Component Identification

[dogman]

Possibly because of the many different uses the control pcb might be put to and they just forgot their own advice, wouldn't be the first time.

I'm assuming from your comments re hoses these are for hose control reels on a fire truck.

Yes, a fire truck.

 

[dogman]

Would solid state relays be a better choice & offer better long term reliability than the electromechanical relays for this application?

 

[augustinetez]

They may be OK but.....

DC solid state relays easily available here in Oz (Jaycar, Altronics) don't like switching voltages below 12V, if motor/clutch start up causes voltage dips below 12V or the battery is getting low, they drop out or won't turn on.

You will need to add back EMF suppression to the clutches. They don't tolerate high voltage spikes, it will kill them.

They ain't cheap compared to mechanical relays, only 100A versions are available from Jaycar/Altronics (or 2A which are no good to you in this application). I haven't checked RS or Element14 but you can expect pricing of at least triple if they have anything suitable.

 

[dogman]

They may be OK but.....

DC solid state relays easily available here in Oz (Jaycar, Altronics) don't like switching voltages below 12V, if motor/clutch start up causes voltage dips below 12V or the battery is getting low, they drop out or won't turn on.

You will need to add back EMF suppression to the clutches. They don't tolerate high voltage spikes, it will kill them.

They ain't cheap compared to mechanical relays, only 100A versions are available from Jaycar/Altronics (or 2A which are no good to you in this application). I haven't checked RS or Element14 but you can expect pricing of at least triple if they have anything suitable.

Sounds like they have there own set of potential issues which we don't need.
The relays used are prone to failure as well, they don't last long.
Just thinking of alternative methods, best to have something that is bullet proof.

 

[Nigel Goodwin]

Would solid state relays be a better choice & offer better long term reliability than the electromechanical relays for this application?

No, relays are extremely reliable, and you would require specific low voltage DC solid state relays anyway - basically MOSFET switches.

If the relays aren't lasting long?, then use higher rated ones - what fails on them?.

 

[augustinetez]

If the relay contacts are constantly burning out, a capacitor across the relay contacts should help but you should add back EMF suppression across the clutches.

Same as the relays, a diode across each clutch coil.

 

[dogman]

If the relays aren't lasting long?, then use higher rated ones - what fails on them?.

Sometimes the contacts weld themselves together & other times the contacts are all black & worn from arcing I guess.
I mentioned before that the Datasheet for the relays says that the maximum switching rate is 6 times per minute but you can throw that out the windows for our application in emergency situations & depending on the operator. Sometimes panic sets in & the remotes & relays are repeatedly & excessively used.

The other big issue is that the relays are not in pcb sockets, they are soldered into the boards so they can't be changed in a hurry if at all in the field, bad news again. Don't have the time to sit down with a soldering iron & fix the damn things.

Edit:
When the system is working it works very well, it just needs modification to enhance reliability, it's one of those things that looks good on paper & for slow casual use but in reality the real application is far from that.

 

[dogman]

If the relay contacts are constantly burning out, a capacitor across the relay contacts should help but you should add back EMF suppression across the clutches.

Same as the relays, a diode across each clutch coil.

Yes, all the diodes will be going in don't worry.
I should have the truck here before the end of next week.
What value capacitor to start with for testing & type of capacitor do you recommend, ceramic or other?

 

[augustinetez]

The attached pdf shows the various forms of contact suppression, you will be looking at the first one and calculating the component values is shown (a resistor and a capacitor).

For the capacitor, X2 Metallized Polypropylene Film Capacitor like this https://www.altronics.com.au/p/r3137a-1.0uf-x2-pcb-mains-suppression-capacitor/ and for the resistor 1W carbon or metal film should be fine for testing (do not use the 5w wirewound types).

Going by the calculations and the current you mentioned previously, the resistor would be 12Ω and the capacitor ~4μF - may mean paralleling several capacitors depending on what values you can source.

 

[dogman]

The attached pdf shows the various forms of contact suppression, you will be looking at the first one and calculating the component values is shown (a resistor and a capacitor).

For the capacitor, X2 Metallized Polypropylene Film Capacitor like this https://www.altronics.com.au/p/r3137a-1.0uf-x2-pcb-mains-suppression-capacitor/ and for the resistor 1W carbon or metal film should be fine for testing (do not use the 5w wirewound types).

Going by the calculations and the current you mentioned previously, the resistor would be 12Ω and the capacitor ~4μF - may mean paralleling several capacitors depending on what values you can source.

