Fan application 2

[krsh]

Hello friends,
Company bought a used fan from an abandoned factory which i was assigned to install in our factory. The problem is when i start the fan(DOL starting) the motor protection trips. The motor is a 3 phs, 5.5 kW 50 Hz 230/380, delta/star, 20.8/11.6 Amps which i connected in star and set the OL to 12 Amps. When i measured the amps with a clamp-amp it showed 60 A and it droped slowly to 40, 30, 20 and finally to running current 10.2 A in about 4-7 sec, the problem is that the OL trips in 3-4 sec. My opinion is to install a soft starter but i am a bit undecided if it can supply the needed starting torque.
Waiting for your opinions.

 

[rbulsara]

High inertia for a centrifugal fan/blower is not a problem but a fact of life. They will have typically longer acceleration time than say pumps or other motor loads. So it is a matter of dealing with it. Lowering voltage will only compound the issue.

As for overload relay you may have Class 10 relay and need to be changed to class 20, which has greater time delay (20 sec at 600X vs. 10sec) at given current. See this link

http://sea.siemens.com/step/pdfs/cc_2.pdf

What you are observing is very normal. After all starting a 5.5 kW fan on DOL should not be a challenge.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

By the way, I should have asked this earlier. Make sure that outlet dampers are closed during the starting, to reduce the starting current. This makes a big difference.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Also there are variations depending on the type of fan:
Radial flow fans draw less torque with damper closed
Axial flow fans draw less torque with damper wide open

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(2B)+(2B)' ?

 

[krsh]

Change to Class 20 OL looks like the best choice.
Thanks rbulsara.

 

[rbulsara]

krsh:
You are welcome. Hope that it works out to your satisfaction. Do let us know as to how it turns out. We all learn from our experience.

Just another note, make sure that the motor selected is of right type (thermally) and/or limit number of successive starts and starts per hour to limit the heat build up. Or you can easily damage the motor.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

A class 10 overload should trip in 10 seconds @ 600% motor rated current. It should also take a longer time to trip at any current under 600% of motor rated current. You have reported that starting current is 60A peak and then falling and the start time is 7 seconds maximum. Well, 60A is 60/11.6*100% = 517% and 7 seconds is less than 10 seconds. Conclusion - the overload is bad since it's not even meeting class 10 requirements. It's also possible the overload is some other oddball lower class rating. A true class 10 overload should work fine.

If you want a class 20 then ensure the motor safe stalled is >20 seconds before installing it.

A soft-starter will provide a less dramatic start but it will not lower the amount of motor heating. It will likely include it's own overload that actually works as rated though.

 

[krsh]

You are right is class 5 trip OL. I will install a class 10 OL to see if it works and post the conclusions.