Aircraft Reverse Current Cutout

J. M. M A R Z O L F A S S O C I A T E A I E E

THE FOLLOWING CIRCUIT is proposed for an air-craft reverse-current differential cutout. It consists

of two units outlined by the dotted lines in Figure 1. To explain its operation, assume that the main contactor

is open and the generator voltage is zero. In this condition, both the triple-pole double-throw switch associated with the main contactor, and the permanent magnet control element will be in the upper position as shown in Figure 1. When the generator voltage increases to approximately 20 volts in the proper direction, the polarized auxiliary relay in the differential coil circuit closes, thus making the cutout opera-tive. Since the battery voltage, however, is still higher than the generator voltage, current will flow through the differen-tial coil in the reverse direction, holding the permanent magnet in the original position, and the cutout will not close. When the generator voltage becomes slightly higher than the battery voltage, the flux from the differential coil is sufficient to overcome the residual flux in the magnetic core of the control element. This causes the permament mag-net to rotate and make instantaneous closure with the lower contact, causing the thyratron to fire continuously regard-less of subsequent grid voltage. This causes the pilot relay and the main contactor to close, and the pilot relay is locked in by its holding coil. When the main contactor closes, it mechanically throws the auxiliary contacts to the lower position and, in passing from one position to the other, instantaneously opens the plate circuit of the thyratron, extinguishing it, but the pilot relay and the main contactor remain closed.

When reverse current flows from the battery to the gen-erator, the voltage drop across the generator series field is impressed on the reverse coil of the permanent magnet control element, causing the magnet to rotate to the upper position and the thyratron will fire again. The plate cur-rent passing through the operating coil of the pilot relay in the opposite direction overcomes the holding coil and allows the pilot relay and main contactor to open. As the main contactor opens, it mechanically returns the auxiliary contacts to the upper position and, in so doing, interrupts the thyratron current long enough for the tube to be ex-tinguished, and the cycle can be repeated.

The purpose of the residual coil is to restore the residual flux in the magnetic core of the control element and thus insure that it will operate at the same point each time. In addition a high value of residual flux is guaranteed to pre-vent spurious operation under conditions of shock or vibra-tion at no-load conditions.

The ballast lamp serves only to protect the heater of the

Digest of paper 48-223, "Proposed Circuit for Aircraft Reverse-Current Cutout," recommended by the AIEE air transportation committee and approved by the AIEE technical program committee for presentation at the AIEE Middle Eastern District meeting, Washington, D. C , October 5-7, 1948. Scheduled for publication in AIEE TRANSACTIONS, volume 67, 1948.

J. M. Marzolf is with the Naval Research Laboratory, Washington, D. C.

0 DIFFERENTIAL ' ^ -

REVERSE " ^

HOLDING

SAFETY £

" ^ I S

Figure 1. Circuit diagram for proposed reverse-current cutout

thyratron tube and prevent damage under the wide varia-tions of terminal voltage that must be tolerated.

The safety coil on the pilot relay is included to insure that the cutout "fail-safe" and would operate the pilot relay only under large reverse currents if the thyratron tube were to burn out while the cutout was closed.

A properly designed cutout based on this principle would be practically independent of temperature, altitude, and terminal voltage (from 20 to 30 volts). The same cutout could be used on any aircraft 28-volt d-c system regardless of generator rating. The components may be located remotely for convenience, accessibility, and so forth. It

DIFFERENTIAL COIL

Figure 2. Schematic represen-tation of magnetic circuit of con-trol element for proposed re-

verse-current cutout

i j> RESIDUAL

could be constructed as a plug-in unit to be removed and a new one installed quickly in case of failure. The power loss is low since no shunts are required in the power circuit. There are no restraining springs on the permanent magnet, thus it can be made quick-acting and will not be subjected to fields which might reverse its polarity accidentally.

Since no attempt was made in the mechanical design of the experimental model to take care of the factors of vibra-tion, shock, changes in physical dimensions due to tempera-ture, and so forth, no tests were performed under actual operating conditions, but sufficient trials were made to convince the author that the principles involved are sound, and that a properly designed cutout based on these prin-ciples would be entirely satisfactory under the extremely wide variations of conditions encountered in aircraft.

DECEMBER 1948 Marzolf—Aircraft Reverse Current Cutout 1183