3-phase motor tripping circuit breaker
Discussion
Can anyone advise as to whether my circuit breaker too sensitive for the well pump motor or is the problem possibly elsewhere?
Motor: 3-phase 3kW
Circuit breaker: 10amp (6kA nominal cut-off short-circuit power)
Backstory: The old motor started running rough and tripping an oversized 65kA circuit protector. I've replaced the motor with a freshly rewound exchange unit and replaced the old bakelite 10amp fuses with the 3-pole circuit breaker above, removing the now redundant 65kA protector. Pressure switch/relay connections all cleaned up and no obvious issues with that. It's all new cabling between circuit breaker and motor.
NB Given that the old circuit protector label suggests it's suitable for a 15kW motor, I very much doubt it was chosen based on any calculated requirement, but more likely someone had one lying around and just used that.
The problem: there's a 40/60 chance that the motor trips the new circuit breaker when it fires up (but never trips once running).
I'm struggling to find any kind of guidance as to what the initial power surge is likely to be and what the short-circuit cut-off rating needs to be. I could replace the circuit breaker with a 10amp / 10kA but that seems about as high as the short-circuit cut-off goes for that size. Would it likely be enough?
Or would a soft start controller help reduce the initial current?
Or is there a chance that a fault in the wiring before the circuit breaker be causing an issue?
All educated guesses welcome. I'm starting to get bored of having it trip mid-shower and having to go outside to reset it when it's -10 outside and snowing. It gets a bit nippy in just a towel and I'd rather keep the timing of any cold-therapy to my terms and not the well pump's!
Motor: 3-phase 3kW
Circuit breaker: 10amp (6kA nominal cut-off short-circuit power)
Backstory: The old motor started running rough and tripping an oversized 65kA circuit protector. I've replaced the motor with a freshly rewound exchange unit and replaced the old bakelite 10amp fuses with the 3-pole circuit breaker above, removing the now redundant 65kA protector. Pressure switch/relay connections all cleaned up and no obvious issues with that. It's all new cabling between circuit breaker and motor.
NB Given that the old circuit protector label suggests it's suitable for a 15kW motor, I very much doubt it was chosen based on any calculated requirement, but more likely someone had one lying around and just used that.
The problem: there's a 40/60 chance that the motor trips the new circuit breaker when it fires up (but never trips once running).
I'm struggling to find any kind of guidance as to what the initial power surge is likely to be and what the short-circuit cut-off rating needs to be. I could replace the circuit breaker with a 10amp / 10kA but that seems about as high as the short-circuit cut-off goes for that size. Would it likely be enough?
Or would a soft start controller help reduce the initial current?
Or is there a chance that a fault in the wiring before the circuit breaker be causing an issue?
All educated guesses welcome. I'm starting to get bored of having it trip mid-shower and having to go outside to reset it when it's -10 outside and snowing. It gets a bit nippy in just a towel and I'd rather keep the timing of any cold-therapy to my terms and not the well pump's!
My main question is, how does it start, ideally its star delta rather than direct on line, and it definitely needs a D curve breaker if it’s the latter.
The starter should have an overload, which is the primary protection to the motor with the MCB suitable for protecting the cable size, which is hopefully 2.5 and ten you can upgrade to a 10A type D. 6A is too tight as depending on various local factors you could be drawing 4-6amps at full load, and while starting direct on line, you can be 5 to 8 times that.
Although if not qualified, you shouldn’t really be monkeying about with three phase kit.
Personally, to avoid pipe shock and to minimise strain on the system and save energy, I’d be fitting a variable speed inverter driven set up.
The starter should have an overload, which is the primary protection to the motor with the MCB suitable for protecting the cable size, which is hopefully 2.5 and ten you can upgrade to a 10A type D. 6A is too tight as depending on various local factors you could be drawing 4-6amps at full load, and while starting direct on line, you can be 5 to 8 times that.
Although if not qualified, you shouldn’t really be monkeying about with three phase kit.
Personally, to avoid pipe shock and to minimise strain on the system and save energy, I’d be fitting a variable speed inverter driven set up.
Edited by Rough101 on Sunday 5th January 09:19
Thanks chaps
Should've seen the state of the electrics when we moved in: cracked and broken cable insulation, exposed terminals everywhere, broken sockets, broken earth circuit, and a DIY pressure tank with welds that had a penchant for popping and spraying water over everything. An arrangement best described as "exciting".
The motor is supposed to be star delta.
Curve information is not exactly forthcoming, but judging by the part number it looks like it's probably a B curve on that MCB so I guess I goofed on that. I'll look for a 10kA D-curve instead.
Rough101 said:
Although if not qualified, you shouldn’t really be monkeying about with three phase kit.
Yeah, I'd rather not be. However, finding an electrician around here that's actually sober... definitely not in Kansas anymore. Should've seen the state of the electrics when we moved in: cracked and broken cable insulation, exposed terminals everywhere, broken sockets, broken earth circuit, and a DIY pressure tank with welds that had a penchant for popping and spraying water over everything. An arrangement best described as "exciting".
