# DC Generator starting inertia

PNW Hobbyist
TL;DR Summary
How to understand the initial resistance fo a DC motor spinning up as a generator
Hello,
I am not sure if this is the correct forum. I was torn between Electrical Engineering and Mechanical Engineering and I thought I would start here.

I have been kicking around an idea for a DIY project and realizing I don't know enough about DC motors to find the parts I need. In this use case I want to use the DC motor to generate power based on a steady but slow wind speed. (My project requires very little power and only intermittently so a slow, gradual charge to a small battery might be enough to power it because there is no source of electricity or sunlight in the area I will install this thing)

I have been 3D printing various rotors and so far the savonius rotor style seems to work the best at the very low wind speeds available. I can mount one and it spins up just fine. However, when I attempt to attach it to the tiny DC generator I am playing with it doesn't have enough torque to break the intial inertia of the motor. I could of course print a larger rotor, but I was hoping to keep the final package as small as possible so I am wondering if I need a different DC motor. The ones I am using were just cheap little things I bought off Amazon for less than $1 for a POC. When I look at the specs for a DC motor (With the intention of using it as a generator) what am I looking for to identify the amount of force required to get it to start moving? I see some that list a "Torque - Max Momentary" value. Is that what I am looking for or something else? Any suggestions for how to search for and understand these specfications for DC motors would be appreciated. Also, if you happen to know of any manufacturers or even specific motors that I should look at that would be awesome! Thank you berkeman ## Answers and Replies Science Advisor Welcome to PF. ... and so far the savonius rotor style seems to work the best at the very low wind speeds available. The savonius rotor is fascinating and useless. It will spin freely without load, but will not extract power efficiently from the wind. Too many beginners are still being infected by the "savonius mind virus". You must put the savonius rotor out of your mind before you waste any more time being distracted. Anything will be better than a savonius rotor. berkeman PNW Hobbyist Well, it does seem to work better than the alternatives I have tried so far. I plan to explore a hybrid design combining the savonius (to get it rotating at all) and perhaps a darrieus style in one rotor (to help it gain RPM once it's spinning at all). I my abandon all of those options and go with something else entirely, but first I need to learn enough to understand how I can get past the inertia of the motor in the first place. While your advice to abandon the savonius design is interesting, I notice you didn't address my actual question nor make any constructive suggestions... The idea here is to harvest small amounts of power from the kinetic energy of low wind speed. Don't need much. A little will do. But so far, I cannot get the motor to spin at all with the amount of torque I am getting from the rotors. Is that a limitation of physics, or is that a limitation of the cheap motors I've been experimenting with? I may have answered my own question by digging deeper into specs on motors, but I am still not sure I understand what they mean. For example, does Starting Torque refer to the force necessary to spin the motor mechanically (as a generator), or the amount of torque it will generate when I apply current to it (using it as a motor)? If I could understand how to identify the amount of force necessary to start spinning the motor at all, the rest would be simple math. I was hoping there was an industry standard way of measuring this so I don't have to just start buying lots of motors and testing them one by one. Science Advisor I notice you didn't address my actual question nor make any constructive suggestions... One day you will realise the good advice that I have given you. Use the motor you have with a different rotor. Connect the motor to a battery. The speed the motor spins will be less than that needed with the motor as a generator, to start charging the battery. If you connect your motor through a diode to a battery, there will be no load or torque on the motor until the speed is sufficient to begin charging the battery. Then the speed will not increase much as the battery is charged. The generator torque will be proportional to the current flowing. Science Advisor https://cleantechnica.com/2014/04/07/vertical-axis-wind-turbines-great-1890-also-rans-2014/ Vertical axis wind turbines (VAWT) continue to get attention, press and R&D funding. Antagonists of mainstream wind generation continually point at them as if they were a superior technology. People perpetually re-invent them and believe that they have found something new and exciting. However, they are undeserving of any significant attention, are an inferior technology and definitely aren’t new. Outside of a couple of niches, they are more of a distraction from deployment of effective utility-scale, horizontal axis wind turbines (HAWT) than anything else. russ_watters Mentor I don't think the starting torque is published(because it is generally not useful information), but if the generator won't even spin you either need a smaller generator or more torque. Mentor The savonius rotor is fascinating and useless. And here is a plot that shows why he said that: It's from Marks' Standard Handbook for Mechanical Engineers, Ninth Edition. Compare the weight and complexity of a Savonius rotor to a standard two blade turbine with the same wind catching area. If you are working with one of those cheap little hobbyist DC motors, be aware that they have cheap plain bearings and cheap brushes, so a lot of friction. You can measure the starting torque of the motor by wrapping a string around the shaft and measuring the force to turn the shaft by pulling the string. Your wind turbine needs that much torque just to get the generator to start moving. berkeman, russ_watters and Baluncore PNW Hobbyist In this case the ability to place