Self Excited AC Single Phase Generator

Hi I'm doing a uni project and need help with a Self Excited AC Single Phase Generator. I'm using an AC Single Phase Induction Motor to act as the generator. Its turned by a DC Motor

The AC motor is 1350 RPM and the DC motor is 2500 RPM

Its not producing any electricity (just a few volts) for some reason. I think I have the capacitors wired incorrectly

I have 2 x 20 mF capacitors connected to the power terminals and the generator turns CW clockwise

The AC motor has 6 terminals

2 for power, 2 for an existing 8 mF capacitor and another 2

See attachments

Any suggestions? Thanks

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Baluncore
2019 Award
Any suggestions? Thanks
The frequency generated will be at resonance of the motor inductance and the external capacitance. It will not begin to generate until the RPM exceeds synchronous speed.
Measure your motor inductance by current with rotor locked. Decide on the synchronous generator RPM, calculate the capacitance required to resonate at that frequency.
How are you going to limit the output voltage ?

DaveE
How are you going to limit the output voltage ?
Hi thanks for the reply and feedback. I have a rectifier to convert to DC then a buck converter to drop the voltage down to a consistent 48V

The frequency generated will be at resonance of the motor inductance and the external capacitance. It will not begin to generate until the RPM exceeds synchronous speed
The DC motor is 2500 RPM and the AC generator is 1350 RPM so sufficiently above synchronous speed

Measure your motor inductance by current with rotor locked. Decide on the synchronous generator RPM, calculate the capacitance required to resonate at that frequency.
I'm not sure abut the locked rotor. In YouTube videos on it, one guy just adds capacitors until it works and the other guys uses three 20mF capacitors in a delta configuration

I'm also not sure if I have the capacitors wired correctly and if they should go directly to the two power terminals on the AC motor or any of the other terminals? See image

Thanks :)

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Baluncore
2019 Award
I have a rectifier to convert to DC then a buck converter to drop the voltage down to a consistent 48V
You misunderstand the situation. AC motors are inductive. Parallel capacitor(s) are used to compensate the Power Factor, PF, of an AC motor, to get the phase of the current close to the phase of the applied voltage.

If an induction motor is overcompensated for PF, by adding too much capacitance, it becomes an induction generator when the AC supply is disconnected. Motor momentum and the LC resonance, can produce a damaging over-voltage.

Without any load, and at an RPM just a few % above LC resonance, the AC voltage generated may rise to the point where it can damage the induction generator and the capacitor insulation.

The DC motor is 2500 RPM and the AC generator is 1350 RPM so sufficiently above synchronous speed
By synchronous speed I refer to the resonant frequency of the motor inductance with the parallel capacitance. If the motor capacitor was a series starting capacitor, which normally determines the direction of rotation, then it will be insufficient for parallel resonance of the main winding.

I'm not sure abut the locked rotor.
The locked rotor test measures the current when connected to an AC voltage. That makes it possible to calculate the reactance, and so the inductance of the motor. The test must be done quickly as the current is the higher starting current. Once you know the inductance you can calculate the capacitance necessary for resonance at the synchronous RPM. Only at RPMs higher than synchronous does it become a generator.

… one guy just adds capacitors until it works ...
Adding more capacitance, lowers the resonant frequency, until that synchronous frequency is below the driving RPM, when it begins to generate.

... the other guys uses three 20mF capacitors in a delta configuration
The delta configuration is used with a three phase motor. That is the easiest to work with because the three windings at 120° on the motor are all the same.

With a single phase motor there is a main winding, and a starting winding at 90° that is normally driven through a series capacitor. The combination of the two fields makes a directional rotating magnetic field that drags the armature around in the required direction while starting.

I'm also not sure if I have the capacitors wired correctly and if they should go directly to the two power terminals on the AC motor or any of the other terminals?
As a motor you would use a parallel run capacitor to adjust the power factor.
You will use a series starter capacitor on the other winding to start it spinning in the required direction.

As a generator you will use parallel capacitors to set the synchronous speed in RPM.
If you load an induction generator while starting, it may overdamp and so prevent the resonance needed to excite the generator.
If you overload an induction generator while it is operating, you will draw away the excitation energy, kill the field, so it will stop generating.

The ideal induction generator is operated a few percent above synchrous while connected to and excited by the three phase grid. Self excited induction generators are more precarious and need to be carefully managed.

DaveE
Hi Baluncore thanks for the excellent detailed explanations. Its getting a bit too technical for me lol but I really do appreciate the help. You definitely know your stuff. I have the battery side sorted with a boosted 48v (see image) I just need to produce about 100v AC on the generator side

LC resonance, can produce a damaging over-voltage
It maybe safer to use the other method where you drill out the rotor and add magnets but that will destroy the motor. I originally decided against this as I thought that capacitor method would be way easier, which its turning out not to be lol

Interestingly, there is another guy on YouTube who uses another method. He connects the AC motor to power and runs it to speed. Then he turns on the DC motor and runs it connected at the same time till over the synchronous speed of the AC motor. The AC motor splutters, he turns it off and it starts running as a generator. I tried this method but the plastic tube motor coupler broke

As a generator you will use parallel capacitors to set the synchronous speed in RPM.
If you load an induction generator while starting, it may overdamp and so prevent the resonance needed to excite the generator.
Ok so from this, I think I need to remove the 8 mF starter capacitor and run the capacitors in parallel into the power terminals. Maybe Ill just use the two 20 mF and one 8 mF capacitor in parallel and see how that goes

The ideal induction generator is operated a few percent above synchrous
Ok I need to get a variable DC speed controller to control the PRMs better and slowly bring it up to speed

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Baluncore
2019 Award
What are you really trying to do ?
Are you inverting low-voltage DC to produce AC ?
There is probably an easier way than a single phase self excited induction generator.
What will you do with the AC once it is generated ?