The motor is supposed to be star delta.
Curve information is not exactly forthcoming, but judging by the part number it looks like it's probably a B curve on that MCB so I guess I goofed on that. I'll look for a 10kA D-curve instead.
Rough101 said:
Personally, to avoid pipe shock and to minimise strain on the system and save energy, I’d be fitting a variable speed inverter driven set up.
It definitely doesn't have a pump-friendly start to it which has forever annoyed me. I was thinking along the lines of fitting something like https://www.automation24.co.uk/soft-starter-schnei... in the circuit to reduce wear and tear. Suitable?It’s the starter that needs to be star delta, if it is it will start at low low torque, then it will clunk again as it changes over to the delta contactor and full power.
Forget about the kA rating, it’s no relevance here, that’s its withstand rating based in the prospective short circuit current of your supply, domestic ones usually 6KA, commercial 10kA, the higher the better.
If you’ve 2.5mm wiring I’d try a 10A type D MCB, which will probably be 10kA rated by default.
I’m assuming where you are is 400/230V, otherwise the above is all nonsense!
Soft starter needs careful selection, you’d either need the full motor spec sheet or have tong tested it working.
Forget about the kA rating, it’s no relevance here, that’s its withstand rating based in the prospective short circuit current of your supply, domestic ones usually 6KA, commercial 10kA, the higher the better.
If you’ve 2.5mm wiring I’d try a 10A type D MCB, which will probably be 10kA rated by default.
I’m assuming where you are is 400/230V, otherwise the above is all nonsense!
Soft starter needs careful selection, you’d either need the full motor spec sheet or have tong tested it working.
Rough101 said:
It’s the starter that needs to be star delta, if it is it will start at low low torque, then it will clunk again as it changes over to the delta contactor and full power.
It's definitely not doing that, so must be direct on line. Just a full power start as the pressure switch relay closes. Rough101 said:
If you’ve 2.5mm wiring I’d try a 10A type D MCB, which will probably be 10kA rated by default.
I’m assuming where you are is 400/230V, otherwise the above is all nonsense!
Yeah, all 2.5mm and 400/230V (sorry, should have said).I’m assuming where you are is 400/230V, otherwise the above is all nonsense!
Rough101 said:
Soft starter needs careful selection, you’d either need the full motor spec sheet or have tong tested it working.
Drat; can't see me being able to get much in the way of motor spec sheets. There are considerations that need to be taken before just swapping out different curve breakers, im not clever enough to explain it properly but the circuit needs to be designed so that it can handle the fault currents correctly.
That being said i dont think it needs to be on a star delta starter personally, its probably a case of trying a C type MCB out but taking the above into consideration.
Full load current is what about 6 amps?
That being said i dont think it needs to be on a star delta starter personally, its probably a case of trying a C type MCB out but taking the above into consideration.
Full load current is what about 6 amps?
Belle427 said:
Full load current is what about 6 amps?
That's about what my schoolboy maths worked it out as.Belle427 said:
There are considerations that need to be taken before just swapping out different curve breakers, im not clever enough to explain it properly but the circuit needs to be designed so that it can handle the fault currents correctly.
That being said i dont think it needs to be on a star delta starter personally, its probably a case of trying a C type MCB out but taking the above into consideration.
So far I've been content with a circuit that simply removes the guaranteed possibility of frying on touching the motor That being said i dont think it needs to be on a star delta starter personally, its probably a case of trying a C type MCB out but taking the above into consideration.
It would be nice to take it beyond the 17th edition "that'll do, it mostly works" electrical standard that this house was built to. We didn't have to buy Xmas lights the first year - the sparking sockets were all the twinkling fairy lights we needed
Belle427 said:
You can buy dedicated motor circuit breakers which incorporate both the short circuit and overload protection in one unit.
May be worth considering one.
Well who'd have thunk it. Looking online for one and thought "those look familiar". Had a rummage in the box of random crap in the barn and yep, there's an old Siemens 5.5 to 7amp variable motor circuit breaker in there. May be worth considering one.
Should have guessed - I mean, who doesn't have an old but functional 3-phase motor circuit breaker kicking around on the off chance it might be needed one day.
Swapped the MCB for it and so far, so good. I may be in for an uninterrupted shower tonight. Happy days
Thanks to all for the advice
jagnet said:
Belle427 said:
You can buy dedicated motor circuit breakers which incorporate both the short circuit and overload protection in one unit.
May be worth considering one.
Well who'd have thunk it. Looking online for one and thought "those look familiar". Had a rummage in the box of random crap in the barn and yep, there's an old Siemens 5.5 to 7amp variable motor circuit breaker in there. May be worth considering one.
Should have guessed - I mean, who doesn't have an old but functional 3-phase motor circuit breaker kicking around on the off chance it might be needed one day.
Swapped the MCB for it and so far, so good. I may be in for an uninterrupted shower tonight. Happy days
Thanks to all for the advice
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