Motor overload protection for SINGLE phase

Any panel I've built with contactors and overloads has been 3 phase.

I'm now quoting on one with some small single-phase loads. Definition of small, one is 0.5Amps FLC and the other 5 Amps FLC.

I've had a look at the Schneider LR97D which has a dip switch on it for single or 3 phase, but it comes with 3 legs. I've seen other discussions online whereby you use a 3 phase contactor but loop the single phase through it so that each contact is seeing the same amperage.

UK, 230V Live-Neutral.

Any other options?

the_msp said:

Any panel I've built with contactors and overloads has been 3 phase.

I'm now quoting on one with some small single-phase loads. Definition of small, one is 0.5Amps FLC and the other 5 Amps FLC.

I've had a look at the Schneider LR97D which has a dip switch on it for single or 3 phase, but it comes with 3 legs. I've seen other discussions online whereby you use a 3 phase contactor but loop the single phase through it so that each contact is seeing the same amperage.

UK, 230V Live-Neutral.

Any other options?

Click to expand...

Doing the loop through the contactor/OL phases is how I've always seen it done. I've also done it on a couple personal projects successfully.

Most documents I have seen usually have a wiring diagram for single phase motors showing looped. If you are looking to save space, the Siemens SIRUS 3RM1 motors starters have overload protection and can save some serious space in a cabinet. They are 3 phase but here is an example of how they show to wire for single phase.

Snyd1 said:

Most documents I have seen usually have a wiring diagram for single phase motors showing looped. If you are looking to save space, the Siemens SIRUS 3RM1 motors starters have overload protection and can save some serious space in a cabinet. They are 3 phase but here is an example of how they show to wire for single phase.

Click to expand...

Your diagram is done a lot in the field, but I've never seen the 2nd phase wired reverse like your diagram shows.


Always looped to the top of the overload (or contactor/overload pair) and have the electricity flow downwards on all 3 contacts.


EDIT: Also it's quite common for the L3-T3 line to have the Neutral run through it and the hot looped through L1-T1 - L2-T2

another example from rockwell

Here is a FAQ specific to Schneider for the Tesys D series no pictures though only words

https://www.se.com/ca/en/faqs/FA117906/

Three phase overloads have a function that checks for phase loss, so if you only put current through two legs, it trips. The 'normal' way of doing it in the UK is to loop the Line side through two legs and the Neutral through on the third leg. It works, no need to reinvent the wheel
For very small motors it can sometimes be that the cost of the overload is greater than the cost of replacing a motor, so sometimes those are just fused/mcb'd to protect against short circuits. It depends on how important the motor is to the function of the machine and how easy it is to replace.

Before you go down that road further, make sure it is necessary. A lot of small single phase motors have built-in thermal OL protection inside of the motor. An external OL relay may be unnecessary.

It is totally normal and the correct method to loop one phase through 2 contacts when you have a 2-phase motor.

BryanG said:

Three phase overloads have a function that checks for phase loss, so if you only put current through two legs, it trips.

Click to expand...

That is called "differential protection". It doesn't trip immediately when a phase is missing. The trip curve is slightly lower so that it will trip at a lower current value than if all 3 phases are present.

jraef said:

Before you go down that road further, make sure it is necessary. A lot of small single phase motors have built-in thermal OL protection inside of the motor. An external OL relay may be unnecessary.

Click to expand...

Do you mean that the overload protection is all internal, i.e. it cuts supply directly ? Or it is a potential-free N.C. that must be connected in series with the starting contactor ?
In the 1st case without a signal back to the motor starter, the control system will not know that the motor is not running which may be serious drawback.
In the 2nd case you need an additional cable, which may be a greater hassle than just using a normal overload.
OLs are cheap.

Agree with Jesper, TH O/L in the motor does not protect over current, examples are water in terminal box or a damaged cable so a trip before cable & motor is essential

I've definitely used the method of looping single phase through a 3-pole device. This also works well for DC devices, like a large DC motor on a wire draw machine for instance. Until one can install the proper breaker or device.

jraef said:

Before you go down that road further, make sure it is necessary. A lot of small single phase motors have built-in thermal OL protection inside of the motor. An external OL relay may be unnecessary.

Click to expand...

I put them in when I want the PLC to know if the OL tripped.


If it's simply an internal protection that stops the motor the control wouldn't know the motor wasn't running anymore - or until it cooled down and restarted.



If it's a required process (a spray nozzle washing residue off a part so the next tank process isn't contaminated) then keeping the line running could be costly and create scrap.

Thanks all. It's in the RFQ spec for contactor and overload, despite that yes, that costs more than the motor.

Ok. So if you buy a traditional NEMA design starter where the OL relay has replaceable heater elements it does NOT need to have power through all 3 phases. That issue only applies to IEC design overload relays. But a number of mfrs have taken to abandoning their older NEMA style OL relays because of changes in international rules on the materials used to make them, called “RoHS” (usually pronounced “Rohas”) that eliminated the use of lead solder. Those rules don’t apply to equipment being used in North America yet, so you can still buy the older NEMA type, just make sure it’s the type with replaceable heater elements instead of dual adjustable ones.

jraef said:

called “RoHS” (usually pronounced “Rohas”) that eliminated the use of lead solder. Those rules don’t apply to equipment being used in North America yet,

Click to expand...

I worked in a circuit board shop that had many US customers specifying RoHS product, and not for export reasons, so it is coming here for no valid reason.


If you think about how much lead is in the solder on one circuit board, how many circuit boards could possibly be scrapped somewhere and how much lead would seep out of all of those there would be more lead naturally in the surrounding land.


Another rant: lead free solder on plumbing. The lead only leaches into stagnant water and would have to sit in a single pipe a few years to get a detectable level of lead and after all that time a person would have to drink gallons of it, then another house the next day.... Just to sell higher priced solder that doesn't work as good. Even the lead-free solder in the circuit board shop was difficult to work with, needed a higher temperature, harder to clean up after and created more waste that needed taken away and treated.