Prevent endless start stop cycling of compressor when cooling water flow fails for my boat

Thread Starter

sdowney717

Joined Jul 18, 2012
648
I have this cruisair heat pump which runs off 120vac.
It is cooled by raw water flow. If the water flow quits, the head pressure of the compressor will head over 400 psi, and click off the safety switch shutting off the compressor.
But the safety pressure switch will as the head pressure drops back down automatically reset, then of course with no cooling flow the compressor will again overheat and pump up the pressure past 400 psi and shut down. It will do this forever until somthing might break.

Now it is on a dedicated GFCI-AFCI 20 amp breaker. I noticed I accidently touched the incoming hot wire to ground with my hand and the GFCI instantly tripped it off. I was thinking that would be a good way to shut off the power to the unit, if a circuit could create a ground fault when the water flow stops, and the head pressure zooms over 400 and trips off the switch, have it also switch a relay? to create a momentary ground fault killing the incoming AC power.

Any ideas on how this could be easily done, or some other way?
The safety pressure switch has 2 terminal, so its just on or off.
The safety pressure switch is to the right of the start relay in the second picture.
20190711_105546.jpg
 

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MaxHeadRoom

Joined Jul 18, 2013
19,029
Why not use the pressure switch to operate a motor contactor that is initially powers the compressor from a standard stop start circuit, the switch would be inserted in the string, it would remain off until manually reset/start sw is pressed.
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
Can you draw out a circuit flow so I can visualize that?
I dont want to lose ability to use the start stop switches inside the boat cabin, except perhaps for a cooling water flow fail, which you have to figure out down in the bilge.

The AC has a start and run circuit potential relay and start and run capacitors for the compressor.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
I still dont understand your diagram.
Is the start then with M under it held on with current flow? So if the current quits, the switch opens?
If true that wont work since I would have to open the hatch and manually start it every time?
Right now I have an off-on-on rotary switch. This first on position starts water pump, the second on starts the compressor. Of course off is off for both circuits. Switch is inside the boat cabin.

Do you mean use something like a manual reset relay?
Where if relay coil looses power, it has to be manually reset?
Any part numbers for that?

I guess what I can imagine is a switch that when it loses power has to be manually rest.
 
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MaxHeadRoom

Joined Jul 18, 2013
19,029
I guess what I can imagine is a switch that when it loses power has to be manually rest.
That is exactly what it does.
M is a N.O. contact on the compressor shut off relay. It may be possible to use a contact that is already present on the rotary switch as the Start function on my circuit.
You would need to examine what the 'second on' position does or how it could be used. I am guessing you could incorporate it in the circuit shown.
Do you have any control circuit diagram from the unit? If so if would make it easier to see what is going on.
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
The way the compressor starts, it sends power to both the run circuit motor winding and start motor winding at same time/
As the motor reaches 3/4 of full rpm, a potential relay measuring the cut off back EMF voltage generated by the winding opens the start relay circuit and then the only energized winding is the run winding.
So it can stop or start with simply providing power of removing power
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
The way the compressor starts, it sends power to both the run circuit motor winding and start motor winding at same time/
As the motor reaches 3/4 of full rpm, a potential relay measuring the cut off back EMF voltage generated by the winding opens the start relay circuit and then the only energized winding is the run winding.
So it can stop or start with simply providing power of removing power
That is just a typical compressor motor starter.
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
That is exactly what it does.
M is a N.O. contact on the compressor shut off relay. It may be possible to use a contact that is already present on the rotary switch as the Start function on my circuit.
You would need to examine what the 'second on' position does or how it could be used. I am guessing you could incorporate it in the circuit shown.
Do you have any control circuit diagram from the unit? If so if would make it easier to see what is going on.
Max.
Maybe I can find something on web that demostrates the circuit, it is the basic start-run capacitor induction motor setup. The 3 position switch off-on-on is progressively on, so first 'on' turns on water pump, second 'on' keeps the first 'on' on, and also turns on the compressor. So the user can turn on the AC, by first turnng on the water pump, then turn on compressor, its just the way the OEM set it up. /instead, It could all come on together.
 

LesJones

Joined Jan 8, 2017
2,317
An alternative approach (As you do not have a normal motor starter as in Max's schematic.) would be to have a flow switch in the coolant water supply. The flow switch would drive contactor in series with the supply to the compressor. So then the compressor could only run if the cooling water was flowing.

