protect 3 ph motor from loss of 1 phase
protect 3 ph motor from loss of 1 phase
(OP)
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?





RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
http://www.ab.com/overload_relay/
for SMP 1, SMP 2 etc.
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
http://www.ab.com/power/prodinfo/smp/pdf/b509.pdf
http://www.ab.com/power/prodinfo/smp/prodover.html
http://www.ab.com/power/prodinfo/smp/smp1f.html#phase
http://www.ab.com/power/prodinfo/smp/
etc. for more info
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
I’d like to resume this thread because it has a good background about the problem I’m focused: a protection for phase loss.
Perhaps one reason for the debate of the effectiveness of this protection is the reasonable “low cost” and to be one of the most common motor failures, as relay manufacturers point out. In particular, I’m concerned about the operation principle, based in voltage measuring.
Actually, phase loss relays have three single-phase undervoltage elements in one unit. When voltage is regenerated at the motor’s lost phase(s), having the right phase and magnitude during motor’s inertia, that wouldn’t produce any damage until the speed decreases and, consequently, the generated voltages, where the undervoltage protection operates. For example, ABB’s model CM-PBE Phase Loss Monitor trips at 60% Vn (This is over NEMA ratings.) So, a further step for motor protection is to consider the imbalance at an earlier stage, still measuring the voltage's phase deviation. But, that’s another thing. My question here is if the undervoltage per phase is the right way for phase loss protection measuring.
Thanks for your input.
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
These are also available with adjustable Class (10, 15, 20 or 30)plus ground fault and jam protection.
We use these often in rotary electric vibrator applications, in some industrial plants with far from ideal electrical service. Devices such as these are often abused, i.e., overloaded, and phase losses are not unusual. By using these, we have significantly extended their reliability.
Hope this helps!
RE: protect 3 ph motor from loss of 1 phase
jbartos recommends it for thermal overload relay backup. Good. Others cases to be analyzed could be:
1. Low power motors, <10HP.
2. Coordination/Setting: Voltage/phase setpoint and accuracy when this is the sole protection and when it isn’t.
3. Usefulness of voltage imbalance detection as a complement of undervoltage. In my opinion, voltage negative sequence detection doesn’t add any further level protection (except for bad wiring), considering that phase deviations aren’t considerable at phase loss.
RE: protect 3 ph motor from loss of 1 phase
Releasing a phase, the free leg voltage imbalance went to 2.7% and no anomalous behavior was noted (as expected.)Then, by burdening a little the rotor axis, the imbalance rapidly went to 4.5%. A simple voltage monitor with adjustment 2-20%Vn was used, which tripped precisely in both conditions.
To conclude, I’d like to point out that it isn’t my intention to endorse this kind of relay as THE phase loss protection; this is just one case.
RE: protect 3 ph motor from loss of 1 phase
I can bring experience to the table.
We used Phase Monitors in the 80s'.
Frequently lost them due to transients, sometimes just a readjustment fixed the problem, but a nuisance.
Sprecher-Schuh was just coming into the US Market. They told me about the built in phase loss protection. More hype I thought, so I came in on that weekend and tested. Sprecher was telling the truth.
We tried it and then went full with that method.
We have never looked back. To the best of my knowledge, we hae not lost one single motor to single phase since.
That would be about 1,500 motors perhaps, where we used the built in protection only, and not one lost so far. I would know because all warrantee comes across my desk, and our dealers and technicians are not allowed to replace a motor without first making the diagnosis as to cause of failure.
In our training class one time a die hard NEMA fan was balking. We pulled a motor and connected it. Dropped the phase and in about 2 seconds the overload dropped out. He was impressed and stopped arguing.
Ok, very subjective data, but that is the truth and we at least know that and profit from it.
PUMPDESIGNER
RE: protect 3 ph motor from loss of 1 phase
Transients... Good point.
Effectively, the isolation, surge withstand capability, ambient protection, also sturdiness, among others, is essential in static relays. In general, certifications and test results are presented in relay’s data sheets by the mfg.
However, harmonic filtering and EMF filtering are characteristics that relate the electronic design. These are essential in voltage relays, but difficult to analyze from the user’s standpoint. For instance, the phase monitor referred in my previous post does not have harmonic filtering; tests were made on the bench, but its performance in hostile industrial environment is unknown. The experience will say it.
Resuming, the electrical, electronic and mechanical design, also ambient characteristics of protection relays are as important as the physical measuring principle.
RE: protect 3 ph motor from loss of 1 phase
I can not see any way voltage monitoring, alone, could be useful for single phase detection under all circumstances. The three phases are 120 degrees apart by design, there is no way to monitor power flow using just voltage, and the harmonic content will not provide any meaningfull information that I can see concerning single phase conditions.
RE: protect 3 ph motor from loss of 1 phase
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.
RE: protect 3 ph motor from loss of 1 phase
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.
RE: protect 3 ph motor from loss of 1 phase
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.
RE: protect 3 ph motor from loss of 1 phase
RE: protect 3 ph motor from loss of 1 phase
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.