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Discussion Starter #1
I am in the process of putting together an inverter generator. The unusual part is it is a "Combined Heat and Power" (CHP) or "micro-co-generator" installation and thus will be governed by building heat demand, not electricity demand. The waste heat from the engine will heat the building. If I need heat the engine speeds up, and if it is warm out, the engine slows down. Whatever electricity that will be generated will be sent to a solar inverter.

I have a couple of options for the generator head. Because the RPM and Voltage will be variable, I am expecting to bypass the voltage regulator and output the wild AC that is generated. This wild AC will be rectified and then fed into the solar inverter.

I am having trouble find some answers. Anyone here know?

Will a brushless 3 phase AC generator maintain its excitation at lower RPM/voltages?
If I use a brushed generator, how can I excite it? Does the excitation voltage need to vary with RPM?
If I use a permanent magnet 3 phase generator (motor), will that handle variable RPM and voltage?

Comments?

Thanks
Bryan
 

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Hi Bryan,

Sorry for the delay, yesterday my account was hacked and it was impossible for me to answer your questions.

I would like to know the power involved in this project, KW, BTU/ hr. and also, how do you think to capture the engine's heat.
About the generator heads: You can obtain a linear response with respect to the RPM's and the output voltage and frequency (Hz), using an external, fixed but adjustable, DC supply. In this way, if the heat demand increases, the RPM will increase too, doing the same the output voltage. This applies to a brushed or to a brushless generator. For a permanent magnet generator, this is automatic, more RPM's, more output voltage.

If you mention "Generator head", it means that an engine will be coupled to it, directly (same RPM) or using belts and pulleys. The first option fixes the generator speed range to the engine's speed. The output voltage will vary the same. With a high excitation level useful electric power can be obtained at low RPM's. Increasing RPM's, a high voltage, higher than the max solar regulator can withstand, will be produced, damaging the controller. This implies the use of a limit speed governor, set to obtain at the max engine RPM's, the max voltage allowed by the solar regulator-inverter, adjusting it with the external DC supply.

Just to mention, if you use a diesel engine, (for prolonged periods) you may consider for your calculations that, approximately, one third of the power obtained is converted in mechanical power in the output shaft, one third in hot exhaust gasses (400-500 °C) and one third in cooling water and engine irradiation.

Well, I hope this help something to your project.
 

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Discussion Starter #3
1. I would like to know the power involved in this project, KW, BTU/ hr.

2. how do you think to capture the engine's heat.

3. With a high excitation level useful electric power can be obtained at low RPM's. Increasing RPM's, a high voltage, higher than the max solar regulator can withstand, will be produced, damaging the controller. This implies the use of a limit speed governor, set to obtain at the max engine RPM's, the max voltage allowed by the solar regulator-inverter, adjusting it with the external DC supply.
Thanks for the feedback Osviur

1. The power I am targeting is around 10 kW. There are commercial products available for larger installations but the only thing at this size that I have found is an extremely expensive package unit that uses natural gas or propane.

2. The engine coolant (85 deg C) will be circulated through a Flat Plate Heat Exchanger which will heat the building Hot Water heating loop (70 deg C). The engine exhaust will be run through the combustion chamber of a boiler which is also a part of the building Hot Water Heating loop. This boiler has easily cleaned gas tubes.

3. With a standard brushed generator head, if I adjust the excitation voltage so that at max engine RPM, the genrator output voltage matches the max inverter input, will I get a roughly linear reduced generator output as the engine RPM drops back to idle? Will I need to adjust the excitation voltage as the RPMs drop?

Permanent magnet generator is likely the easiest and most appropriate but they are very expensive.
It is my understanding that brushless generators have a small exciter built onto the rotor which is nice but how will it respond to different frequencies and in particular, low RPM?
 

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Bryan,

I see that your project is a complex one. Good luck!

A Brushless generator, has a little alternator mounted in the same shaft as the main rotor, with the exciter winding stationary. At this point the DC external supply creates a fixed magnetic field similar to the rotating field found in the brushed generators but , due to the multiplier effect, a brushless generator requires less power in the exciter field.