Hi, You are trying to use an AC induction motor as a generator?Well, maybe my knowledge of electric motors may be outdated by now but I have NEVER heard of an AC single phase motor being able to be used as a "generator". Firstly the term "generator" as I know it refers to a machine designed to "generate" a DC output.
They are generally built with field coils to generate a magnetic field and have an "armature" which is wound with wire coils and connected to a commutator. The armature is rotated by some power source(turbine, diesel etc) and the rotation of the armature in the magnetic field generates a voltage which is collected by sets of brushes at the commutator.
A machine which outputs AC is called an ALTERNATOR( to my knowledge). Very similar except the rotating part is called a "rotor" and is wound with a coil connected to sliprings which are connected to a DC source to provide a rotating magnetic field. The magnetic field couples to coils wound in the "STATOR" and the rotors magnetic field generates a voltage in the stator coils which is the output.
An AC Induction motor does not have any wound coils in the rotor but depending on type has a "squirrel cage" of bars (usually aluminium) imbedded in rotor laminations. The coils of the stator windings generate a moving magnetic field which connect to the rotor and "induce" a voltage and current(basically act as a transformer). This induced magnetic field reacts to provide the torque reaction to spin the motor shaft and do work.
Now from what I read of your question I cannot see how you can generate ANY useful output since there is no rotating magnetic field or wound "rotor" in your machine. Any voltage you are seeing(at the U1, U2 terminals I presume) is only from a slight residual magnetic field left in the rotor generating a slight voltage at the motor terminals as you spin the rotor.
Looking at your pic of the motor terminals,it looks like a straight forward AC single phase motor with two field stator windings. Two windings are required to provide a "rotating " field. One winding is connected directly to the ac power supply and the other winding has the 8Mfd capacitor in series. The makes two AC fields at a "phase angle" to each other and appears as a "rotating field" to spin the rotor. Not quite sure about the device V2. It could be a start switch which disconnects the capacitor field winding once up to speed. The different connections shown are the way to change motor rotation by changing the links.

Not sure what all the talk about synchronous speed is. Synchronous speed refers to the speed required to bring parallel connected ALTERNATORS to the SAME speed and PHASE angle to allow connection to the grid.
I very much doubt you will get ANY useful output trying to use this setup. If you want to generate any useful AC I suggest driving a car alternator which will give you three phase output at about 15 volts and 40 plus amps(depends on alternator and drive unit). Actually looking at the power ratings you would be better off using the AC motor to drive the DC motor as a generator. So you want to rectify the AC and then convert it to stable DC? By using a dc motor to drive an AC motor? see previous sentence. Love to see how you get on.

anorlunda
Staff Emeritus
I have NEVER heard of an AC single phase motor being able to be used as a "generator". Firstly the term "generator" as I know it refers to a machine designed to "generate" a DC output.
Sorry, much of what you said in that post is wrong. This is a good place to start study:
https://en.wikipedia.org/wiki/Induction_generator

However, it remains unclear what the OP is trying to accomplish or why he is using an induction generator.

From Wikipedia:

An induction generator produces electrical power when its rotor is turned faster than the synchronous speed. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, the synchronous speed is 1800 rotations per minute (rpm). The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM. The motor normally turns slightly slower than the synchronous speed; the difference between synchronous and operating speed is called "slip" and is usually expressed as per cent of the synchronous speed. For example, a motor operating at 1450 RPM that has a synchronous speed of 1500 RPM is running at a slip of +3.3%.
An induction machine requires an externally-supplied armature current. Because the rotor field always lags behind the stator field, the induction machine always consumes reactive power, regardless of whether it is operating as a generator or a motor.

A source of excitation current for magnetizing flux (reactive power) for the stator is still required, to induce rotor current. This can be supplied from the electrical grid or, once it starts producing power, from the generator itself. The generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual magnetization. In some cases, that residual magnetization is enough to self-excite the motor under load. Therefore, it is necessary to either snap the motor and connect it momentarily to a live grid or to add capacitors charged initially by residual magnetism and providing the required reactive power during operation.
Required capacitance
A capacitor bank must supply reactive power to the motor when used in stand-alone mode. The reactive power supplied should be equal or greater than the reactive power that the machine normally draws when operating as a motor.
For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.
That "complex function of machine parameters, capacitance used for excitation, and load value and type. " is quite a chore to compute. That may be one reason that stand-alone induction generators are seldom used outside of the laboratory.

Edit: On thing I do agree with you on is, " I very much doubt you will get ANY useful output trying to use this setup. "

Baluncore
Baluncore
2019 Award
If you want to generate any useful AC I suggest driving a car alternator which will give you three phase output at about 15 volts and 40 plus amps(depends on alternator and drive unit).
That will give you AC at 400 Hz or more. To get 50 Hz AC you must slow the car alternator, while greatly increasing the field current to compensate.

Here are a couple of good YouTube videos about the Self Excited AC Generator. These guys seem very excited and knowledgeable about it

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What are you really trying to do ?
Hi the project is a wind farm simulation with 48v DC micro grid. The DC motor simulates the wind fan, the AC motor acts as the generator to produce about 100v AC. Its then rectified to DC and bucked down to 48v. The 12v DC battery is boosted up to 48v DC to provide power when there is no wind. The aim is to produce a constant 48v DC. Image attached

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