Contactor overload

[copey84]

Hi all, had a blocked pump that caused a overload on motor, ocpd went before motor overload. Should the motor overload always discriminate with ocpd and operate first?
The motor overload was set to auto but I don't think it ever operated under fault. Motor draws 6amps under load and the motor overload is set to 7amps, prob should go to 6.5amps but 7 is lowest available.
Tried the manual trip and it works fine, so dont think it's faulty. Think it comes down to time current graph difference, not sure,any advice greatly appreciated.

 

[David M]

What type & rating is the ocpd?

 

[Marvo]

With a B curve OCPD it might be faster than the thermal overload under locked rotor conditions especially if it's a single phase motor, you'd need to look at the manufacturers published curves. Not sure why the thermal would be set to auto... Which order they operate in may or may not be an issue, depends on that specific installation but usually the OCPD is sized to protect the supply circuit which is normally designed to be large enough to handle the starting current without excessive volt drop so generally the motor thermal overload will win the race unless it's a direct short that causes instantaneous tripping with the mag side of the MCB.

 

[Darkwood]

What is the condition of the motor now... did the ocpd trip through a short as apposed to an overload?
Dependant on the control circuit config' a short can weld the overload contacts closed thus the ocpd takes over.. this can be common where the coil is 400v or 230v and tapped of the motor supply through the start/stop circuit...

 

[copey84]

It's a 15amp rewirable fuse, wired in 2.5mm2 singles. do you think the manual reset would be best, does the auto just close the circuit again once the fault has cleared?

 

[David M]

Assuming that the re-wireable was wired using 15amp fuse wire then it is highly unlikey that it would operate before the motor overload, if an overload fault occurred.
So my guess is, its either a short circuit fault or the motor overload is not operating to its set rating.

 

[copey84]

Motors working fine now. Think it was run dry and there was a blockage, all clear now.
Did ir tests and measured the resistance of windings all good. Also flushed out pipes before connecting everything back up, was working fine this evening.

 

[Marvo]

^^ As per David. Also if it's a centrifugal pump a blockage can actually cause reduced run current, not overload, depending on the individual pump.

 

[copey84]

Hi there David m was thinking the overload should have went first, the contactor is by C&S electrics there an Indian company, there was poor instructions with overload, asummed the auto setting had to be pushed in, anyone familiar with this type of overload?

 

[copey84]

Hi Marvo, the motor has two capacitors so I think it has a centrifugal switch, had noticed a drop of in current of about 1.5amps

 

[Marvo]

.....the contactor is by C&S electrics there an Indian company, there was poor instructions with overload.....

C&S are from the Indian sub continent, I've got first hand experience of there products. Their contactors are okay but I don't like their thermal overloads, I've had more than one that I've bench tested and they've been operating far outside of their published curve.

Here's some info I have immediately available. I've got more on my office server, if I've got time to dig it out tomorrow I will.

View attachment TC Overloads Technical Data.pdf

 

[copey84]

Thanks Marvo, can't access link on tablet, will have to use the laptop.
Can you remember does the auto setting on there overloads need to be pushed in or remain out?
Also is there any other reason to select auto reset other than convenience?

 

[Lucien Nunes]

do you think the manual reset would be best, does the auto just close the circuit again once the fault has cleared?

Probably, but depends on the circumstances. Auto reset will re-energise the contactor as soon as the overload heaters have cooled down, so if the likely causes of a stalled motor are ones that need manual intervention to correct, then manual reset is much better as the motor will otherwise experience repeated overload conditions until something breaks or someone arrives to sort it. OTOH if the typical cause of an overload is only temporary, such as a compressor trying to restart against backpressure, then auto-reset will get it back in action without a service callout. Single-phase capacitor start motors are vulnerable on auto-reset because the start cap can overheat with repeated prolonged start attempts.

It does sound like the overload relay wasn't responding correctly there, unless the fuse had the wrong wire in or had been weakened in the past.

 

[copey84]

Darkwood it's a 230v coil taped off as you said. Contacts are ok, open and closing on test trip. Just not sure if the auto setting is active when button is in or out.

 

[copey84]

Thanks Lucien, will change to manual reset. Fuse wire was the correct 15a type, think your right that the overload was not active.
Will sort it tomorrow thanks again.

 

[davesparks]

It sounds like it's got the wrong overload relay anyway if its lowest setting is 7A and for your motor it should be 6A.


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[Deleted member 72666]

Assuming that the re-wireable was wired using 15amp fuse wire then it is highly unlikey that it would operate before the motor overload, if an overload fault occurred.
So my guess is, its either a short circuit fault or the motor overload is not operating to its set rating.

Unless it had been weakened over time from the higher start currents.

 

[copey84]

Hi all, motor started tripping again today this time it was the contactor overload. Think it has kicked in now because I have had to lower the overload rating due to the current demand of motor decreasing, think Marvo mentioned last night that this can happen not sure exactly how though.
Could anyone explain why a capacitor motor would decrease in current demand, think the abuse it has taken has done damage just like to know if I can fix it.

 

[Marvo]

Head pressure of the pump will have an effect on motor run current. What type of pump is it? Make and model number? What is the pump doing ie what application?

 

[copey84]

Hi Marvo don't have the pump details now, it's used to feed sprinkler system in glasshouses. It has 2.5 to 3 inch pipe coming in from tank actual pump is around 18 inches tall by around 10 inches wide, outgoing pipe is around 2.5 to 3 inches. Think it is a basic pump assembly nothing fancy.
Seen plate on motor today with 9.5a run current, never seen motor draw this much current only 7.5amps then down to 6.
Owner thinks pump is ok, not sure what else could explain current changes in motor though, any idea?
The motor has two caps and two windings, one has a resistance of over 3 ohms the other is over 5ohm, seems ok.
Hope this was of use.