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Permanent Magnet AC Alternator - No Load

  1. Jul 3, 2013 #1
    OK... so my understanding is that a DC generator under no load generates high voltages without any current, but does the same hold true for an AC alternator?

    What happens with current when the RPMs of the alternator exceed what is necessary for the current requirements?
    Last edited: Jul 3, 2013
  2. jcsd
  3. Jul 8, 2013 #2


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    Generated voltage is proportional to the rate the magnetic field crosses the windings.

    The open circuit voltage produced by a permanent magnet DC generator is proportional to the RPM of the generator. When you load the generator at a fixed RPM, it's voltage is reduced by the output current flowing through the resistance of the generator windings.

    A permanent magnet alternator will produce an AC voltage that has amplitude proportional to RPM. (The frequency will also be proportional to RPM). As the RPM rises, the voltage will rise, so the load will draw more current. Unless there is some form of regulation or control the power dissipated in the load will rise in proportion to the square of the RPM.

    If there is no field current control, the RPM of an alternator should be regulated to produce the supply voltage needed by the load. In a motor vehicle the RPM varies, field control is needed to regulate the voltage that charges the battery.
  4. Jul 8, 2013 #3


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    This doesn't answer the question in the title but it's worth noting that the inductance of an alternator under load can help to regulate the voltage with speed. The humble bicycle dynamo protects the lamp bulbs in this way when you are going fast. This only works for a particular value (range) of loads and some form of regulation is needed (series) to cope with different loads. This may take the form of a mechanical speed regulator where a wind turbine is involved.
  5. Jul 8, 2013 #4


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    @ sophiecentaur.
    There was no question in the title.

    The first question in the post started "...so my understanding is that a DC generator under no load generates high voltages without any current,.." which seems to suggest there is no sensible voltage limit. That statement is false, and is therefore a misunderstanding of the voltage generation process. “but does the same hold true for an AC alternator?” can be answered as No, it holds for neither DC nor AC. The first line of my reply explained the situation.

    The second question in the post “What happens with current when the RPMs of the alternator exceed what is necessary for the current requirements?” is answered by the statement that the output voltage continues to rise. If the Permanent Magnet AC alternator is satisfying the current requirement of the load then a tiny increase in RPM will increase the output voltage a tiny amount.

    There was no reply to this question for several days so I did my best to cover it quickly. IMO you should have posted your answer days ago and not left an unanswered post hanging.

    I expect a refinement of the question or a request for further explanation will be posted by SonnyBoy based on our replies so far. That will draw others in to answer the better defined question.
  6. Jul 8, 2013 #5


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    That is cheeky!!:wink: or defensive? I don't think I was criticising any answers in the thread. It doesn't do to be too literal in responses or we'd spend all our time arguing, I think.
    Some of us have better things to do than to sit at their computers all day. I have had a busy and entertaining weekend with dogs, boats and family.

    The idea of unloaded performance was introduced and I 'addressed' it. But we're far from 'ideal circumstances' for most alternators.

    The wording in the OP - "current requirements" is not a description of a typical load, which will be pretty much Ohmic, with I proportional to supply V - except in the case a large number of loads, each one being thermostatically controlled, for instance and producing a net, mean, constant power draw. This will only apply to big, regulated alternators, though.

    Permanent Magnet alternator applications are rare and I suspect they're limited to wind turbines and bicycles (I have to be wrong there, so put me right, please), where they are often supplying less than would be required. The advantage of having fixed coils and a rotating magnet with no need for brushes and slip rings is clear (although my Rutland 503 appears to have fixed magnets - I wonder why).
  7. May 28, 2014 #6
    This is not quite correct. Even if rpm were held constant, i.e. cruise control engaged, as the load current varies, the filed current must vary to keep terminal voltage constant.

    If the alternator open circuit voltage is measured (no load), then loaded, the voltage drop incurred is due to more than just the winding resistance of the stator, but stator winding inductance. The inductive reactance is generally much larger than the resistance.

    For an alternator the inductive reactance is typically 100% plus or minus 50%. The resistance is maybe 2% to 5%. When a load is placed across the terminals, the voltage drops from its open circuit value to Voc - (R + jX), where X = Lω.

    If load is held constant while speed increases, then power goes up with speed squared only until X equals/exceeds R. At that point power levels off. Although the open circuit voltage increases, the fraction of open circuit voltage, Voc, that is impressed on the load decreases. This is due to X increasing so that a smaller fraction of open circuit voltage appears at the load. Also, current asymptotically approaches constant value since increase in Voc and increase in L are equal.

    Think about conservation of energy. If power equals Tω, T=torque, then raising ω results in higher power, varying as ω to 1st power if torque is constant. If power at electrical load increased with the square of ω, CEL (cons of energy law) is violated. In order for electric power to increase w/ square of ω, T must increase as well.

    Just some clarification. Thanks.

    Last edited: May 28, 2014
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