Electric Fan Current Surge - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 17 Old 08-17-2005, 02:45 PM Thread Starter
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Electric Fan Current Surge

Have dual 11" Spal fans turned on by a single 60 amp Bosch power relay and the alternator is a new PA Performace 3G (130 amp). When the fan is switched on the system voltage drops below 10 volts and the engine momentarily dies. Current draw once the fans are up and running is okay (about 30 amps) and the voltage is 14.2. So its the spinning up phase that is nasty I haven't measured the current inrush but I have seen mention of currents in excess of 100 amps. If I disconnect one of the fans the voltage drop isn't enough to upset the ECU.

So is there a way to get the fans turned on slowly so the current draw is smoother? Say a PWM setup? Or better yet something simplier like an inductor inline?

Thanks


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post #2 of 17 Old 08-17-2005, 03:07 PM
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Have you looked at the Delta Current Control? Instead of turning on the fan at full voltage, it runs the fan at a speed proportional to the coolant temperature. I have one and it works well - not sure about compatibility with dual fans but I don't think there would be an issue.

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post #3 of 17 Old 08-17-2005, 04:20 PM
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Large inrush currents when starting DC motors isn't unusual. I'd verify what the source for the fan motors actually is. A constant 30A draw when running and large in-rush point to the need for a direct battery feed to the relay through something like a 100A fuse. I wouldn't use the existing fan power circuit in the car for these fans. As well, a sizeable supply wire will be needed...like 10-AWG. I'd be very surprised if a direct battery feed would have trouble supplying the needed inrush while maintaining a half-decent output voltage.

You can also consider staging the fan-turn on using a timer element on one of the relays so only one is in high-draw mode at a time.

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post #4 of 17 Old 08-17-2005, 04:27 PM Thread Starter
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One of the issue complicating matters that I neglected to mention is that I have the MAD electrical battery relocation kit. The connection between the alternator and the battary is via an 8ga wire. The starter gets it own 0/1 lead that is hot only during cranking.

I am assuning that the 8ga wire reduces the ability of the battery to function as a large instantanous current source? I have not ever heard of a timer element for relays. Do you have any information concerning them?

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post #5 of 17 Old 08-17-2005, 06:44 PM
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Quote:
Originally Posted by killerbee
...So is there a way to get the fans turned on slowly so the current draw is smoother? Say a PWM setup? Or better yet something simplier like an inductor inline...
DCC is a PWM.
http://www.dccontrol.com/index.htm

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post #6 of 17 Old 08-17-2005, 09:24 PM
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"connection between the Alt and battery is an 8ga wire"???!!!!! Are you crazy? I'd say a minimum of a 4ga wire and 2ga would be better and alot safer. I hope you have a fuse in between tha alt and battery, the 130 amp alt has the potential to easily overpower that 8ga wire. The smaller the wire, the higher the resistence to current flow, so a larger (4ga or 2ga) wire will have much less resistence to current flow and could help the problem your having, along with being safer. Like stated above, a two relay setup would also help. The first relay would start one fan as well as energize a "timed delay" relay for the second fan. That way the first fan would be up to speed before the second fan comes on.
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post #7 of 17 Old 08-18-2005, 01:41 AM Thread Starter
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The arrangement is from MAD electrical and is used fairly widely by many members of the board. The alternator supplies the current required for the varaible loads, lights, fan heater etc. The 8 ga wire is used only to maintain battery charge and does have a fuseable link. In general the current flow is not high and is limited by the resitance of the longish 8 ga lead. The high current capability of the alt does not go to charge the battery but for accessories. And does not have to "go" to the battery first as there is a terminal post where the starter relay used to be. As an example I seem to remember that GM products once started can continue to run if the battery is removed.

As to the problem that I am experiencing it would seem that a varaistor might do the trick and I am looking into that. I also contacted DCC and am waiting on an email response.

Thanks

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post #8 of 17 Old 08-18-2005, 08:19 AM
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I don't think the varistor will prevent your problem. Varistors are typically used in conjunction with minimizing the damaging effects of voltage "spikes" generated when large inductive loads are energized. A fan motor is an inductive load, but it doesn't sound like your problem is being caused by a spike.

The DCC is a slick unit - I believe it will do exactly what you want.
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post #9 of 17 Old 08-18-2005, 09:25 AM
 
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A large part of the problem may be:

You have apparently taken the battery out of the circuit as a buffer by allowing it to ONLY be charged and dedicating the alternator to the accessories.

Believe it or not.
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post #10 of 17 Old 08-18-2005, 12:57 PM
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post #11 of 17 Old 08-18-2005, 02:33 PM
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Quote:
Originally Posted by killerbee
...The alternator supplies the current required for the varaible loads, lights, fan heater etc. The 8 ga wire is used only to maintain battery charge and does have a fuseable link. In general the current flow is not high and is limited by the resitance of the longish 8 ga lead. The high current capability of the alt does not go to charge the battery but for accessories. And does not have to "go" to the battery first as there is a terminal post where the starter relay used to be. As an example I seem to remember that GM products once started can continue to run if the battery is removed.

