Requirements of 12 Volt DC Motor

If you use an 18-VDC motor, there should be little or no chance of damage from overvoltage. If the sunshine is less intense (off angle or clouds), the motor will just run slower; but this should be okay since the roof is also receiving less sunshine.

It's a 12 volt dc motor

Directly driving other than very small motors is tricky. Even if you short cirquit the panel at full sunshine you won't get more than 8-9A, on other times of day much less. It resembles a current limited voltage source. It's available power curve is narrow and requires some conversion and possibly storage to get good efficiency. An 136W panel will not start a 20W DC motor directly without help most of the day. Once it starts it will propably overrev. You can at least charge a big capacitor through a diode, then use a switch to the motor. But charging a small to medium Pb battery with a MPPT dedicated charger is the way to get the most of your panel. S.M.

Most photovoltaic panel suppliers can distribute, or direct one to where one may find, voltage regulators that can be used with their devices to condition the generated power before it is transferred elsewhere. Check, oh, say, yacht chandleries for suitable equipment, as well as electronic distributors for suitable retail outlets.

It's not possible to give a precise answer on this without knowing more about the fan/motor. Most small PMDC motors can easily be run at 125% rated voltage, and I've run 12v motors at 24v (checked with supplier first!) without problems. Is it a PMDC motor or a brushless motor (most small dc fan motors are brushless these days).

Also consider that, even if you can run it at 18v, the speed will be greater (probably not as much as 150%), and the load even higher, possibly double, which will mean even greater current draw.

I agree with PWSlack. Voltage regulators are cheap, readily available and easy to use. If you google 12v regulator you'll find lots of info. Here's one on ebay.

Got more information on the fan motor. It uses a max of 15a on start up then goes down to 10a and it requires a min. of 55w to operate. I guess that means that it won't work with this solar panel. Or does it? Will the motor just run slower if it gets less than normal power? I'll try a solar distributor to see if they have the appropriate motor but I was hoping to get an inexpensive one and I doubt a solar dealer would be inexpensive. The motor I'm talking about was on ebay. Thanks again for your help. Mike

The smallest current the DC motor will start depends on it's consruction, and among other details the angle the core slots are axially twisted. If they're not, you will feel the resistance when trying to move it, and that it peferes to stop at certain 'detent' positions within a revolution, because it's magnets apply forces only to core material and not to winding slots. To start at all you must overcome this applying a minimum current specific for every PM motor type. S.M.

If you already have the panels then there are two options to use these 12V motors:

1/ Put the motors in series. That way each will get about 9V which should allow them to run and the current drain is the same as only one of them.

2/ Find a suitable PWM (switch mode) motor controller for 24VDC input, 12VDC (nominal) output. The motor controller will act as a buck converter thereby reducing the input current.

I ran a DC brushless fan at 18 Vand it worked for several hours then it got overheated and burnt. it was a 40mm computer fan.

Very commendable and a good move.

Yes you could damage the motor with running it at a higher voltage than it needs, but that is dependant on the style and build of the motor. A 24 volt may fix that problem in a simple manner and run quieter......

Using a 12 volt lead acid battery and a charger (that can be adjusted to stop/start charging at specific voltages) will accomplish all you need to do. I may be able to assist with some circuits if you wish.

The battery will also help in spreading the time for available power so that even after dark, you will be able to ventilate the attic properly.

May I recommend brushless DC motors in the fans as they will run for many years with little or no maintenance so long as they never get wet......many small ones are better than a few large fans, backup in the case of one failing and cheaper to repair/replace.

Also, less chance of irritating noise. But you may need to have a circuit to control the final voltage to the motors to reduce noise. A very simple method is to be able to switch two motors at a time is to use a double pole changeover switch so that they are either in parallel or in series. Most good motors will still start in series mode, but if they don't, start them in parallel and switch to series to run for quietness. I use such a circuit on my caravan ventilation....one such a circuit for two 2 x 220 VAC and one for 2 x 12 volt DC fans as I sometimes camp away from the mains supply.

Also, note the mainly prevailing wind direction that you usually have around your house (especially in hot weather) and make sure that the fans are not working "against" that wind.

Sadly brushless DC motors are not easy to reverse without a lot of electronic knowledge and access to the stator coil connections.....

This setup is fine, but you should add a few pieces of equipment. That is actually the correct voltage for a 12V charging system (if open circuit is around 18V)

You will need

1 a very small charge controller with a load output and a low voltage disconnect. You can use a very small one, and they are inexpensive. You can use the small sunsaver and get it for less than $30.

2 a small 12V motorcycle, garden tractor or kiddie car battery. Some are expensive. Some are cheap. You can use a cheap one, less than $30. If it is a typical flooded lead acid (it has removable caps), you should check the water level at least every few months.

Connect the panel to the charge controller solar terminals. Connect the battery to the charge controller battery terminals. Connect the fan to the load output terminals of the charge controller. Use 10 Gauge wire, and keep them as short as possible. Enjoy.

Install this.

https://docs.google.com/viewer?url=http%3A%2F%2Fwww.axiomatic.com%2Fisolated-power.pdf

Although this looks like it will work, it does not contain circuitry to deal with insufficient current even though you may have sufficient voltage. It will still subject the fan to substandard operating parameters. The OP needs to condition the power supply, not shape it, or find a fan that will operate effectively at a vary wide range of voltages and currents, and be a brushless type that will not be damaged by variations. As some previous posters have noted, he may in fact have one, but didn't tell us.

a couple of links for you to consider if you are of the tinkering sort (which I presume you are.).

http://www.mdpub.com/555Controller/

http://freesolarenergy.1colony.com/555charger.html

Remember that 18V is the MAXIMUM it will supply under controlled LABORATORY conditions, under most circumstances it will supply less, possibly MUCH less.

