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Thread: Correct circuit breaker for small single phase pump motors

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    Correct circuit breaker for small single phase pump motors

    I am building a household water purification skid. There are four pumps used for various functions during the process.

    I need to understand how to choose the correct type, curve and rating of the circuit breaker for the system. I prefer using EATON products.

    Each motor is in the order of 1 kW on 230VAC with a stated running current of 5A.

    My second related question is if you would use a relay or a contactor to activate the motor?

    If you need more info please ask and I will dig the specs for the motors up.

    Thanks in advance!

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    Circuit breakers are designed to protect circuits from overload so if a circuit is installed with a 2.5mm˛ cable then a 20 Amp circuit breaker would usually be an appropriate device to protect the circuit cabling from overload damage. The size of the circuit breaker protecting the circuit that supplies your purification skid will be determined by the wiring size of the circuit that's installed, not by the pumps that are being supplied.

    If you design your purification skid so that multiple motors don't start simultaneously then a C curve breaker should be adequate. If for some odd reason you can't use best practice design and multiple motors must start simultaneously then a D Curve breaker may be necessary but you should test the supply circuit to establish whether the fault disconnect time will be adequate with a D curve device before fitting one.

    To protect motors you would usually use a thermal overload device that fits onto the contactor that controls the motor. The thermal overload (if set correctly) is specifically designed to trip when the motor draws too much current but it can accomodate the relatively high start current a motor draws without nuisance tripping. Thermal overloads can also be incorporated into a DOL starter which also has stop/start buttons to control the motor. Another variation is a manual motor starter which usually panel mounts but also has Stop/start control buttons.

    Thermal Overload
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    Thermal overload and contactor assembly
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    Manual Motor Starter
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    DOL Starter
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    Diamond Member AndyD's Avatar
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    In answer to your second question, relay or contactor is potatoes potartoes. Relays and contactors perform the same function but relays tend to be only available in smaller sizes. If you're going to use thermal overloads to protect the pumps then the form factor of contactors would make life easier for assembly. If you're using manual motor starters then either relays or contactors would work equally well.

    To be honest I wouldn't generally worry about over-current protection on small pumps. Usually it's just a waste of money because their demise is almost always caused by an undesirable condition on the wet-end of the pump (dry run, no flow etc) and all these conditions won't usually result in a high enough run current to cause a trip until long after catastrophic machanical damage has already occurred. I'd usually protect against dry run and no flow using a purpose-built pump protection relay that can monitor phase angle and under-load in conjunction with the usual pressure, flow and level switches.
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    Thanks for the detailed reply @AndyD.

    I am very aware of the cable and upstream supply switch gear limits imposing on what happens downstream. In this case there is a dedicated 4mm˛ three wire supply from the supply DB to the skid. The circuit is protected by a C-curve 32A dual pole circuit breaker in the supply DB.

    As per your reply and the pictures posted, I keep running into the situation where we all are familiar with three phase design and installation and herein lies my problem. This is a single phase design which needs to take into account that home owners will have the skid installed and operate it.

    The best practice design in this case must allow for all four pumps to run simultaneously as each pump has a dedicated yet different function. Here are the basic functions:
    1. Tank cross feed pump
    2. Greywater irrigation pump
    3. Slow sand filter feed pump
    4. Potable water pressure pump

    The pumps are all float level or pressure controlled.

    I have watched the wife operating the first version and she definitely wants a manual start with an automatic/manual stop function on the irrigation and the cross feed pumps.

    So in my opinion the following should be installed:
    1. D-curve circuit breaker for each pump - which rating? Otherwise recommend what you would like to see.
    2. Manual stop/start override for each pump

    And now I come to relay or contactor and thermal protection.
    1. I would prefer contactors, but DIN rail relays are a lot cheaper and in this case thermal protectors that fit to the bottom of a contactor is not of any (or too late) use as you pointed out.
    2. Pump protection relays should be the way to go, but do you get dedicated single phase ones? If there is a dedicated PLC measuring levels, and current in the cabinet, are these then required?

    Thanks again for your time and effort!

