Fuse protecting Fans in the Inverter
Fuse protecting Fans in the Inverter
(OP)
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").
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.
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").
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.





RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
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.
RE: Fuse protecting Fans in the Inverter
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.
RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
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.
RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
Do keep posting suggestions if you have more.
Thanks to jwerthman, redtrumpet and electricpete for their valuable suggestions.
Prav.
RE: Fuse protecting Fans in the Inverter
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
RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
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.
RE: Fuse protecting Fans in the Inverter
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."
RE: Fuse protecting Fans in the Inverter
RE: Fuse protecting Fans in the Inverter
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?
RE: Fuse protecting Fans in the Inverter
http://www.chipcenter.com/power/powp177.html
where new Siemens circuit breakers with motor overload relay function have been presented.
RE: Fuse protecting Fans in the Inverter
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.
RE: Fuse protecting Fans in the Inverter