Q-57. What is the disadvantage of using a double receiver instead of two individual receivers?
Q-58. What is the purpose of "stickoff voltage"?
If synchros are to work properly in a system, they must be connected and aligned correctly with
respect to each other and to the other devices with which they are used. The reference point for alignment
of all synchro units is ELECTRICAL ZERO. The mechanical reference point for the units connected to
the synchros depends upon the particular application of the synchro system. Whatever the application, the
electrical and mechanical reference points must be aligned with each other. The mechanical position is
usually set first, and then the synchro device is aligned to electrical zero.
There are various methods for zeroing synchros. Some of the more common zeroing methods are the
voltmeter, the electrical-lock, and the synchro-tester methods. The method used depends upon the
facilities and tools available and how the synchros are connected in the system. Also, the method for
zeroing a unit whose rotor or stator is not free to turn may differ from the procedure for zeroing a similar
unit whose rotor or stator is free to turn.
The most accurate method of zeroing a synchro is the ac voltmeter method. The procedure and the
test circuit configuration for this method vary somewhat, depending upon which type of synchro is to be
zeroed. Transmitters and receivers, differentials, and control transformers each require different test
Regardless of the synchro to be zeroed, there are two major steps in each procedure. The first step is
the coarse or approximate setting. The second step is the fine setting. The coarse setting ensures the
device is zeroed on the 0º position rather than the 180º position. Many synchro units are marked in such a
manner that the coarse setting may be approximated physically by aligning two marks on the synchro. On
standard synchros, this setting is indicated by an arrow stamped on the frame and a line marked on the
shaft, as shown in figure 1-37. The fine setting is where the synchro is precisely set on 0º.