# If I spin a house fan backwards

Will I pump electricity back into my house? Is the motor that runs my fan essentially a servo?

When you move a wire through a magnetic field, some current in induced. How the motor is constructed and if it’s a/c or d/c will determine how much actual power is produced. For a a/c motor, I don’t think it matters which way you turn it by hand, for DC the voltage would change + to - if you reversed direction.

You may pump a little power back, but it will most likely be out of phase to the power co’s, and anyway be so small as not to do anything.

I just got a big headache thinking about this one!

Theoretically, it is possible to produce electricity from the fan under certain conditions.

As a practical measure though you would have to turn the blade at the right speed and precise position to put power back on to the grid (your house)

But it wouldn’t matter which direction you turned the fan.

Of course, in order for you to put power into the grid you would have to be attached to it, and if that happens, you’d already be running the fan. You could cut the main power line you your house though…

How hard would it be to hook up an incandescent light bulb directly to a fan’s plug? They’d form a simple circuit and you could spin the fan forward and back in a dark room.

Could this be done with a simple extension cord with multiple outlets? Would I need to short the extention cord’s plug?

Your fan may use an AC induction motor, which won’t generate electricity when manually spun by your hand.

Bolding mine.

I don’t think that the fan would produce at that point. I admit this is a SWAG but I think that the opposing magnetic fields in an electric fan both require house voltage. I don’t believe that permanent magnets are used. So you would have to somehow induce voltage in the fan before it could itself create voltage. Otherwise you would just be spinning two sets of wound wires near each other.

The motor that runs a house fan is an induction motor. It runs a bit slower than synchronous speed. (which at 60 Hz would be 1800 or 3600 rpm for a four or two pole motor respectivly)

If you connect this to the power line, and blow air through the fan such that the motor is driven to slightly more than synchronous speed, you have an induction generator, and the “motor” will be delivering power to the grid.

It is possible to make this work without a line connection. This would be done by connecting large capacitors across the line. The frequency will not be well regulated in this case, though.

I’m not at all sure this is correct. An induction motor has a rotor with a number of shorted turns on it. The reason the rotor turns slower than the theoretical synchronous speed is that it must do so in order that there be a changing magnetic field surrounding the rotor turns to generate a current in them. This rotor current produces a magnetic field that interacts with the stator field produced by the power from the source to make the motor turn.

In order to generate a current in the rotor that would induce a current back into the stator you would need to run the rotor faster than synchronous speed and I’m not sure that you can do that.

It’s going to requsire some more thinking.

Why could you not turn the rotor at any desired speed within the physical limits of the rotor materials to resist centrifugal force?

Well, I’m not sure about this but it seems to me that if you try to spin it faster than synchronous speed you develop a strong braking force through the interaction of the rotor field and the stator field. And the harder you push the harder it pushes back. It is this effect that makes me not at all sure that you can’t generate current back into the source with an induction motor.

I’m still thinking, not constantly but off and on, and I don’t intend to lose any sleep over it.

I have to agree with AndrewL on this one. It’s been a long time since physics class, but the basic principles behind motors (and transformers) are that a current carrying conductor will generate a magnetic field and conversely, a conductor moving through a magnetic field will create a current. An AC induction motor uses the (house) current in the stator to generate a moving magnetic field which induces a current and therefore a magnetic field in the rotor. Magnetic forces being what they are, the magnetic fields induced in the rotor will attempt to align with those of the stator. The stator fields are rotating due to the alternating frequency of the incoming power, so the rotor follows along and spins the fan.

If you spin the fan WITHOUT the exciting field being present in the stator, nothing will happen electrically since there is no magnetic field, but you can make air move.

What David Simmons is pondering is whether spinning the motor even faster than normal while it is running would generate electricity - that is, current would stop flowing FROM the grid, and instead flow INTO it. As Kevbo pointed out, this is an induction generator and is in common usage in the wind turbine industry See this link for detailed info.

As far as turning the fan backwards, all that does is put a VERY high load on the motor, which just causes the motor to get very hot and stop working permanently, unless it is thermally protected.

Your cite seems to show that you can generate electricity into an already energized grid with an induction motor. Now if I could only read the print.

This link from ConEd gives considerable detail on the power industry’s view on induction generators and what it expects for you to put one on their grid.

Also, the average induction motor is emphatically not designed for generator duty. Of course, you already knew that.

Driving home from down south today I was thinking about this and the armature reaction of a motor convinced me that you could generate power with an induction motor, however inefficiently, connected to an energized grid. Any time armature current flows in a motor that current affects the field current so, I thought, if you hook a gasoline engine to the rotor on an induction motor and turn it faster than synchronous, that extra power input will react back into the field and thus the grid as a power input.

Frankly this sounds like a great idea. We’ve already heard why the AC motor may not function for this purpose, but if you bought a 12V DC fan (for use in a car or boat), it seems like this should work.

A parallel connected dc motor will work as either a generator or a motor. All you need to do is connect the output to a load and turn the rotor and it will generate electricity. How this happens is sort of interesting. There is always a little residual magnetic field in the field poles of the motor. If the load isn’t to much, i.e. if not too much current is required, that small residual field will generate a small output voltage which is applied to the parallel field. This output voltage produces a stronger field which produces a bigger output voltage which produces a stronger field which produces a bigger output voltage and so on. The output voltager increase continues until a stable point is reached where no more voltage buildup is possible and the generator output will be constant for a given load current. Such an arrangement is referred to as a self-excited generator which sounds sort of racy, but that’s the name.

And to answer Nurse Carmen. It isn’t the direction that you turn the motor so much as it the speed with which you turn it. Synchronous speed for a two pole motor is 3600 rpm with 60 Hz line voltage. Faster than that you put power into the line, slower you take power from the line. The direction of rotation doesn’t matter (but there are some types of induction motors where it does matter but that’s too complicated to worry about.)

… but of course if you do it fast enough, the house will go backwards through time and actually get younger.

Yup, the shorted turns in an induction motor armature require excitation via the stator field, and a finite slip frequency. The slip can be positive or negative, however, and this determines if the machine is a motor or a generator. If the rotor has some risidual magnitism, then a capacitor bank can supply the excitation power instead of the electrical grid.

Induction motors are in common use as single machine motor-generators, used to convert single phase power to three phase power where such is not economically available, my garage being the example with which I am most familiar. You can buy them commercially as a rotory converter, or build your own from a motor.

Single phase induction motors work pretty well as induction generators. The starting windings take up some slot space that could otherwise be taken up by heavier wire, resulting in reduced copper losses.

There is an issue with three phase motors, which are frequently delta connected. In generator service, there can be issues with circulation of harmonic currents, so a star or wye connection is preferable to reduce noise and losses by that mechanism.

Motor armatures are compromise designs between high starting torque, speed stability, and good running effiency. In a generator, only effiency is a concern, so more effiecient rotors are almost certainly possible.

On the fan as wind-generator thing: One issue is that the camber of the blades is opposite what is desireable/efficient for a wind turbine.

If you spin your house fan backward, you’ll hear the hidden Satanist messages.