What is a Redundant Power Supply and How Does It Work?
The electrical components inside enclosures that house industrial automation systems like
PLCs often operate on a 24 volt power supply. So in these enclosures we usually need one
or several power supplies that each have a 24 volt output voltage.
But in some applications where ultra-reliability is essential, and an unexpected loss of power
would be disastrous we need to use “redundant power supplies”.
For instance, each one of these enclosures that you see here is going to be used to control
and monitor the parameters of gas turbines in one the biggest oil refineries in the world,
so in order to provide the necessary power for electrical components, we have used a
set of redundant power supplies. In this video we’re going to learn exactly how these types
of power supplies work from a practical perspective. Ok, so we have a set of redundant power supplies
in this enclosure. As you can see each one of them has a set of terminals here.
The lower terminals are used to connect the input power. This is usually a 110 or 220
volt AC supply. So we can turn on the power supplies by connecting AC power to the input.
First we connect AC power to these terminals; the power will travel through wires that are
routed through these wire ducts; the wires terminate in a connection to the input terminals
of the power supplies. However, this input power is not directly connected to the power
supplies and as you can see we have used a series of protective devices such as surge
protection, a harmonic filter and fuses before the power supplies. This yellow wire here
is the ground wire. So to turn on the power supply we connect
a 110 or 220 volt AC power here. These upper terminals are the output terminals.
When we connect AC power to the input terminals, it gives us a 24 volt DC voltage here.
We can access the output voltage through these two red and blue wires that are connected
to the output terminals. So these two wires are the input AC power and these two are the
24 volt DC output. In addition to the output voltage, another
important parameter of power supplies is the current rate. The current rate of each one
of these power supplies is 5 Amps as you can see here. This shows the actual capacity of
these power supplies, it means that the maximum current that each one of them can supply at
the output is 5 Amps. The more current the electrical devices within the enclosure draw,
the higher the current rate of the power supply needs to be.
Ok now let’s see how they work. Here each power supply is capable of powering the entire
enclosure but by setting the output voltage of the primary supply 0.2 volt higher than
the backup supply, only the primary supply delivers current to the load.
With this setting, if the primary supply's output voltage decreases by more than 0.2
volts for any reason, the priorities are reversed, and only the backup supply delivers the load
current. So let’s test this on a practical level.
While both of the power supplies are operating, we are going remove this terminal in order
to cut the input power of the primary supply. With the terminal removed the primary supply
is not working anymore and the backup supply is operating and supplying the load current.
Now, how does this happen? The outputs of these power supplies are interconnected
through a diode switching arrangement. This diode can detect a fault, isolate the primary
supply from the system output, and then pass only the output of the other supply with no
interruption of output power during the transition. So the diode is a key element for operating
these two power supplies as a redundant system. If we had two normal power supplies without
an internal diode, we could use an external redundant module that is designed especially
for this kind of application, in order to make the two supplies operate as a redundant
system. So there are two methods of utilizing redundant power supplies: using power supplies
with internal diodes and using normal power supplies with an external redundant module.
Ok, within each one of these power supplies we see here, there is also an “open contact”
which closes whenever the power supply fails. We've connected the two ends of this contact
to the PLC input, so that when the contact closes, a signal will be sent to the PLC to
let us know about the problem. Now we replace the input terminal, and as
you can see, normal operating conditions are restored.
Ok so in this video we learned when to use redundant power supplies and we also conducted
a practical test of the operation. Okay if you haven’t already please subscribe
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