I need to sense 3 phase 240 volts

janner_10 said:

Not it is not. If you took the time to read the original post, you would have seen he has ruled out a phase relay.

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I did read it, i discounted it as a comment from somebody who has not actually used a phase failure relay before.

If that won't operate in certain circumstances then what difference is a relay going to make ?

I agree with Bryan on installing contactors so if a change is required and voltage from another source is applied the other source has been removed such as with a house generator, this way you wont get backfeed. To monitor with a relay use 2-220 vac relays L1-L2 & L2-L3 won't let a single phase occurrence trick you. Just a thought

GaryS said:

This thread takes me way back to when I first started in the industry

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VERY NICE!!!

I remember the old timers talking about 'the way it was', when I first started out I worked in a printing plant... over 1 mil sq feet, they moved from San Francisco to Newark and they were always talking about the 'old plant' it was the clock tower, they had 4 buildings in downtown SF and the rigging they had to do back then was amazing, most of the guys I worked with started there in the 50's and not much was electrical, they had one printing press that was just about all mechanical and it was a piece of art.... 3 stories tall and about 60 yards long, it took two days to do a lube job

The clock tower is now housing https://www.youtube.com/watch?v=Atqva_5ApzY

One of the helpers I worked with had to go up and oil the clock every day, I think he just loved the views

Trainelectric said:

I was thinking about using a phase relays, but there are situations where it will not activate,which would result in the plc trying to put two sources together.

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I'm just curious. What is a circumstance under which a phase loss detector would fail to detect a phase loss?

Three relays put in delta on your phases or in star to the neutral. Take one contact of each and put these in series: indication that all three phases are on (1 on PLC input). Take another contact of each and put these three in parallel: indication for all three phases off (0 on a second PLC input). Both inputs 1: everything OK and power on. Both inputs 0: everything OK and power off. Any other combination: check out things before switching.

I reviewing this post I just realized something
We are talking about a 3ph motor moving cars with the power source coming from 1 of 3 different power sources. As we all are aware that 3 phase motor rotation direction is dependent on the supplied power sequence. So just knowing if the power is present is not enough you also need to phase match all 3 power sources in order to ensure that the motor run the intended direction no matter which power source is used.
While it in possible to design a DC power source that could connect to all 3 power sources. In face they could all be connected at the same time the dc output would be powered from whatever source has power at the time somewhat like as UPS system.
With an AC power source it can be dome but the 2 generator sources must be synchronized to the AC line source in order to work. This same thing is done with every power station that is connected to the grid.
If you setup a system the used multiple power sources and disconnect one and then connect another it could cause an unexpected reversal of the motors putting personal at risk.

As solution that may work would be to control all the motors using a VFD connected in a common DC buss configuration. Each power source supplies power to the DC buss while connected.
If all the power sources are at the same voltage the buss would not care what the source is.
This would simplify the controls and eliminate the power source problem.