I’ve always heard that you have to spin it faster than the speed it wants to turn as a motor… What if I just wanted to couple a 3 phase induction motor to a gasoline engine to make electricity? What would it take to get the magnetic field started? And once started could the power source be disconnected and would the generator keep producing electricity provided there is an electrical load and the engine is still spinning?
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Also part of the reason for creating this thread is I read this document yesterday. Redirect Notice
I’ve spun inductuon motors by hand before and they “freewheel” without any drag so I didn’t think they generate any voltage while doing so. I guess I had never heard that you had to start spinning the rotor without a load attached first before you could start generating electricity?
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running the motor as a motor. see, with an induction motor, the armature (rotor) is completely passive. it has no electrical connection to anything; the magnetic fields set up by the stator induce a current in the armature cage, and that current sets up its own magnetic field which is “dragged around” by the stator’s rotating magnetic field. when you use an external motive source to “overspeed” the armature, the armature’s magnetic field induces a current in the stator windings.
when I say “overspeed,” I mean spin it at the desired AC output frequency. Induction motors have “slip;” in order to generate torque the armature must spin slower than the rotating magnetic field. for 60 Hz AC, the stator’s rotating magnetic field is rotating around the motor at 3600 rpm. Typical “slip” at rated load means the armature is spinning at roughly 3450 rpm. If you use external mechanical means to force the armature back up to 3600 rpm, it’ll induce current in the stator.
nope. once the stator current is gone, there’s nothing to induce current in the armature and it’s just a spinning chunk of iron and copper/aluminum.
if you want a pure AC alternator, you can do it like it’s done for the alternator in your car. The rotor becomes the field, and is energized by current fed through slip rings. then the mechanical power of the engine spinning it inside a set of stator coils causes the field to induce a current in the stator, which can be drawn off as electrical power.
Induction motors can’t generate the reactive power that they need to get the field going, so if you just have a motor and spin it, it won’t build up a field and it won’t generate power.
If you connect it to an AC line and spin it faster than its synchronous frequency, then it will build up a field and generate power. The AC line will supply the reactive power necessary for the field.
If you want to operate it as a standalone generator, you still need to supply reactive power to it or it won’t build up the field. You can do this by connecting capacitors in delta between the three windings. The drawing in that pdf shows the capacitors but it isn’t very clear that this is what they are doing.
Note also that induction motors don’t supply reactive power, so not only do they not supply the reactive power needed to generate the field, but they also won’t supply reactive power to a load. If your load is reactive, you’ll need to increase the size of the capacitors to compensate for it.
ETA: Just to be clear, the capacitors will only allow the motor to run as a generator AFTER you’ve used external power to get it going. If the motor ever stops, or slows to the point where the field collapses, you’ll need to supply external power to get the field going so that it can generate power to supply the capacitors.
I presume when you say “if your load is reactive”, you mean “if your load is inductive”? Capacitors themselves are reactive, too.