Using a fixed excitation, it is possible to adjust just one point where the voltage output curve crosses the excitation current . This point must be the upper voltage. An AVR fed with a fixed external supply, will try to generate the set voltage even at low RPM's, down to the point where decreasing the speed, will reduce linearly the output voltage.

As the electrical load is an inverter, the power taken at different input voltages will not be linear as a resistive load. Due to this, an engine RPM increase will not load it in a linear mode. A resistive load will load the engine in a quadratic form, because Power (Watts) = voltage squared divided by resistance, and voltage is proportional to RPMs.

A permanent magnet generator has not a controlled voltage output. In this case, using the characteristic voltage output curve vs speed, you will need to use pulleys and belts to obtain the max voltage allowed at tha max engine RPM.

Best regards.
 

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Discussion Starter #5
A Brushless generator, has a little alternator mounted in the same shaft as the main rotor, with the exciter winding stationary. At this point the DC external supply creates a fixed magnetic field similar to the rotating field found in the brushed generators but , due to the multiplier effect, a brushless generator requires less power in the exciter field.
Does this mean that a brushless generator (without an AVR) will output a quadratic curve voltage according to the RPM?
 

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Discussion Starter #6
With the varying RPMs and associated varying frequency, the AVR will freak out. My plan is to bypass the AVR and use the wild AC that is generated. Rectify it, smooth it with some capacitors, and then feed the solar inverter.

Just trying to sort out the excitation for the varying RPMs.
 

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Discussion Starter #7
Obviously, the voltage generated at max RPM will have to match the max input voltage of the solar string inverter so I don't let out the magic smoke ...
 

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Discussion Starter #8
A Brushless generator, has a little alternator mounted in the same shaft as the main rotor, with the exciter winding stationary. At this point the DC external supply creates a fixed magnetic field similar to the rotating field found in the brushed generators but , due to the multiplier effect, a brushless generator requires less power in the exciter field.

Best regards.
Does a brushless generator have "DC external supply" that controls the excitation voltage? And if so, what DC voltages are typically used?

Thanks for letting me pick your brain ...!
 

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No, a linear output voltage will be generated with respect to the engine RPMs if a constant DC voltage is applied in both generator types.

The power consumption will increase as the squared voltage, in a resistor load, not in the inverter case.

To generate a voltage, you need a winding, "cut" by a moving magnetic field. In a PM gen, the magnetic field in constant and permanent so it is just necessary movement. In generators with an electromagnetic field, the rotor, is necessary to feed the windings with a DC voltage to create the magnetic field that will cut the stator winding, inducting a voltage proportional to the RPMs, field intensity in the rotor and number of turns cut in the stator. To vary the voltage output, just vary any of this factors.

You mention "wild AC", this means to me an uncontrolled, auto excited directly fed generator. This configuration will produce an explosion in a few seconds. Lets see: starting at low speed, the remanent magnetic field in the rotor, will produce a very low voltage, once rectified, fed to the rotor, increasing the rotating magnetic field. This will increase the voltage induced in the stator which will generate more voltage in the stator up to destroy the rotor winding, or the exciter coil, the weakest components in the loop. The load connected to the output will suffer similar damages.

Normally the DC voltage applied to the excitation circuit, is taken from an auxiliary coil in the stator and then rectified, but in your project, you need to generate even at low speed. This condition cannot be satisfied with the auxiliary coil. that's why I suggest you to excite the generator in an independent form, using a separate power supply, which always will feed the excitation rotating field.

As I said before, A brushless gen will require less power in the exciter field than a brushed gen . The exact values are a little hard to obtain, they usually don't figure in the technical specs. (talking about small generators), but as an example a 15 KVA brushless generator will require 1.5 A @ 15 volts DC. A similar brushed type needs some 5-6 A @ 40 or so volts. The latter is just approx.

Hope this info be useful.
 

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Discussion Starter #10
No, a linear output voltage will be generated with respect to the engine RPMs if a constant DC voltage is applied in both generator types.
So a brushless generator does have an exciter DC input? And if I apply a constant ~15VDC, I will get a linear output voltage proportional to RPM?
 

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Yes Sir. You will get a linear voltage output with respect to the RPM. But before to apply any voltage, please get the specs corresponding to that generator, then reduce the DC and increase the RPM to the max, then adjust the DC external excitation supply up to obtain the nominal voltage. The voltage generated at min RPM is not adjustable.

