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