protect 3 ph motor from loss of 1 phase 1

[montyoconnor]

We have 3 ph 208/120 motors that fail if one phase drops out. Can the cuircuit breaker be sized to protect from this? How much does the current go up when one phase drops out?

 

[FACB25]

The Eaton/Cutler-Hammer (usta be Westinghouse) Advantage Starters are supposed to cover this as well. I don't have practical experience yet with how effective they are.

Another cheapie option I've seen is using 3 individual relays with the control run series through them. No data on how well that works either.

 

[betkro]

buzzp's comment “_before they start_” is exactly that of a friend of mine, asking to advice him in the matter herein. That's right. A voltage measuring relay cannot DETECT phase loss because of voltage regeneration at the lost phase terminal. However, a [good] undervoltage relay can PROTECT against phase loss IF & ONLY IF the voltage decreases at the lost phase terminal. This condition can be true when the motor is loaded. The disadvantage of some [bad] undervoltage relays is that they cannot sense small regenerated voltage changes produced by lightly loaded motors. Still, [good] undevoltage relays can accurately detect imbalances within NEMA’s secure range.

Regarding harmonics interference, I’m testing & simulating 3ph and single-phase detection circuits. The results show that third harmonic contents interfere in single-phase, but not in 3-ph detection. This can be understood because of third harmonic contents of 3ph rectification used. Anyway, I don’t know if harmonics are actually present and/or would affect any motor’s performance.

 

[OperaHouse]

A couple years ago I was designing phase monitors and tested quite a few. There are many tricks that were used to derermine phase reversal and imbalance. Unfortunantly that caused many of these to not truly meet the performance spec. One for example had a specification which said it tripped at 5%. Unfortunantly, in half the possible combinations of phase imbalance it tripped at 10%

The simplest phase loss/reversal relays are only good for protecting a motor at startup. The true phase imbalance devices, when set to around 5%, are fairly good at detecting regeneration if the motor is reasonably loaded. One problem is many plants allready have an imbalance of 3% and you can get false tripping.

Some applications like shakers have a problem with breaking wires to the motor. These applications are best served with a monitor that measures current imbalance. This would also be the case with lightly loaded motors where protecting the process is more important than protecting the motor. This checking before a load would protect the process when a motor single phases and once loaded would stop a critical application.

There have been dramatic changes in the last 5 years with the newer microprocessor monitors. They are now vastly superior at a very low cost and do wat they say.

 

[jbartos]

Comment: The integration of protective functions in one device causes the discrete protective devices to be less attractive since they are consuming more space, protection per protective feature is costlier when more protective features are specified, and reliability may be lower.

 

[buzzp]

You will here many companies saying their devices will protect against single phasing regardless of regenerated voltages. This would likely be true if they monitor the current but if they only monitor voltage then forget it they can not protect all motors in all applications by just monitoring the voltage.
Equally important is where these voltage monitors are in the system. If they are tapped off from the line side of the starter (most are) then if you have a bad contact on the starter that does not close all the way, the voltage monitor dont care since it gets its inputs from the line side of the starter.
It is possible, in my mind, to have a case where a fuse blows and the motor continues to run, regenerating the lost leg, who's amplitude is high enough not to cause an unbalance trip (based on user setting it to 5% unbalance or higher as typically is the case). The motor will still be damaged even though the voltage relay said everything was just fine. In most cases, the overload protection will protect the motor but in the case of unbalances (negative sequence currents) the overload protection will generally not prevent motor burn out.