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?

 

[cbarn24050]

Hi, the overload protector should trip if correctly set, or you can get special phase loss relays, there is some debate as to how effective these relays are.

 

[electrifire]

The circuit brreaker will not protect the motor. A three overload heater arrangement will normally protect the motor when the overload heaters are sized at motor full load amps. The two overload heater arrangement will not protect the motor. I have had reasonable luck with a three phase loss of phase monitors similiar to those manufactured by Time Mark,

http://www.time-mark.com.

 

[RadarRay]

About 12 years ago, when IEC parts first started to be used, one of the benefits that were touted for MSP's (Motor Starter Protectors) was that they would protect against single phasing. I don't know if all MSP's were designed this way or if they still are. Several vendors make them, so I would check on a couple of them.

 

[jbartos]

Suggestion: Some smart or solid state or electronic overload relays, e.g. SMPs by Allen-Bradley, protect the motor against single phasing among other motor protection features. Visit

http://www.ab.com/overload_relay/

for SMP 1, SMP 2 etc.

 

[buzzp]

There is much debate as to the effectiveness of voltage type single phase protectors due to the regenerated voltages caused by the inertia of the load/motor when it loses power. Based on my research and discussions in this forum, current monitoring all three legs is the best method. However, voltage monitors will work fine as long as the regenerated voltage does not have the same rms voltage of the source voltage (when it is present). This seems like it would be extremely rare if the motor is matched well with the load. If the motor is not working much to run the load then the current method would likely be best. I have tried to find some actual test data on regenerated voltages for any application. I have had no luck. If someone knows of any data I would be real interested in getting a hold of it. One thing everyone agrees on is that current monitoring all three phases for a single phase condition will always work(if set-up correctly and the current monitor is not a piece of junk). Thanks and good luck.

 

[sparking]

without one phase may happen at the downstream side of the starter, e.g. loss wire connection at motor terminal box. In this case, monitor via regen. voltage does not help which i believe. correct me if i am wrong.

 

[jbartos]

Suggestion to the previous posting. This may depend on the electronic overload relay principle of operation. Check with the manufacturer of the overload you intend to use.

 

[jbartos]

Small encore to the previous posting: Visit

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

 

[buzzp]

Whether the voltage monitor would work, depends on where the taps in the circuit are made to monitor the voltage. Usually this is made at the starter, so you are right, any problems downstream would not show up.

 

[jbartos]

Suggestion: It also depends how the smart electronics is designed. Certainly, the open circuit downstream from the overload needs the more sophisticated design, since all three phases appear at the overload relay location. Then, one may monitor a current in each phase. If the current in one phase is substantially lower, this will signal one phase conductor is open, i.e. single phasing.

 

[buzzp]

If monitoring current in each phase seperately, to detect a single phase, it will not matter where the conductor is going through the current monitor at or through the CT's. Current needs a closed loop to flow, if it is broken at the source or the load, will not matter.

 

[betkro]

Hello,

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.

 

[jbartos]

Comments on the previous posting: It depends. Supposing that the current transformers are aligned with the motor overload relay, then the manufacturer will tend to base the single phasing motor protection on the electrical current relationships. Since the motor overload is often protected by the motor overload relay, then a special relay based on voltage principles to protect the motor for singlephasing may potentially and justifiably be used as a backup to the overload relay.

 

[bklauba]

Check out Sprecher & Shuh CEP7 line of O/L's. They are designed to trip within 2 seconds if a phase loss occurs on a 100% loaded motor, and only slightly longer if the motor is lightly loaded. If the single phase condition is present on startup, they will trip within 8 seconds (if the motor is loaded > 80%). A 50% phase imbalance is treated the same way as a phase loss. These times are much faster than any electromechanical relay I have seen.

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!

 

[betkro]

I appreciate your answers. Excellent discussion has already been carried out in this forum about voltage regeneration regarding phase loss, also, qualified comments covering practically every possible and actual application. However, my intent is limited to the usefulness and accuracy of voltage measuring when a phase is lost, rather on a comprehensive motor protection. In other words, how valuable a “voltage phase monitor [protection]” is, and when it should be used.

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.

 

[betkro]

This is just to show the test results made on a small 220/380V 3HP general purpose motor.

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.

 

[PUMPDESIGNER]

betkro,

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

 

[betkro]

pumpdesigner,

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.

 

[buzzp]

The only usefullness of voltage monitors as far as motors are concerned is BEFORE THEY START. After the motor is started, current monitoring can provide overload and single phase (or unbalance) protection. Most modern devices will not be affected by ambient temperatures or transients. You should look for standards identified in the data sheet that reflect what types of transients, etc the device was tested too.
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.