4.3.5.4. Minimum current

The minimum current can be regulated is important when performing the microstep control, or when using the multi-level control. When attempting to regulate the flow, it is very small compared to the peak current produced when the motor is directly connected to the supply voltage.

Figure 4.40. Transfer function depending on the type of control, showing the loss of linearity for low currents.

Figure 4.41. Minimum current through the winding and the minimum duty cycle executable for the chopper by inhibition and phase.

With the chopper by chopper inhibition or a phase the only problem is the loss of regulation of the current below the minimum value. Figure 4.40. shows typical response curves for the output current based on the reference. This minimum is due to the characteristics of the engine, mainly by the winding resistance, supply voltage and the minimum cycle duty enforceable by the control circuit. The minimum current that can be supplied to the winding well determined by the minimum duty cicle. Below this value, the regulation is impossible. With the chopper by inhibition, the minimum current through the coil also responds to the minimum cycle duty, but it returns to zero during each cycle, see figure 4.41. When using the chopper by phase, the current never returns to all zeros, so always have a residual component.

When using the constant frequency control, the duty cycle (duty cycle) is basically the minimum duty cycle oscillator, the dwell time of the activation impulse acting on the (SET) flip-flop. Regulators in stopping time (OFF) constant, the minimum output is determined by the propagation time (delay of the sequence of unemployment).