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post #1 of 16 (permalink) Old 04-06-2019, 02:56 PM Thread Starter
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Generator Theory Question

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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post #2 of 16 (permalink) Old 04-07-2019, 05:19 PM
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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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post #3 of 16 (permalink) Old 04-08-2019, 03:36 PM Thread Starter
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Originally Posted by Osviur View Post
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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post #4 of 16 (permalink) Old 04-08-2019, 05:13 PM
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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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post #5 of 16 (permalink) Old 04-08-2019, 05:41 PM Thread Starter
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Originally Posted by Osviur View Post
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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post #6 of 16 (permalink) Old 04-08-2019, 05:50 PM Thread Starter
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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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post #7 of 16 (permalink) Old 04-08-2019, 05:52 PM Thread Starter
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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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post #8 of 16 (permalink) Old 04-08-2019, 05:59 PM Thread Starter
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Quote:
Originally Posted by Osviur View Post
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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post #9 of 16 (permalink) Old 04-08-2019, 07:38 PM
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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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post #10 of 16 (permalink) Old 04-09-2019, 01:01 AM Thread Starter
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Originally Posted by Osviur View Post
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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