Les.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
An alternative approach (As you do not have a normal motor starter as in Max's schematic.) would be to have a flow switch in the coolant water supply. The flow switch would drive contactor in series with the supply to the compressor. So then the compressor could only run if the cooling water was flowing.

Les.
Any examples of such a switch?
Other thing is there is not much water pressure involved with this centrifugal pump.
These pumps are basically free flowing no restriction, output is measured in gph.and they dont lift more than a few feet, you want volume of flow not pressure, although there must be some small measurable pressure. So when its flowin free, the PSI is going to be very low.

Here is a typical water pump
https://www.amazon.com/Seaflo-Conditioning-Seawater-Circulation-Submersible/dp/B074TXL7YW/ref=pd_cp_200_1?pd_rd_w=blYYP&pf_rd_p=ef4dc990-a9ca-4945-ae0b-f8d549198ed6&pf_rd_r=QSZ8ENXQ2C02CDJNSN10&pd_rd_r=dea63096-a3fc-11e9-bc59-bd9b9921c68b&pd_rd_wg=XYbQk&pd_rd_i=B074TXL7YW&psc=1&refRID=QSZ8ENXQ2C02CDJNSN10
 
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LesJones

Joined Jan 8, 2017
2,317
On of the flow switches in this ebay search may suit your requirements. If it was me doing it I would replace the existing three position switch with start and stop buttons with the normal starter contactor as Max has shown. The normally closed pressure switch on the compressor would be connected in series with the stop button.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
Maybe I can find something on web that demostrates the circuit, it is the basic start-run capacitor induction motor setup. The 3 position switch off-on-on is progressively on, so first 'on' turns on water pump, second 'on' keeps the first 'on' on, and also turns on the compressor. So the user can turn on the AC, by first turnng on the water pump, then turn on compressor, its just the way the OEM set it up. /instead, It could all come on together.
I would also look at re-vamping the circuit, if it was all done with relay logic, the sequence could still be maintained, i.e. the water pump would be sequenced and interlocked with the compressor for the right mode of remote operation.
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
Does a switch exist that is magnetized on. So when you push it down to on, a magnetic field generated by current flow holds the switch down and on. And then when the power is interupted, the magnetic field is broken releasing the switch?
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
Some AC motor contactor stations and the Chinese shop machines use this method of a cheap form of the ON PB circuit I showed, instead of the ON P.B. there is a insulated rod that physically pushes the contactor armature over, resulting in it retaining itself through the retaining contact on the relay.
Of course, the ON/OFF circuit shown does this also, but electrically.
Max.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
648
Some AC motor contactor stations and the Chinese shop machines use this method of a cheap form of the ON PB circuit I showed, instead of the ON P.B. there is a insulated rod that physically pushes the contactor armature over, resulting in it retaining itself through the retaining contact on the relay.
Of course, the ON/OFF circuit shown does this also, but electrically.
Max.
What is an 'ON PB' circuit in longhand?
And I was thinking any logic would have to account for if the water pump was actually moving water. See on a boat usually the intake screen clogs so the impeller will be powered on and not pumping anything. Rarely would the pump electrically quit spinning its impeller.

The whole situation OEM resolved by changing the safety pressure switch to needing a manual reset in newer models and of course lots of semiconductor circuitry, my old system is very reliable electrically. . Its not too bad if your there and notice it cycling, whats bad in unattended operation of the AC, no one is there to shut it off. So I would worry a lot about leaving it on when away from the boat.

Earlier you said,
'M is a N.O. (normally open) contact on the compressor shut off relay.'
Since the unit has no shut off relay are you saying somehow add one?
I still dont understand how that can work with this compressor start run cycle the way it starts and runs..
The potential relay never has power until power is supplied to all the circuits. and it is just a cutout relay for the start circuit. So power ON, start and run windings are on, then cut out relay shuts down start winding but run is always on when power is on.
If you energized just run winding all the time, motor would hum, never spin up and eventually burn up. And you cant possibly control the start winding manually with a button properly.

If you know how, I need more exact detailed description, just pretend I am a dumb computer.program needing an operation programming.
 
