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Can DC Motor Operate at Different Amperages?

11/29/2011 5:00 PM

I am using a small DC motor with a fan on it to to move air from a solar air heater to my house. I intend to power it with a harbor freight 45 watt solar panel. I believe the panel will produce a maximum of 3 or 4 amps. Of course depending on the sun the output of the panel can vary. The DC motor says it draws 4.3 amps. It is a 12 volt motor. Will the motor just run slower in partial sun or is this not a good idea. Thanks for your help. Mike

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#1

Re: Can DC Motor Operate at Different Amperages?

11/29/2011 6:30 PM

"Will the motor just run slower in partial sun or is this not a good idea."

Yes. no.

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#2

Re: Can DC Motor Operate at Different Amperages?

11/29/2011 6:32 PM

Basically the answer is yes it can run on varying voltage. As the voltage gets lower the motor slows down. You can damage the motor if the voltage get too high above its rating. Do you know what the output voltage of the panel is? If the output is only 4 amps, I doubt that is a problem.

Free Information http://www.tigoe.net/pcomp/code/circuits/motors

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#6
In reply to #2

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 11:53 AM

This is how Harbor Freight describes their solar panel kit:

The solar panel kit comes with three 15 watt solar panels

Maximum current: 3000 mA 15 watts max per panel

Peak voltage: 23.57 volts open current

I think they mean that the maximum current is 3 amps total for the 3 panels, not 3 amps per panel. If the open current voltage is 23 volts is that a problem for a 12 volt motor?

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#3

Re: Can DC Motor Operate at Different Amperages?

11/29/2011 8:45 PM

This is not a reliable or economical way to run a fan...If on the other hand you are just interested in setting up a solar power system for experimental or observational purposes, that's different...In that case you will need the whole setup, including storage battery or batteries, low voltage cutout, and so on, probably with more panels if you want reliable power...

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#7
In reply to #3

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 11:58 AM

Why do I need batteries? I only want the fan to run when the sun is out.

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#9
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Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:11 PM

The battery is to store and regulate the voltage....Running the fan on low voltage will overheat the motor, causing premature failure....Your fan control should be based on temperature, using a thermostat..

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#10
In reply to #9

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:25 PM

I did intend to use a snap switch to turn the fan on and off based on temperature. So you're saying that as the sun gets weaker the voltage ( as well as the current) will decline and that will damage the motor?

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#11
In reply to #9

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:28 PM

Could I just use a device to cut off power if voltage drops below 12v rather than getting involved with batteries?

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#12
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Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:38 PM

You might run into a rapid cycling problem...Do you have such a device?? Is it time delay?

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#13
In reply to #12

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:41 PM

Found one here...The 12 volt shop...this bilge blower looks good too..

http://www.12volt.com.au/

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#14
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Re: Can DC Motor Operate at Different Amperages?

11/30/2011 3:12 PM

If that's the Rule 240 bilge blower it is motor I'm talking about. Snap switch turns on at 110 degrees and off at 90 degrees so I don't think rapid recycling would occur.

That voltage controller says it will not cut off current until voltage gets too low for 60 seconds. Will the motor be ok for 60 seconds?

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#15
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Re: Can DC Motor Operate at Different Amperages?

11/30/2011 3:37 PM

yes..

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#16
In reply to #11

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 10:44 PM

We built something similar. Used a 12 volt (3W) computer fan running off a panel rated at 22w. We used a 7812 IC to avoid cooking the motor in direct sunlight. When the sun first comes up, the fan twitches for a bit until it finally starts turning. Motor doesn't seem to mind low voltages; we installed this two years ago (2009) and it's still going strong. Motor claims 50000h mtbf, so it should be good for a while, but it is often -20C where we have it, so it may not do so well at these low temperatures.

I'd say try it and see if the motor turns at low voltage (Go up slowly and watch).

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#18
In reply to #9

Re: Can DC Motor Operate at Different Amperages?

12/01/2011 6:38 AM

GA

That is what will happen in the real world!!!

