NEVER connect a 400-Hz synchro to 60-Hz voltage. The reduced impedance
results in excessive current flow and the windings quickly burn out.
Q-16. What type of equipment normally uses 26-volt 400-hertz synchros?
OPERATING TEMPERATURES AND SPEEDS
Standard synchros are designed to withstand surrounding temperatures ranging from
67º F to +257º
F (-55º C to +125º C) at the terminal board. Prestandard synchros operate in a range of
13º F to +185º F
(-25º C to +85º C). When a synchro is energized and not loaded, its temperature should stay within
prescribed limits. Loading an energized synchro causes it to generate more heat. Similarly, overloading
causes a synchro to generate much more heat than it would under normal loading conditions and could
possibly result in permanent synchro damage. To meet military specifications, all standard synchros must
be capable of continuous operation for 1,000 hours at 1,200 revolutions per minute (rpm) without a load.
A prestandard synchro has one of two specifications, depending upon its use in a data transmission
system. Low-speed prestandard synchros must be capable of continuous operation for 500 hours at 300
rpm without a load. Low-speed prestandard synchros must be capable of continuous operation for 1,500
hours at 1200 rpm without a load.
Q-17. When will a synchro generate more heat than it is designed to handle?
THEORY OF OPERATION
Synchros, as stated earlier, are simply variable transformers. They differ from conventional
transformers by having one primary winding (the rotor), which may be rotated through 360º and three
stationary secondary windings (the stator) spaced 120º apart. It follows that the magnetic field within the
synchro may also be rotated through 360º. If an iron bar or an electromagnet were placed in this field and
allowed to turn freely, it would always tend to line up in the direction of the magnetic field. This is the
basic principle underlying all synchro operations.
We will begin the discussion of synchro operation with a few basic points on electromagnets. Look
at figure 1-8. In this figure, a simple electromagnet is shown with a bar magnet pivoted in the
electromagnet's field. In view A, the bar is forced to assume the position shown, since the basic law of
magnetism states that like poles of magnets repel and unlike poles attract. Also notice that when the bar is
aligned with the field, the magnetic lines of force are shortest. If the bar magnet is turned from this
position and held as shown in view B, the flux is distorted and the magnetic lines of force are lengthened.
In this condition, a force (torque) is exerted on the bar magnet. When the bar magnet is released, it snaps
back to its original position. When the polarity of the electromagnet is reversed, as shown in view C, the
field reverses and the bar magnet is rotated 180º from its original position.