do you need overload protection with a DOL

[Spectric]

Hi

This diagram shows what I would call a std DOL, but I can see how your MEM DOL functions without the need for an auxilary contact block to latch the contactor. My choice of starters has mainly been Moeller, so nice to see how other OEMs do things differently.
Your DOL uses the START contact as the latch, START button temporarily links it out to energise the coil which pulls in the main contacts and closes contacts 13 & 14 which maintain coil voltage when START button is released. The STOP/OL simply breaks the supply to the coil and the contactor drops out. For a 230 volt coil the N connection needs to be linked to neutral. I cannot see any reason for the "CONNECT" cable link, why is the load not just connected on 2 & 4 , ie opposite side of the Live, Neutral supply. So once you get the OL unit it should all be plain sailing.

 

[guineafowl21]

I cannot see any reason for the "CONNECT" cable link, why is the load not just connected on 2 & 4

The idea is to ensure the load runs through all three overload elements, allowing some kind of balance. I’ve never been quite convinced, and there’s no need here as there’s no overload.

 

[--Tom--]

Running it through all three was my understanding too, overload is ordered so should all follow as per the diagram now.

thanks for the help all

 

[Spectric]

The idea is to ensure the load runs through all three overload elements, allowing some kind of balance. I’ve never been quite convinced, and there’s no need here as there’s no overload.

When you dig deeper the overload device in three phase operation can also detect loss of phase, so you have to assume that in single phase it needs current in all three elements to prevent spurious tripping, new one on me because when loss of phase was important I ould use a phase detection relay in the Stop line.

 

[NetBlindPaul]

You should not calculate the motor current, you should read the current from the motor rating plate for correct selection of the overload.
I would recommend that you use all 3 main poles on the contactor by doubling back the live or the neutral through the 3rd pole of the contactor.
13 & 14 are the identification numbers for the first pair of auxiliary contacts on a contactor in accordance with IEC standards. So effectively the contactor has the auxiliary built in. Its a 4 pole but with only 3 poles which are current carrying.
Generally overloads detect phase loss by increasing current in the remaining phases.
One of the failings of the thermal overload is that it will often fail to detect phase loss on a running motor under light load, which ventilation fans and similar often are which is unfortunate!