# DC motor as a generator: characteristics

I am using a DC motor as a generator for my small toy/experiment. The problem is that the motor/generator can not provide enough counter EMF or torque or whatever the correct word is. So the rotor on the other side just keeps spinning at too high RPMs. I would like to control the rpm of the rotor to do so I will change the motor. My question is what is the right characteristic to check while buying the new one. Power supply (Watt), the speed constant (RPM/V) , the supply voltage (V) and torque constant (mNm/A)

Clarification:

I am using it with a small wind turbine model. It is connected to a circuit where I can play with the resistance. I change the resistance and read the voltage output by that I get the power extracted. It is being extracted, so it should be connected correctly I guess. But the resistance range of 10 to 10000 Ohm at a constant incoming wind speed, the rpm of the rotor only changes from 5000 to 5200. surely The RPM goes higher with an increasing incoming wind speed but the change of rpm with same resistance range is very limited. so somehow the counter EMF is not enough need to buy a new motor?

Right now the characteristics of the motor I am using:

• Maxon RE 10 Ø 10 mm,
• Precious Metal Brushes,
• 0.75 Watt Values at nominal voltage
• Supply voltage 12 V
• No load speed 11500 RPM
• No load current 5.37 mA
• Nominal speed 2790 RPM
• Nominal torque (max. continuous torque) 0.731 mNm
• Nominal current (max. continuous current) 0.081 A
• Stall torque 1.01 mNm
• Starting current 0.106 A
• Torque constant 9.55 mNm/A
• Speed constant 1000 rpm/V
• Speed / torque gradient 11900 rpm/mNm
• could you explain what you are trying to achieve in more detail. I'm confused as to why you can't control speed. Dec 24, 2013 at 10:58
• Are you asking how you can use a DC motor as a brake? Or how you can use it to generate electricity? Or how you can power it and control its speed? Or what?
– user16324
Dec 24, 2013 at 11:34
• I worked on a project where we used a small dc motor as a brake for a scale-model hydraulic turbine. If that's what you are trying to do, please clarify and perhaps the question will be reopened. Dec 24, 2013 at 12:02
• The driving motor high RPM's mean it's not seeing enough load. Questions; is the generator turning the right direction to produce power? Is there an electric load on the generator? Dec 24, 2013 at 14:10
• Is the stator terminals left unloaded? if that is the case then yes it will just accelerate off based upon the shaft torque and any additional frictional loads. Do some quick sums and put a power resistor (right R, right W) across the stator terminal and you should see the speed settle out. Then you need to change that resistor into an active powerload
– user16222
Dec 24, 2013 at 15:48