The OP needs a circuit to switch the motor off at a particular drop in voltage and to switch it on again when the voltage rises above this. Some hysteresis is needed to handle the sudden voltage drop/rise as the motor starts or stops.

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#20
In reply to #9

Re: Can DC Motor Operate at Different Amperages?

12/01/2011 8:31 AM

you can't store voltage

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#21
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Re: Can DC Motor Operate at Different Amperages?

12/01/2011 10:11 AM

More precisely stated, store energy and regulate voltage...

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#4

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 2:12 AM

If you were using a solar powered fan to cool an attic, this would all make sense; more sun = hotter attic = faster fan to cool it.

What you have looks like a runaway positive-feedback loop.

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#8
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Re: Can DC Motor Operate at Different Amperages?

11/30/2011 12:03 PM

This is equivalent to an attic fan in that when the sun is out the solar heat collector gets hotter and the heat needs to be moved by the fan through a duct to the house. When the sun goes down no heat is being collected and there is no need for the fan to run.

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#5

Re: Can DC Motor Operate at Different Amperages?

11/30/2011 3:57 AM

The current is determined by the torque. The torque is determined by the load. Just 'cos it says 4.3A at 12V doesn't mean it will draw 4.3A at 12V. It draws 4.3A at 12V at maximum load.

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#17
In reply to #5

Re: Can DC Motor Operate at Different Amperages?

12/01/2011 2:41 AM

PWSlack has got it, the fan needs only the amperage to run at the speed the voltage determines, (DC motor's speed is determined by voltage) and probably the fan is not working as hard as it could because the solar panel when cool will put out a higher voltage and that is likely to be morning or evening when the light intensity is not enough for the rated amperage to be produced, - then the fan will not meet the voltage "expectation", and may even not turn. I used to supply simple systems using a small solar panel and a computer fan for home food drying and also composting toilets, and it was important that the fan could handle not turning when having very low amps, (usually they could). Basically that is using margins built into the product probably for other reasons, - probably not good engineering, technically speaking, but the fans lasted for years and are very in-expensive and designed to be easy to replace. Cheers, Geoff.

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#19

Re: Can DC Motor Operate at Different Amperages?

12/01/2011 8:20 AM

Most of these are good answers, but surprising, because they don't show much experience with the actual question. I do, and the combo will work fine. I would use a thermo snap switch, because you will experience premature operation of the fan in the morning if you do not. (The collector will not be warmed before the fan starts)

For all practical purposes, the voltage output of the panels will be around 17V whenever sunlight is on the PV panel. (Open circuit is not relative to this discussion, as it means that the snap switch (if there is one) has removed the load from the circuit. If the circuit is closed, the voltage will immediately drop, and the motor will do a fine job of matching load to available amps, inducing only minor voltage fluctuations. A 12V motor like yours has no issue with this.

It is the amperage output that varies, an an almost linear fashion with insolation (sunlight intensity)

This DC motor will run very slowly with very little current. A slow turning fan requires very little amperage, as the torque (load) in the motor is almost non-existent. As the PV panel amperage output increases, the load increases, and the fan motor turns faster, increasing torque on the motor. It is a fine loop, with no danger of short cycle damage.

Many thousands of these systems are in place, we have built a bunch from components, but you can buy kits as well. BTW, 45 watts is 17 V at about three amps. Those panels will be wired in parrallel, because each is around 1 amp. It's a good match. And the system will run just fine at 1 amp. (25% sunlight, or 250 Watts per square meter) If you can, I would aim the panel (in the northern hemisphere) at about 210 degrees (SW) Try it.

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#22

Re: Can DC Motor Operate at Different Amperages?

12/01/2011 11:29 AM

Thank all of you for taking the time to answer my question. I enjoy this stuff and only wish I knew more about it. Thanks again. Mike

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#23

Re: Can DC Motor Operate at Different Amperages?

12/02/2011 3:04 PM

Your characteristic of the enlightened solar panel looks like follows:

Depending on your load you move on the red line having the best power efficiency when you meet the working point. Remind that the 45W output will only be available when the solar radiation is about 1 kW per m², which is a lot !! So, most of the time you`ll get only a fraction of the power the module manufacturer wants you to believe to get.