the turbine blade horizontally and be able to elongate it for more blade surface area if needed seemed like a good option because of the small space in which it will operate. A traditional prop style blade doesn't fit the space available very well, but it sounds like the recomendation from everyone here is to find a way to make that work. Message received. Thank you for the feedback. Baluncore, jrmichler and berkeman Science Advisor Message received. Thank you for the feedback. Congratulations on your intelligent and quick recovery from the savonius mind virus. It is sad, but there are many others who will never recover. They will be handicapped for all of their life, by what could be termed "long savonius". Tom.G, russ_watters, jrmichler and 1 other person PNW Hobbyist Congratulations on your intelligent and quick recovery from the savonius mind virus. It is sad, but there are many others who will never recover. They will be handicapped for all of their life, by what could be termed "long savonius". Rotor aside, I am still stuck with how to identify a motor with a low torque start-up requirement. I have a continuous, but slow wind speed to work with, and low power requirements. Seems like there should be plenty of kinetic energy available, but how to convert it to electricity is the challenge. It's entirely possible what I am trying to do isn't possible with the available energy, but seems like there ought to be a way. I was only interested in the Savonius because I came across some whitepapers claiming it had the lowest wind speed to start spinning, but as this discussion has made clear, the drawbacks outweigh the benefits. I'm not trying to do anything earth shattering here, just trying to avoid having to swap out batteries on a regular basis. I was originally working from the perspective of: how much power do I need? And designing from there. But recently I thought it might make more sense to first determine how much power is available, and work the other direction. In the end, all I might end up doing is proving to myself that the energy available isn't sufficient to power the device. That's OK too. At least I will learn something, have fun, and get this voice in the back of my head to stop nagging me that I need to work on this idea. :-) Maybe I phrased the question wrong. How would YOU go about harvesting small amounts of power from relatively low wind speeds? The best analogy I can think of is charging your cell phone off the air that flows out of the heater vent. Even a cell phone battery is more than I need. Half that? Appreciate any suggestions. Even if that's: "go read this book". PNW Hobbyist And here is a plot that shows why he said that: View attachment 319160 It's from Marks' Standard Handbook for Mechanical Engineers, Ninth Edition. Compare the weight and complexity of a Savonius rotor to a standard two blade turbine with the same wind catching area. If you are working with one of those cheap little hobbyist DC motors, be aware that they have cheap plain bearings and cheap brushes, so a lot of friction. You can measure the starting torque of the motor by wrapping a string around the shaft and measuring the force to turn the shaft by pulling the string. Your wind turbine needs that much torque just to get the generator to start moving. Thank you @jrmichler for that diagram. That really helped me to "get it". jrmichler and berkeman Mentor Appreciate any suggestions. Even if that's: "go read this book". Marks' Handbook for Mechanical Engineers has a section on wind power. That section is useful, readable, and appropriate for your needs. I know that both the 8th and 9th Editions have the wind power section, I would assume that other editions would also. The latest edition is expensive, but earlier editions are available much cheaper from Amazon: And there is also interlibrary loan. Science Advisor I am still stuck with how to identify a motor with a low torque start-up requirement. Turn the motor over by hand. Can you feel the permanent-magnet fields passing the poles? Is that the problem you perceive? A DC motor produces a voltage proportional to RPM. Your battery voltage therefor decides the wind speed needed to reach the required minimum RPM to begin charging. That wind speed should be more than sufficient to overcome the initial torque. If the wind speed is insufficient to reach the critical RPM, then there is no point spinning the turbine or motor until the wind picks up. In post #4, I explained how to estimate the RPM needed to charge a particular voltage battery. For the lowest speed generator, compute the RPM/volt of the available motors, then select the lowest. One experimenter I know was so concerned with starting torque that he mounted two motors on the same shaft, with the phase adjusted for minimum starting torque. It was an unnecessary waste of effort. shonuff Mentor I was originally working from the perspective of: how much power do I need? And designing from there. But recently I thought it might make more sense to first determine how much power is available, and work the other direction. The correct approach is to determine both and see if they match. So, how much power do you need? How much wind do you have available? anorlunda Science Advisor And here is a plot that shows why he said that: View attachment 319160 It's from Marks' Standard Handbook for Mechanical Engineers, Ninth Edition. While, qualitatively, I agree with this plot in that the Savonius is a pretty useless design unless the goal is absolute maximum simplicity with minimal concern for actual power collected, this plot was clearly done by someone who has no idea whatsoever about the operating conditions of wind turbines. Modern multiblade wind turbines tend to operate at tip speed ratios around 8-10, which is well off the right side of this chart, and for this to show them falling off dramatically above 2 is so wrong as to be totally laughable. Honestly, you'd be hard pressed to find a single horizontal axis wind turbine designed in the last 3 or 4 decades that wouldn't be designed to operate at either the far right side of this chart or off the right side of this chart. I suppose you could try to claim that all modern designs fall under "high