Linear Torque-Speed Curves of DC Motors

[Crabstiks]

I'm looking to purchase a dc motor but need to accurately know the torque-speed characteristics of it. I don't really have $$ for a good quality one that would come with accurate info (like I'm led to believe maxon motors do).

When buying a cheap dc motor (i.e. from ocean controls. For example this motor: http://www.oceancontrols.com.au/MOT-212.html ), the only specifications given are the stall torque and no load speed. I know that these can be used to make a linear estimation of the torque-speed characteristics of the motor but how accurate is this?

I was looking at motors from Baldor ( http://www.baldor.com.au/product_view.php?PROD_ID=26 ) and the performance curves they have listed are linear. Am I right in assuming these graphs are created in the same manner and as such are only an approximation?

**If you can answer the above questions great! If you are curious about the application or just a really helpful guy/girl then below are some more details about the application.

I am doing honours in engineering and my project is to investigate the characteristics of a small scale vertical axis wind turbine (VAWT). There has been some research previously into the turbine and to measure the power output they used a Prony brake to measure torque and an encoder to measure angular velocity. My supervisor believes that the Prony brake was inaccurate and that to make the project worthwhile a better torque measurement system must be used. There is also an issue in that the bearing friction seemed to swamp the torque generated at higher rpms. The previous people working on the project spent a lot of time optimising the bearings but they are still not good.

The shaft is only 12mm so fitting strain gauges to it is an issue and I don't have the budget for a small pre-packaged torque sensor.

My supervisor has suggested attaching a small motor to the base of the shaft that is attached to the frame of the turbine only through a load cell at a known distance. The motor will then be used to get the turbine up to speed (while the turbine is under a wind load), to overcome the bearing friction. The load cell can be used to measure the total torque acting on the assembly and by subtracting the torque of the motor (hence the need for accurate motor torque characteristics) the torque that the turbine is producing can be determined. (The only issue is that this will also include bearing friction however this can be estimated through a spin-down test.)

At least I believe that is how it is supposed to work. I'm still trying to understand it but my supervisor seems confident it can work.

If you can come up with a better way of measuring the torque that would be much appreciated, if not then any info on the original question will be great!

Thanks in advance for any responses!

 

[Achy47]

Thank you for your post. I understand the importance of accurate torque-speed characteristics when purchasing a dc motor. It is understandable that you may not have the budget for a high-quality motor with precise specifications, but there are still ways to obtain accurate information about a cheaper motor.

Firstly, using the stall torque and no load speed can give you a rough estimation of the torque-speed characteristics, but it may not be accurate enough for your project. This is because the motor's performance can vary depending on factors such as load, temperature, and voltage. Therefore, it would be best to obtain more detailed information about the motor's performance.

One option is to contact the manufacturer of the motor and request more detailed specifications. They may have additional information that they can provide to you, such as performance curves or data sheets. It is worth reaching out to them and explaining your project and the need for accurate torque-speed characteristics.

Another option is to conduct your own tests on the motor. This could involve using a dynamometer to measure the motor's torque and speed under different conditions. This would give you more accurate data that is specific to your motor and your application.

Regarding the performance curves provided by Baldor, it is possible that they are only an approximation. However, this may still be a useful tool for estimating the motor's characteristics. It is always best to have more data and information available, even if it is not 100% accurate.

As for your project, it sounds like your supervisor has a good idea for using a motor and load cell to measure the torque produced by the turbine. However, it may be worth exploring other options, such as using strain gauges or a dynamometer, to measure the torque directly on the turbine shaft. These methods may provide more accurate data and eliminate the need for estimating bearing friction.

I hope this information helps you in your project. Best of luck with your research!