As your motor has only one impedance when connected directly you can use only one point on the red line which is a poor efficiency under most conditions. So one would use, as others said, a battery, loaded by an adaptive regulator which automatically seeks the best working point which is determined by the solar radiation.

First advantage of using a battery with loader is that you always have the right motor voltage. And if you want a temperature dependant regulation then there are cheap electronic boards which can be wired between battery and fan. They measure temperature and modulate the motor voltage so it runs faster at higher temperatures.

The second advantage is as follows: When the sun comes out first the temperature is still low for some time and you can store the energy which is not necessarily used that moment. On the other hand, if the radiation goes down, temperature may be still high because of storage effects. So it might be useful to let the fan run a bit longer even if no radiation is available to power the module.

Regards Uwe

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#24
In reply to #23

Re: Can DC Motor Operate at Different Amperages?

12/02/2011 5:24 PM

Interesting. I think I will consider a battery and regulator. Thanks much. Mike

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#25
In reply to #23

Re: Can DC Motor Operate at Different Amperages?

12/03/2011 9:41 AM

Mike,

Uweka's chart does not reflect an operating PV system. The voltage curve is relatively flat, under load, it is the amperage curve that is so variable, based on insolation. His working point is incorrect, as the panel will operate effectively at a range of less than .5 to 3 amps, making the range wide. The point should be a line, located over most of the arc. He is correct when he says suboptimal output (hence lower fan speed) will occur much of the time. This is not problematic. Insolation is driving your thermal collector, is it not?

1 What is the goal of your system? It is to energize a fan, so that as the air in the thermal collector reaches a useful temp, (but not below) air is moved into your home.

2 Are there correlations between the thermal production and the (oversized) PV production? yes, in fact many of these systems operate on 20 watt panels.

3 Is there a penalty for what Uweka calls inefficient operation? not to the effectiveness of the system, taken as an operating unit. In another words, does the system perform?

4 There are three penalties for adding a battery and regulator. The first is cost, both up front and recurring. The second is the reduced temperature efficiency you will experience unless the battery is in conditioned space (the very thing that is driving Uweka) Third, and most important, the robustness of the system is reduced, which also can be factored into efficiency, as down time and surveillance adversely impact net energy. Go back to question one. Is this relevant? How simply, and at what cost can you build a robust system that fills your program requirements.

5 Finally, please note that the design goal of driving the fan efficiently is not relevant. The design goal is to move the appropriate amount of air, at a time when that air is heated properly. Good luck! Most things in life do not operate at maximum efficiency, especially me.

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#26
In reply to #25

Re: Can DC Motor Operate at Different Amperages?

12/03/2011 2:16 PM

PFR, Thanks for your input. Mike

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#27
In reply to #26

Re: Can DC Motor Operate at Different Amperages?

12/03/2011 4:51 PM

no sweat Mike, good luck. Come back and post your experience, I'm one of those people who gets a lot more than they give from CR4.

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#28

Re: Can DC Motor Operate at Different Amperages?

12/16/2011 3:10 PM

I use solar panels and dc computer fans to cool my greenhouse. The brighter the sun the faster they run until the panels reach peak voltage which is the same as rated voltage of fans. I like the addition of a snap switch, but I couldn't find just the right temp. The beauty of this simple setup is, if it's cloudy or dark, I don't need fan to cool. I want to keep heat in when it's cool. This has proven to be maintenance free and built of all recycled/ reused materials. I don't use storage batteries because they are not needed. No sun, no heat, no need to cool. Good luck my friend.

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#29
In reply to #28

Re: Can DC Motor Operate at Different Amperages?

12/16/2011 3:53 PM

I purchased a 60 watt solar panel and hooked it up to the 12v fan. Unfortunately, although it runs fine it will not have enough power to move enough air for the setup I have so I'm going with an AC blower. Mike

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AKdude (1); Andy Germany (1); Lookfar (1); lyn (1); mikesoneill (10); PFR (4); PWSlack (1); Roland Burton (1); SolarEagle (6); Tornado (1); uweka (1); wareagle (1)

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