Motor Single Phasing

[brian john]

Having a discussion regarding Time Delays for Phase Loss Relays (single-phase, UV, OV, or Reverse Phase) in regards to Motor Starting. Assume a facility has a single-phase condition and an attempt is made to start a 3-phase motor, what would be the maximum delay you would want for the Phase Relay? Some are saying instantaneous, I think some time delay is possible as the motor can handle inrush currents on start-up and single-phase currents would be somewhat similar to inrush but only for a much longer duration if the motor is not removed from service. Another point was brought up that the Thermal Overloads should protect the motor and it was cited that Bussman literature that states their Low-Peak or Fusetron fuses if properly sized will protect motors from Single-phase conditions

The issue arose around the practice of installing Phase Loss Relays on the Main (Fused Pressure Switch or CB manually operated to close) for a facility with no downstream motor phase protection, other than fuses and overloads. I believe phase loss for motors (UV, OV Single Phase) protection should be installed in MCC, a single Phase loss with controls too prevent startup and stop any running motors or Phase protection for individual motors in the starters. The advantage is an EMS system can restart the motors once the single-phase condition is resolved. With protection at the main fused BPS or CB would require an electrician to come to the facility, determine the cause of why the Main opened and close the Main. With overloads and fuses, it has been my experience that they are more often than not improperly sized, using Name Plate in lieu of actual full load running current, which would negate the protection.

 

[Jraef]

There is no reason to have ANY time delay for acting on a single phase condition. Why would you? Delays on protective elements are generally for the purpose of determining if the condition is actually there or if it is a transient event. There is no transient nature of a phase loss, it is either there or not there.

A 3 phase motor will not reliably start on single phase and if it happens to, it might run in the wrong direction. That alone could be VERY dangerous or detrimental to the machinery.

No delay.

 

[Cow]

The only time delay I'd want in regards to a phase monitor, would be a short delay after power is restored, if I had the option.

 

[brian john]

Jaref, We have several sites that experience momentary dips several times a month the current setup dumps the complete building, a bad design, but I am not the engineer that specified the protection, I am trying to minimize outages that require an electrician to be dispatched. If I had my way there would be protection for the MCC or at each starter.

 

[Russs57]

No way I’d want to take out the main breaker.

You could consider changing to electronic overload relays on the motor starters.

 

[ramsy]

Jaref, We have several sites that experience momentary dips several times a month the current setup dumps the complete building, a bad design, but I am not the engineer that specified the protection, I am trying to minimize outages that require an electrician to be dispatched. If I had my way there would be protection for the MCC or at each starter.

Sounds like a job for an ATS when a secondary power source is needed.

 

[bwat]

There is no reason to have ANY time delay for acting on a single phase condition. Why would you? Delays on protective elements are generally for the purpose of determining if the condition is actually there or if it is a transient event. There is no transient nature of a phase loss, it is either there or not there.

I know of several rural electric co-ops in my area that operate in a single phase basis for their 3 phase distribution. Meaning that they have 3 single phase reclosers that can temporarily open up just a single phase if there’s an event like a tree branch falling that only affects a single phase for a second or two... depending on the operation that may be something you’d like to try to ride through without shutting down..

Just taking exception to the statement that there aren’t any reasons to ever have a phase loss delay and that it would never be temporary/transient.

 

[Jraef]

Ok. I hereby rescind my absolutism... these are good reasons.

 

[brian john]

Sounds like a job for an ATS when a secondary power source is needed.

We are discussing 12 story commercial office buildings with 2-4 2000-4000 amp services, low-end office buildouts, if they were going to spend money they would install Phase protection at each motor starter.

 

[Hv&Lv]

Motor starting as jraef mentioned shouldn’t have any delay at all. The last thing yo want to do is try to start without all three phases.
Motors that are already running can have a delay in the LOP. As mentioned POCOs use single phase OCRs for protection. The curves determine the clearing time, so say a setup with 2A and 2B curve operation can operate usually within 10 seconds. Chances are if the fault hasn’t cleared by the first B curve, it isn’t going to clear and you might as well have your relay shut the motors down.

 

[paulengr]

Motor starting as jraef mentioned shouldn’t have any delay at all. The last thing yo want to do is try to start without all three phases.
Motors that are already running can have a delay in the LOP. As mentioned POCOs use single phase OCRs for protection. The curves determine the clearing time, so say a setup with 2A and 2B curve operation can operate usually within 10 seconds. Chances are if the fault hasn’t cleared by the first B curve, it isn’t going to clear and you might as well have your relay shut the motors down.

Mixing terms. Utilities use ANSI or IEC or some recloser curves that are not a standard but are sort of a de facto one. 2A and 2B are UL curves, not necessarily the same.

 

[Hv&Lv]

Disagree.
I’m not talking about miniature circuit breaker curves.
OCRs don’t use UL curves.

 

[kwired]

I'd say load characteristics come into play in the decision. A higher inertia load that is only interrupted briefly (say a single phase recloser operates just once) there should be just fine remaining or returning to running status after the event is done. Many HVAC loads also can automatically resume without a manual reset, some of them maybe still need a delay before restarting though.

Process loads where there is critical timing of events, those may need to shut down and be manually reset/restarted.

In general the motors themselves can handle single phasing without damage if they have proper overload protection. If the motor is driving higher inertia load and the phase loss is brief, it will ride through it. If motor actually stops because there isn't enough torque/inertia, then it will draw high current if the phase loss persists, but overload protection should still stop it before it becomes damaging. If it happens frequently damages will become accumulative though.