3 phase contactor with overload

[Akanami]

Hi all,

Could some one explain the correct way in connecting a 3phase contactor with overload protection.

I have the supply going into l1, l2, l3 and motor connected to the load terminals, t1, t2, t3. I also have the 230v connected to the a1 coil and neutral on a2. Just a bit confused with over load connections. I'm pretty sure it has to be on normally closed.

Thanks

 

[Darkwood]

Need more info is it a stand alone unit or contactor mounted O/L ...
Your supply to your start stop control circuit needs to run through the n/c of the overload first if its a contactor mounted unit thus when activated it cuts power to your control and start button remains isolated until reset is done this stops a start button pulling the contactor in when pressed but not retaining it while O/L is in trip.
N/O is often utilised as a warning contact like a trip light or buzzer option as it closes on trip.

 

[pepparz]

The correct way is to mount the overload unit to the underside of the contactor with the 3 phases passing through both units to the motor. The control side is wired before the contactor coil usually in terminals 95 and 96 which are normally closed and the overload set to no more than 125% of the full load current written on the motor name plate.

 

[Darkwood]

The correct way is to mount the overload unit to the underside of the contactor with the 3 phases passing through both units to the motor. The control side is wired before the contactor coil usually in terminals 95 and 96 which are normally closed and the overload set to no more than 125% of the full load current written on the motor name plate.

125%? --- this is always a bone of contention on this forum, we all have our habits but we should be explaining the O/L is set to the motor duty +20% and if the motor is the correct size for the job it should still leave it set below the FLC of the motor concerned (if in doubt copy the motor FLC rating), this has then the added benefit of seeing other problems like bearing issues long before the motor is stressed. 125% can see a motor in use with a long term overload without tripping this can damage the windings over time. I would never suggest over the FLC of the motor on a forum no matter what personal practices were on site.

I can't count the number of motors ive had rewound or replaced because the O/L was too high by even as little as 15% ... any tolerances the motor may have are usually in the O/L design and yes some motors may be good for a 25% overload long term but its not a blanket rule of thumb you can apply to all.

 

[pepparz]

motor duty + 20% = 120%, but in an ideal world just set to FLC of motor unless its star/delta then set to 58% of FLC on name plate.

 

[Darkwood]

When i said motor duty i meant its actual working load not what its capable of -
So if motor was loaded to 60% then set overload to 80% obviously the nature of the load needs to taken into account, the old teachings did use rules of thumb of 10-20% or other values dependent where you were taught but its now taught that you set to FLC of the motor and no more which IMHO is how it should be done... if a motor is running at 110% then there is something wrong with the set-up be it incorrect specified motor or other issues, as i said in my job i set the motors FLC to 20% over its running load and if i get a call out that its tripped i increase it to FLC of motor plate to get it running again this limits down-time and schedule a maintenance at a convenient time. If id have set the O/L at 125% the issue would have needed to be addressed and rectified their and then at a much higher cost.

 

[pepparz]

can u please explain how to work out exactly how much load it will take in order to set the set point lower than the flc, and how mechanical load (( bearing wear ) increasing with time) will effect the machines current usage?

 

[snowhead]

can u please explain how to work out exactly how much load it will take in order to set the set point lower than the flc,

Use a clamp ammeter then set the overload as required.

and how mechanical load (( bearing wear ) increasing with time) will effect the machines current usage?

It won't have enough effect to lift the current to +20% of normal (not plated FLC) current until the bearings fail

 

[pepparz]

hmmm ok, so what do you do when you cant measure the load current after installation? or when the motor is on a agitator stirring a liquid of greatly differing viscosity? buy a house near the installation? ask the client to take ammeter readings and alter the overload themselves? i think not. this is why u set the motor to FLC for DOL 58% for star/delta.
#epicfail.

 

[Darkwood]

As snowhead has politely answered you can use the O/L setting as an early warning that can be quickly reset and increased to keep production running giving time for a scheduled less costly maintenance/service. This is a set-up that can't be applied across the board its dependent on load characteristics + sometimes the motors can be oversized for work load and can if allowed to damage the machinery if its current setting isn't limited. I had a bloke alter my O/L setting on a panel i built because it had tripped twice so he ramped it up to full approx 120% and started giving it large that i had set it up wrong... 2 days later the bearing seized shattered the card roll moved 10mm and caused 15k of damage, lucky for e this full of himself bodgit sparky had slagged me off to everyone and cleared my name in the process for liability because he changed a crucial setting.... it would have been evident had they called me out to the O/L trip that something was a miss.

 

[snowhead]

hmmm ok, so what do you do when you cant measure the load current after installation? or when the motor is on a agitator stirring a liquid of greatly differing viscosity? buy a house near the installation? ask the client to take ammeter readings and alter the overload themselves? i think not. this is why u set the motor to FLC for DOL 58% for star/delta.
#epicfail.

You do it at commissioning, which would involve watching a process through it's whole cycle. (even more epic fail)??

Pepparz, You need to move out of Utopia and into the real World,

 

[pepparz]

so you watch the process through all its differnt batches? how long do you do this for? what about new factories where a manufactoring process hasnt started?
"early warning" ha do it once and do it right.

 

[pepparz]

any way who gives a ****. think you two need to get a room.lol

 

[Darkwood]

hmmm ok, so what do you do when you cant measure the load current after installation? or when the motor is on a agitator stirring a liquid of greatly differing viscosity? buy a house near the installation? ask the client to take ammeter readings and alter the overload themselves? i think not. this is why u set the motor to FLC for DOL 58% for star/delta.
#epicfail.

I did say this isn't suitable for all set-ups that was made clear i merely commented that 125% is not the way i work and it does cause a strong point for debate, where current will be dependent on the product load or speed etc then the current monitoring will be done with a fully loaded machine duty before setting up as such.

You kind of contradict yourself you say you would do a DOL at 125% yet mention star/delta at 58%..The currents through the winding are 1/root 3 (58%) of the current in the line on a star/delta set up so here the 58% reflects the true plated FLC of the motor without any extra % added. So you add 25% to DOL overloads yet stay with 58% on a Star/Delta 6wire arrangement ... why not approx 72% thus accounting for the 25% you add to you DOL O/L?

 

[snowhead]

so you watch the process through all its differnt batches? how long do you do this for? what about new factories where a manufactoring process hasnt started?
"early warning" ha do it once and do it right.

So do you think manufacturers supply plant worth millions and let the buyer just switch it on?

Commissioning can take days or even weeks on some plant