# Is it good idea to charge a battery from a starter-capacitor of an AC generator?

Some friends and I took a working electric motor out of an electric lawn mower.

We suspect it is a single-phase AC induction motor (correct me if I'm wrong) with a starter capacitor (pictures above). Our plan is to use this motor as a generator for a wind turbine. We tested the motor on mains voltage (230V 50 Hz) to prove its functionality. We also tested the motor twice as an generator. We used an air compressor to turn the cooling fan of the motor.

First test, we attached our multimeter to the original terminals of the motor. We got around 5-10V.

Second test, we attached our multimeter to the terminals of the starter-capacitor. That time we got pretty close to 30V. Obviously we want to get as much power as we can out of this thing, to "impress" our teachers (it's a school project).

Finally my question is: Is it a good idea to power something like a battery to these capacitor-terminals? (Of course we need to convert the AC to DC.)

I would also want to know if this generator can actually be used for a wind turbine?

• The problem with an induction motor is that it's very 'difficult' to use as a generator. The output you happened to get from it is down to the residual magnetism in the rotor. The way an induction generator should be used is to drive it with an AC voltage. When it's turning at less than synchronous speed, it's a motor absorbing electrical and delivering mechanical power. Spin it mechanically faster than synchronous speed, and it will absorb mechanical power and deliver power from its terminals. But you need the external AC excitation to control it. Ignore the capacitor, once started. Mar 6, 2019 at 10:47
• No, not a good idea. Mar 6, 2019 at 10:50
• For those sceptics, I would like to inform that I have a professional made single phase genset that works exactly in the manner described. If the motors and generators aren't your strong field, then you should avoid giving false satements. Mar 6, 2019 at 13:19

There are several challenges to using a single-phase induction motor as a generator driven by a wind turbine.

1. The output voltage will be proportional to speed. The motor referenced in the question, would need to be driven nearly 3000 RPM to produce an output of 220 volts. Sufficient mechanical speed increase to achieve that speed may not be very efficient.

2. The required capacitance varies with load and speed. The 16 uf capacitance value shown is likely too small. It may need to be as much as 10 times that. Wind speed and load variations could cause the system to cease generating.

3. It may be necessary to go through a manual starting procedure it initiate generation successfully. The generator may not build up voltage if it is started with a load connected.

4. The motor depends on residual magnetism to initiate generator action. You may need to be prepares to remagnetize sometimes or perhaps every time it is started.

With some effort, you may be able to successfully demonstrate wind-turbine electrical power generation using the motor you have as an induction generator, but it may not be as impressive as you would like it to be.

Try to connect the capacitor directly on auxiliary winding. In such may the remanence of magnetic field would induce a voltage in the winding, but since the capacitor is connected, the current will flow 90 degrees out of phase, making a suitable excitation field in the stator.

simulate this circuit – Schematic created using CircuitLab

Please note that the a large current can flow trough aux winding. Use some clamp meter. If the voltage at the nominal speed exceeds the nominal voltage, then the capacitor is too big, analog, if the voltage is small then you should put a second capacitor in parallel.

EDIT: If you apply to much load on this kind of generator, the rotor will demagetize and you will loose the power. In such scenario you can magnetize back by connecting a 12V battery on main output for a very short time.

An this is how it is wired now:

simulate this circuit

• Hi Marko, Would you be offended if I fix a couple of spelling/grammar issues, which might be making it difficult for readers to fully understand your answer? Although I can improve some other parts, I couldn't understand what you tried to say here: "In such may the remanence of magnetic field would induce a voltage in the winding" Did you mean to say something like "In such a way [meaning something similar to "as a result"], the remanence...". I'm not an expert on motors, but I'd like to help make the answer easier to read, if you think the time spent doing that would be worthwhile. Thanks. Mar 7, 2019 at 15:49
• By "remenance" he meant "residual". AC induction motors use ELECTROMAGNETICS, so when there is no electricity on them, there is no magnetics, hence they don't make good stand-alone generators; a "chicken vs egg" issue. Residual magnetism means that the iron (steel) inside of the motor is retaining SOME small amount of magnetism when not powered. In his design, the small amount of electricity that the motor can produce with this can be stored in the capacitor to build up the magnetic flux in the motor and make it generate. It's not terribly efficient in terms of the input vs output but it works Mar 8, 2019 at 3:11
• @SamGibson You can correct it. I am not a native english speaker, so I believe there are many mistakes. Mar 8, 2019 at 9:53
• @J.Raefield - Thanks, yes I understand the term (unsure why you capitalised the word). However the grammar issues in the quoted sentence make the intended meaning unclear to me, and I don't want to change the intended meaning. For example why "may" followed by "would" - "may" is conditional (but conditional on what?) whereas "would" is unconditional. That discrepancy between conditional & unconditional is part of what I was trying to clarify, in order to avoid a subsequent edit changing the intended meaning. The motor-related terminology isn't the main problem :-) Thanks anyway. Mar 8, 2019 at 10:46