Familiarization with the Reversible DC Power Supply

DISCUSSION

The reversible dc power supply

A reversible dc power supply is a device whose voltage and current are reversible. This means that the polarity of the voltage across the dc power supply can be either positive or negative, and that the current can flow through the dc power supply in either direction. F8-1 shows the symbol to represent a reversible dc power supply whose voltage (E) is adjustable. The arrow beside letter E in the symbol points towards the positive terminal of the reversible dc power supply when voltage E is positive.

F8-1: Symbol representing a reversible dc power supply whose voltage is adjustable.

F8-2 shows four simple circuits in which a reversible dc power supply is connected to a fixed-voltage dc power supply.

In F8-2(a), the polarity of voltages E and E₁ is positive, and the voltage E is greater than the voltage E₁. Therefore, the current in the circuit (I) flows in the direction shown in F8-2(a). The polarity of this current is positive because it flows in the direction indicated by the arrow over the letter I in F8-2(a). therefore, the polarity  of the voltage E and current I is positive and power flows from the reversible dc power supply t the fixed-voltage dc power supply.

In F8-2(b), the polarity of the voltages E and E₁ is still positive, but this time the voltage E is lower than the voltage E₁. Therefore, the current I flows in the direction shown in F8-2(b). The polarity of this current is negative because it flows in the direction opposite to the direction indicated by the arrow over the letter I in F8-2(b). Therefore, the polarity of the voltage E is positive, the polarity of the current I is negative, and power flows from the fixed-voltage dc power supply to the reversible dc power supply.

F8-2: Simple circuits using a reversible dc power supply.

In F8-2(c), the voltage E has been adjusted so that its polarity is negative and the connections of the fixed-voltage power supply have been reversed so that the polarity of the voltage E₁ is also negative. Furthermore, the voltage E is lower than the voltage E₁. For example, the voltage E and E₁ could be equal to -30 and -20 V, respectively. Therefore, the current I flows in the direction shown in F8-2(c). The polarity of this current is negative because it flows in the direction opposite to the direction indicated by the arrow over the letter I in F8-2(c). Therefore, the polarity of the voltage E and current I is negative and power flows from the reversible dc power supply to the fixed-voltage dc power supply.

In F8-2(d), the polarity of the voltage E and E₁ is still negative, but this time the voltage E is greater than the voltage E₁. For example, the voltage E and E₁ could be equal to 10 and 20 V, respectively. Therefore, the current I flows in the direction shown in F8-2(d). The polarity of this current is positive because it flows in the direction indicated by the arrow over the letter I in F8-2(d). Therefore, the polarity of the voltage E is negative, the polarity of the current I is positive, and power flows from the fixed-voltage dc power supply to the reversible dc power supply.

F8-3 is a four-quadrant diagram of the current I versus the voltage E which summarizes the operation of the dc power supply. Dot A in this diagram represents the voltage E and current I related to the reversible dc power supply in F8-2(a), dot B represents the voltage E and the current I related to the reversible dc power supply in F8-2(b) and so on. This figure shows that the reversible dc power supply can operate in either one of the four quadrants. For each quadrant, the figure indicates whether the reversible dc power supply sources or sink power.

F8-3: Four-quadrant diagram summarizing the operation of reversible dc power supply.

In brief, current can flow in either direction in the reversible dc power supply regardless the polarity of the voltage across the reversible dc power supply. Furthermore, the reversible dc power supply can either source or sink power.

Implementing a reversible dc power supply

A reversible dc power supply can be implemented using a separately-exited dc motor/generator mechanically coupled to a synchronous or asynchronous motor/generator. F8-4 shows a reversible dc power supply in which the separately-excited dc motor/generator is coupled to a three-phase squirrel-cage induction motor (asynchronous motor).

 F8-4: A reversible dc power supply implemented with a separately-excited dc motor/generator and an asynchronous motor.

The output of the reversible dc power supply is taken across the armature circuit of the separately-excited dc motor generator. The polarity and value of the armature voltage, and therefore, the polarity and value of the voltage E provided by the reversible dc power supply depends on the polarity and value of the current flowing in the exciting coil of the dc motor/generator and the rotation speed of the dc motor/generator. The rotation speed of the dc motor/generator is equal to the rotation speed of the asynchronous motor, which varies little. Therefore, the polarity and value of the voltage E provided by the reversible dc power supply especially depends on the polarity and value of the current flowing in the exciting coil of the dc motor/generator.

The current flowing in the exciting coil of the dc motor /generator depends on the voltage provided by a fixed-voltage dc power supply and tandem rheostats. The tandem rheostats are rheostats which share a single shaft. The tandem rheostats allows the voltage applied to the exciting coil is –E_x. On the other hand, the voltage applied to the exciting coil is E_x when the cursors are set to the lower end of the rheostats.

The direction of the current flowing in the armature circuit, and therefore, the direction of the current I flowing in reversible dc power supply depends on whether the dc motor/generator source or sinks power. When the dc motor/generator sources power, the squirrel-cage induction motor drives the dc motor/generator which operates as a generator. Conversely, when the dc motor/generator sinks power, it operates as a motor and drives the squirrel-cage induction motor which operates as a generator and provides power to the ac power supply.