Phase monitoring with Brx

[PLCwannabe]

I'm working on a 30 year old transfer switch, and would like to add an in-phase monitor to the transfer process, however because the switch is obsolete, this part is no longer available. So my idea is to use a Brx plc to build this monitor.
  I think programming the logic will be quite easy, but I'm a bit stuck on how to measure the voltage curve. Are there any analog input modules available to measure instantaneous voltage, so i can compare the sine wave between the 2 sources? Or maybe ther e is a simpler way to do this. Any ideas?

[BobO]

I'm working on a 30 year old transfer switch, and would like to add an in-phase monitor to the transfer process, however because the switch is obsolete, this part is no longer available. So my idea is to use a Brx plc to build this monitor.
  I think programming the logic will be quite easy, but I'm a bit stuck on how to measure the voltage curve. Are there any analog input modules available to measure instantaneous voltage, so i can compare the sine wave between the 2 sources? Or maybe ther e is a simpler way to do this. Any ideas?

BRX onboards and -3 modules are right at 1ms. That would get you close, I'd think.

I would think you could just use a discrete input though. You just need the zero cross, right? You'd need a few external components to cap the voltage at what the input could tolerate, and a diode to lop off the negative half (BRX inputs are full wave), and then just use a high speed timer or interrupt to find the edge and time the phase.

[Garyhlucas]

I have worked on generators and implemented in phase transfers because an out of phase transfer is really nasty when driving 3 phase motors during the transfer back to the utility from the generator.  If the the utility is running behind the generator the motors try to back up for an instant and bad stuff happens, like huge voltage surges that trip breakers and in one case broke couplings on 20hp motors!

If you need to do this transfer automatically then you need a controller.  However, a pair of neon lamps connected A to A and B to B will let you see the correct moment very well.  They go out completely when the phasing is perfect.  My customer didn't want to trust this and hooked up a 3 channel oscilloscope to  watch what was happening and was amazed at how well the neon lights indicated correctly.  They never bothered doing anything better.  If the speed of the generator isn't exactly matching the utility the lights blink on an off. as speed gets closer they blink slower and slower so the window of opportunity for the transfer can be quite long.

I suspect that this suggests using a pair of isolation transformers to connect the two sources and sensing for zero output voltage from each would do the job and the PLC could then know if the speeds are close enough as well not to cause a big bump.

[PLCwannabe]

Thanks for the ideas. I think I'll try the diode/resistor with discrete inputs. That should get me really close. The nice part is, no way to make a screw-up. It really can't get worse than the random switching its been doing.
  I did think of the neon lights, but this is an auto-transfer switch which gets exercised every week, so it has to be done automatically.
  So if I wanted to catch the top of the peak as opposed to the zero point, are the brx inputs "on-threshholds" consistent enough to capture both peaks at the exact same voltage level?

[BobO]

I would time the period. Half is the peak.

[PLCwannabe]

I've been trying to get this to work with an ac input, but not having any luck. Do I actually need a dc input to make this work?

[BobO]

I've been trying to get this to work with an ac input, but not having any luck. Do I actually need a dc input to make this work?

Yes. You'll need to make sure the filter time is set low as well.

[PLCwannabe]

Works great with dc input module. I'm using a 20 kohm resistor in series with a diode to limit the current from a 120vac source. I'm never buying ac modules again. P.S. Is there some kind of a trick to use the ac modules with a 24vdc supply?

[BobO]

P.S. Is there some kind of a trick to use the ac modules with a 24vdc supply?

I'm not sure understand the question.

[PLCwannabe]

What I meant was can a 120vac input module be used with a 24vdc supply? The above posts kind of explain how to use a dc input module with a 120vac supply. I have accidently purchased 120vac input plc's a couple times, and then realized that they don't work with the 24vdc supply voltage.