We have in our NGL Plant, 3 explosi

[Abuomar]

We have in our NGL Plant, 3 explosion - proof small motors rated 1.5 HP,230 VAC, 3 Phase.Those motors have experienced repeative failures which led to 2 motors burnt out to-date.

Further, our investigation revealed that the motor failure occurred during mechanical seizure in the pump unit, which caused a locked-rotor condition. It failed due to exposure to a prolonged or sustained overload, since the the control circuit is only protected by a 15 Ampere Circuit Breaker, with no additional motor protection.The high load current experienced at this condition, possibly reaching as high as 34 Amperes based on the motor design, which caused deterioration of the winding insulation and eventual motor burnt out because of overheating.

Is it possible to review the existing control circuit and provide an additional motor protection against prolonged overload condition?

 

[electricpete]

For a large motor, the overload protection should be sensitive enough to prevent damage to the motor during a locked rotor condition. Specifically the relay time-current curve is selected to fall underneath the motor thermal damage time-current curve. If nothing else this makes sense based on economic considerations since motor replacement cost is high compared to the cost of the relaying and analysis.

For a small motor such as yours, I don't think the same economic considerations apply, but I would think that code (fire safety) requirements would dictate protection which is sensitive to trip off the motor before potentially damaging currents occur. I'm sure there are others on this board that can provide more insight.

 

[peterb]

You definitely need some form of thermal overload protection, as the circuit breaker really only provides short circuit protection for the circuit.
Is there a motor starter in the circuit? If yes, then contact the manufacturer for details of available thermnal overload. If no, then consider adding a contactor + thermal overload for this purposes.
I just found this site

http://www.pbeng.co.uk

(good to see that they are still around) on one of the other forums, visit it & look at the Microvision relay, which has integral CTs for protection of motors with FLA 1-15A. Might be an idea for a retrofit - note that it includes protection against running stall (trips if motor current increases dramatically after a period of running at or below FLA, sounds like your case).

 

[jbartos]

Suggestion: Some manufacturers, e.g. General Electric Co., etc., offer 3-phase circuit breakers from very low amp ratings (GE Co from about 3A rating). The properly sized circuit breaker should protect your motor adequately. Your motor draws approximately 4.5A FLA. If you experiment with smaller 3-phase circuit breaker sizes from about 12A downward, your motor will be safely protected. If you happen to obtain your motor characteristic curves (starting, thermal, etc.), then the circuit breaker selection will be faster and more accurate.

 

[redtrumpet]

jbartos - circuit breaker is generally not allowed for overload protection unless the motor is 1 hp or less and certain other conditions apply. If this was Canada or the US, I can't see the respective electrical codes allowing this installation for a 1.5 hp motor without separate motor overload protection. From the standpoint of fire protection and protection of equipment, a thermal overload is required.

peterb is correct in his assessment of this problem.

 

[jbartos]

Suggestion: Yes, I essentially agree with the overload relay as the best motor protection; especially with the sensitively sized circuit breaker. However one cannot be blind to see that the modern solid state circuit breakers are somewhat approaching to overload relay protection curves, thus combining old electromechanical circuit breakers and thermal protection relays. It appears that this trend will continue to reduce the cost of electrical installations.

 

[TheDOG]

I think the best solution would be a motor protection circuit breaker. These provide both adjustable thermal and magnetic release, and are designed specifically for this type of application.

jbartos,
A circuit breaker alone cannot provide adequate protection for a motor. If you look at the trip curves of a circuit breaker Vs an overload, you will see that they are completely different.

 

[redtrumpet]

The Dog - I'm not familiar with "motor protection circuit breakers" with both thermal and magnetic adjustable trips. Who makes these?

I've only seen motor circuit protectors (MCPs like Cutler-Hammer HMCP, Square D Mag-Gard, etc.) with adjustable magnetic trip only.

 

[TheDOG]

Redtrumpet,
I'm sure that there are quite a few manufacturers (I only know of two):
Allen Bradley Bulletin 140
Schneider (Telemecanique) GVN

 

[teco]

abuomar:-you could fit thermisters into the winding.these are embedded into the end windings (3# one in each phase) when rewinding or during manufacture of new machines. for a class "f" insulation system ptc130 should be used.thermistors moniter temprature rise with in the machine ,when the machine reachers critical temprature the resistance on the thermistoer goes high and drops out the thermister relay, this in turn drops out the main motor supply contactor.
kind regds.

 

[jraef]

Although Aboumar did not identify what part of the world he is in, it appears to be the USofA since he refered to HP instead of kW for the motor size. That being the case, "motor protection circuit breakers" will be difficult to come by readily. For those in the US not familiar with worldwide electrical equipment, these are thermal-only circuit breakers specifically designed to protect a particular motor, i.e. 3.2A, 4A, 6A etc. The trip curves are essentially the same as Class 10 overload relay curves. Unfortunately they are not typically UL listed as "circuit breakers" but rather as "suplemental circuit protectors", meaning that by code, a fuse or circuit breaker must be providing the branch circuit protection. IEC manufacturers also have "Motor Protective Switches" available, described earlier as the AB Buletin 140 et al. Although they also provide magnetic short circuit protection, these too are not typically UL listed to provide branch circuit protection by themselves, so they are UL listed as "manual motor starters", an accurate description by US standards. The benefit in both cases of this type of product is that they will open the power circuit themselves, as opposed to an overload relay that must also have an associated contactor to cut power to the motor. Since Mr. (or Ms.) Aboumar already has a circuit breaker available , any one of these devices would suffice if his local supplier has them available. Try Siemens, ABB, Schneider (Sq. D), Moeller Electric, S & S, or any other european mfgr as a source. If they can't be found, you will need to add a properly sized motor starter w/ overload and a control circuit for it to protect your motor.

 

[jbartos]

Suggestion: Please, notice that the electronic solid state trips of circuit breakers can configure the circuit breaker characteristics close to the thermal relay. Visit

http://www.geindustrial.com/products/applications/DET-118A.pdf

for GE MicroVersa Trip

http://gess.ge.com/pdf/gea-400low.pdf

etc.
Generally, if the protective device I-t curve is below the thermal overload relay curve, then the motor is consider more sensitively protected.

 

[redtrumpet]

jbartos - this is a 1.5 hp motor we are talking here. The GE breaker you referenced is an insulated case power breaker! If a motor is large enough to require an 800 A or larger circuit breaker, don't you think the client is going to want dedicated motor protection? Do you think the best use of an insulated case breaker is for motor switching? How many companies have the staff to do the breaker maintenance, assuming the contacts can even be adjusted and cleaned on the insulated case breakers? You need a separate control power source, probably a battery system, another maintenance headache. Sure, I've seen it done with ACBs, but not in the last 20 years. And most of the companies who did use ACBs are upgrading to vacuum contactors.

 

[jbartos]

Comment to the previous posting: The MicroVersa Trip was used to evidence electronic adjustable band circuit breaker concept only. However, there are electronic circuit breakers that come in lower amp ratings. Visit

http://www.pge.com/002_biz_svc/002c1c_understanding.shtml

(Under "Single Phase Protection&quot

http://www.prodigit.com/e2000c.htm

etc. for more info.