# Low RPM, Very efficient DC motor/Generator, 200w to 1Kw

• Fizzics
In summary, you may be able to power a flywheel up to 50 RPM using a 200 watt or less motor/generator. However, for a DIY project with a goal of recovering less than 5% of the energy used, a higher RPM on the flywheel is required. Additionally, cost is an issue.

#### Fizzics

Hi
I am looking to purchase a Low RPM, Very efficient DC motor/Generator between 200w and 1kw for a low friction fly wheel project which involves powering up a 10kg flywheel to about 50 RPM over say 30 seconds then recovering the energy back through the motor/Generator with minimal losses, ideally with losses less than 5%.
I would be grateful for any pointers and as usual cost is an issue.

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Fizzics said:
Hi
I am looking to purchase a Low RPM, Very efficient DC motor/Generator between 200w and 1kw for a low friction fly wheel project which involves powering up a 10kg flywheel to about 50 RPM over say 30 seconds then recovering the energy back through the motor/Generator with minimal losses, ideally with losses less than 5%.
I would be grateful for any pointers and as usual cost is an issue.
I'm not sure if you can do that without gearing...but also I would think it should take way less than 200W. Unless that 10kg is on a shockingly thin disc or support structure, this is much less than 100 watts -- possibly less than 10.

So, why so slow?

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On the generator end, AC? DC?

Also, during recovery the RPM and the voltage will be dropping, ending at zero zero. A simple resistor will capture all the energy regardless of RPM or voltage. If you're using something else to consume the electric power, it may have voltage and or frequency characteristics that make it difficult. So choosing how to consume (dissipate) the energy is as important as selecting the motor and generator.

Edit: A resistor turns the electric energy to heat. So do losses in the motor and generator. So if you account for those heat losses, it's not so important that those losses be small. In other words, the losses are more a matter of definition and bookkeeping, not physics.

The tiny amount of energy that is regenerated by decelerating that slow moving flywheel will disappear in the noise. Servomotors can do what you want. Here is an example from one of the best known servo manufacturers: https://literature.rockwellautomation.com/idc/groups/literature/documents/pp/mp-pp001_-en-p.pdf.

The regenerated energy from your small, slow system will just disappear in the drive. For larger flywheels at higher speeds in hard accel/decel applications, you can get drives that pump the regenerated energy back into the power lines. One manufacturer found the hard way that a hard accel/decel cycle made the lights in the entire plant blink from the resulting voltage variation.

If cost is an issue, just spin up the flywheel by wrapping a rope around it and pulling. Then decelerate it using a bicycle generator connected to a light bulb. Turning the light on and off will clearly show the relationship between mechanical work and generating electricity.

Have you tried to estimate how much energy can be stored in your flywheel (it will depend on a radius, for practical reasons I doubt you can go for something substantially larger than 1 m)? For how long can you recover this energy at 1 kW? 200 W?

Fizzics said:
I would be grateful for any pointers
As an energy (electricity) storage device a flywheel is usually works within an RPM range, since it's quite problematic close to 0 RPM.

Fizzics said:
with losses less than 5%.
20% loss would be quite good already for a DIY project. 5% loss is just unrealistic.

Fizzics said:
That's too low for a motor/generator. You will need some transmission (more losses) - or higher RPM on thy flywheel.

Fizzics said:
as usual cost is an issue.
I did this with a HDD spin motor from an old double sized 5.25 MFM drive, between 500 and 2000RPM. Of course for 0.2-1kW you will need something beefier.

## 1. What is a low RPM DC motor/generator and how does it work?

A low RPM DC motor/generator is a type of electrical machine that converts mechanical energy into electrical energy or vice versa. It operates at a lower rotational speed (typically less than 1000 revolutions per minute) compared to traditional motors/generators. It uses a DC power supply and a commutator to convert the direction of current flow, which in turn creates a continuous rotation of the motor/generator.

## 2. What makes a low RPM DC motor/generator more efficient?

A low RPM DC motor/generator is more efficient because it operates at a slower speed, which reduces the amount of energy lost to friction and heat. Additionally, the use of permanent magnets and advanced electronic controls results in a more efficient conversion of energy compared to traditional motors/generators.

## 3. What is the power output range of a low RPM DC motor/generator?

The power output range of a low RPM DC motor/generator can vary from 200 watts to 1 kilowatt, depending on the size and design of the motor/generator. This type of motor/generator is typically used in applications where a moderate amount of power is needed, such as in small wind turbines or electric vehicles.

## 4. What are the advantages of using a low RPM DC motor/generator?

One of the main advantages of using a low RPM DC motor/generator is its high efficiency, which results in lower energy consumption and cost savings. It is also more compact and lightweight compared to traditional motors/generators, making it ideal for portable and space-constrained applications. Additionally, its steady and smooth operation makes it suitable for sensitive equipment that requires a stable power supply.

## 5. What are the potential applications of a low RPM DC motor/generator?

A low RPM DC motor/generator has a wide range of potential applications, including renewable energy systems (such as wind and hydro power), electric vehicles, robotics, and medical equipment. It can also be used in industrial and commercial settings, such as in conveyor systems and pumps, where a low RPM and high efficiency are desired.