What is the control mode of the DC servo motor?

To understand the control mode of the DC servo motor, you can first look at a set of speed formulas of the DC servo motor: n= (U_a-I_aR)/C_eФ. Where: U_a is the armature voltage, I_a is the armature current, R is the total resistance of the armature loop, C_e is the EMF constant, and Ф is the magnetic flux of each pole. According to the above speed formula of a DC servo motor, changing the armature voltage U_a and each pole magnetic flux Ф can change the motor speed, so there are two control modes of a DC servo motor: one is armature control, the other is magnetic field control.

DC servo motor control mode - armature control

The armature control is to keep the excitation flux constant and change the speed of the motor by changing the voltage of the armature winding. When the load torque of the motor is unchanged, increasing the armature voltage, the speed of the motor will increase; Conversely, the speed is reduced; When the armature voltage is equal to zero, the motor does not turn; When the armature voltage changes polarity, the motor reverses. Figure 1-5 illustrates how the motor speed changes under the control of the armature. At t₁, the armature voltage of the motor changes from U_a1 to U_a2, the current increases rapidly from I_a1, and the torque increases, so the speed increases from n₁. As the speed increases, the back potential increases, the current decreases, and the torque decreases, so the speed is stable. At time t₂, the speed stabilizes at a higher speed n₂, and the current also returns to I_a1.

Figure 1-5 Change process of motor speed during armature control

Control mode of DC servo motor - magnetic field control

The magnetic field control is intended to maintain the voltage of the armature winding constant and to change the voltage of the excitation loop to control the motor's speed. When the excitation voltage is increased, the excitation current increases, the main flux increases, and the motor speed decreases if the load torque of the motor remains unchanged. In contrast, the speed increases. By changing the polarity of the excitation voltage, the motor steering changes accordingly. Although the magnetic field control can also achieve the purpose of controlling the size and direction of rotation speed, the relationship between the excitation current and the main magnetic flux is nonlinear, and its mechanical characteristics become soft with the decrease of the excitation voltage, and the adjustment characteristics are also nonlinear, so it is rarely used.