speed two or three bladed turbine", but even then, two blade designs likely are operating at 10+ TSR, and the falloff above 5 is totally incorrect, plus that brings into question what the point of the "modern multiblade turbine" curve even is, since nearly all modern turbines are a 3 blade horizontal axis design. This really, really should be updated, or at least used with caution. Science Advisor Marks' Handbook for Mechanical Engineers has a section on wind power. That section is useful, readable, and appropriate for your needs. I know that both the 8th and 9th Editions have the wind power section, I would assume that other editions would also. The latest edition is expensive, but earlier editions are available much cheaper from Amazon: View attachment 319243 And there is also interlibrary loan. If the above plot comes from this book, I'm really not impressed. It contains some very serious errors. I'd highly recommend something like the Wind Energy Handbook instead. Science Advisor I'd highly recommend something like the Wind Energy Handbook instead. At least the Wind Energy Handbook gives the savonius rotor the attention it deserves. Unfortunately, that gives some readers the idea that vertical axis wind turbines are something new and exciting. PNW Hobbyist Turn the motor over by hand. Can you feel the permanent-magnet fields passing the poles? Is that the problem you perceive? A DC motor produces a voltage proportional to RPM. Your battery voltage therefor decides the wind speed needed to reach the required minimum RPM to begin charging. That wind speed should be more than sufficient to overcome the initial torque. If the wind speed is insufficient to reach the critical RPM, then there is no point spinning the turbine or motor until the wind picks up. In post #4, I explained how to estimate the RPM needed to charge a particular voltage battery. For the lowest speed generator, compute the RPM/volt of the available motors, then select the lowest. One experimenter I know was so concerned with starting torque that he mounted two motors on the same shaft, with the phase adjusted for minimum starting torque. It was an unnecessary waste of effort. Yes, the resistance I was referring to is the permanent magnet passing the poles. The prop I was using simply didn't have enough torque to get it spinning with the wind speed I have available in this particular application. That wind is a fixed speed and never changes so I either have to work within those parameters or give up on the idea. Right now I'm at the stage of "Can I even generate enough power here to do anything with it at all?" I was able to find a small prop meant for model airplanes at the local hobby store that finally had enough torque to spin up the motor. I posted my initial question because I don't know much about DC motors and I don't know if this cheap little thing I bought off Amazon for a couple dollars is just such poor quality that it has a higher starting torque than a quality product might have. Looking at the spec sheets for various motors I don't see any metric that would tell me how one differs from another in that regard. Or, if that's just a fundamental function of all DC motors. If for example I could locate a DC motor that has a very low start up torque to get past the attraction of those magnets, does that mean I sacrifice something else in exchange? Is this just the way DC motors work and the higher the output voltage at a certain RPM, the higher the starting torque will be? I am no electrical engineer if that isn't abundantly obvious from my questions. :-) I will go back and read your earlier post and see if that helps me move forward. Thank you for your responses. Science Advisor A propeller is twisted along its length, so when it is running and the plane is flying, the angle of attack will be productive over the full length of the blades. Another reason why starting is hard is that, when you want to start in a fixed wind, only a small length of the blades near the hub is near a reasonable angle of attack. Only once the propeller is spinning, can more of the blade length can be productive. The best and most expensive propeller blades have an asymmetric airfoil profile. That is more efficient than a symmetrical section blade. Unfortunately, you are building a turbine to be driven by the wind, not a propeller to drive the wind. The asymmetric section of an optimum propeller is the opposite of an optimum turbine, so a propeller does not make the best turbine. It will do for your application, but avoid expensive asymmetric section propellers. Yes, the resistance I was referring to is the permanent magnet passing the poles. That is called cogging. https://en.wikipedia.org/wiki/Cogging_torque The more magnetic poles your motor has, the easier it will be to start. More powerful simple motors have stronger magnetic fields and require more starting torque. For your experimental application, you might consider another identical motor on the same shaft, with phase adjusted to cancel the starting torques. The DC voltage generated by the motors will vary over the cycle, so one could remain disconnected, or you might get away with connecting them together in parallel. But that is an experiment that can wait until the turbine starts reliably. russ_watters Science Advisor Gold Member There are also DC motors available that do not have any metal in the rotor. These are used in some instruments to avoid the cogging problem. One such instrument is a Laser used in the construction industry. When a suspended (acoustic tile) ceiling is being installed they of course want it level. They use an instrument that has a Laser pointing upward and a motor driven right angle prism mounted above it. The prism rotates to continuously sweep the Laser beam around. The prism drive motor has a coreless rotor so that when they stop the motor, the beam can be manually aimed in any direction without the motor fighting for a preferred position. These instruments are sold in the larger "home improvement" chain stores for around USD$100. The motors are pretty low power so you won't get much power out of them. You may be able to track down the motor manufacturer to see if they have anything bigger.