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    My 2 cents worth...

    Create a scope of work...description of what it is you want to achieve ...then make up a simple sketch with the layout ...equipment layout...create a flow diagram.

    If a circuit has a manual stop start motor (not soft start or VSD) you must use a D curve breaker/Curve 1 (same thing just different brands) ...just beware of using these breakers where there are inverters ...they hum.

    A general rule of thumb...

    Curve 3 breakers ...generally used in domestic installations (the white lever heineman breaker...not so popular anymore due to their ridiculous pricing)...offices etc.

    curve 2 breakers ...used for inrush current ...but more for things like inductive loads like magnetic tables etc.

    Curve 3 or D curve breakers ...used for inrush current...but more for manual motor starting.

    Protection and control for the motors ...it all boils down to the budget.

    Sometimes 3 phase motors are cheaper and you can use a simple inexpensive VSD even if you only have only have a single phase 230 V supply...they convert the single phase to 3 phase (check the motor is compatible with the drive...and star delta connections)

    To save money ...make sure the motor has thermal protection (like on a pool pump ...the button on the side of the motor terminal box) and a circuit breaker...

    or you can use a manual motor starter breaker...in place of a breaker and thermal cutout...you can use 3 phase equipment( contactors etc) ...just make sure you wire it up correctly.

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    To prevent "thumping" caused when pressure switches are used ...make sure you fit a bladder or time delay.

    Float levels ...so long as you are using a high level... low level and cutoff ...you shouldnt experience nuisance switching ....so long as the floats are installed at the correct level.

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    Quote Originally Posted by ians View Post
    Curve 3 or D curve breakers ...used for inrush current...but more for manual motor starting.

    Protection and control for the motors ...it all boils down to the budget.
    Thanks @Ians. While I was editing you posted.

    The flow diagram and logic diagrams are done. My issue is that this is a single phase, budget conscious with no knowledge users/operators. So yes, I can use three phase equipment, but if there is a quality single phase product that can do the same job at a lower price then that is what I would like to know about.

    The pumps are quality industrial pumps (Grundfos or Wilo). I will check if the specific models have thermal protection on-board.

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    Quote Originally Posted by ians View Post
    To prevent "thumping" caused when pressure switches are used ...make sure you fit a bladder or time delay.

    Float levels ...so long as you are using a high level... low level and cutoff ...you shouldnt experience nuisance switching ....so long as the floats are installed at the correct level.
    Yes, there is a bladder type pressure vessel designed in for this specific reason, to safe pumping costs and eliminate water hammer.

    Yes, two separate floats per control point. I always design these so that in case one fails the other serves as an emergency safety in the logic control.

    I have more than 35 years of experience designing the water side, but never for a household application which makes it a bit of an interesting yet fun challenge (specifically in the electrical side) to say the least.

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    Quote Originally Posted by mygoggie View Post
    .....So yes, I can use three phase equipment, but if there is a quality single phase product that can do the same job at a lower price then that is what I would like to know about.

    The pumps are quality industrial pumps (Grundfos or Wilo). I will check if the specific models have thermal protection on-board.
    The thermal overloads and motor starters and even the pump control relay can all be wired for single phase or 3-phase applications. A contactor and thermal overload is the cheapest form of protection I know of unless you fit Klixon type bi-metalic....but I'd caution about their accuracy and reliability.

    One other thing I meant to caution about, when you spec your contactors or relays be sure to use their AC3 ratings, not the AC1 rating.
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    Quote Originally Posted by AndyD View Post
    when you spec your contactors or relays be sure to use their AC3 ratings, not the AC1 rating.
    Thanks for this wise piece of knowledge!

    So for a 4A (at P1 power rating of pump) the AC3 rating will be in the order of 20A to 25A. What will the thermal protection unit as fitted to the contactor be rated at?

    This will obviously then be protected by a dual pole D-curve circuit breaker at which current rating? Just for interest sake, the pump manufacturer specifies a requirement of a dual pole circuit breaker with an air gap of 3mm or more between contacts.

    I am learning a lot. Interesting!

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