Micro DC Motor as generator - regenerative braking of the shaft for RPM control

Discussion in 'The Projects Forum' started by Roshu018, Jul 6, 2015.

  1. Roshu018

    Thread Starter New Member

    Jul 6, 2015
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    Hello everyone! I am new here and I did go through a lot of posts trying to find the answer but couldn't find the specifics for what I am trying to do. I have a small horizontal axis water turbine and I am using a micro PMDC motor as a generator (this is all for experimental purposes so the conversion efficiency from kinetic energy of water to the electrical energy is not very important). Now, at around 500 RPM (I have two blades attached to the shaft of the motor/generator) I am reading 0.45V. All I am trying to do is to electrically control the RPM of the motor, nothing else. So what I did, I added a zero ohms resistor across the motor hoping that the huge load will slow down the generator however that didn't work. Apparently the torque on the shaft from water energy is high enough so that I can't really load the generator enough to cause the RPM change. Any ideas on how I might be able to do this? Also, I need to keep the flow rate of the flow constant so I can't alter the RPM that way.

    Thanks,
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What is the rated rpm and max voltage of the motor, .45v at 500 rpm seems unusually low?
    Are you saying that the motor is turning too fast?
    One obvious way is to reduce the impeller size if too high a RPM?
    Is the reason for this to regulate the output regardless of rpm?
    Max.
     
  3. Roshu018

    Thread Starter New Member

    Jul 6, 2015
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    Well the nominal voltage for this motor is 7V and no load speed is 10400 RPM. Another thing I didn't mention is that I can not change the size of the blade. They have to stay as it is :(
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So you have plenty of room as far as RPM is concerned, I am still not sure what you issue is?
    Unless you want the RPM to remain constant or at a certain voltage output, regardless of water flow?
    Without any control as to the water flow over the blade it would seem your only hope is electrical regulation.
    This would be very easy to do with a wound field motor/generator.
    Max.
     
  5. Roshu018

    Thread Starter New Member

    Jul 6, 2015
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    Yes, I am actually looking for electrical regulation. The reason for that is for example, I have a flow rate of lets say 0.45 m/s which for my blade design gives me a certain torque on the generator at an rpm of 500. What I am trying to do is to electrically load the generator so that the shaft is harder to spin thus reducing the RPM. Everything else stays the same: flow rate, blade design, all of it. I know that with lower RPM I will have a lower voltage output but that is fine because that is what I need. I tried loading the motor but simple putting resistors across it but it did not effect the rpm. I was looking into regenerative breaking, basically supplying power to the motor in the opposite direction to slow it down. Do you think that might be a possibility? Also can you elaborate a bit more on a wound field motor/generator? I am a mechie so I did have some EE classes but long time ago. And off course, thank you so much for the feedback I really appreciate it.

    Uros
     
  6. MaxHeadRoom

    Expert

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    A wound field generator can be any DC motor with a shunt wound (parallel) field, or such as an automotive alternator, which BTW has the regulator built in, which also can be customized similar to a DC shunt motor.
    What is done essential in both cases, is the output is monitored and the field voltage/current is regulated to whatever the required DC armature/ALT stator output voltage is desired, regardless of rpm.
    Max.
     
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  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Basically you need a larger motor that runs at far lower RPM's as your generator. Right now expecting any amount of drag from a 10,000+ RPM motor running at 500 RPM as a generator is unrealistic.
     
  8. Roshu018

    Thread Starter New Member

    Jul 6, 2015
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    Again thank you for all the help :)

    @MaxHeadRoom just to make sure I understood this, by controlling the field voltage/current I can load the rotating shaft electrically which would then cause the change in the torque thus allowing the shaft to either spin easier or harder?

    @tcmtech I can't use a larger motor that runs at lower rpm because of the size constrain. However, I am not trying to use this generator for power production, I am using it in experimental purposes. The motor/generator is just being used in a model marine hydrokinetic turbine to provide a mean of energy extraction from the water (I am actually researching fluid dynamics aspect). But in order to do that I need to have a realistic model of a turbine that acts like a real mhk or even a wind turbine, operating at certain rpm.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So you are trying to control rpm and not generated voltage??
    It sounds as though you want to control RPM regardless of voltage?
    Max.
     
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  10. Roshu018

    Thread Starter New Member

    Jul 6, 2015
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    Yes, I would like to control the RPM regardless of voltage. I know that if the rpm goes up or down a different voltage will be induced in the motor and that is fine because I still use that in my analysis. I just need a way to electrically load the motor shaft so that the kinetic energy of the water can not spin it as fast.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Very difficult to do, for one you would need to know the maximum torque exerted on the motor shaft, starting with almost locked shaft, or at least the minimum rpm you want to record at in order to calculate a suitable motor that can be electrically loaded at that torque.
    It sounds as though the motor you have is pitifully small for the job.
    Max.
     
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