When wiring a single phase starter, they tend to jumper the overload such that all phases of the overload are used. Now, considering that the overload has 3 separate bimetal strips all connected to the same tripping mechanism, it shouldn't matter if you only connect 2 or even just one phase through that overload.

As I was reading up on this, I saw the terms "phase balance" come up a lot. This is understandable, but is this a feature of all thermal overload relays? Will they all trip if current isn't balanced through all phases? If not, then why can't we just wire one phase through the overload relay, in the case where we only have a 3 pole overload, that is.

enter image description here


Thermal overload relays are sensitive to the heating due to current flowing in the heater elements. The thermal sensing element in a three-phase overload is calibrated to trip based on the current flowing in all three heaters. If only two heaters are used, the overload will have a lower temperature for a given current, so the trip level will be higher than it should be. If the relay is used for a single phase motor the current must flow through all three heaters for the sensing element to trip at the proper current.


This actually is only an issue for IEC style bi-metal OL relays that have a feature called "differential biasing" that is used to change the trip point if there is a phase loss on a running 3 phase motor. The mechanism is basically a balancing spring on the trip bar that is looking for force being exerted against it from all three bi-metal strips equally. When one strip is not bending under load, it no longer exerts its force against the spring, so the spring moves the trip bar closer, resulting in the OL relay requiring LESS excess current in order to trip. The reasoning behind this is that if you have a 3 phase motor and lose a phase while it is running*, the motor will keep spinning, but the lost phase will create negative sequence current in the rotor, which can cause it to overheat disproportionately to the stator current. So in order to avoid damaging the motor, the trip point of the OL relay is biased, by this mechanism, to trip earlier.

If however you HAVE a single phase system, that differential biasing mechanism will not "know" that this is a normal situation and act to cause nuisance tripping. So you run the current through all three poles in order to have them all exert the same force resistance against that spring.

Eutectic melting alloy OL heaters and NEMA (North America) design bi-metal OL relays never had this feature, so single phase operation has no effect on them therefore you do not need to run current through all 3 poles on those types. Most Solid State OL relays will have a setting or switch that allows single phase operation however.

  • If there is a single phase condition on a 3 phase motor BEFORE it tries to start, it will not start.

Graphic depiction of the differential biasing mechanism; enter image description here

  • \$\begingroup\$ The eutectic overloads trip points are influenced by ambient temperature and by heat from adjacent elements. The heat from adjacent elements may not be a big issue, but I believe use of all three elements is recommended by some people. \$\endgroup\$ – Charles Cowie Oct 17 at 1:03

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