If I spin a turned off ceiling fan with my hands does it put electricity back into the grid?
If your ceiling fan is turned off, it’s not connected to the grid.
No, for all practical purposes.
What it will do depends on whether the fan is on or off.
If the fan is off, it won’t do anything but go around backwards. The stator coils will not be energized, so you won’t get any output at all.
If the fan is on, you will burn out your motor. Before you do that, you will disrupt the grid power insignificantly. I say disrupt, because to put power back into the grid, you would have to match the grid frequency (probably 60 Hz, if you’re in North America) or a harmonic of it. But, since your fan motor wasn’t designed as a generator, it probably won’t be able to handle the excessive heat and will (if it is a modern fan) blow a thermal fuse.
Now, if you were on a DC system and your fan had a permanent magnet motor, you could, but assuming you live in the US or Canada and the fan is less than 75 years old, that isn’t likely.
excavating (for a mind)
I was wondering if this was possible when I saw an unplugged standing fan turning in the breeze.
So for some designs of motors, it’s possible? But no modern fans use such motors?
The two word answer is . “Not possible”.
Not possible since the power produced by turning the fan blade by hand is just too small , and its at the wrong frequency. There’s no inverter which is so efficient that it could operate on such low power and still return power to the grid.
Oh forgot to say: how about not into the grid? How about charging batteries?
Ah…windmill comes to mind. Different design parameters but basically a fan none the less.
Spin it forward or backward makes no difference. ( As the OP did not specify a direction. {I don’t think he was necessarily thinking backwards with the 'back into the grid} )
RPM = 120 F/n where n=number of poles.
Can anyone derive that for me? There is a 60 to convert frequency in Hertz to RPM,
but I can’t come up with the rest.
RPM of a dc motor is a function of voltage, is it not?
I don’t know what kind of electric motor is in ceiling fans. If the motor in question has permanent magnets in it then, yes you will be able to generate electricity by spinning the motor by hand. In order to charge batteries you would need to rectify the AC output into a DC signal using a bridge rectifier.
Chances are there are no permanent magnets in the motor, meaning that electricity is used to create an electromagnet for the field coil. In this case you can spin all you want but unless the coil is powered, not electricity will be generated.
Squirrel cage motors become generators when rotated faster than their intended RPM, and I believe will feed power back into the grid when that happens. I think these are unlikely to be used in a ceiling fan.
if by “squirrel cage” motor you mean an AC induction motor, this isn’t how they work. The armature is not electrically connected to anything so spinning it manually will get you nothing. an example of an AC generator is the alternator in your car; the armature spun by the engine becomes the field when energized through slip rings/brushes. The spinning field induces current in the stator windings.
It IS how they work. If the stator is connected to the grid, and the rotor is driven above synchronous speed, (negative slip) then you have created an induction generator, which will supply power (a tiny amount) to the grid.
The rotor is energized any time it’s speed differs from synchronous speed, either above or below. The difference between the operating speed and synchronous speed is referred to as “slip”=(Sync speed/operating speed) -1 in efficient industrial motors it is a few percent. A ceiling fan usually operates at very high slip rates,* (even more so at the lower speed settings) so to get up above synchronous speed you would need to drive the fan much faster than normal operating speed. It might be a good idea to remove the blades if you intend to try this, as they may well not be rated for such abuse, and will soak up a great deal of your driving power regardless.
*I tried to put a number on this, but fan manufactures don’t seem to specify how many poles their motors have, and many of them don’t even publish operating rpm.
It is possible to turn a motor’s shaft and generate electricity. It’s what they do at Tom Sauk Mountain in Missouri. They use motors to pump water up to the top of the mountain and later, let the water run down through the motors to generate electricity.
No, it won’t work with your fan, for reasons already given.
I haven’t taken many fans apart to see what kind of motor they use but I was under the impression that they generally use a split capacitor induction motor these days. This type of motor is simple and cheap and the speed is easy to control by switching in and out different capacitors.
If you have an AC synchronous motor, it can be used as a motor or a generator. If you tie two of them together, they tend to lock in frequency. If you spin one faster, it will start to supply current to the other generator, causing it to act like a motor and also speed up. This puts a bigger load on the one you are trying to spin though, since you are not only spinning it but are also spinning the other machine through electricity and magnetic fields. The more motor/generators you connect together, the harder it becomes for one motor to affect the speed of others, and after a handful of motors are all connected together there is so much force involved in trying to change the speed that each individual motor/generator can’t supply enough current to change it without burning out. At that point you can effectively think of the entire system as an infinite bus, which is effectively a “power grid”. At that point, trying to spin each individual motor faster will result in it adding more power to the grid but it can’t affect the actual speed of the grid. Changing the speed of the entire grid then becomes a lot more difficult as it has to be a coordinated effort among all off the motor/generators, and this is what happens on the real power grid.
At this point, if your AC synchronous motor/generator spins at exactly the speed of the grid, it exactly balances out what is on the grid and doesn’t generate or consume power. If you decrease the mechanical force spinning it, it becomes a motor. If you increase the mechanical forces on it, it becomes a generator. So it can operate anywhere between completely being powered by the grid (as a typical motor) to generating as much current as it can without burning out its coils, but it can’t change speed.
You might find an AC synchronous motor in a really old fan, but I would be very surprised to find one in a modern fan.
The induction motor in a modern fan will add current back to the grid if it is already on and running and you force it to spin a bit faster. Modern fans are designed to very easily slip though, and what is probably going to happen is that if you try to spin it faster it will just slip past the point where its coils are in sync with the grid and you’ll very quickly lose its ability to function as a generator. So while it is theoretically possible to generate some power with one, as a practical matter you aren’t likely to ever make it work.
During each sine wave, the voltage increases, effectively turning the coils into electromagnets, then decreases back to zero. These magnetic forces are what spins the rotor and it moves one pole during every part of the cycle. But each sine wave also then goes negative and then back up to zero for a second set of charging and discharging of the electromagnetic coils. This makes the rotor move to a second pole locations during during the second half of one cycle of the AC sine wave. In other words the rotor moves two pole locations for each one AC cycle, so you have to multiply by 2. Then multiply by 60 to convert seconds to minutes and divide by the number of poles.
So it’s 2 x 60 x F / n or 120 F/n
A DC motor basically spins as fast as it can until the applied voltage balances out with the internal voltage it generates itself as it spins. This can result in fairly high rotation speeds though so quite often the speed is determined more by friction and the load on the motor rather than its unloaded speed. Increasing the voltage will increase the current as well, which increases the torque and push the motor to spin faster.
This isn’t a very efficient way to control the speed of a DC motor though. More often something like PWM control is done. PWM is pulse width modulation. Basically instead of feeding the motor a constant DC voltage you turn the supply voltage on and off very quickly and vary the on time of the pulse compared to the off time of the pulse to generate more or less average torque. The speed of the motor is monitored and fed back into the PWM controller so that it constantly tries to generate exactly enough torque as necessary to balance out the load and achieve the desired speed.
Thanks, Comp.
Good info, thanks! It was just a question that occurred to me as I was going through the house switching the direction of fan rotation for warmer weather. …and no, didn’t even occur to me that the off switch in GENERAL wouldn’t allow for it. Silly.
Great info though, and for whatever reason when I think of electric motors I only tend to think of permanent magnet motors, need to change that thinking. 