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Counter torque, voltage and current

  1. Jan 17, 2014 #1
    When an ac generator produces high current the counter torque increases, but if voltage increases would the counter torque decrease? Is counter torque related to counter EMF?
  2. jcsd
  3. Jan 17, 2014 #2


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    Counter torque is required by conservation of energy. Mechanical power is equal to the product of torque times rotational speed. Torque (in n-m) times rotational speed (in radian per sec) gives power in Watts.

    Thus the minimum possible counter torque on a generator is equal to the output power divided by the rotational speed. Of course the actual torque is typically somewhat greater than this due to the presence of both electrical and mechanical losses.
  4. Jan 18, 2014 #3
    is counter torque affected by voltage and current phase shift?
  5. Jan 19, 2014 #4


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    Yes, look at what I said above about power and counter torque. Since power can be influenced by all of those variables then the counter torque can be influenced too.
  6. Jan 19, 2014 #5


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    "is counter torque related to counter emf?"

    Consider that when voltage is induced in the armature of the generator, a connected load will draw a current that creates a magnetic field which opposes the source magnetic field (lenz' law)

    The more load connected (parallel) the lower the overall resistance. The current draw increases, and the magnetic field which opposes the source field is stronger.

    The counter torque is the reaction between the source field (which induces voltage in the armature) and the field created by the armature current
  7. Jan 20, 2014 #6


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    Yeah, that's actually a very pertinent question.

    Not all machine models include an internal "back" EMF (induction motor for example), but for those that do (synchronous and DC machines for example) the energy flowing in/out of this internal EMF is precisely the energy being converted (from/to electrical to/from mechanical).

    So yes, this counter EMF (or more particularly the power in this EMF) is extremely closely connected with the counter torque. Specifically it is this power, which when divided by the rotational speed, gives precisely the counter torque.
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