Fuse protecting Fans in the Inverter

[KnicksJets]

Hi,
This is a very specific question and it might require more information. I won't be able to provide any drawings or documentation; if you have experienced similar problem or have any hint on solution please recommend.

We have inverter that takes DC input voltage of about 105VDC to 140VDC and outputs AC voltage of 118VAC single phase. 5 fans are used to cool the components and are mounted a top of the inverter unit(height 77.75", width 60&quot.

The Rotron CL2T2 fans @115VAC input has input current of 0.87 amps, with a "locked rotor amps" of 1.4 amps. Five of the Rotron CL2T2 fans running in parallel, would have a fan current of 4.35 amps. The vendor recommends that we should use fuse rated 5 amps, even though we insist because of high in-rush current we prefer to increase the rating by 6 amps. We cannot go in conflict with the vendor now, and the Fan fuse keeps blowing out. The fan has recommended life expectancy of 2 years but is also recommended to replace on failure basis. Fan being expensive we resorted to latter option. Even if the dust build up on the fans can slow the rotors thus increasing the "locked rotor amps", we expected the fan fuse not to blow.

Do you see any problem here(expect the last sentence I mentioned?). Do you have any solution?
Please help.
Prav.

 

[redtrumpet]

Does the fuse blow on fan start up or after the fans have ben running for a while? Are you using a fuse with a time-delay characteristic? Why can't you modify the cabinet to put in additional fuseholders and individually fuse each fan? Why can't you go against the vendor now? At this moment, somewhere, someplace, a maintenance electrician is modifying a piece of vendor equipment to correct a poor design. The vendor is not always right.

 

[electricpete]

two quick comments...
I'm not sure exactly what type of motor it is. But for induction motor, increasing the inertia or load will not increase the locked rotor amps (just may prolong the starting time... which can also be a problem).

In your discussion you mention the vendor wants to protect at 5A which is silightly above the running current. But you want to allow ~10A which is slightly above starting current. As Redtrumpet mentioned a time delay fuse can meet both these requiremnts.

If the fuses are blowing I think you have to fix it.

 

[KnicksJets]

Redtrumpet, thanks a lot for your response.

Yeah, the fuse's time-delay characteristic prevents it from blowing out due to high in-rush current(5*1.4=7amps). Thing is the problem happents almost every 2 months and fans are always running. Only the fuse blow up.
Vendor did all the required testing and we now agree with vendor that the rating is correct.
Any more suggestions?
thanks.

 

[jwerthman]

I suspect that the marginal rating of the fuse compared to the full load current of the 5 fans coupled with the high ambient temperature in the inverter cabinet is probably enough to eventually cause the fuse element to melt. Running fuses close to their rating in high ambient temperatures is always a bad idea and usually results in the problems similar to yours. I agree with Redtrumpet that the best solution would be to separately fuse the motors, or at least separate them into 2 or 3 groups.

 

[KnicksJets]

Thanks Jwerthman and Electricpete for the responses,

Jwerthman, the ambient temperature outside the fuse is tolerable for the fuse according to the manufacturer. We are reluctant on seperating each motors with its own fuse as the inverter comes as guarantee from manufacturer unless we make changes to the circuitry or use different ratings.

Please keep posting if you have any suggestions/ comments.

 

[redtrumpet]

Okay, then I think you are back to working with the vendor. If the ambient temperature and fuse size and type are both in accordance with the vendor's recommendations, then hound the vendor to come up with a solution - it is ultimately their responsibility and this is a new unit that is under warranty. Do you buy a lot of equipment from this vendor? The threat of loss of future business can be a powerful motivator for some, but not all, vendors.

 

[KnicksJets]

Thank you guys.
Do keep posting suggestions if you have more.
Thanks to jwerthman, redtrumpet and electricpete for their valuable suggestions.
Prav.

 

[akhaider]

I assume followings.
1) The fan motors are 3 - phase Induction type.
2) You have been using time delay fuses.

The vendor is trying to protect your five fans with fuses. Since the value of fuses is very close to the running amps of the fans therefore they get heated up and ultimately get blown. Normally fuses are used for short circuit protection and thermal protection is provided for overloads. Moreover the starting torque of blowers / fans is longer as compared to pumps, therefore value of fuses should be 3 - 4 times of running amps.