Why not try an automotive radiator fan? You can probably pick one up at the local junk yard for the price of admission, it will even have a shroud.

As for the voltage, as others have pointed out the voltage shouldn't be a problem. As for the start, you could use a large (1 farad or so) cap to provide the surge for starting.

This sounded like a good plan to me. I went to a junk yard and picked up a used radiator fan. I hooked it up to a 12 volt battery and it puts out a lot of air. Now I have to complete my solar panel and I'll see if it works directly or I have to use a capacitor or what. I'll check back with an update when I finish. Probably not for 2-4 weeks. Thanks for all the help so far. I love this stuff. Mike

May I suggest that the probably size of a cap to do what you what may be be very, very expensive.

A lead acid battery is in some ways a cheap HUGE capacitor!

A leisure type battery, especially if kept cooler than your loft obviously gets (place it near to the cooling fan!) will allow you to use "more" of its capacity without damaging it.

If you go for the car battery type LA, keep its voltage between say 13.4 volts max and 12.6 volts minimum. That is, turn off fan when the voltage reaches 12.6. Though there is a little latitude provided it does not sit too long at 12.6 volts or less, say less than 12 hours. Leaving it undercharged for long periods will cause sulfation and early life failure.

Never charge it to anything over around 70% capacity or gas (which is basically the same thing!).

Make sure that the battery is in a well ventilated position, any gas produced is explosive.

Use fuses or similar to stop over current damage.

Use well dimensioned cables, properly and safely installed.

Enjoy!

GA JWthetech.

Just add a 12volt chargeable bettery in all you got till now. Wire them as they should be, and ventilate your fears with that plus the attic.

Have fun.

Running a DC motor at a higher voltage will cause it to run faster as the speed that a DC motor turns is directly proportional to the applied voltage. It is OK to run a DC motor faster if the mechanical load is reduced. Mechanical output power (hp) is calculated as torque load (in-lb) X rotational speed (rpm).

The problem with running a fan motor faster than its rating is that fan loading goes up as the cube of the speed. The bottom line is that the motor will overheat and the motor insulation will eventually break down.

I'm not enough of a motor guy to know whether you're right or wrong. The problem I see here is that the solar panel nominally supplies 18 volts, but the current is limited.

If the current is limited, I don't think the fan speed will even achieve rated speed, and, with limited current, will not overheat (or, will not heat as much as might be expected running connected to an 18 volt source).

OOPS! Rather than sharing knowledge, I shared ignorance.

I have to agree with you you that the motor will not be damaged. I know a fair amount about motors and know nothing about solar cells. If the power output is limited to 135W or the current limited to 7.5A the voltage supplied to the motor would be reduced. The motor would probably run safely at a reduced speed rather than at the 150% overs peed and over loaded as I suggested above.

Without knowing the characteristics of the solar cell, I don't know how to predict the the voltage output and therefore the speed that the motor would be running.

If the solar cell is inherently power limiting so that it would not be overloaded and damaged, I would give it a try.

Perhaps someone could share their knowledge on operating solar cells at or above their ratings.

Thanks for the followup!

My (admittedly limited) understanding of solar cells is that they are current / power limiting.

In other words, for a given amount of (sun)light, they will put out only a certain current, depending on the characteristics of the cell (including its area).

As I'm pretty sure others in this thread (or in similar threads) the more traditional and proven way to use solar cells in a case like this is to use the solar cells to charge a battery, and use the battery to drive the motor, with suitable protection in both cases to the battery to avoid undercharge and overcharge.

But, trying it without the battery would be an interesting experiment. ;-)

GA

Because unless the motor manufacturer says differently, the only safe way is to do EXACTLY as you say - 12 volts for a 12 volt motor.

An LA battery can have a slightly higher voltage (generally accepted to be around 13.2V as a middle value) this should still be within the acceptable range for a nominally 12 volt motor.

18 volts may not be..........(there are some others here who patently do not understand that!!)

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I wonder if the car fan motor the OP has bought is brushless or brushed, he did not say....in my limited experience, such motors are generally "brushed".....but I am fully prepared to be completely wrong!!

Being automotive it's a safe bet it's a brushed motor and actually designed for 14.7 V, that being the system voltage with a running engine/alternator.

It may however draw more than the design current in free air, and may produce more air/noise than desirable, so voltage regulation for speed may be a good idea.

Or it may well be the two fans in series are just right on 18 V

But the main reason for regulation is, PV cells don't actually produce a 'rated voltage' unless attached to the specified drain.

As said previously, at great length, in a past thread, PV's are a Watts device.

Meaning if the fan load is not equal to the panel current, the voltage will be the Watts result. E.g. 135 W / 3.6 A = 37.5 V ^{_{(about, depending on a list of variables)}}

However there are many solar powered motor devices that simply use a PV matched to the load. Fans are particularly common, as they match 'inherantly'.

So if the OP is building the PV, why not just match it to the fan.

Never mind, I believe I've found it Post #17 in Re: Solar Cells/Build Panel

Re: As said previously, at great length, in a past thread, PV's are a Watts device.

Can you give more clues to that past thread? I've started searching, looking for posts by you with PV and watt in them. Only found two, one of them being this, the other being about search mode for an inverter. Was it your post? Other clues?

Yes, also #3 & #25 - which might be of interest to the OP

^{_{(hence "at great length")}}

Thanks!

A good charge controller will however regulate the incoming voltage down to the proper charging voltage which is perfectly safe for a 12 V nominal motor. Also the 18V rating I suspect is open circuit voltage. If you put any load at all on the system i suspect the voltage will drop quite a bit. it is also a theoretically perfect voltage based on complete and perpendicular illumination.