Need help on 3 phase and current sensing ckt.

Thread Starter


Joined Jul 8, 2011
Hi all,

I am designing circuit which has over current, under current, short circuit sensing facility and it also sense whether any phase is ubsent from 3 phase input supply.

As per my knowledge we can design such circuit using CT and OPAMPs, but don't have exact idea so kindly help me in this regards.

I am in urgent need.

Thanks in advanced.



Joined Oct 26, 2011
I have done the phase loss and phase rotation functions via several methods in the past because this is a necessary function in three phase to DC thyristor drive controls. All have used voltage sensing techniques.

The simplest involved an AC relay, capacitor and resistor--it worked well determining phase rotation, but did poorly at determining phase loss on one of the 3 legs--proper location of the control transformer solved this deficiency.

Another system used the three phase output from a low voltage power/synchronization transformer--all three phases summed together provided a node with low AC voltage--to obtain phase loss out of this, one of the phases was inverted and phase shifted via a low-pass filter and summed into the same node.

Still another system sampled each of the three phases via a high input impedance attenuating differential amplifier, then summed the signals together as in the previous system.

A system developed by another did this function via a sequence of logic signals.

Now what you are talking about appears to be a higher level power monitor--such are available from Allen Bradley, Siemens, Square D etc. Some are stand-alone, while others are integrated with motor starter contactors with integral overload feature. In the past, the OL feature has been done with overload heaters and bimetallic thermal switches. This, of course, has the disadvantage of having to be sized for a narrow band of load currents. The higher level types are now microcontroller based and are adjustable for a broad range of applications. Currents are generally sensed and isolated via current transformers--I would not be surprised if more modern, cost-effective designs used hall effect current sensors. Regarding the under-current function, this would be an extension of the power monitor technology--I have not seen this before.

Standard, commonly available current transformers have historically employed a 5A secondary rating--other current ratings may be now available--often are unreasonably expensive--actually CTs are a simple design and custom versions that are more compatible with power monitor applications should not be difficult to obtain.

Other features would include various 'thermal' curves for allowing normal motor (load) starting, and as you mentioned short circuit sensing. In this regard, the difference between a power monitor and circuit breaker is becoming blurred, so a clear specification of the task at hand will be required. As a circuit breaker, the interrupting capacity is the real "hooker".

Technically, a power monitor design task is relatively easy compared with testing and the obtaining of agency approvals (CE, UL/CSA etc).

Hope that this rambling piece is helpful--it may add more questions than answers.

Thread Starter


Joined Jul 8, 2011
Thanks for the reply.

Can you/anybody please share the such circuit which make me easy to solve my problem. :)