Paulcs:
Pumps can fail and/or liquid can be drained. I would use a flow switch as "permission" for the SSR to activate the heater. I also like indicator beacons, so someone can tell you over the phone what is, or is not, happening.
I also like a MOV across the SSR power terminals when switching an inductive load, such as a pump. It helps to suppress the voltage surge at turn OFF of the pump.
I do plan on a flow sensor and a non-invasive current sensor, I was going to use those as a safety/sanity check at the software level. Should I hard wire the flow sensor into the system as you've shown? ( I haven't settled on a solution for flow sensing and don't know that what ever I come up with wont need processing anyway )
I was going to use low voltage led's at the panel to indicate when things were "supposed" to be happening, I'll use 120v indicator lamps instead as you suggest (so that I know how things are actually working.)
(edit) Reading up on MOVs I find "As MOVs wear out from use, they will become more sensitive to common voltage spikes, causing them to wear out quicker. When they entirely fail, they will become a short circuit, potentially creating a fire hazard." Would a mechanical relay on the pump be a better choice or would it require an MOV too just because of the inductive load. I was thinking of using the SainSmart 2-Channel relay just on the pump as I have an extra one already.
@dlloyd, thank you for that. Something to consider, may come in handy to be able to take physical control over the pump like that. Though, if that situation came up I could just plug the pump directly into the wall. Hmmm....
(edit) Reading up on MOVs I find "As MOVs wear out from use, they will become more sensitive to common voltage spikes, causing them to wear out quicker. When they entirely fail, they will become a short circuit, potentially creating a fire hazard." Would a mechanical relay on the pump be a better choice or would it require an MOV too just because of the inductive load. I was thinking of using the SainSmart 2-Channel relay just on the pump as I have an extra one already.
I've used 100's over the years (mechanical relays switching transformer loads) ... haven't had one fail yet. For 120VAC, I size them for 150VAC working voltage. MOV's are even more important on mechanical relays because they switch anywhere on the AC cycle and have contact bounce. Also look at snubber circuits.
A zero-crossing type solid state relay would work well for the heater and should last a lifetime (no MOV required).
For the motor, if the PF > 0.75 zero-crossing type is OK, otherwise this article recommends random-fire type relays.
Of course, you could easily protect the 120VAC circuit for external surges/lightning by using a surge protected power power bar or similar (these have multiple MOVs on the internal circuit). As a bonus, some have power switch, power indication and circuit breaker for overload protection.
Yes it's supposed to be run with a load. It's not self priming, and running the pump dry can damage the pump it self. I'm working on getting more specs from seller other than what I posted at replay 15.
dlloyd:
Power Factor. For your pump, I'm sure its greater than 0.75 because the motor is always being operated with a load (not free running).
Well the manufacture is refusing to hand over any kind of data sheet or additional specs. He did say it's small shaded pole motor with no power factors.
It's probably a case I don't even know the right questions to ask.
Well the manufacture is refusing to hand over any kind of data sheet or additional specs. He did say it's small shaded pole motor with no power factors.
It's probably a case I don't even know the right questions to ask.
Based on his reply, he doesn't know what power factor is. Anyways, its nothing to worry about.
When the relay manufacturer mentioned PF > 0.75, this means 75% PF.
Your pump would be designed to operate near 100% of its rated load, and it will always see near rated load while running.
If you scroll to the bottom of this link, you'll see a graph of power factor vs motor load for a typical motor. Motor Selection
Your pump is probably running at 85-90% power factor.
Can anyone help me pick the right fuses for this application? I would like a common size that is easily replaceable. I need a protection fuse for my pump and a fuse at the mains entrance into my box.
I have browsed digikey but I don't even know where to start.
Again, I'm looking at 1.4A for the pump (continuous), 11.5A for the heating element, and whatever the arduino plus sensors will need (5vdc 1A)
This is the latest iteration of my plan:
(I'm still on the fence about putting the pump on a relay)
There are some 20A Toggle Switches here that could eliminate the need for a contactor. These are DPST type so you could parallel the contacts for 40A. Some are rated for up to 250VAC.
I would recommend a 4A slow blow fuse for the pump.
Yes, good call. A 20A double pole will simplify this.
Why slow as opposed to fast? How critical is my fuse choice? I've been told at least a dozen different things as far as amperage, slow vs fast, casing size, etc. I hate to just resort to the 'any fuse is better than no fuse' mentality.
Why slow as opposed to fast? How critical is my fuse choice? I've been told at least a dozen different things as far as amperage, slow vs fast, casing size, etc. I hate to just resort to the 'any fuse is better than no fuse' mentality.
Within the first second or so of the pump starting up, there will be inrush current perhaps 3 to 4 times rated current. That's why its good to have a slower acting fuse ... it'll still protect the circuit against overload but won't blow during startup.
After 10 sec or so, they're all the same, but check the difference here at 1 second:
What would cause an overload on the circuit there? Do I really need the pump fuse? I've been told that these shaded pole motors have very little stall current, if that's the case would the fuse really serve any useful purpose?
Your right about the shaded pole motor. Low starting inrush, low torque and low stall currents. From experience, as an electrician, there is little to be gained in protecting this type of motor with an additional over-current device. The best would be an internal thermal overload built into the motor but I wouldn't even worry if yours doesn't have one of these either. The most common failure of SP motors is the bearings/bushings wearing out after after years of use.
Quick question: if you are going to use an emergency stop button will it be the two position de-tented type
that will lock in the off position? A momentary type as your diagram indicates would not be very useful.
Please make certain that all of your components are rated for 20 amps at 120v. A Tslot duplex receptacle would be ok, but watch out for your switch rating. Also what type of enclosure are you looking to use for all this?
macgregor_sailor:
Quick question: if you are going to use an emergency stop button will it be the two position de-tented type
that will lock in the off position? A momentary type as your diagram indicates would not be very useful.
I haven't incorporated the e-stop into the drawing yet. I'm not using any momentary switches either. The switches that I'm using are on-off switches, one for power to the pump, the other to enable power to the ssr and in turn to the element. (did the cheesy missile switch covers throw you off? I'm not using those.)
Please make certain that all of your components are rated for 20 amps at 120v. A Tslot duplex receptacle would be ok, but watch out for your switch rating. Also what type of enclosure are you looking to use for all this?
Right now the plan is to use a metal ammo can, portable, water resistant, and a good price for the size. I plan to install some small bolts in the lid and the body of the can for grounding posts. All openings will be caulked and everything sealed up as much as I reasonably can. I'll use some rubber 'boots' over those toggle switches. The big SSR heatsink will be mounted on the exterior of the box and I was planning on relying on passive cooling. Does this plan sound safe?
Can you elaborate on switch rating? I don't plan to switch the element faster than once per second.
Should I have more protection in here in the event of ssr failure? I've seen the horror stories how they can fail in the on state and overheat causing a fire.
I will already be monitoring the current draw of the system to detect that everything is drawing the amps that it should. I could put another mechanical relay in series with the ssr to act as the master or fail safe relay, though that is software dependent. Maybe I should use a thermal fuse in series and next to the ssr?
