Inductive resistance influence

on functioning of thrust module simulator

V.A.Lesnevskiy, V.V.Sokolov

EDB "Fakel"

EDB Fakel develops and manufactures electric propulsion systems (EPS). EPS is a complicated functional system which consists of a stationary plasma thruster (SPT), xenon flow controller (XFC), propellant storage and supply system (SSS), and power processing unit (PPU).

PPU generates the required modes of electric power supply for all the elements that constitute EPS, either of which differs in a mode and consumed power, as well as in the nature of processes that run in the paths of power transmission. In this connection, for each type of EPS consumers it is necessary to create generators of the required mode of electric power supply and separate paths of power transmission in PPU. Also, PPU provides receipt and operation of coming control commands from the on-board digital computer complex of a spacecraft (S/C), generation and transfer of telemetry information to the telemetry control system [[1]].

To perform autonomous tests and incoming inspection of PPU at the enterprise-manufacturer of EPS, it is necessary to develop an electrodynamic thruster simulator (EDTS) that simulates functioning of SPTs and EPS elements as a whole at different operation modes. To provide high integrity of the data obtained during the activities with EDTS, rather severe requirements for static and dynamic accuracy of EPS simulation are placed on the articles of this type. Dynamic characteristics are specified as frequency responses of internal impedance of EDTS module. EDB Fakel has developed a simulator that consists of the following simulative units [[2]]:

- discharge gap simulator;

- magnetic system simulator;

- simulator of cathode-compensator heaters (SoCCH);

- XFC simulator;

- ignition discharge simulator.

During the incoming inspection of the PPU manufactured by OJSC AVEKS for лazSat, Express-MD S/Cs using EDTS the measured value of the cathode heater current turned out to be above normal. Since the PPU supplied a.c. to the cathode heater, additional inductive resistance emerged in the SoCCH of EDTS. This work presents the influence of inductive resistance of the line and transformers on the impedance of SoCCH, the assessment of influence of the higher harmonic components on a value of the actual current that passes though the heaters.

The feature of SPT as an object of power supply is a high consumed power (up to 50% of the power generated aboard the S/C) and multi-channeling of electric power consumers. The main elements of SPT power supply are: anode, heater of the cathode, igniter, magnetic coils and thermothrottle.

SPT consists of two plasma cathode-compensators which contain the heaters designed for the startup heating. The simulator of cathode-compensator heaters simulates their resistance that several times increases while heating. The possibility of manual regulation of heaters resistance is provided for.

It follows from an analysis that to decrease output impedance it is necessary to decrease internal impedance of the power amplifier, increase amplification factors and to extend frequency band of the operational amplifier, power amplifier and voltage amplifier.

Measurements of leakage inductance of the transformers were made due to the decrease of reactance of the line during the incoming inspection of the PPU, which transformers are intended to correlate input circuits of the cathode heater simulators with the load unit in EDTS.

Additionally, inductive resistances of double-wire lines of 0.5 m in length were measured, which lines were simulating the connecting lines between the input connector in EDTS and the transformer primary winding.

These tests were carried out to determine the influence of inductive resistance of the lines and transformers on impedance of the cathode heater simulators in the electrodynamic simulator of thrust module.

As a connecting line the following options were considered:

- non-twisted parallel wires;

- twisted wires;

- insulated wire pulled into the braid shielding рнм, where the shielding functioned as a current-carrying return wire.

- parallel connection of two lines of the wire in the braid shielding рнм.

Due to deep influence of relative position of the wires in a pair of two non-twisted wires, the inductance of the lines was taken as a mean value of the seven measured - 0.53 μH. For the lines, the нзыч wire with a cross section of 1 mm² was used.

To determine total/accumulative leakage inductance of the transformer windings we relied on the fact that in the event of a short-circuit of the transformer secondary winding, its primary winding represents inductance that is equal to the sum of leakage inductance of the primary winding and leakage inductance of the secondary winding reduced to the primary one.

To measure inductances, a resonance method was used, where a condenser is connected to the in-parallel sought inductance, and then sine voltage is supplied to the resulting parallel oscillating contour. The equality of environmental expose frequencies and contour eigen frequency leads to the increase of amplitude of oscillations in it, what is recorded by an electron oscillograph.

Noticeable differences in a value of leakage inductance of transformer #3 are caused by a change in the technique of winding insulation manufacture, and of transformer #5 - by particular thoroughness of performance of all the operations related to the manufacture.

To calculate inductive resistance of the heater circuits simulators of EDTS, the sum of the transformer leakage inductances and different option line inductances is taken.

That the increase of the cathode heater simulator impedance in EDTS due to inductive resistance of the transformers and supply line is:

- approximately 0.28% in the best case when the line of the wire pulled into the braid shielding рнм is used,

− ~ 1% in the worst case when the line of twist-free wires is used.

Given that the higher harmonic components of oscillations are several times lower than the value of the first harmonics with their amplitudes decaying while the harmonic number grows, the influence of the higher harmonic components of oscillations is much lower than that of the first harmonics.

Therefore, the reactive component of the cathode heater simulator impedance in EDTS does not significantly influence the value of a current that passes through it. To exclude the reactive component of the simulator resistance, the supply lines in EDTS should be made of a twisted wire which represents a wire pulled into the braid shielding рнм, where the braid should be used as a current-carrying return wire.