Sunday 2 March 2008

Converting an induction motor to a generator

if your trying to do this, I think i can help you.

Getting Started

You need to understand polyphase circuits because most common induction motors have at least two phases. www.allaboutcircuits.com is an excellent reference.

You have an induction motor, from somewhere.

you have to work out what kind of motor it is ?

some common types are:
  • Three Phase, is very popular for conversion.
  • Single phase, rare
  • Two phase, capacitor start and capacitor run (most common for single phase power sources)

Three phase conversion is what all the literature is written about.

But I wanted to convert an ordinary induction motor from a washing machine. My dad had this washer and the computer failed, so it was cheaper to buy a new washer.


I'm talking about 240v mains power at 50hz, because in Australia we like high voltage. Of course the same applies to 120v.


Two phase induction motors have two induction coils with their poles offset by 90 degrees inside the stator winding.

They use a capacitor to create a leading current in one of the phase coils. this causes the rotor to start spinning in the direction of the starting coil.

see lessons in electric circuits to fully understand how the two phase induction machine works.

How to do it

The trick is, treat the two phase machine the same as a 3 phase machine with a missing phase.

Connect the capacitors in a Y (Star) (minus one phase) configuration in parallel with the coils. Make sure you connect the neutral wire to the centre of the Y between the coils and the caps.


You need one capacitor per coil.

the other tricky part is determining what size capacitors to use.

I've seen lots of graphs that show cap sizes and things, but it didn't make much sense.

Capacitor Sizing

What you need to understand is that a resonant LC circuit needs to be set up by wiring each inductor and capacitor in parallel

You can easily calculate the capacitance required for resonance if you know the inductance and the frequency.

c = SQUARE(2*PI()*F)*L

where
L is inductance
F is frequency

Yeah yeah, my background is computer software so I like my functions written in plain text thanks very much.

Frequency is really easy, its the design frequency of the machine, 50 hz in Australia.

You can use any frequency you want to run the machine at, but i suggest making it compatible with all your appliances. Also you need a voltage source running at that frequency for the measurement.

Best one i can think of without buying a function generator is the mains, it's pretty reliable(just hope there's no switching going on), it for sure will be close enough to get an usable value from the equation.

Inductance is a little tricky to measure. You can just fudge it a bit and it will get you close enough

Connect the motor to the mains, wired as a normal motor, measure the voltage and the current with the motor running without mechanical load( ideally spin up the rotor to synchronous speed with another motor, don't need to, just fudge it).

You also need to know the power factor of the voltage source, but dont' worry, just fudge it, assume 100%

calculate the impedance Z = E/I

measure the resistance of your motor across the terminals,

Z - R = Xc + Xl

subtract that from Z, if there's any capacitors in the circuit subtract

Xc = 1/(2*PI()*F*C)

Where
F = frequency 50 hz
C = Capacitance (Farads)

Xl = Z - R - Xc

Ltot = Xl/(2*PI()*50)

Lphase = Ltot / 2

Where Lphase is the inductance per coil in the two phase induction machine.

now to determine the capacitance required

Cphase = 1/(POWER(2*PI()*50,2)*Lphase)



now you know the approximate impedance of your coils (all of them) in a two phase machine the coils are connected in parallel so they divide the inductance.

If your motor is capacitor run, it probably has equal inductance coils, so multiply Z by 2 to get the inductance of each coil.

If it is capacitor start the start coil is probably smaller than the run coil, so it will not be a simple half.

Now you can approximate C by solving the resonance equation.

How to get the capacitors

You need the motor run caps! cause they can handle the voltage. They need to be high voltage cause they could be exposed to phase to phase voltages. that would be

SQRT(2)*Vln

Where Vln is voltage line to neutral.

339 volts, for a 240v 2 phase motor.

get the 400v caps

you can get them from Jaycar electronics, about $12 a piece.

Don't worry if you can't get the exact microfarads, it just means the LC circuit will resonate at a slightly different frequency, it will still work.

In fact varying capacitance is the primary method of voltage control for the induction alternator.


Starting the Alternator

a lot of jokers reckon that the induction generator can be started from remnant magnetism in the rotor, thats bullshit. Maybe for large machines it works, but for these small ones it doesn't.

You need to charge the caps.

The easiest way is to briefly connect the capacitors to the mains power.

Obviously this is not possible in remote, so you can use a battery, you just need to get the voltage up. What piece of hardware does most remote installations have? an inverter, so charge the caps with the inverter

Another point to make is that you need to switch the caps in once the machine is spinning.

Or as I do now, just give the caps a kick from the mains when the machine is spinning, you only need to flick the mains on and immediately off again.

I have successfully started the alternator using both methods.

And wow what a surprise your generator will come to life. Its not that hard!

Ideally I would like to have a 400v dc supply to charge the caps with.





As can be seen in this photo there are three wires coming out of the induction machine.

the white wire with the red stripe is the neutral. The blue and the purple are the two active phases.

The capacitors are connected in serial with each other and the neutral is connected in the middle of the serial capacitors. The two active phases are connected on the each side of the serial capacitor, forming a two phase Y (wye) connection.

Those capacitors are motor run capacitors rated to 400v of 30 microfarad each.

This generator produces 240v phase to neutral at 50 hertz

And 240v phase to phase at about 33 hertz

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