As to the problem that I am experiencing it would seem that a varaistor might do the trick and I am looking into that. I also contacted DCC and am waiting on an email response.

Thanks
An alternator does not produce DC. It is a multi-phase AC generator. The battery acts as a capacitor to "smooth" the AC. Once the battery is charged, all the current supplied to the vehicle comes from the alternator. 8AWG may be adequate but is not advisable. The difference in cost is minimal, use 4AWG. Would you want to put 100A through 8AWG? I would not.

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post #12 of 17 Old 08-18-2005, 03:05 PM
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Quote:
Originally Posted by Blown88GT
An alternator does not produce DC. It is a multi-phase AC generator. The battery acts as a capacitor to "smooth" the AC.
Actually, it does provide DC. The rectifier bridge in the alternator converts the 3-phase AC to DC (albeit with a fair amount of ripple...)

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post #13 of 17 Old 08-18-2005, 05:14 PM
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Quote:
Originally Posted by KanuckiStang
Actually, it does provide DC. The rectifier bridge in the alternator converts the 3-phase AC to DC (albeit with a fair amount of ripple...)
Heres a picture for everyone.
Top is without diode pack (rectifier bridge).
Bottom is alternator output with no battery. This is a 3-phase output. Alternators are 4 or more phases.

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post #14 of 17 Old 08-18-2005, 05:28 PM Thread Starter
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Blown88GT: My understanding the function of the alternator is that it produces DC with a ripple voltage on it (how much ripple I do not know). A good explination is here: http://www.alternatorparts.com/under...lternators.htm. The figure you provided looks like DC to me

The alternator is rated at 130 amps but under normal operation the current flow to the battery won't be anywhere near that high. I gather that the current flow is determined by its impedance and the leadwires. If the 8ga lead was shorted then this might be possible. That is why both ends of the 8 ga wire have fuseable links installed.

After talking to Brian from DCC it would seem that alternator is not able to cope with the sudden change in load when the fan is switched on. In addition as noted by bronzesink8 the battery has been partially isolated from the alternator so it can not effectively act as a buffer/accumulator to aid the alternator.

After talking to Brian I will install the DCC box and think that it will solve the problem.

Thanks

92 LX, Vortech S-trim, decent brakes, griggs & MM stuff

Last edited by killerbee; 08-18-2005 at 05:31 PM.
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post #15 of 17 Old 08-18-2005, 05:39 PM
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Just so you know Blown88GT = masters in electrical engineering.
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post #16 of 17 Old 08-18-2005, 05:56 PM
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Quote:
Originally Posted by killerbee
Blown88GT: My understanding the function of the alternator is that it produces DC with a ripple voltage on it (how much ripple I do not know). A good explination is here: http://www.alternatorparts.com/under...lternators.htm. The figure you provided looks like DC to me

The alternator is rated at 130 amps but under normal operation the current flow to the battery won't be anywhere near that high. I gather that the current flow is determined by its impedance and the leadwires. If the 8ga lead was shorted then this might be possible. That is why both ends of the 8 ga wire have fuseable links installed.

After talking to Brian from DCC it would seem that alternator is not able to cope with the sudden change in load when the fan is switched on. In addition as noted by bronzesink8 the battery has been partially isolated from the alternator so it can not effectively act as a buffer/accumulator to aid the alternator.

After talking to Brian I will install the DCC box and think that it will solve the problem.

Thanks
Yes, it's DC with lots of ripple, just like the link you provided:


If you ever see 130A, you've got a big problem. If it's shorted, you've got 600A (battery CCA) through the charge wire.
130A alternator can only produce full rated output for a very short time, 130A rating is peak rating, more like 100A continuous rating.

Alternator response time is determined by the type of voltage regulator. Some respond faster than others. It's called LRC (Load Response Control). Heavy-Duty regulators do not have this feature and will be slow to respond.

8AWG is rated for 68A inside a hot, enclosed space.
4AWG is rated for 136 inside a hot, enclosed space

Brian is THE MAN. We both did some tests. Mark VIII inrush current with relay = 100+A; Mark VIII inrush current with DC35 = 65A. Fan power wires are 10AWG.

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post #17 of 17 Old 08-19-2005, 02:15 PM
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Quote:
Originally Posted by Blown88GT
Heres a picture for everyone.
Top is without diode pack (rectifier bridge).
Bottom is alternator output with no battery. This is a 3-phase output. Alternators are 4 or more phases.
Um..isn't this what I said? The green "resultant" of the full-wave recitification is the ripple-laden DC output of an autmotive alternator. You originally wrote:

"An alternator does not produce DC. It is a multi-phase AC generator. The battery acts as a capacitor to "smooth" the AC."

I was just clarifying the apparently erroneous descriptions of both the output of an automotive alternator (it's DC, not AC) and the implication the battery itself acts as a rectifier (it doesn't, although it may help smooth the ripple via its inherent capacitance... Maybe that's what you meant.)


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Last edited by KanuckiStang; 08-19-2005 at 02:19 PM.
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