Lots of “Ifs” here…
First, it depends on IF the motor was running at the time of the event or not.
- IF it was not running, and IF the phase that was lost was not affecting the control power, and IF something called for the motor to run, receiving single phase will not allow the motor to rotate, so it stays in the locked rotor condition. But that’s without the full measure of locked rotor current, it’s only about 58% of LRC because it is single phase now. The OL trip curve will be based on full 3 phase LRC, so the trip time is essentially more than doubled (it’s a logarithmic curve). That is usually long enough to damage the windings.
- IF it WAS running when the phase was lost, then it depends on the LOAD on the motor at that moment. IF the motor is running at less than half its full load amps, it may not be damaged and will chug along as if nothing happened. The current will increase, and because of “negative sequence currents” in the dead phase, the negative torque that creates will effectively “fight” the normal torque, so the motor will run a LOT hotter than normal, which is what can damage the windings. But because you started out so low to begin with, the motor may be able to handle it. Any more than around half of the FLA and the effect can be damaging. See below about the OL relay for more.
Usually, that’s due to the lost phase being one of those that was supplying control power, PLUS there were other motors running across the line at that time, which, like a “rotary phase converter”, create a “phantom” voltage on the missing phase so that instead of the control power completely dropping out, it flutters around at a low voltage level that leads the coil to chatter, destroying the starter.
Again, it depends on the first issue; loading and the type of overload in play;
- If the motor was running at half or less than its rated load, it may be fine. If more than that, the heating effect (from the negative sequence currents) will cause disproportionate heating in the windings, but the two active phase currents will not increase as much as that heating effect, so the OL relay may not think anything is wrong.
- The sub variant to this is whether or not you are using a NEMA type OL relay, or an IEC bi-metal type. IEC bi-metal OL relays usually have what’s called a “differential trip mechanism” that deliberately skews the trip curve downward toward that 58% curve rather than the full one. So they might trip fast enough to protect it. Still not guaranteed though, it too depends on the motor load at the time of the event.
Bottom line, you can’t really expect a simple electro-mechanical OL relay to protect against phase loss because of all of the IFs involved. I switched to Solid State OLRs years ago when they became almost the same cost as good NEMA type OLs. SSOLs almost always include true Phase Loss protection based on current (not voltage because voltage base phase loss protection can be fooled). I gave never looked back.
As to “phase loss relays”, my issue with those is what I just said, most if them are only looking at voltage, not current, and can be fooled by the amount of regenerated voltage on the lost phase by already spinning motors. So those are good for DISALLOWING systems to start with a phase missing, but may allow an already running motor to continue running if the phase was lost after it started. I’ve seen some claim to have kicked this problem, I have not seen one that actually did it under all circumstances to where I would trust them over current based phase loss protection.
