L/R is defined as the time constant

L/R is defined as the time constant of the circuit, which is the time required for the current in the circuit to reach 63% of the maximum current in Imax. When t=t:, the circuit is disconnected from dc voltage source V and shorted. The current in the circuit starts to decline at the initial rate of V/L. The current variation rule is:I (t) = Ve - R/L/R (t - t1)Rectangular wave voltages of different frequencies are applied to the circuit, and the current waveform is shown in figure 2.9. The current can reach the maximum value at low frequency (a); When the frequency of rectangular wave rises to a certain critical frequency, the current begins to decrease as soon as it reaches the maximum value ((b) : after the frequency of rectangular wave exceeds this critical value, the current in the winding cannot reach the maximum value (c). Since the size of the step motor torque is proportional to the winding current, so the motor is running at low speed, the motor can achieve its rated torque, and above a certain frequency runtime, winding current gradually with the increase of the frequency decrease, motor torque also decreases accordingly, which reduces the high-speed operation with load capacity.