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LesJones

Joined Jan 8, 2017
2,317
an 'ON PB' is an on push button. There may be a way to use the existing switch. If the off to on transition of the set of contacts on the switch was used to generate a short pulse (Say 0.5 seconds) that could simulate the press of the on button. This would be easier to do if it was just switching a low voltage DC signal so that some electronics generated the pulse. It could also be done using a DC relay, a bridge rectifier, a capacitor and a resistor. The bridge rectifier would feed the relay via the capacitor so the relay pulled in just while the capacitor was charging causing the relay to pull in for a short time. A resistor would be needed across the output of the bridge rectifier to provide a discharge path for the capacitor when the switch opened. The set of contacts that now switches the pump could act as the stop button contacts. This could be directly or if they still switched the water pump via a relay.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
d
What is an 'ON PB' circuit in longhand?
And I was thinking any logic would have to account for if the water pump was actually moving water. See on a boat usually the intake screen clogs so the impeller will be powered on and not pumping anything. Rarely would the pump electrically quit spinning its impeller.

The whole situation OEM resolved by changing the safety pressure switch to needing a manual reset in newer models and of course lots of semiconductor circuitry, my old system is very reliable electrically. . Its not too bad if your there and notice it cycling, whats bad in unattended operation of the AC, no one is there to shut it off. So I would worry a lot about leaving it on when away from the boat.

Earlier you said,
'M is a N.O. (normally open) contact on the compressor shut off relay.'
Since the unit has no shut off relay are you saying somehow add one?
I still dont understand how that can work with this compressor start run cycle the way it starts and runs..
The potential relay never has power until power is supplied to all the circuits. and it is just a cutout relay for the start circuit. So power ON, start and run windings are on, then cut out relay shuts down start winding but run is always on when power is on.
If you energized just run winding all the time, motor would hum, never spin up and eventually burn up. And you cant possibly control the start winding manually with a button properly.
If you know how, I need more exact detailed description, just pretend I am a dumb computer.program needing an operation programming.
The ON PB is the Start PB in post #4.
The motor and start winding current relay is considered a separate entity and would not enter in to any external control logic.
This is just to sense motor start up, and it does this automatically, it automatically comes into play when the power is supplied to the motor
The idea would be to incorporate all the other sensors, etc and set up a relay/contactor control circuit, which may be the ideal.
The control circuit would use the 400psi PS and all other sensors to control the circuit relays etc, instead of controlling it direct as it may do now.
Ideally sensor etc should switch low current control circuits, rather than switch motors/compressors direct, it is easier on the sensors!
Doing it this way makes it easier to use flexible or configurable logic.;)
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
https://www.ebay.com/itm/AC-220V-3A-22mm-Plastic-Vertical-Horizontal-Magnetic-Water-Flow-Switch-w-Sensor/371932003336?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20131003132420&meid=5ea0aee4fb9646c8a982c12d7be41505&pid=100005&rk=3&rkt=3&sd=183859556798&itm=371932003336&pg=2047675&_trksid=p2047675.c100005.m1851

What do you all think about using a water flow switch like this one?
I am assuming when water flows, the switch turns on?
Have it connected to a relay coil.
The relay l connects the compressor AC power as an on-off switch.
If water flow stops, the relay clicks off cutting power to the compressor.
It seems really simple? Of course need a power relay to handle the high amp draw of the motor. Somewhere I do have some contactors.

I use 3/4 inch rubber hose and this is 20 mm threads. Could just glue and clamp hose to the threads. I dont know if those threads are NPT.
Normally people use 1/2 to 5/8 hose because the pumps are that outlet size, but my pump outlet is bigger.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
648
It might not work right, the flow rate is very low.
These pumps can fill a 5 gallon bucket in one minute, restrict the flow and its no good for cooling.

This one looks like the other one.
Minimum starting flow ≥ 0.75L/min
The maximum flow rate ≤5L/min

https://www.ebay.com/itm/Magnetic-Plastic-Water-Flow-Sensor-Switch-G1-2-for-Laser-Welding-Cutting-Machine/113665940729?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20131003132420&meid=87697e81f7b94c6faceab672d273ca6c&pid=100005&rk=6&rkt=12&sd=371932003336&itm=113665940729&pg=2047675&_trksid=p2047675.c100005.m1851
 
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