The pricing starts under \$2.

Cheers,
Tom

Gold Member
The savonius rotor is fascinating and useless.
You can see a number of small leisure craft with vertical axis wind generators . Even if they produce little output, I think they have an advantage in that they don't vibrate so badly. I had a small Rutland 503 (?) regular axis wind turbine on my boat and I had serious vibration problems. But that won't have been anything fundamental.

Variable vane pitch or pole spacing would allow any turbine to get up to speed until the pole to pole torque steps could be evened out. A bit like a Joule Thief circuit, the power out would be tiny and mostly only just worth while going for. A bigger turbine would probably do a better job.

Vibration problems can come from rotor dynamic imbalance, or from local wind shadows with too few blades. Near wind farms, you can hear the blades passing the towers, which is not a resonance you want to encourage.

Fixed pitch turbines need some form of speed limiter. That can be rubber flap spoilers that are thrown out at high speed, or an off-centre mount with a swinging tail like the farm pump windmills that turn aside in strong winds.

I think it is easier to reduce the generator magnetic cogging forces by cancellation, than it is to make a three-bladed centrifugally-feathered turbine that will start in a low wind and limit maximum speed, all while remaining balanced.

Gold Member
Vibration problems can come from rotor dynamic imbalance,
That's only half the problem, though; it was mechanical resonance of the mounting / whole boat. The mounting cannot be totally massive and rigid. I discovered that fibreglass spars give far better damping than the equivalent weight / cost in steel. Either I actually managed to improve things or just came to terms - I can't be sure. But that throbbing was a great high wind alarm.

The speed regulation was with an electrical resistor and you could hear it kicking in during gusts.

Fixed pitch turbines need some form of speed limiter. That can be rubber flap spoilers that are thrown out at high speed, or an off-centre mount with a swinging tail like the farm pump windmills that turn aside in strong winds.

It can also, at least on horizontal axis turbines, be what's known as a "stall-regulated turbine". Fascinatingly, through some clever design of the blade and generator, you can have a turbine that just by the nature of its design, has parts of the blade start to stall above a certain wind speed, and thus the power will drop off naturally without any geometric change.

sophiecentaur
It can also, at least on horizontal axis turbines, be what's known as a "stall-regulated turbine".
It is one thing, stall regulating the maximum power that can be generated in high winds, to that which can be handled by the alternator and grid connection. But when there is no load connected and no power is being extracted, the speed of the unloaded rotor must somehow be limited in the extreme winds.

Staff Emeritus
Most big wind turbines have variable pitch blades. They can then be "feathered" in high winds and rotation stops.

There are too many esoteric turbine possibilities for beginners. This thread needs to focus on simple fixed geometry turbines for DIY construction. Once the turbine is operating in a fair wind, all is good. The problems come at the ends of the wind speed range.
How can a simple fixed-geometry turbine start a cogging alternator in a low wind?
How can such a simple turbine survive a wind storm without destruction?

sophiecentaur
Staff Emeritus
How can such a simple turbine survive a wind storm without destruction?
Wind turbines used on boats use a rope tether to anchor the blade during high winds. It's hard to get simpler than that.

Gold Member
Wind turbines used on boats use a rope tether to anchor the blade during high winds. It's hard to get simpler than that.
But only if you're on board at the time or remember (amongst all the other chores) to do the tether every time you disembark.

anorlunda
Staff Emeritus
But only if you're on board at the time or remember (amongst all the other chores) to do the tether every time you disembark.
Like everything else in boat life, if you still survive after the 3rd time making every possible mistake, you become a seasoned salt.

russ_watters and sophiecentaur
Wind turbines used on boats use a rope tether to anchor the blade during high winds. It's hard to get simpler than that.
So, to restart the turbine in the calm after a storm, the operator should first remove the tether, and then give a turbine blade a push with their hand.

I believe the OP is looking for a less hands-on solution.

Gold Member
Those poor bearings!

Staff Emeritus
I believe the OP is looking for a less hands-on solution.
I think you're right. The OP does not want the KISS solution.