PTO Generator

[TugboatEng]

I have an idea that requires a generator that can produce 10-20kW of power over an available speed range of 650-1800 rpm, it's essentially the same application as an automotive alternator. I can use a step up gearbox if necessary. I eventually need it to be 480V 3 phase. I'm looking for guidance with regards to the best type of generator to achieve this output. Bonus if it can be motorized.

 

[waross]

Some suggestions come to mind;
1> Look for an inverter that will accept a variable frequency input.
2> I haven't used one but I have seen literature for a variable speed hydraulic drive. Variable speed pump input. Controlled speed motor output. See what google brings. This would allow a conventional alternator to be driven at a constant 60 Hz (or 400 Hz, if you can use that).
3> A DC to AC inverter with a DC generator similar to an automotive alternator.
I am thinking about the phenomena of "Load Dump" in alternator circuits.
With the engine at higher revs, and the loss of a large loads, the alternator output may reach or exceed 100 Volts.
I am thinking a 100 Amp alternator geared up from your 650 RPM prime mover until 100 Volts output is reached. Use a modified AVR.and feed the alternator output to an inverter.
I'll leave the details to you if you like this solution.

 

[TugboatEng]

Aircraft generators a looking like a possible source for an off the shelf solution. The smaller private jet sized units appear to be rated up to 12kva but have very high shaft speeds and 400 amps DC sounds expensive to work with. Larger airliner type units are gearbox driven so their shaft speeds are likely more inline with my application. They are a bit oversized at 45-90kva.

 

[waross]

What load are you supplying?

 

[TugboatEng]

~10kW base load with momentary spikes to 75kW. Future base load may increase to up to 20kW. The goal is to use a battery to cover the spikes.

 

[waross]

Motor loads?
Heating loads?
A/C?
Refrigeration?
Critical, Thrusters, Mooring or Anchor Winches?

 

[MintJulep]

A Google search for "3 phase 480 volt pto generator" shows that this is thing you can buy from several manufacturers.

 

[waross]

A Google search for "3 phase 480 volt pto generator" shows that this is thing you can buy from several manufacturers.

Maybe in there somewhere, but the search brings up dozens of agricultural PTO generators, designed for constant speed PTOs, either 540 RPM or 1000 RPM.

 

[waross]

What will you be driving this with, Tug?
It's hard to offer specific help when you have provided so little information.

with momentary spikes to 75kW

That needs some further explanation before offering a solution.

The goal is to use a battery to cover the spikes.

More explanation please.
You may be looking for a complex and expensive solution to a problem that may be much less than you imagine.
If you really have a 75 kW spike (maybe not) and are willing to use an inverter to supply this, then use the inverter at all times and use as many automotive alternators (modified) as you need for your base load. You can float batteries (probably more than 12 Volts) to supply the spikes.
With this system, when the load increases, the 75 kW inverter will be adequate but you may need to add alternators.

Another question:

an available speed range of 650-1800 rpm

Is this a varying speed or is it,"Pick any steady speed in this range"?

 

[waross]

If the speed is varying, you may find a suitable "doubly fed machine" in wind turbine hardware.

 

[MintJulep]

Maybe in there somewhere, but the search brings up dozens of agricultural PTO generators, designed for constant speed PTOs, either 540 RPM or 1000 RPM

Power, speed, torque, frequency......

All related.

A system that outputs constant voltage, frequency and power while the mechanical input can vary is.... complicated

 

[TugboatEng]

This is a variable speed application. Available speeds will vary from 650-1800 rpm. I need to be able to profuce 15+kW continuous at any point within that speed range. AC or DC are unimportant, this will be charging batteries/powering a bus, inverters and DC to DC converters are likely to be involved. I have plenty of PTO power available at all engine conditions. Yes, I will chose an inverter and batteries capable of supplying 75kW under continuous operation.

 

[waross]

Do you have a target voltage for the batteries?
Can you use 24 Volts?
I may have found your solution;
Catalogue Page
24V nominal (over 27 Volts actual) @ 600 A = Over 15 KVA
Add that to your batteries and inverter and you should be good to go.
These have the advantage of being brushless. An advantage for marine applications.
Prestolite/Leece Neville

 

[LittleInch]

Have you looked at CVT type PTO gearboxes?

