High Efficiency Low RPM Generators: Powering Your Needs at Just 20-50 RPM

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Discussion Overview

The discussion revolves around the feasibility and options for generators that operate at very low RPMs, specifically in the range of 20-50 RPM. Participants explore various aspects including power output, voltage requirements, and design considerations for applications such as wind turbines.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants inquire about the existence of generators that can operate efficiently at low RPMs, emphasizing that power is a factor but not the primary concern.
  • One participant shares a personal experience with a hand-cranked generator that operated at approximately 15-16 RPM, suggesting that such generators do exist.
  • Another participant notes that output voltage of a generator scales with speed, suggesting that users should define their voltage needs and find a generator that meets those requirements.
  • There is a discussion about how to measure power for a generator, with one participant providing two methods: using output current and voltage or calculating mechanical input power based on torque and RPM.
  • Concerns are raised about the size and weight of generators needed to produce usable power at low RPMs, with one participant suggesting that gearing the generator to a higher RPM might be a viable solution.
  • Another participant emphasizes that very slow-turning generators tend to be larger and require more windings of thinner wire, which can limit their maximum amperage and total power output.
  • Participants discuss the constraints of weight in their project, specifically in relation to a wind turbine design, and express uncertainty about the power output and voltage of generators suitable for low RPMs.
  • There is a suggestion that a gearbox could significantly reduce the size of the generator needed for the project, with a specific mention of a 1:10 gearbox being a feasible option.
  • One participant proposes estimating power output based on blade size and wind speed, questioning whether this could inform the choice of generator.
  • Another participant estimates a desired power output of 70-100 W and queries about the appropriate gear ratio for achieving this from a 20-50 RPM input.

Areas of Agreement / Disagreement

Participants express a range of views on the necessity and practicality of using gearboxes versus low RPM generators. There is no consensus on the best approach, and multiple competing ideas regarding generator design and power output remain unresolved.

Contextual Notes

Participants mention various assumptions about the relationship between RPM, voltage, and power output, but these are not fully explored or resolved. The discussion also reflects uncertainty about the physical constraints of generator size and weight in relation to the desired application.

ual8658
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Are there generators out there that run on very, very low rpm? We're talking in the range of 20-50 rpm and no more. Power is not the biggest concern, but still is a factor.
 
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ual8658 said:
Are there generators out there that run on very, very low rpm? We're talking in the range of 20-50 rpm and no more. Power is not the biggest concern, but still is a factor.
Of course.
 
ual8658 said:
Are there generators out there that run on very, very low rpm? We're talking in the range of 20-50 rpm and no more. Power is not the biggest concern, but still is a factor.
There are some hand-cranked generators which operate at this low speed.

When I sailed on a cargo ship during my student days, I was assigned to help the ship's radio operator with testing the emergency radio. The radio used a simple Morse key and was powered by a hand-cranked gen. I got to turn the gen while the operator tested the transmitter. I believe the speed was about 15-16 RPM, and the cranking had to be done at a constant rate.
 
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What output voltage do you need?

For a given generator, output voltage scales directly with speed. Define the voltage per RPM (or 1000 RPM, as it is often stated) and search for one that is close or a bit higher. Then you reduce the output voltage as needed with a regulator.
 
Power is based on voltage and current right (rusty physics knowledge here)? How would we measure power for a generator with an arbitrary voltage?
 
Either:

1) The output current drawn from the generator times the voltage at the time the current is drawn.

2) The mechanical input power in Watts is approximately Torque(in.lbs.) * RPM / 84

Of course the two answers won't be the same due to mechanical and electrical inefficiency.
 
Based on the minimal information provided the answer is "yes". I recommend you give us some more info on what you are trying to achieve or the moderators might close the thread on the grounds that the question has been answered :-)
 
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Why does the generator have to turn slowly? Isn't the option of gearing it faster available?

Typically speaking, very slow turning generators are much bigger than faster turning counterparts... Slower generators need more windings of thinner wire to generate the same voltage, which reduces their maximum amperage, and thus total power output.
 
Weight is a constraint for us right now with this project (the easiest description is a wind turbine). We were hoping to avoid putting a gearbox into it to reduce the weight. The basics are we expect this device to have an input of between 20-50 rpm at most from wind speed. We're still lost as to what the power output and voltage is because we have yet to even come across a generator of any output that would take such a low rpm and still not be the size of a whale.
 
  • #10
The size/weight of the gearbox will be minimal in comparison to the size of the generator required to get usable power out of the system. a 1:10 gearbox is easily made with a planetary gear, or even a belt drive and will greatly reduce the size of the generator required.

What sort of power output in volts and watts are you looking at?
 
  • #11
ual8658 said:
Weight is a constraint for us right now with this project (the easiest description is a wind turbine). We were hoping to avoid putting a gearbox into it to reduce the weight. The basics are we expect this device to have an input of between 20-50 rpm at most from wind speed. We're still lost as to what the power output and voltage is because we have yet to even come across a generator of any output that would take such a low rpm and still not be the size of a whale.

Can't you estimate the power from the size of the blades and wind speed (or whatever the power source is)?
 
  • #12
Based on the wind speed and blade length, we're looking at between 70-100 W of power. So would finding a gearbox and a higher rpm generator be a more viable choice? How much of a gear ratio should we aim for if we're getting 20-50 rpm for ideal weight.
 

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