Please proceed as follows.
1) Fuses are for short circuit protection. Your fuses are blowing because their value is very close to the operating amperes. Select fuses as following.
0.87 X 5 X 3 = 13 Amps.
Select time delay fuses rated at 12 Amps.

2) Thermal protection is recommended for overload protection. Select thermal relay with its trip value set at 85 - 90 % of the rated amps. Thermal setting at 95 % is calculated as following.
0.87 X 5 X 0.85 = 3.7 Amps
For 90 % setting
0.87 X 5 X 0.9 = 3.9 Amps
So install a thermal with current setting at 3.7 to 3.9 Amps.
Best regards

 

[jbartos]

Suggestion: Since the motor nameplate data are not posted, any suitable fan motor current monitoring could reveal the proper protection scheme for the fan motors. The motor thermal relay protection is better than the fuse protection since the time-current, I-t, characteristics are specially designed to fit the motor thermal characteristics to envelope them as closely as possible, and leaves the fuse protection for short circuits or similar faults. Also, it shall be noticed that some thermal relays (not ones based on smart electronics) have a wide margin of the error, e.g. 20% or so.

 

[jwerthman]

I don't disagree that some type of overload relay will provide better protection for the motors than a simple fuse. However, these muffin fans are almost throw-away items and it just isn't worth the investment to protect this type of inexpensive motor with anything fancy. Fuses are quite normal for this type of application. The problem here is that the fuses are just undersized for the 5 motors running together. As long as the fuse size doesn't exceed the rating of the connecting wiring, there shouldn't be any problem increaseing the rating.

 

[KnicksJets]

Thank you all for the valuable responses.
I also agree on not using the thermal overload relay protection. However the fuse rate cannot be changed either. If fuse rating were simply wrong, it wouldn't blow up after 6 months, it would do it in considerably short duration of time.
I thank all of you for your support and answers. I hope I can do same for you if you have any questions.
I declare the closing of this forum.

 

[busbar]

I know it’s late, but aren't this size of "Rotron/Papst"/et cetera motors considered impedance protected, with very marginal overcurrent in locked-rotor/failed bearing conditions? That may make current-based overload sensing more difficult.

Seems hard to imagine inverter design becoming even more "price competitive."

 

[redtrumpet]

I think you're right, busbar, compare the 0.87 A running current to the 1.4 A locked-rotor current. LRA is less than 2x FLA.

 

Is this the only inverter you have of that model that does this?

It does sound like the fusing is entirely too close to the running amps. Like it was stated before, fuses are used to protect against short-circuits, not overloads. And if I read the other replies correctly, the only people who disagree are the manufacturer/vendor.

As far as it running 6 months then failing. This is entirely possible for a fuse sized so close to the borderline. It could be that the right conditions to help the fuse blow are rare, such as a day or two here and there where the temperature is a little warmer than normal, or like one case I personally have seen where vibration was the culprit (which was being caused by a couple of flights/day from a nearby airport). Of course that was not on an inverter, but you see my point that there may be other external elements causing a slow degredation of the fuse and then finally causing it to fail, but ONLY because it is sized SO close.

I think the problem lies more with the vendor/manufacturer. What reasons did they provide you for fusing the motors so closely?

 

[jbartos]

Suggestion: If the overload relays are not preferred by the requestor, then perhaps special type of circuit breakers might be considered, e.g.

http://www.chipcenter.com/power/powp177.html

where new Siemens circuit breakers with motor overload relay function have been presented.

 

[akhaider]

Your motors are under protected with fuses only. The ventilation of inverters depend on the cooling and ventilation thru fans. Fans also carry away the carbon particles from electronic circuitry, which are generated by effect of heat on insulation. Therefore you should not neglect the importance of fans. Some inverters trip when cooling fans trip.
Thermal overload protection in any way is essential for AC induction motors. There is variety of motor starters available in market. But ALL motor starters, of all makes, available in market, incorporate thermal overload protection. I have found fuses excellent for short circuit protection, but they have a NUMBER of problems, when used alone for overload protection. Similarly thermal overload protection cannot be used for short circuit protection. To avoid using fuses, some motor starter units, have built in thermal and short circuit protection. These are small compact units with thermal and short circuit protection buuilt in. You will be amazed to see their small size and the protection they can offer, in terms of current rating.

 

[Laplacian]

Have you put a clamp-on current probe on the circuit to measure actual operating current, or are you going by the fan motor nameplate data?