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]

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

With DC, frequency doesn't matter.
Variable voltage?
An AVR will take care of that.
I have seen diesel generators where the voltage will vary from 100% at no load to 50% at full load with fixed excitation.
Add an AVR and the voltage will be stable within a few percent.
Why do aircraft use 400 Hz?
Size and weight.
A example 10 Volt transformer may supply a maximum load of 1 KVA at 60 Hz.
That same transformer will supply a maximum load of (400Hz/60Hz = 6.67) 6.67 Hz at 66.7 Volts and 400 Hz.
This is based an a safe Volts per Hertz Ratio. (1:10 The voltage in Volts shall not exceed 1/10 the frequency in Hz.)

What to expect in operation?
This example is based on the internal voltage drop characteristics of a conventional diesel generator.
The speed ratio from no load to full load capability may be less but the principle is the same.

With fixed excitation, the voltage of an alternator is speed dependant.
In this example, we will use a target voltage of 50 Volts from a 24 Volt automotive alternator.
In our example this alternator no load output voltage will reach 24 Volts Nominal/27.6 Volts actual at an engine speed of 550 RPM.
Using the 2:1 ratio, the alternator will reach 24 Volts nominal at 1100 RPM.
At speeds above 1100 RPM, the AVR will hold the voltage steady at 24 Volts and the available output power will no longer increase with increasing speed.
What happens at 825 RPM or 50% of the speed range?
The alternator will put out 50% of rated current before the internal voltage drop causes the terminal voltage to drop.
The actual current may depend on the state of charge of the batteries.

Now what happens if we want to run this alternator at two times rated voltage?
ALL of the speeds given in the example will be doubled also.

The other parameter to consider is the minimum frequency.
The Volts/Hertz ratio (to be determined by you) must not be exceeded.
You may have to add a circuit to inhibit excitation when the V/Hz ratio is too low.

More on voltage step-up and transformer sizing later.