Thread: Is a motor start relay switch needed for a 3ph 2.8kw pump motor?

I don't kow sorry. But why not have the relay in the shed? or is that not allowed?

Stuart

Good question Stu! Not sure on the rules on that one. The problem is reversed if I'm working on the pump ( not often, after floods or when their is air in the line) .. I have to start it at the workshop and walk 5mins to the pump., stop it there, adjust and walk back up to start it again.... I like the idea though... Will have a think about that one!

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it pays to switch all 3ph motors via contactor...switch's being mechanical operation can work fast or slow depending on how quick one wants to be...the slower the more risk of damage to switch thru arcing of contacts...where as a contactor is always fast..well at least it should be else its stuffed

fit new stop start contactor at switchboard and replace old contactor at pump with isolating switch if none there.

By relay do you mean a just a simple "no-volt switch " or a full on "3P starter with excess current protection"?

If the latter, one setup I've seen on a situation like this is a bypass switch on the starter at the convenient operating location and another switch at the motor.

Once everything is honky dory the bypass is switched off and motor switch left on.

Excess current protection is pretty handy for things that get turned on-off often and operate unattended.

It is only drawing 2.2 amps per phase (no doubt less when running), although no doubt more on startup..

Depending on how many times you will be flicking the switch, I would just install a 20 amp or 32 amp 3 phase switch/socket on the wall and use that...

Eskimo, is on the money. ( as always )

You can have the Start/Stop wherever you like, at the switch board if that's what you want. Then install an isolating switch at the pump end, so you can isolate it without having to walk all the way back to the other end.

Depending on the cost of a replacement pump and motor, you can get very basic pump protection, with a current monitoring relay that will detect loss of prime, by detecting that the current is too low, or a motor overload with current too high and shut down.

Originally Posted by eskimo

switch's being mechanical operation can work fast or slow depending on how quick one wants to be...the slower the more risk of damage to switch thru arcing of contacts...where as a contactor is always fast..well at least it should be else its stuffed

I thought most switch gear was designed in such a way as to remove the slow opening slow closing. i.e. once the contacts start to move its spring pressure doing the work.
or is it just that contactors are "even quicker"?

Stuart

I knew the brains trust would guide me on a better path!

I will look into it a bit more. If the relay fails between now and then, I will just bypass it.
It only gets turned on less than 20 times a year and runs for about 12 hrs each time.

Loss of prime can ruin a pump pretty fast...will look into it some more!

Thanks fellas!

A 3 phase starter has Thermal Overload Protection. The TOL drops the contactor out if the motor draws too much current. If the contactor is welded closed then the TOL protection will not work.
Install a new 3 phase starter near the switchboard and as has been suggested an isolating switch at the pump. Just plugging it in to an outlet or using only a switch means that if the pump gets something caught in the impeller or a bearing heads south then the motor will continue to draw excess current until it burns out.

Originally Posted by Stustoys

I thought most switch gear was designed in such a way as to remove the slow opening slow closing. i.e. once the contacts start to move its spring pressure doing the work.
or is it just that contactors are "even quicker"?

Stuart

that is supposedly the way they are made but for some reason you can manipulate it to switch a lot slower
My experience has been that if you dont tell an apprentice to manually operate the switch without hesitation he will get arcing....stuffing something up...definitely more noticeable on larger motors...even i have done it...not stuff anything up but the audible noise from the isolator is unmistakable

Originally Posted by Stustoys

I thought most switch gear was designed in such a way as to remove the slow opening slow closing. i.e. once the contacts start to move its spring pressure doing the work.
or is it just that contactors are "even quicker"?

Yes, but the other advantage of contacts is that they can be set up to have a no volt release as well as delayed or remote starting and even via PLC - so that you can run a low (or lower) voltage cable to control something and several hundred amps of start up current are not running through the switch in your hand (we have a motor at work with a steady state draw of around 230A - start up is around 850A).
I was 'mildly surprised' to discover when I first saw it that the control circuit for my lathe contactors runs at 415V...

As Ian says, isolator by the pump and remote start at the switchboard.

Michael

Originally Posted by Michael G

I was 'mildly surprised' to discover when I first saw it that the control circuit for my lathe contactors runs at 415V...

l

they generally do that to save wiring in a nuetral

Brains trust has done it again!

The pump is $1400 and not worth skimping on..

Will put a relay switch with a thermal overload at the board. Isolator is already next to the pump but might swap it over....it's a tired looking switch.

I was wanting to experement with a psi gauge plumbed in to the water lines to see if I can monotor the tank volume when the pump is off. I might be able to see if the pump needs priming and when it's pumping well... I could not find any info on the net about this yet. Must be looking in the wrong places.

Stu, you were on the money from the first post!

Originally Posted by Ben Dono

Brains trust has done it again!

The pump is $1400 and not worth skimping on..

Will put a relay switch with a thermal overload at the board. Isolator is already next to the pump but might swap it over....it's a tired looking switch.

I was wanting to experement with a psi gauge plumbed in to the water lines to see if I can monotor the tank volume when the pump is off. I might be able to see if the pump needs priming and when it's pumping well... I could not find any info on the net about this yet. Must be looking in the wrong places.

Stu, you were on the money from the first post!

a psi gauge wont tell tank volume...a ft head gauge will give some indication tho
why not a simple float to a small indicator...lots of ideas on the net