Contactor overload

[Lucien Nunes]

The current will vary with the load but on a centrifugal pump the relationship might not be obvious. Unlike a positive-displacement pump where the motor has to work harder with increasing total head, many kinds of centrifugal actually decrease in power consumption with increased head / throttled flow because of the fluid dynamics inside the impeller. Pumps of this type can overload their motors by letting them pump too freely e.g. full bore against zero head, but will not be overloaded even when the flow is completely shut off. The motor itself will always draw more current with increasing shaft load though.

 

[copey84]

Hi lucien, the motor I have has two capacitors so I assume it is a cap start cap run type, does this type use a centrifugal switch to change between windings? Also I just realised that when I ran the motor and it was showing 6amps I had the outgoing pipe disconnected, wanted to flush out pump at the time perhaps that's why the current dropped.

 

[copey84]

Hi all, will keep this short as my last post deleted itself when tried to send, very annoying.
Anyway motor still tripping clamped it this morning and it is now pulling 7.5amps, set the overload to 10% higher and it tripped 20min later.
Done all I can getting service engineer out on monday, just like to know when setting a overload is it best to clamp motor and see exactly how much it is pulling, then set overload or should I set 10% higher than current rating stamped on side of motor which is 9.5a?

 

[Darkwood]

Hi all, will keep this short as my last post deleted itself when tried to send, very annoying.
Anyway motor still tripping clamped it this morning and it is now pulling 7.5amps, set the overload to 10% higher and it tripped 20min later.
Done all I can getting service engineer out on monday, just like to know when setting a overload is it best to clamp motor and see exactly how much it is pulling, then set overload or should I set 10% higher than current rating stamped on side of motor which is 9.5a?

You shouldn't set the O/L higher full load current rating on the plate although in certain set-ups you can set it lower by knowing the loading of the motor as this can pre-empt issues.

Two things here, if you have had a fault current then it may have damaged the O/L making it unreliable also if the motor is cycling on and off in a short period then it may heat up the O/L and cause it to trip. Without clamping the motor or testing it etc its really just guess work from our position.

 

[copey84]

Hi darkwood thanks for answering my question, always thought it best to set a motor overload to its measured current. Have done a few tests and all appears ok, doesn't seem to be anything obvious will leave it for service guys on monday.