How to find high current ?

[dhanasingaraja]

Hi,
In my village, 3 phase supply comes for 4 hours, and single phase supply comes for remaining hour. I wanna switch 5HP,30amps motor for every time phase changing.My problem is how can i sense 3 phase supply.can i use resistors? If power comes i should control the connection with relay

 

[ljcox]

I suggest that you connect a relay (connected through a rectifier bridge) from the neutral to one of the phases that is disconnected when the supply changes from 3 pahes to 1 phase.

Thus the relay will be operated when the 3 phase supply is on & released when it changes to 1 phase.

Of course, you need a relay that is designed for the mains voltage.

As an alternative, you could use a transformer to reduce the voltage for the relay.

 

[dhanasingaraja]

I want to monitor/control this switching through gsm.so i should use micro controllers.How can i sense/control this with micro controller?
pls help me

 

[alec_t]

In the interests of safety, use an isolating transformer plus bridge rectifier, or a mains adapter, fed by one of the phases that is active only during the 4-hr period to obtain a low DC voltage (say 12V, which could be used to power a contactor) and apply the low voltage, via a potential divider if necessary, to an input of the micro.

 

[user_88]

I second the usage of an isolation transformer ... phase to neutral.
You are courting disaster if you do not.

The transformer you use must be an isolation type transformer.
You must insure that there is no electrical connection or conductivity between the primary and the secondary.

Frequently, you will find the 1:1 ratio isolation transformers that are available on the market.
You can use one of these followed by another transformer, in parallel, that has a turns ratio appropriate to your application.

I would assume that you do not need excessively large power transformers ... 50 Volt-Amps (VA) should be sufficient for your purposes.
Just be careful selecting the particular type.

 

[Reloadron]

You do not mention the voltage? Because of the motor requirement I assume 230/240 volts? You also do not mention if the 3 phase power is a Y (WYE) or a Δ (Delta) configuration. Because of the way you state it I assume a WYE configuration where phase to neutral is used, would that be correct?

I would come off each phase to neutral with a small line to 24 VAC transformer as was mentioned. Then I would use opto couplers to run the signal(s) into the uC you plan to use. A simple opto configuration would give you a 50 Hz (I assume as you don't mention mains frequency) pulse into the micro controller. Write the uC code around that.

If this is what you have I'll give you a drawing if you want to go that route.

Ron

 

[tcmtech]

I don't see the exact need for anything other than a few line voltage powered DPDT or similar relays and three larger power contactors to make this work.

Its just a simple relay logic circuit to choose which line is not the dead one of the three and thusly switch the load to it if its not already connected.

 

[user_88]

... A variation of something previously mentioned:

Arrange three light bulbs ... incandescent or whatever ... to indicate which particular phase has power at the changeover.
You would have one light bulb on each phase, powered by a phase to neutral connection.

Each light bulb would turn on a particular photo-transistor ... indicating the active phase, and sending a signal to the controller.
The controller would turn on the appropriate relay that would provide power to the motor, while turning the two inactive relays off.

The logic condition that would indicate the switching of the three phase would simply be the state in which you do not have all three light sources turned on.

 

[Reloadron]

I don't see the exact need for anything other than a few line voltage powered DPDT or similar relays and three larger power contactors to make this work.

Its just a simple relay logic circuit to choose which line is not the dead one of the three and thusly switch the load to it if its not already connected.

That was my first thought, until I saw where the OP wants to feed a uC. Not that the uC matters much. Personally for a circuit like this I wouldn't worry about transformers initially either but there is likely a half dozen ways to go about something like this or more. For simple, something like the attached. Unfortunately the OP leaves a few things out. It can be simple or get complex.

Ron

 

[ljcox]

... A variation of something previously mentioned:

Arrange three light bulbs ... incandescent or whatever ... to indicate which particular phase has power at the changeover.
You would have one light bulb on each phase, powered by a phase to neutral connection.

Each light bulb would turn on a particular photo-transistor ... indicating the active phase, and sending a signal to the controller.
The controller would turn on the appropriate relay that would provide power to the motor, while turning the two inactive relays off.

The logic condition that would indicate the switching of the three phase would simply be the state in which you do not have all three light sources turned on.

This is a good idea. You could use opto couplers in lieu of the light globes & photo transistors.

The outputs of the optos would be connected to the microcontroller.