There seem to be a few around that could do what you want if you pick an output rpm of say 940 or similar mid range.

There are quite a few tractors now which are CVT, but they still need a PTO which is fixed speed.

Maybe talk to a few Tractor or agricultural suppliers?

I think it's easier to modify the speed than deal with variable frequency or voltages.

 

[waross]

You probably know this Tug, and my experience is based on the internal voltage drop of small (up to about 50 KVA) diesel generators.
The principle and reality of internal voltage drop is the same but the 2:1 ratio may be off.
Many diesel generators will produce close to rated voltage, open circuit, when excited with 12 Volts. (testing with a lead acid automotive battery)
They will typically need around 24 Volts excitation at full load due to internal voltage drop.
At that 2:1 ratio, if a generator starts outputting rated voltage at 650 RPM, it may take 1300 RPM to put out full rated current at rated voltage.
If you want to double the voltage from a variable speed generator expect double speeds; that is 1300 RPM for full voltage, open circuit and 2600 RPM for full voltage at rated Amps.
That is based a a voltage divider in the AVR circuit so that the excitation is still rated excitation, not double excitation voltage.
If you use a more complex AVR, to get double voltage on the excitation, you may be limited by field winding heating.
Years ago there was a hack available to get 100 Volts from an automotive alternator for use with power tools with universal motors.
The engine had to be run a quite high RPMS but the 12 Volt alternators would put out 100 Volts.

 

[MintJulep]

So, I'll assume that the engine is governed to hold a constant rpm regardless of load.

Any generator or alternator that outputs 15 KW at 650 rpm will output "much more" power at 1800 rpm.

If you don't have something useful to do with the extra power then you'll need to burn it off as heat.

@LittleInch beat me to the punch. You need a CVT between the engine and the generator or alternator to run it at constant speed.

 

[waross]

This is a variable speed application. Available speeds will vary from 650-1800 rpm.

So, I'll assume that the engine is governed to hold a constant rpm regardless of load.

This may not be a valid assumption.

Any generator or alternator that outputs 15 KW at 650 rpm will output "much more" power at 1800 rpm.

There are two constraints on a generators KVA output.
1> The maximum safe current in the windings. Increased cooling at higher speed may add some capacity but it will be much less than an increase based on speed ratios.
2> The working voltage or rated voltage.
Thus, an alternator rated at 125 Amps and 240 volts will accept a load of 30 KVA, regardless of speed.
I discussed a special case of an alternator turning at too low a speed to develop enough voltage to overcome the internal voltage drop at full load.
Once that alternator is at or above the speed for maximum output, there is no further increase in output.

If you don't have something useful to do with the extra power then you'll need to burn it off as heat.

Really Mint. I have restrained myself up to now, but this statement along with prior ill advised and misleading statements and assumptions shows that it is time for you to step aside from this discussion.
You are only adding confusion to the discussion.

 

[TugboatEng]

The target voltage for the batteries is going to be 700-1000v.

 

[waross]

You may be able to take the three phase output of that Reece-Neville alternator and feed it to a step-up transformer.
There would be a minimum safe speed/frequency to feed the transformer to maintain a safe Volts-per-Hertz ratio.
The transformer size/rating will be inversly proportional to the voltage ratio.
eg: a transformer rated at 100 KVA at 480 Volts and 60 Hz will handle 200 KVA at 960 Volts and 120 Hz.
Most dry type transformer insulation will safely withstand 1000 Volts.
Automotive alternators are inherently higher frequency devices, so 2x or 3x or even 4x 60 Hz may be doable.
10 KVA would be 3.3 KVA per phase. At 180 Hz, that would require 3 of 1.1 KVA transformers,
This is an example subject to more information of the characteristics of the equipment available.
If you go all the way to 400 Hz, your transformer size drops by a ratio of almost 7 to 1.

 

[waross]

Or, you may go for a conventional alternator run overspeed.
If the output will be rectified to DC then frequency is less important.
However, given the mechanical forces resulting from overspeed, an automobile alternator may be the better choice in the end.