Single Phase Preventer Circuit

Everyone knows that for working heavy electrical loads three phase power or AC is needed to be able to produce the working economical and viable. But yet this demands the existence of all the three stages at all situations. If any of the levels fail, may cause devastating effects to the linked systems. The following article provides a good alternative for tackling the above problems.

As mentioned above, a three phase load for example an industrial heavy motor will need the occurrence of all the three input AC mains phases for efficient and right procedures.

If there is certainly any conflict with the existence of the input phases, the motor may well commence working under greatly demanding and irregular problems.

This could result in large current intake, warming of the winding and eventually burning of the motor parts.

The circuit of a single phasing preventor demonstrated below may be efficiently useful for removing all sorts of unwanted outcomes which may derive from an unusual three stage problems.

In the diagram we are able to notice the utilization of three transformer/relay driver stages.

The transformers could be the regular step-down types, graded properly for changing the associated relays.

Certainly one of the input primary terminals of all the transformers are created normal and associated with the basic line.

While the other terminals of each transformer are attached to the specific first, second and the third phases of the input mains.

In spite of this the above connections are carried out skillfully via the relay N/O contacts of the following relay assemblies for applying the needed single phasing prevention.

At first when the set-up is built-in with the the three phases as per the presented connections, the phases are remain cut off from the output load, simply because the relay contacts are all open.

On pressing the provided push button, the specific step in the line is permitted to achieve the second or the middle transformer primary winding.

The middle transformer immediately works its own relay, whose contacts exactly like the above relay attaches the second respective phase with the primary of the bottom transformer, which lastly functions its relay running the top transformer.

As soon as this occurs the whole method gets latched via the N/O contacts of the relays such that now even if the push button is introduced the system proceeds and maintains the voltages across the outputs and to the transformers.

Right now guess if any of the stages turn out to be low or fails, the specific transformer in line instantaneously deactivates its relay and the whole process of relays break down in series, instantly avoiding and disconnecting the output loads.

Hence the process efficiently stops the loads from functioning under the loss of any of the levels rendering it sure nothing goes out in fumes.

The circuit was developed solely by me, I assume so, if it’s previously been found out please supply me with the link:



Another simple single phase preventor circuit can be seen the following diagram:


simple single phase preventor circuit using an opamp


As can be seen in the design, the voltages from all the 3 phase are fed to the non-inverting pin#3 of the opamp, via a rectifying diode, so that a DC becomes available at this pin of the IC.

Pin#2 which is the inverting input of the opamp is held at some constant reference level through the voltage divider network using a 3k3 and a 10k resistor.

As long as all the 3 phases are present, the sum of their phases produce a zero potential at pin#3 of the IC, this happens since we all know that sum of the phases when joined end up producing a zero potential.

Therefore pin#3 is held at 0V, which tends to be lower than the pin#2 of the IC, resulting in a 0V at pin#6 of the IC which keeps the relay switched OFF.

However in a case where any one or two phases are absent allows the pin#3 potential to climb a much higher level than pin#2, resulting in a high at pin6 of the IC, the relay responds and toggles ON, shifting contacts to the N/O position.

Now since the relay contacts become responsible for disconnecting or connecting the load with the 3 phase supply, due to an absence or a missing phase. Therefore it becomes imperative to use a 3 contact relay, through which the load may be enabled with the 3 phases for its operation.

With such connection whenever the relay toggles, it either connects the lad or disconnects the load from all the 3 phases depending on whether the 3 phase is good or bad, thus implementing the intended 3 phase prevention for the load.