- Assum that I have a DC electric motor, and I have five solar panels to power it. In the available light, each solar panel produces 5 volts max at 1 amp. Does it make any difference in how the motor will operate if I hook up the solar cells in parallel or serial? Does one way give better torque, while the other gives better top speed? (it seems like more current at a lower voltage would give better torque but less RPM’s, but what do I know?) - MC
On paper, either arrangement you’re describing yields 25 watts of power, however motors are current driven devices so I would suggest the parallel arrangement for 5VDC @ 5A. Keep in mind, it’s the load that demands a certain amount of current to operate properly, and it is up to the power source to furnish it. And just because the supply can deliver 5A doesn’t mean the load will draw it. Bottom line is: it’s hard to give a definite ruling because we don’t know the motor’s power requirements. If your motor is rated for 12V @ 2A (consumes 24 watts), the series connection won’t work even though it’s good for 25 paper watts. The motor will load the cells down as it attempts to draw 2 amps from a source that can supply only 1A.
Okay, okay… it might spin at some feeble rate, but it won’t operate at its rated capability.
Simply put (Attrayant is correct), the solar cells are like batteries, which are like capacitors, they are summative in parallel. Resistors are summative in series.
I’m certainly no expert in this area, but I’ll give my two cents anyway.
First of all, what voltage is the motor rated to operate at?
Solar cells do not behave as voltage sources throughout most of their power rage; they behave as current sources. Therefore, if you hook them in series, the current will be low and the voltage will be high. Possibly too high. If you hook them in parallel, the current will be high and the voltage will be low. Possibly too low.
That is why you must first determine the operating range of the motor. That information will dictate how to hook up the solar cells. (Note that five solar cells can also be hooked up in a variety of parallel / series combinations, allowing a variety of voltage / current characteristics.)
Also, the power requirements of a motor spinning freely and one under load ( i.e. driving something ) will differ. In the latter case it will draw more current, I believe.
Batteries are summative in series or in parallel. In series, their voltage can be summed, in parallel their current can be summed.
This might be a little more clear if you started from basics. Voltage is a force, electromotive force to be exact, compare it to the pressure in a water pipe. Current is rate of electron flow. Multiply voltage by current in a given circuit and you have power in watts.
When like devices are conneted in series, voltage is additive and current is common. This is true of batteries or the voltage drop across resistors. Three 1.5v flashlight batteries end to end give 4.5volts but current flow is the same through the entire circuit. Put three identical reisistors in series and apply the 4.5 volts and each reisister will have a drop of 1.5v.
When in parallel voltage is common and current is additive. Three 1.5V batteries in parallel are still 1.5V total. Resistive loads are a bit different. Divide the product of the the resistors Ohm values by the sum to get total reisistance. Two ten Ohm resistors in parallel is (10 * 10) / (10 + 10) = 5 ohms.
You have too many unkowns to know what is the right setup. As crafter pointed out you can exceed the range of the motor. Also a motor is an inductive load, not a reisistive one so the physical load on the motor makes a huge difference.
That’s true except with one qualifier: batteries can be put in parallel as long as they all have the same voltage. For example, you (obviously) shouldn’t put a 1.2V NiCad in parallel with a 2.4V NiCad. Furthermore, even if both batteries had the same number of cells (example: two 1.2V NiCad’s), I would check to make sure the voltage on each was about the same before connecting them, else you’ll get a large current transfer.
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- Okay, more accurately:
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- I want the maximum low-RPM torque from the motor at startup, but I also want the maximum RPMs, but the maxRPMs doesn’t have to have nearly as much torque as the lowRPMs does. I am wondering if the cells should be in parallel at startup, and then switch to serial. Presume that I have a motor (15V @ 3A max) and a switching arrangement set up that could re-orient the connections of the cells (5V @ 1 amp).
So the motor’s max rating is-----15V @ 3amps.
The serial power connection is—25V @ 1amp.
The parallel power connection is-5V @ 5amps.
Would there be any benefit or not? - MC
Your best bet by a long way is to connect the solar cells up to a battery charger unit and run the motor from the battery via a relay which would break the connection from the cells as well as make the connection to the motor.
I’m wondering if this motor is going to operate continuously or just when you turn it on.
There is one way to get maximum low speed torque out off a DC motor but if you try it it is imperative that you make sure that there is some mechanical load connected at all times when it is running or it will overspeed and self-destruct this might even cause injury.Usually the load is bolted to the output shaft rather than using belts which might wear and eventually snap.
Most DC motors are connected with the windings on the rotor in parallel to the field windings, sometimes there is an additional field winding in series with them.
Look at the connections going into the motor, chances are that they are joined at the brush holders.
To change into a series rotor and field connection you need to go to one brush and separate the wire leading to it.
Run one wire to the now free connection and one wire to the brush holder.
This will cause power to pass through the field and out again to the brush you haven’t touched, then it will go throught the rotor and out throught the brush you have just connected a wire to.
The voltage will be dropped across both windings so you will probably have to increase it.
That’s a very good point, casdave. And if you intend on running the motor 24/7, then you (obviously) must have a battery in the system.
Not to toot my own horn, but I designed the electronics for a solar-powered refrigerator called the “Vaxicool”:
The entire system consists of a PV cell (e.g. 70W Solarex or eq.), deep cycle lead-acid battery, and refrigerator w/ internal electronics. (The electronics consisted of a couple custom-designed PCB’s I designed in-house, an OTF switching power supply, and OTF charge controller.) I did a lot of research into charge controllers, and I finally down-selected to a model manufactured by a small company located in Missouri called “B.Z. Products.” The B.Z. Products charge controller was inexpensive, efficient, reliable, and worked great.
But it must be kept in mind that the “battery / charge controller” route has advantages and disadvantages:
Advantages:
- Constant-voltage power source
- Can power loads 24/7 (assuming you have a net energy gain from the PV cells)
- Might be a better impedance match for the load.
Disadvantages:
- Might not be as electrically efficient (because of diode drops, switching transistors, extra circuitry, etc.), though this depends on how well the PV’s are matched to the load.
- Must have one or more batteries. Batteries are expensive, heavy, dangerous, toxic, and must be replaced every now and then.
So here’s my opinion: if you only need to run the motor infrequently or during daylight, then I would do everything possible to try to run the motor directly from PV cells (i.e. without a battery and charge controller). But if that proves to be unworkable, then I would look into getting a battery and charge controller.