Limiting initial inrush current from fan

matt_sd

Member
Join Date
Jan 2007
Location
Australia
Posts
96
Hello,

A DC/DC converter (20A) provides the power to the control system. We also have recently put in a fan to cool the machine that is 24VDC and draws 7A when running. The problem is that for a split second when it starts, there is 22A that is so high, the voltage drops causes control system problems.

Now is there a device I can buy that will limit the current of the fan of say max 10A, just to get it started?

Thinking of something like a DC/DC converter at the fan that is current limited to 10A or a resister (but that would be a big resistor).

The fan was not in the design as a last minute addition to remove heat and the wiring has been done and cant be changed.

Regards

Matt
 
i mean by changing the fan by install 3 or 4 smaller fans and start then simultaniously ( like 5 sec delay) one after the other,and u said 7 amp,24 v that what u have which is almost 150 w i can imagine how big it is , so u can use 24 v or 110 v ac fan as well or 220 v ac with lower current starting would be max 1 amp,cos if the voltage is going higher the current would be lower.
 
Thanks for your replies. Unfortunatley, this size fan is required for adaquate cooling to the machine and there is no AC - only this one supply we have and in this application, no way of getting any additional power provided.

So the only options I see (and not good ones) are either, a current limiting component inline with the fan. The fan run all the time (but need to start/stop to limit noise and runtime). The smaller fans are less efficient (current/airflow) and this one was ordered as well within current. It was listed as 8A full load current fan but startup current was not taken into account.


Any ideas?
 
Use a Starting Resistor

The simplest and cheapest way will be to use a starting resistor, which gets shorted out by a contact which closes a few seconds after the fan is started. Choose a resistor which will limit the current to an acceptable level (approx. 10-12 Amps). Since the resistor will only be in the circuit for a few seconds, it does not have to be rated to carry the fan current continuously. Be sure the contact is rated for at least 7 amps.
 
Thanks Vic, we have ended up doing that - will get the parts from RS Electronics delivered tomorrow,t - hope that the fan has enough power to get it spinning with the resistor in circuit for a few seconds - will find out tomorrow.

Hi zankorel,
yes a DC/DC convertor is an option as long as it current limits, but at 10A it will be a big beast
 
The problem is that you need some extra current for fan starting. After that, everything okay.

Suppliers of extra DC current: 24 VDC capacitor, 24 VDC battery. For the battery you probably should add a 24 VDC battery charger, then run the fan off the battery. When starting it will pull extra current from the battery, but the charger will pull only its usual amount.
 

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