            Figure 1 shows the performance of the same motor driven by bipolar   1.0  0  600  1200  1800  2400  3000
            stepper drive with different current ratings. In this comparison all drives
            have the same supply voltage. Note that high speed performance is not   1.0  1.5 x Rated I
            appreciably affected by the different current ratings. Low speed running
            torque, however, varies considerably with changes in the current rating.   1.0
            It is important to understand that when current over the rated current of   0.8  1.0 x Rated I (Reference curve)  STEPPER MO
            the motor is applied, the increase in torque will not be proportional to   NORMALIZED TORQUE (UNITLESS)  .5 x Rated I
            the increased current. Furthermore, applied current levels above rated   0.6
            current will likely result in damage to the motor from demagnetization      .25 x Rated I
            and/or overheating.                                     0.4                                                 T
            Effects of Available Voltage                            0.0  0   2000   4000   6000    8000  10000          OR GENERAL
                                                                                SPEED (FULL STEP/SEC)
            Figure 2 shows the performance of the same motor driven by bipolar
            stepper drive with different supply voltage ratings. In this comparison,   Figure 1
            all drives have the same current rating. Note that low speed running
            torque is high and not appreciably affected by supply voltage
            differences. High speed performance, however, varies considerably with   SPEED (RPM)                        TECHNIC
            changes in supply voltage. Caution must be exercised when increasing   0  600  1200  1800  2400  3000
            supply voltage. Higher voltages will result in increased motor heating   1.0
            regardless of motor speed.                              0.8                                                 AL GUIDE

            Effects of Motor Inductance                             0.6        4 x V 3 x V

            For a given supply voltage, a low inductance motor will give better   NORMALIZED TORQUE (UNITLESS)  2 x V
            performance at high speeds than a high inductance motor, but will   0.4
            operate at a higher temperature. This is true because current will           1 x V (Reference curve)
            increase faster in a low inductance winding, each time the winding   0.2
            power is switched. High inductance motors yield higher maximum
            torque and operate cooler, but their top speed is limited and torque falls   0.0
            off more rapidly as speed rises, versus a lower inductance motor.  0  2000  4000  6000  8000  10000
                                                                                SPEED (FULL STEP/SEC)
                                                                                     Figure 2
            Full-Step, Half-Step, and Microstepping

            The terms full-step, half-step and “microstep” are commonly used in the discussion of step motors. A 1.8° step motor, for example, has 200 discrete
            positions in a full 360° revolution. Since 360° divided by 200 equals 1.8°, the motor shaft will advance 1.8° each time the motor is given a digital
            command to take one step. This is known as a full-step. The term “half-step” indicates a 0.9° step angle (half of a full 1.8° step). This is achieved with
            a switching technique that alternately applies positive current, no current, and negative current to each winding in succession. The term “microstep”
            refers to a more sophisticated form of control which goes beyond the simple switching of power between phase A and phase B of the motor windings,
            and takes control of the amount of current being sent to the individual windings. Microstepping permits the shaft to be positioned at places other
            than the 1.8° or 0.9° locations provided by the full-step and half-step methods. Microstepping positions occur between these two angular points in
            the rotation of the rotor. The most commonly used microstep increments are 1/5, 1/10, 1/16, 1/32, 1/125 and 1/250 of a full step. A major benefit of
            microstepping is that it reduces the amplitude of the resonance that occurs when the motor is operated at its natural frequency or at sub-harmonics of
            that frequency. The improved step response and reduced amplitude of the natural resonances result from the finer step angle.

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