System current-to-DC converter - Electric drive

System current-to-DC converter

Functional diagram of the "converter-motor (P-D) system with current cutoff" is shown in Fig. 6.2.

The input voltage with any physical nature (electromachine, magnetic, semiconductor converter, etc.) is supplied with the reference voltage . The inverter controls the motor along the armature winding circuit by changing the voltage. The current sensor UA - in the simplest case, the resistor - produces a voltage proportional to the current of the motor armature M. The diode VD and the source of the reference voltage , which is removed from the potentiometer RP. The feedback signal is formed on the resistor R.

Functional diagram of the system

Fig. 6.2. Functional diagram of the system converter-motor with current cutoff

The family of electromechanical characteristics of the open system PD is shown in Fig. 6.3. Each of these characteristics corresponds to a specific value of the reference voltage , which in the open control system is

Electromechanical characteristics of the system

Fig. 6.3. Electromechanical characteristics of the system converter-motor with current cutoff

Let the motor run on an electromechanical characteristic corresponding to the control voltage . When the engine is idling, the current sensor voltage and the current connection does not work. As the load increases, the current sensor voltage rises and when the armature current becomes becomes equal to the reference voltage . A further increase in the load, for example, up to the current , causes the voltage values ​​ to exceed the reference voltage values ​​(). In the feedback loop, a current begins to flow through the diode VD . The voltage of the converter control when the feedback action is equal to the difference between the reference voltage and the feedback voltage , decreases. The motor goes over to the electromechanical characteristic corresponding to the new value of the voltage .

Further increase in load, due to the action of feedback, leads to a decrease in the control signal , which subsequently takes the values ​​, which corresponds to the transition It represents the points of the excavator characteristic according to the corresponding electromechanical characteristics of the open system. The motor stops when the current reaches the value . With the fixed motor armature, the converter's EMF is balanced by the voltage drop in the motor's anchor circuit.

By changing the voltage with the potentiometer RP , you can change the current , at which the negative feedback, and therefore adjust the motor torque limit.

The need for galvanic isolation of the control circuit and power circuits of the electric drive makes it possible to use the active resistance as a current sensor only in electric drives with electromotor and magnetic amplifiers or as a primary sensor. In thyristor electric drives, the signal proportional to the armature current is obtained, as a rule, with the help of a current transformer TA , which is connected to the AC power circuits of the thyristor converter (Figure 6.4).

The current sensor made on the current transformer

Fig. 6.4. Current sensor made on a current transformer

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