Will 3 Phase Motor Overloads trip if a 3 Phase Motor runs Single Phase for a time

[Rob S.]

Good Morning ,

I should know this , but will a 3 Phase overload trip if a 3 Phase motor runs single phase for a long period of time ? Example , like a 5 hp motor. Full load amps is 7.6 ( 480 vac ) . The motor overload is set for 7.6 amps. A single phase condition happens , and phase C is missing , will Phases A and B , increase above 7.6 amps and trip the overload ?

Thanks so much . Have a great day.

 

[Rob...]

Yes most (not all) thermal overloads will have imbalance protection.

 

[Rson]

Yes most (not all) thermal overloads will have imbalance protection.

Agree. This is the reason when you use an overload or circuit breaker in single-phase you have to run the neutral or hot through two poles in order to prevent them from tripping.

 

[jraef]

Most IEC style OL relays have a "differential trip bar" in their design that skews the overload trip pickup point to be lower than normal in order to account for additional motor heating from negative sequence currents running in the rotor as a result of the single phase condition. But that does NOT mean it will trip. It is still dependent upon the actual load. So if your FLC is 7.6A and your OL relay is set for 7.6A, the "pick up point" of the OL relay will be somewhere in the neighborhood of 8.5 to 9.5A depending on the manufacturer's design*. If you get a single phase condition WHILE the motor is already running, that pick up point will be skewed down to approximately 60% of that value, so between 5.1 and 5.7A. If the actual LOAD on the motor is only 5A, the OL will never trip.

And this only applies to IEC style bimetallic over load relays. Older NEMA type OL relays do NOT have this feature.

Solid State OL relays often have a fixed value of minimum current on any leg to trip on phase loss, i.e. 20% or 30%, making them better at protecting against a single phase condition.

*In North America, OL pick-up points cannot ne GREATER than 125% of the motor FLA for a motor with a service factor of 1.15 (which is most of them), so most OL relays designed for use here will have that as a pickup point. But because IEC motors do not have a service factor, OL relays from IEC manufacturers (who do not have a separate product for North American use), will have a pickup point between 112 and 117%. The can be used here in North America because they don't exceed 125%.

 

[g.mccormick]

If you have an expensive motor to protect (semi hermetic refrigeration compressor for instance) your best bet is to use another means of phase detection to ensure that you do not have a single phase situation.

Phase detection or "motor protection relays" are your best bet. You can get models that have 1 set of inputs to monitor Line side or you can even get ones that are designed to monitor the line and load side of the contactor.

https://www.icmcontrols.com/product/icm450a/

 

[rupej]

Most IEC style OL relays have a "differential trip bar" in their design that skews the overload trip pickup point to be lower than normal in order to account for additional motor heating from negative sequence currents running in the rotor as a result of the single phase condition. But that does NOT mean it will trip. It is still dependent upon the actual load. So if your FLC is 7.6A and your OL relay is set for 7.6A, the "pick up point" of the OL relay will be somewhere in the neighborhood of 8.5 to 9.5A depending on the manufacturer's design*. If you get a single phase condition WHILE the motor is already running, that pick up point will be skewed down to approximately 60% of that value, so between 5.1 and 5.7A. If the actual LOAD on the motor is only 5A, the OL will never trip.

And this only applies to IEC style bimetallic over load relays. Older NEMA type OL relays do NOT have this feature.

Solid State OL relays often have a fixed value of minimum current on any leg to trip on phase loss, i.e. 20% or 30%, making them better at protecting against a single phase condition.

*In North America, OL pick-up points cannot ne GREATER than 125% of the motor FLA for a motor with a service factor of 1.15 (which is most of them), so most OL relays designed for use here will have that as a pickup point. But because IEC motors do not have a service factor, OL relays from IEC manufacturers (who do not have a separate product for North American use), will have a pickup point between 112 and 117%. The can be used here in North America because they don't exceed 125%.

Very interesting. I've tested the common Allen-Bradley 193-E series solid state overload relay, and found that it did not trip on loss of phase if the load is relatively low- even up to about 50% of the trip setting IIRC.

 

[JesperMP]

What is the OPs concern ?

Is it that there is the risk of fire or damage to the motor because it runs overloaded for a long time when a phase fails ?
If so, a differential trip will go a long way for protecting motor and cable, but wont be perfect. Running slightly overloaded will usually 'only' mean that the motor lifetime is reduced rather than immediate damage.

Or is the concern that a lost phase will cause other problems due to the drop in speed, and the OL shall then function as a detector of lost phase to shut down before damage is done ?
In that case I think you need a dedicated detection of phase loss.
We use a couple of methods to detect phase loss. One method is electronic OLs that cover many more protection scenarios, another method is a small CT in each phase giving a signal to the PLC.