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Question:

How to connect an AC brushless motor to a transformer?

If I have an AC electric motor with 3 wires instead of two, how do I modify the wiring to be able to connect it to a home-built transformer? (to boost the voltage created by the motor acting as a generator)

Answer:

I don't think this motor will work very well as a generator. .
You need to identify the motor. What are the nameplate ratings. A brushless motor implies a DC motor with electronic power supply that uses 3 windings, white similar to a small AC generator. Used in video recorders etc. An AC motor without brushes is usually some sort of induction motor, with a squirrel cage rotor. It could be a 3 phase motor (usually bigger) or a single phase motor with some provision for a phasing capacitor or starter switch or klixon. All these can be used as generators in some way, and if the RPM is right they will generate voltage similar to their motor voltage. Induction motors need excitation by using capacitors or connecting to a supply. that both excites and receives the output. If it is a shaded pole motor it will be a little disappointing. A transformer would boost the voltage assuming it is designed for the frequency of the generator and the voltage and power levels involved. Look up the motor types and you may find explanations and pictures that help you identify the motor type, and size. This activity has the potential to be hazardous, but I have no idea exactly what it is you intend to do. It makes sense to get someone knowledable to supervise the activity to make sure you are not at risk.
If this really IS an AC motor and has 3 wires, it's 3 phase. If it's an AC motor single phase, 1 of the wires is for another purpose e.g. capacitor connection. So if it is marked brushless, it could be a 3-phase model aircraft type motor, which will require its mating speed controller/ 3- phase (battery or DC fed) controller. The other possibility is you have a 3- wire computer brushless DC fan motor, 2 are for connection to DC, and the 3rd. is a rotation confirmation signal wire. Further. Transformers output AC. A motor running as a generator cannot possibly boost its voltage. I'm glad you haven't mentioned joule thief''' this time. You still have not stated EXACTLY what you are trying to achieve, or maybe this is the start of another brand new misconception re electrics/ electronics?
What you have is ether a 3-phase motor, which is no good on your transformer, or anywhere else indoors either, OR if the motor has brushes, then it may be designed for CW and CCW direction. I don't think you will generate any higher voltage than what you are putting into it, that would be like perpetual motion.

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