Thank you for the help with that.
Do you think the picture attached of the circuit I have simulated would be an ok representation of the relay & clutch operation?
It is interesting without the diodes in place.

 

[augustinetez]

My experience with sim is limited, but my experience in real life has shown there is a big gap between theory and what actually happens in practice - you can't simulate human interaction.

Depending on how much work you want to do and if there is the budget for it, you might want to think about replacing the relays with 12v DC heavy duty contactors (pdf is just one example). They aren't expensive but I would be looking for ones with spade connections rather than screw down type.

 

[dogman]

you can't simulate human interaction.

Exactly my thoughts!

 

[Les Jones]

I think adding diodes in parallel with the clutch coils will solve the problem with the relay contacts. If you look at the back emf spikes with an oscilloscope without the diodes and then again with the diodes you will see a very large difference. If you can take the cover off the relays you will see a reduction in the amount of sparking.
The diodes will slow down the release of the clutches slightly but I don't think this will be enough to cause aproblem.
If the value of resistance in post #50 of the clutch coils is correct then the current is only about 3.5 amps. This should not be a problem for the relay contacts.

Les.

 

[Tony Stewart]

I just reviewed the entire thread and have seen a lot of insufficient and inaccurate details in schematics regarding the motor load Rs and estimates on each coil Rs with missing datasheets. Rs is often specified as DC is the series R is measured at DC. The time constant of each inductance is Tau=L/R can predict how hot the PTC will get along with the faster than normal start stop operation, raises the PTC false triggering. Each motor should have it's own PTC and not shared. We need the datasheets with P/N, Model for every component in this design including the motors to make this 100% reliable. Clamp diodes absorb the overvoltage and stored energy in the load coil which inturn slows down the stop time and extends the arc duration in the contacts.

There are a lot of false assumptions in the question, and this is why there have been 53 comments. So accurate data is needed now.

Contrary to opinions given, any human activity can be modeled on electronic components if you know how. It is well known that DC motors draw ten (10) times the rated current on start up, +/-20% and thus you will likely see failures due to false triggering of the PTC fuse.
We need to know the worst case timing of start/stop and forward/reverse which can draw 20x the rated motor current.(+/-20%) since the back EMF is added to the battery voltage.

If you want a reliable system, then we need reliable data. Then I can recommend what is best from soft-start (anti-surge), over-temperature (PTC).

I wonder who designed this in the 1st place and decided two receiver hose pumps could be supplied by 1 PTC. Now I've forgotten if these are pump motors and/or winding motors. arghh.

Releasing inductance from motors, relay coils always creates a flyback voltage. We do not know if the loads are ground switched or Vbat switched.

Photos would also help so we can recommend the easiest and best solution that is 100% reliable for 10 or 20yrs or as long as you specify, as relays are consumeable parts.

 

[Les Jones]

Hi Tony, I think we all thought at first thet the two relays were powering motors. It was quite late in the thread that we were told they were driving electromagnetic clutches. We don't know the source of the rotational power. It could be a hydraulic motor or the truck engine. I agree that the clamp diodes extend the stop time as they maintain the current through the coil after the power is removed. I do NOT agree that they will extend the arc time as there there will only be about 0.7 volts (The diode forward volts drop.) across the relay contact during that time. Adding a resistor in series would speed up the release but would increase the voltage across the contacts. The value of the resistor could be chosen to limit the voltage (To say 20 volts). This should reduce the arcing to a large extent. I imagine that the mechanical inertia of the hose reels would have more effect than the decay of the coil current. I don't know if there is anything in the system to brake the hose reels when the clutch is not engaged.
Edit. I now realise that I was wrong when I said that there would only be about 0.7 volts across the relay contacts. There would be the supply voltage added to the 0.7 volts so there would be 12.7 volts across the contacts.

Les.

 

[dogman]

The confusion would appear to be my fault & not explaining things correctly in the first place but in the beginning I was just trying to locate a short in a component & then it expanded from there.

The hose reel system has a diesel engine that runs the water pump via a main shaft, another idler shaft rotates freely being chain driven from the main shaft.
When either of the two or both reel drive clutches are engaged they drive the reels in the "hose retract direction only" from the idler shaft.
The reels are free spool out & they have an adjustable spring loaded friction disc on each to stop over run & to hold in position.
Single direction drive for the reels in the retract direction only.