I have an AC motor from a toilet fan, and i want to use it as an air turbine to generate electricity.

all i know about the fan/motor is the following:

  • Input Voltage:220-240V AC
  • Input Wattage: 15W

how can i know the Output Voltage if rotated at the rated RPM?

i tried rotating it very fast (dont know how much RPM but it was visually as fast as if it was connected to the mains), and i only generated a tiny 0.7V AC. why is it that low?

thank you.

  • 1
    \$\begingroup\$ What type of motor is it? \$\endgroup\$ – Hearth Apr 29 '17 at 15:41
  • \$\begingroup\$ i do not know exactly. all i know is that its a small AC motor. 2 inches long, and 1 inch thick. so its quite small \$\endgroup\$ – Mohdak Apr 29 '17 at 15:43
  • \$\begingroup\$ Does it have a part number? \$\endgroup\$ – Hearth Apr 29 '17 at 15:52
  • \$\begingroup\$ Most of this type of fan motors are small squirrel cage motors. They will NOT work as a generator without modification, though you will see experimentation with capacitive self excitation. The results are hit and miss and the motor speeds need to be high. \$\endgroup\$ – Jack Creasey Apr 29 '17 at 16:34
  • \$\begingroup\$ but as i mentioned, i did generate 0.7V. my question is, why is the voltage too low? can you please give me a technical answer. I am an electrial engineering student, so i can understand the engineering terms. \$\endgroup\$ – Mohdak Apr 29 '17 at 18:25

These motors are usually squirrel cage induction motors. That means the rotor is excited by transformer action from the stator, and consists of a few shorted windings on a transformer-iron (soft iron) core. It has nominally no magnetism of its own, though the soft iron may have a little residual field from its last use.

If you spin it, there is little magnetic field in the rotor, so little voltage generated at the terminals.

You can use an induction machine to generate power, but it must be connected up to the voltage source first. When you connect it up, it will run as a motor, with a small amount of slip. If you apply a load, the slip will increase, and it will draw power from the supply to feed into the load, as you'd expect from a motor.

If instead of slowing it down with a load, you speed it up mechanically, the slip will decrease, and go negative when its speed exceeds synchronous speed. Under this condition, it will take mechanical power from what's driving it, and push power into the electrical connection.

  • \$\begingroup\$ perfect! thank you. but if i have a 240V AC 15W motor with a permanent magnet. how can i can i know (or calculate) the voltage i can generate at the rated RPM. \$\endgroup\$ – Mohdak Apr 29 '17 at 19:22
  • \$\begingroup\$ An AC machine with a permanent magnet rotor is called a synchronous motor. They are fairly rare. There are some 1 watt ones that drive mains powered wall clocks, but they are not used for general purpose motors. If you spun one of these, the voltage, speed and frequency would all be proportional, and voltage as a generator would be less than that required to drive it as a motor. There are applications for much larger synchronous machines, but these tend to have wound rotors, to allow the field to be controlled. \$\endgroup\$ – Neil_UK Apr 30 '17 at 3:58

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