Motors used as generators

Discussion in 'General Electronics Chat' started by strantor, Jul 2, 2012.

  1. strantor

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    This set of questions comes up a lot (in my head) and on occasion here on the forum. The definitive answers seem to always lurk just around the corner but I've never seen them spelled out. So I want to get to the bottom of it.

    1. Is the HP of a motor given as the mechanical HP or electrical HP? It can't be both, right? or else it would be 100% efficient.

    2. A DC permanent magnet commutator motor can be used as a generator, so long as the brushes are not aligned in a way for use in only one direction (in other words, a bi-directional motor) - correct? I think everybody will say yes, so

    2.A. In this case, with your bi-directional DC permanent magnet commutator motor, does it have the same HP as a generator as it does as a motor? Let's say your motor has an electrical HP rating of 2HP and operates @ 180V at full RPM, can you draw 8A from it (2hp)? What if you try to draw 20A? Will the voltage drop or will it burn up or both?

    2.B Will the bi-directional DC permanent magnet commutator motor generate the same voltage per RPM as a generator as it would generate RPM per voltage as a motor?


    3. How does regenerative braking work? Let's say an electric car has a battery voltage of 300V and max speed of 120mph. When full 300V is applied to the motor, it spins 120mph (ignore gearing, example only). Say this car is going only 40mph (1/3 of max), and coasting, the motor is generating only 100V (1/3 of max) so how can this energy generated by the motor be transferred back to the battery, when it is of lower potential than the battery?

    4. how on earth does polyphase induction motor function as a generator? I know they use it in windmills. I have read about it before and I know that the motor has to be spun faster than synchronous speed, but I did not understand the material. It made me think that current flows in 2 different directions simultaneously in the same winding; once to induce current into the rotor, and then again in the opposite direction as the rotor induced current back into the winding. I do not understand.
     
  2. #12

    Expert

    Nov 30, 2010
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    1) it's the shaft horsepower. Electric motors always use more energy from the power grid than the nameplate horsepower rating.

    After that, I am in the wrong pond for the type of swimming I do.
     
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  3. praondevou

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    4. Ok, read here about windmills.

    Oops, I meant here: http://www.electrodynamics.net/documents/electrodynamics_power_gen2002.pdf

    page 6 to 8 may help. Essentially you need a magnetic field in the rotor, the way to get it there is via the excitation current from the stator.

    3. My guess: provide a turning magnetic field that is slower than the speed of the rotor (which depends on the cars speed). That would be similar to the windmill generator even if it's a different type of motor.
    The speed of the drive motor is controllable from 0 to whatever the max speed is right?

    I don't have the HP answers.
     
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  4. THE_RB

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    1. Yep the hp rating should correctly be the output (shaft) power. But you will see some "Chinese honesty" more and more now with output power claims, on router motors they sell many as "3&1/2 hp" routers and the label electrical input power is 1800W. :eek:

    2. Yes, provided brushes work ok which is normally true.

    2A. The DC perm mag motor as a generator should have very similar power rating as a generator. It's internal resistance and ability to extract heat are the main limiting factors and should remain the same in generator use.

    Your question re the output current is a good one, if you draw too much current in generator use the IsquaredR losses increase exponentially and you risk exceeding the max power dissipation of the motor and can kill the motor. The same problems exists too in motor use with excess current.

    2B. Yes the KV rating (kRPM:Volt) remains the same in generator use, the motor voltage is tied to RPM and that principle is used in backemf speed controllers and regenerative braking.

    3. Regenerative braking requires a way to draw lowV highI power from the motor and return highV lowI power to the battery. Fortunately if the PWM motor controller is a full bridge (or half bridge with sync rect) then PWM and the motor inductance itself acts like a SMPS and will do the voltage and current conversion.

    4. I have no idea how to use a polyphase induction motor as a windmill generator, however on the other forum a gentleman was modifying his 3phase squirrel cage induction motor rotor by installing permanent magnets in it to use it as a windmill generator. His thread gave me the impression this was a "typical" mod so maybe what you have seen are *modified* induction motors?
     
