NO-BREAK POWER SUPPLIES
PRINCIPLES OF OPERATION
A no-break power supply is designed to provide
The input 440-volt, 60-cycle, 3-phase power is
uninterrupted electrical power by automatic takeover
stepped down by transformers and applied to an SCR
should the normal supply fail or momentarily deteriorate
(diode) 3-phase bridge. The output voltage of the bridge
beyond the system demands. No-break power supplies
can be controlled by varying the phase that the SCRs are
are provided for communication systems, computers,
triggered. The output is then filtered and sent to the load.
navigational equipment, automated propulsion systems,
and related equipment where a momentary loss of power
The voltage across the load is compared with an
would cause a permanent loss of information resulting
internally generated, temperature-compensated,
in the need to recycle or reprogram the equipment. Since
reference voltage. Any difference between the actual
equipment requiring no-break power normally requires
load voltage and the desired load voltage, indicated by
closely regulated power, no-break power supplies are
the reference voltage, is amplified by the voltage control
designed not only to provide uninterrupted power, but
circuit. This amplified voltage is combined with a
also to provide power that is regulated to meet the needs
properly phased ac signal and applied to the trigger
of the equipment it serves.
circuit of the SCRs. The ac signal controls the time of
firing of the SCRs. By controlling the time of firing of
the SCRs, the output voltage is also controlled.
The SCRs are fired earlier if the output voltage is
The no-break "uninterrupted" power supply system
too low. They are fired later if the output is too high.
consists of two major assemblies plus the storage bat-
Special frequency-shaping circuits are used to ensure
teries. The control cabinet and motor-generator set are
shown in figure 1-41.
stability and prevent oscillations or hunting.
Figure 1-41.-No-break "uninterrupted" power supply system components.