Phase of Voltage Obtained From RL Phase-Shifter

Author: E.E. Kimberly

It is necessary when using a simple phase-shifter of limited range, such as that shown in Fig. 28-14 (a), to connect the phase-shifting elements R and L in the proper relation to the applied voltage in order that the resulting control voltage will be variable in the proper phase range of plate voltage. This proper relationship will be demonstrated by means of Fig. 28-15.

Let it be assumed that ec (negative from cathode to grid) is large enough to block conduction in the tube when it lags 180 degrees behind the voltage eb, as in (b). That condition is shown vectorially in (d) and by the vector Ec in position qp in (c). To permit the tube to conduct late in its positive half-cycle, it is necessary for ec to lag less than 180 degrees behind eb in.(b), in order that ee will lose control of eb before eb falls to zero as in (e). The phase change of Ec in (c) that can be accomplished by changing R is a phase lag a with ideal R and L.

Fig. 28-15. Triode Controlled by RL Phase-Shifter

In any inductance coil there is resistance, and Ec must always lead the position qp in (c). With an RL phase-shifter, therefore, conduction cannot be started in the first few degrees of the half-cycle because of the resistance remaining in the inductance coil. The RC phase-shifter allows control to start practically at the beginning of the conducting half-cycle because a condenser of high quality has negligible series resistance. If the connections of the phase-shifter to the transformer T₂ were reversed, the locus of Ec would be as in (/). In that case the tube grid would always be positive at the beginning of the conducting half-cycle and could effect no control of the plate current. The same lack of control would result if the connections to T₂ were not reversed but R and L were merely interchanged in the circuit.

In Fig. 28-15 (a) a convention known as the dot convention is used to indicate relative instantaneous polarities in different parts of the circuit.

Fig. 28-16. Phase Shift Using Variable-Position Rotor in a Three-Phase Stator

A dot at the terminal of a transformer such as T₁ means that when this terminal is instantaneously positive any other dotted transformer terminal in the circuit is also positive with respect to the other end of its coil. In Fig. 28-15 (a) when the plate is made positive by the dot terminal of transformer T₁, point m of the RL circuit connected to transformer T₂ is also positive. All points in R and L between m and p are negative to m and therefore negative to the plate of .the tube. The degree to which point n and the grid are negative to m depends on the value of the variable resistance R which controls the phase relation of the voltage Ee between q and n with respect to the voltages pm and st. The grid voltage Ec would be constant in magnitude, regardless of its position in its range of shift, if the inductance L contained no resistance. That resistance, however, causes Ec to decrease somewhat as R in the phase-shifter is increased.

Fig. 28-16 shows the use of rotatable elements, known as "selsyns" or "auto-syns," for obtaining the full 360 degrees of phase shift either single-phase or polyphase, as used in multi-electrode tank-type power rectifiers.