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  5. strantor

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    LOL
    Thanks, that was an informative document. It touched on a lot, but still didn't mention how the excitation current and the output current can be present in the same winding at the same time. And why does the excitation current not flow straight to the output?
     
  6. strantor

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    Thanks RB, good info. I have a question about this though:
    So PWM on the low side causes breaking of the current path through the inductive motor, and like breaking the current path through any inductor, it generates a HV spike which exceeds the potential of the battery, therefore charging it?
     
  7. praondevou

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    I think I know what you would like to see, somebody explaining the whole thing with a schematic, indicating current flow... Maybe when I have time I can try to figure that out, it's an interesting subject.

    In the meantime you can read also this, it gives some more details of the theory of induction generators (under "induction generator operation")
     
  8. bertus

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    Apr 5, 2008
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    Hello,

    @ praondevou
    As you might have noticed, the link is not allowed by the forum.
    The site probably does not take good care about the copyrights.

    Bertus
     
  9. steveb

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    For questions 3 and 4, consider that you need some starting energy to get the generation process going. Let's assume you are talking about a standard induction machine and not a dual-fed (or doubly-fed) system.

    First, you need some type of starting field in the rotor, or else spinning does nothing. Either you have to excite the rotor with a field, or hope that there is a residual field that is sufficient to start the process.

    Second, you would like to have some stored energy on bus voltage caps so that you can control the output rectifier switching (usually done as an inverter with reverse power flow).

    With the stored energy you are able to control the voltage on the output of the generator. This means you can control both frequency and amplitude and actively keep the system in a generating mode. Often this is done in a current controlled vector mode in which the flux current and the torque controlling current are independently controlled. Essentially, the rotor flux and the torque are precisely controllable and the operation at a negative torque implies electrical power flow out, rather than flowing in as in the case of a motor.

    Essentially, just as you can do precise vector control on a motor to allow direct torque and/or speed control, the torque controlling current can be forced to a negative value, which reverses the direction of electrical power flow.

    A key piece of the hardware is the standard 3-phase inverter switches used to drive motors. Due to the freewheeling diodes that are part of the transistor switches (usually MOSFETs or IGBTs), the inverter acts as a rectifier when the power flow direction is reversed. Voltage can be controlled in either forward or reverse mode, and hence the dual current control loops works in either case.
     
  10. strantor

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    So are you saying that the excitation current induces current in the rotor, and then once current is flowing in the rotor and producing output current, the excitation current is removed and the output current harvested? So excitation current is not present at the same time as output current? If that is what you're saying, then once current is flowing in the rotor, how is it sustained? Is it perpetuated by cutting lines of flux in the rotor which are present because of output current?
     
  11. steveb

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    Yes, that is my understanding. A field is needed to start the process and then once current is flowing in the stator, given enough mechanical input energy, the process can bootstrap itself.

    Of course nothing is free in the real world, and the process implies a mechanical torque which tries to stop the rotation, but assuming we have a mechanical drive input, beyond a critical speed and existing current level, it should be self sustaining.
     
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  12. strantor

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    Praondevou, I think I found the document you were trying to post, it was from scribd site?
    Induction Generator Theory and Application byJ.E. BARKLE and R.W. FERGUSON, 12 FEBRUARY 1954?

    I wanted to print it, but scribd wants me to pay for the priveledge. I believe the books hould be out of coypright by now so I checked archive.org and no luck. I did find a bunch of stuff on induction generators though, and as always I'm taken aback by how far back the technology goes. There was a book published in 1908 on this topic!
     
  13. WBahn

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    Mar 31, 2012
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    Assuming no cheating, it is the rated amount of mechanical power you get extract from the shaft.

    Let's say you have a motor that outputs 2HP. It will require something more than 2HP (let's call it 3HP for simplicity) of electrical input because it is not 100% efficient. The same will be true of the system acting as a generator. If you put 2HP of mechanical power into it, you will only be able to extract something less that 2HP out of it, say 1.5HP.

