Induction Electric motors, most of them, are made with a metallic core that
have some magnetic retentively capabilities ( residual magnetism) therefore
if we do some experiments ( This I did when young) coupling a running motor
to another induction motor without any electric power and placing a
voltmeter to the motor power terminals we can determine if a voltage is
generated, the experiment demonstrated that around 8 - 15 volts were
At that time I read a French article indicating and suggesting what was
needed to convert an induction motor to a generator by resonating the
windings with running capacitors , also the basic formula and steps were
given, lastly the article indicated that this was found out around 1938
when some experiments were done to improve the power factor of the motors.
By resonating the winding with a capacitor a magnetic field is generated in the
windings and the rotor cutting these magnetic fields produces the Vac with
the power level that the turbine is capable to supply to the now generator.
In some cases, the magnetic field is not generated because the motor may
have been idle for a long time and the residual magnetism fade away, a
simple trick of momentarily applying a DC current to revive the magnetic
field is what is needed, or to solve this problem by inserting in the
winding stator area some small magnets, in my case I used small Alnico
magnets taken from small radio speakers that were bad,
Induction motors as generators need to start rotating or stop unloaded
otherwise the residual magnetism may disappear .
Presently , some small generators commercially available have the same
problem, where there is the need to apply a DC current to revive the field
for the voltage regulator circuit to get enough voltage to feed back some
low power to the rotor to start the generation of higher voltage in a
positive feedback procedure, bringing the output voltage to the required levels.
These commercial Alternators often have a second smaller motor which in
reality it is a rotating transformer, in a common shaft, that take 3 phase
voltage and 3 phase current from the alternator output via transformer
action to generate a 3 phase rectified voltage that feeds the alternator
rotor to generate the necessary magnetic field to activate the output
Those interested in seen how the set up is, visit the Marathon web page and
search for any generator data sheet.
There are alternators = generators that do have a different set up with an
electronic AVR to regulate the output voltage to cover a wider output range.
At around 15 KW the cost of a motor crosses the cost of a true generator so
it is better to use a generator.
There are other tricks to convert the induction motor as a generator --
using the available electrical power on the motor and applying the turbine
torque to the rotor, when the RPM of the motor is equal to the rated RPM of
the motor PLUS the Slip % the motor converts into a generator and start
dumping power into the Grid that is connected,
Lastly, the hydro electric turbine using an induction motor as generator
may produce a frequency that often is a bit higher than the national Grid
but this is not a problem if the Vac is not used to run electric clocks.
This hydro system is generally not used to run motors -- some of the
problems is that motors do have inductances as well, changing the resonant
frequency of the generator and some times causing a field collapse-- though
one can do some other tricks to solve this problem and one of them is to
resonate the motor that is going to be connected to the hydro generator to
keep the frequency stable, also the ELC having additional capabilities to
read the frequency and to insert/remove biasing capacitors to bring the
frequency to the desired point.
----- Original Message -----
From: Benjamin Bof
To: microhydro@yaho... Sent: Tuesday, October 04, 2011 06:49
Subject: Re: [microhydro] iNDUCTION MOTORS AS GENERATORS 3 PHASE AS A SINGLE PHASE
Well, but how do you create magnetic field to excitate ?
> Induction motors as generators, specially the 3 phase can be run as a 3
> phase generator in Star or Delta connections.
> But few people realize that a 3 phase motor that can be wire Star or Delta
> can be as well wired single phase double voltage with a center tap
> The USA Grid system uses the basic 115 VAC and the 230 Vac so called two
> phase system where the voltage is 115-0-115 = 230 Vac .
> If the motor can be wire DELTA ( it could be a 6 or a 12 Wire motor) - 6
> wires for a single Voltage operation or with 12 wires for a dual Voltage
> To run the motor as a Generator two phase system, wire the motor DELTA
> connection -- Select one phase as the primary phase, let's say A phase then
> connect phase B and C in series,
> Delta connection normally done is :
> A1 - to - C2
> A2 - to - B1
> B2 - to -C1
> Where is A1 - OUTPUT primary Phase ; B1 OUTPUT second phase ( 120 degrees)
> ,; and
> C1, OUTPUT third phase ( 270 )
> Open connection A1 To C2 to have a two phase generator !!
> Generated output will be A1 = Phase 1 ; A2-B1 = center tap ; C2 = phase 2
> In USA terms = A1 to A2-B1 = 115 volts , C2 to A2-B1 = 115 Volts at 180
> degrees from A1 and A1 to C2 = 230 Volts with A2-B1 = center tap = Neutral
> In this case one needs to GROUND A2-B1 as the neutral in the USA system.
> In other systems like 230 Volts with one going to neutral, one needs to
> select A1 or C2 and do a neutral connection, and in this case leaving A2-B1
> not connected.
> One needs to remember that the connection that has two phases in series ( B
> & C) has two winding in series which add mathematically to a voltage that
> equal to A1 phase but with 180 degrees offset with DOUBLE winding
> resistance -- which reflects into a higher losses .
> Comments or questions ?
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