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Join Date: Feb 2017
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3 Phase Motor Starter Won't Stay Engaged. Details Inside.

02/20/2017 8:31 PM

Hi ladies and gents. First time posting. Here is the issue. I am diagnosing a piece of oilfiels equipment and its giving me hell.

Inside the cotrol box there is 3 phase 480 coming in to a motor starter and overload. These run the electrical motor which turns the centrifuge. THIS ALL WORKS AS IT SHOULD. From here there are jumper wires for L1 L2 L3 that go to a smaller motor starter and overload. Which runs the pump. The pump can only be turned on once the centrifuge is running. Here lies the issue. Once i have the machine running and go to engage the pump i press the start stop switch and the motor starter and motor engage and run. But as soon as you release the start button it disengages.

I have good continuity through all three phases.

All wires have good connection throughout the box

All fuses show good

I replaced starter and still does this.

Any help would be greatly appreciated

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Pathfinder Tags: 3phase contactor motor starter
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#1

Re: 3 phase motor starter wont stay engaged. Details inside.

02/20/2017 8:48 PM

If you have a Start-Stop button, it must be set up as what's called "3-wire control". That means there is a Normally Open auxiliary contact on the contactor that is wired such that when the contactor closes, that contact jumps around the Start button contacts to "seal in" the contactor coil. That way once the contactor coil is closed by the start button, you can release it and power continues to the coil of that contactor through its own aux contact. Without that, if you let go of the Start button, there is no power to the coil. So if you wired it yourself and didn't do this, that's why it's not working. If it was working and no longer is, most likely that aux contact is bad and needs replacement.

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#2
In reply to #1

Re: 3 phase motor starter wont stay engaged. Details inside.

02/20/2017 9:32 PM

I did not wire it originally. It does have an auxiliary contactor that seems to be working right. But as i dont have a replacement to swap it with at the moment im kinda stuck i guess.

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#5
In reply to #2

Re: 3 phase motor starter wont stay engaged. Details inside.

02/21/2017 2:45 AM

If you can't replace the faulty contacts, you could use another relay with the same coil voltage to perform the auxiliary function. The relay only needs to be a small one as it is only carrying coil current.

Connect the relay coil across the starter coil, and connect one of the relay's N/O contact sets across the start button.

Another trick if you're really stuck is to use one of the main contacts as the auxiliary - this must only be used as a short term fix to get you out of trouble as it will leave one leg of the motor alive even when the starter is off if the coil is across 2 phases but not so if the coil is phase to neutral.

To do this you MUST use the main contact that is on the same phase as the start button and you will likely need to reorient the start/stop buttons from the standard configuration so that the current goes first through the start button, then the stop button and then the coil as shown below. I haven't shown O/Ls etc in the circuit

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#3

Re: 3 phase motor starter wont stay engaged. Details inside.

02/20/2017 9:51 PM

It sounds like a (common/normally open) connection is missing/loose/lost from a relay that feeds the available power to it's own holding coil when the start/stop button is pushed and subsequently lost when the centrifuge looses power.

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#4

Re: 3 phase motor starter wont stay engaged. Details inside.

02/20/2017 10:17 PM

Trace the circuit, it sounds like you don't have a latching contact for the start control in the circuit and hence the motor only runs if you hold the start button down.

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#6

Re: 3 Phase Motor Starter Wont Stay Engaged. Details Inside.

02/21/2017 11:12 PM

As all others have said the holding/latching coil isn't energised. This usually entails a small guage wire that may be missing or broken. It is also possible that it is inserted too far into the terminal and the clamp is on the insulation. Maybe also put into wrong terminal. There should be a diagram inside the cover.

Jim

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#7

Re: 3 Phase Motor Starter Wont Stay Engaged. Details Inside.

02/22/2017 7:40 AM

Do you have a hold in contact on the contactor - you will also need a stop switch, NC contact which 'breaks ' the hold in line.

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#8

Re: 3 Phase Motor Starter Won't Stay Engaged. Details Inside.

02/22/2017 6:02 PM

Thank you for all the help. We just ended up swapping the control box so i can find the issue in the shop. Got an idea what the problem is but have to wait for parts

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#10
In reply to #8

Re: 3 Phase Motor Starter Won't Stay Engaged. Details Inside.

02/23/2017 4:55 PM

Would love a pic to pick.

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#9

Re: 3 Phase Motor Starter Won't Stay Engaged. Details Inside.

02/23/2017 2:07 PM

Have you checked the safety interlock circuit?

I had a similar situation on a centrifuge and the AmpTrap was blow on the phase that supplied the interlocks and they could run in local on the vfd but not remote. I also see that on other machines where the interlocks are series in with the overloads instead of the start/ stop switches.

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