    Now, one question is what defines the limit that establishes the rating? Is it the electrical considerations or the mechanical. My guess is that, more often than not, it is the electrical considerations. So, given the example above, can we safely increase the mechanical power that we put into the generator until we are extracting the same electrical power as we would have had to put in to get the rated mechnical power out? In our case, acting as a motor, we could put as much as 3HP of electrical power into it. So, since 2HP of mechanical in produces 1.5HP of electrical out, could we probably put in 4HP of mechanical power in order to extract 3HP of electrical out? My guy feel is that, within limits, the answer is yes.

    Another interesting question is whether the efficiency as a motor is the same, or at least strongly related, to the efficiency as a generator?

    It should generate the same (or nearly so) open circuit output voltage at a given RPM as the voltage required to achieve that same unloaded RPM.

    In any regenerative braking system (be it electrical, hydraulic, whatever) you have to have some means of converting the generated power into a form that can be stored. If you can store the generated form directly, great. Otherwise, you'll have to condition/transform it and loose some in the process.
     
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  14. strantor

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    Interesting perspective, I had never thought of it that way. I believe you are right, and I hope you're right , because that would mean that my brushed DC motor has more potential as a generator than I previously though. I was locking myself down to the (ex.) 2HP mechanical number and thinking I would have to just accept the fact that it will produce less power as a generator.
    yes, another good question; I neglected to ask it, but I have read elsewhere that the efficiency is less when used as a generator. I have also read that it is the same, but I have less faith in that answer.
    does the motor itself act as the potential shifting device as I described in post #6? I'm still unsure about that. I also get confused between back EMF and the current produced by spinning the motor. To my shaky understanding, the current produced by spinning the motor is not back EMF, and the only thing that could be harvested by regenerative braking is the back EMF.
     
  15. GetDeviceInfo

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    I think what you'll find is that the constraint is heat energy. Shafting aside, you could input any amount of HP into the motor, as long as the heat rise is within tolerance. I've worked with high power density motors that deal with this with liquid coolants.

    I'm not sure if this is what your trying to say, but it basically comes down to creating leading vars. These vars give rise to the rotor bar magnetics. In single speed machines this can be done with capacitors, but in variable speed it is done with the switching elements. There are many other methods employed. The name escapes me, but one version is a synchronous motor seperately driven to provide over unity power for generator excitation.

    One thing I like about this forum is that is has me reviewing older material. Leblanc exciter is what I was thinking of.

    There are two voltages at work, the applied, and the EMF. When motoring, the generated voltage (EMF) counters the applied. When generating, the applied is reduced, or removed as in a PM type motor. What generated voltage is available? In a PM motor, it will largely be applied (less efficiencies), decaying with speed. In excited generation, we can increase the generated voltage by increasing the excitation. You may have encountered this with VFDs going into bus overvoltage errors on decel ramps. If your loads inertia overruns the ramp, the motor becomes a generator with a realtivley high degree of excitation, causing high voltage returning to the DC bus.
     
    Last edited: Jul 4, 2012
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  16. THE_RB

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    Reagrding regenerative braking on DC *permanent magnet motors* the motor when spinning (when in generator mode) acts as a DC power supply which has a series inductance (the motor winding).

    In a spinning motor, if your PWM FET shorts out the motor terminals the current builds up in the motor winding (inductor) and when the FET opens it is the same as a boost SMPS, and the winding (inductor) delivers a higher voltage but lower current pulse back to the battery.

    As a simple explanation;
    if PWM duty > motor speed, current is fed from battery to motor, is a "motor"
    if PWM duty = motor speed, no current is needed, the motor freewheels
    if PWM duty < motor speed, current is fed from motor to battery, is a "generator"

    So the same PWM controller can provide full control of how much power is fed to the motor when accelerating and also power drawn from the motor when decelerating.

    To get this effect you need a full h-bridge PWM circuit or a half-bridge PWM capable of synchronous rectification.
     
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