Electronic Overloads

[Tim Ganz]

When using overloads I am familiar with the old heater type and those would open the overloads contact points and prevent power flow if I remember right? I was always taught that the neutral of the contactor coil should run through the overload aux contacts so the contactor coil will drop out when the overload kicks.

Now here on a new job they have the electronic type of overload and the other day one was found where a motor winding opened and did not trip the overload but tripped stuff upstream.

The guy here found that the contactor coil cam from the plc and did not go through the overload aux contacts but the aux contacts are a loop to a plc input that shows the overload tripped on the HMI screen.

Our electrician said that on electronic overloads that when they trip they don't break power on the 3 power leads they just open the aux contacts and that if the neutral or power wire for the coil for the contactor does not go through those aux contacts then the motor will never stop.

Is this true? Do electronic overloads not break 3 phase power in themselves? They only change the aux contact state?

 

[rdrast]

They don't break the aux contact either, they open (in the event of an NC arrangement) the overload contact.

I always run the aux contact, and the overload contact back to the PLC, and wire the coil directly to an output.

If the OL trips, the coil drops out, and flags an alarm.
If the OL is good, and the coil is energized, and the AUX doesn't pick up, I flag another alarm of "Contactor Failure".

It also depends on exactly what kind of overload you are talking about. MSP's (Motor Starter Protectors) typically will actually interrupt the 3 phase power, and are often used despite the cost since they can also be used as manual lockouts. They also have an actual OL (Trip) contact, and an optional AUX contact.

 

[Tim Ganz]

So the electronic overloads do or don't physically break the 480 to the motor?

 

[bce123]

no they dont and you should break[open] your neutral to the coil with this. Then run another aux contact to your plc for the alarm . the old style did not break the thre phase power ether it.s just when the aux on the overload trips it drops the coil out thus breaking the 3 phase. sometimes i see unqulifed personal reseting these old style as you call them overloads to the point where the overload or the motor burns up if you bring another aux into the plc you can write code to prevent the number of times this can be done before you lock it out. there are lots of ways to use the overload input to the plc.

 

[iant]

Traditionally the old type overloads were for 415V (Aus. 3 Phase)control systems. the design tripped one side of the contactor coil.
As the coil voltage was reduced the trip contact remained in series with the coil. The Neutral is commonly switched, however it is arguably not good practice either. If the coil goes down to earth it can bypass the neutral o/load trip.

before the new electronic range there was two O/L auxiliaries.
1 N/O and 1 N/C


Electronic overloads..
these have many options and additional contacts available.
At the end of the day the coil should always be switched by the overload set. other auxiliary contacts should be utilized into the PLC.
some overload sets have a tripped contact for indication.

 

[milldrone]

The Neutral is commonly switched, however it is arguably not good practice either.

I believe the reason for switching the neutral was so that a single contact could be used for reversing starters.

 

[iant]

I do agree Vaughn, but still it was never good practice.
In remote start/Stop FWD/REV applications - Yes.
But in a single enclosure - Why not do it right