These circuits can also be used to improve the damping and noise characteristics of a step motor. However, the torque at higher pulse rates (frequency) can be reduced so careful consideration must be exercised when selecting one of these methods.
1. Diode Method Fig. 8-1 (a)
2. Diode + Resistance Method Fig. 8-1 (b)
3. Diode + Zener Diode Method Fig. 8-1 (c)
4. Capacitor Method Fig. 8-1 (d)
A step motor operated at a fixed voltage has a decreasing torque curve as the frequency or step rate increases. This is due to the rise time of the motor winding which limits the value of the coil current. This is determined by the ratio of inductance to resistance (L/R) of the motor and driver as illustrated in Fig 8-2 (a).
Compensation for the L/R of a circuit can be accomplished as follows:
a) Increase the supply voltage and add a series resistor, Fig 8-2 (b), to maintain rated motor current and reduce the L/R of the circuit.
b) Increase the supply voltage, Fig 8-2 (c), improving the time constant (L/R) of the circuit. However, it is necessary to limit the motor current with a bi-level or chopped supply voltage.
1. Constant Voltage Drive Fig. 8-1 (e)
2. Dual Voltage (Bi-level) Drive Fig. 8-1 (f)
3. Chopper Driver Fig. 8-1 (g)
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