As explained before a self excited, not controlled generator is auto destructive.
 

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Discussion Starter #13 (Edited)
a self excited, not controlled generator is auto destructive.
sorry for my lack of understanding ... I am only aware of three types of generators.
Brushed, Brushless, and Permanent magnet. I am assuming the self excited type is a brushless. Do some of the brushless have an accessible external field and others are just internally self excited? Or are ALL brushless generators self excited with no control of the field?

Thanks again!
 

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Hi Bryan,

All commercial generators are internally self excited and generally use an AVR. This means that all commercial generators have an auxiliary coil in the stator that feed the rotating field using an AVR to control the output voltage within certain limits.

For the special use that you are planning for a gen set, I'm suggesting to modify the internal generator wiring to obtain a different voltage output curve, because the AC wild production is not feasible without some form of control.

I have drawn some diagrams to show you the differences between a brushed and a brushless generator and the same generators modified to obtain a straight linear response.

I hope this help to clarify the ideas.

Regards.

PS : Which engine type do you think to use?
 

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Discussion Starter #15
Thanks! That helps a lot. I will be using a diesel engine converted to waste vegetable oil. I have an Onan 8 kW marine genset that I would like to use. I have not looked to see if it is brushed or brushless. One other issue I have with it is the governor. I need to change the type of setup to act more like an automotive application rather than a fixed RPM generator type. I need to find out if I can alter this governor or if another governor/injector pump is available for the kubota D905. More research ....
 

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I found this page:

https://www.dieselenginemotor.com/kubota/super05/D905

May be useful, and the performance curves and specs attached.
If your generator is a 10KW output, with a 80% efficiency, will require 12.5 KW input put in the shaft. In the performance curve, under net continuos this Kubota gives 12KW @ 2800 RPM . Check the generator's operation speed if it is a 1800 or a 3600 RPM.
 

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Hi, in answer to your question about the voltage at slow speeds, a standard self excited AC Alternator will not be able to generate its field until you get to about 1350 RPM, so it wouldnt be suitable. You would need a https://en.wikipedia.org/wiki/Permanent_magnet_synchronous_generator - distinct from a AC alternator which has a PMG fitted to boost the excitation under transient conditions.
 

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Hi Charlie,

You are right: "a standard self excited AC Alternator will not be able to generate its field until you get to about 1350 RPM". That's why I said:" About the generator heads: You can obtain a linear response with respect to the RPM's and the output voltage and frequency (Hz), using an external, fixed but adjustable, DC supply.". Using this DC EXTERNAL supply to feed the rotor,
(please see the above diagrams) the magnetic rotating field will stay fixed, independent from RPMs, so the output voltage will be only a function of the RPM. This is a gross aproximation, not includind minor voltage losses as stator IxR or inter demagnetization etc. In this way the operation will be very similar to a PM generator.

Due to economic reasons, Bryan is not considering a PM generator head, as he has a three phase gen head available to be used in this interesting project.

Thanks anyway for your observation.

Regards
 

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hi Speedy,

An inverter generator uses a permanent magnet in the rotor to induce electric energy in the stator so at minimum RPMs the full rotating magnetic field will "cut" the conductors in the stator, generating some variable frequency AC voltage. It is important indicate that the voltage generated is proportional to the intensity in the inductor field (rotor), to the number of turns in the stator winding and the rotor speed (indutor field RPM).
A standard generator uses the the magnetic remanence in the rotor to initiate the generation process, increasing the magnetic field as the voltage generated self feeds the rotor winding, increasing it in a loop that is cut by the AVR (automatic voltage regulator) when the nominal voltage is reached .

As we normally need a fixed frequency and a fixed voltage, in an inverter generator, with variable speed, we will obtain a variable AC voltage (Three phase) and a variable frequency (both depending from the RPM) but this form of electric energy is fed to an electronic converter (Inverter), which rectifies this AC power, converts it to DC voltage and then produces an AC voltage at a fixed frequency (60 or 50 Hz) and a fixed voltage to supply "clean" power at its outlets.

A standard generator runs at a fixed speed and controls, with an AVR, the output voltage that is directly applied to the outlets.

I hope this help you.

Regards
 
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