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  • stuck on theory

    Ive been reading books on theory alot lately trying to understand certain things. In my theory book it states that when the voltage is doubled the current is doubled for a fixed resistance,I got that. If voltage is halfed, current is halfed and so on. I draw out series and parallel circuits test with ohms law and all is well.
    But I noticed today that when you double the voltage on motors the current is halfed. And when you double the voltage on a 1500 watt heater the current is halfed. This is confusing Ive been reading for hours trying to figure this out so I'll be able to sleep tonight....
    I was thinking maybe theres different taps for different resistances for 120/240 heaters, one with lower resistance for 120 and 1 with higher for 240?
    Could that be the case for motors too, different taps at a different point on the motor windings to vary the resistance.
    I would really appreciate some help with this.
    Thanks

  • #2
    Re: stuck on theory

    Think of having more than one winding. You can connect them in parallel for the lower Voltage and in series for the higher. In the case of a heater it could have more than one winding, but normally they are made for the Voltage it will be used on. The resistance would be based on desired Voltage and Wattage.

    In electric motors you connect the run windings either in parallel or in series.

    Hope this helps.

    Comment


    • #3
      Re: stuck on theory

      Originally posted by Old Grunter View Post
      Think of having more than one winding. You can connect them in parallel for the lower Voltage and in series for the higher. In the case of a heater it could have more than one winding, but normally they are made for the Voltage it will be used on. The resistance would be based on desired Voltage and Wattage.

      In electric motors you connect the run windings either in parallel or in series.

      Hope this helps.
      Old Grunter/Woussko,

      You need to pick a user name and stick to it. I hate keeping score.

      Mark
      "Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony

      I never lost a cent on the jobs I didn't get!

      Comment


      • #4
        Re: stuck on theory

        You seem to be mixing apples and oranges.


        For a given resistance..for example a heater element....if you doubled the voltage, the current would also double. from I = E/R

        But for a given WATTAGE ...for example you want 1200 watts of heat....if you are going to double the voltage, you need to increase the RESISTANCE times 4, which will DECREASE the current by a factor of 4. from P = E squared/R

        If you just doubled the voltage on a given heating element, the current would double, but the watts would be times 4.

        Comment


        • #5
          Re: stuck on theory

          WHOO!! hold on there. You guys are WRONG. The problem is when you double the voltage you half the current NOT DOUBLE IT!!! When you half the voltage you double the current. You must have read the book wrong. ANYTIME you have a fixed resistance or impedance like a heating element or a motor winding. If you go from 120v to 240v the current draw will be HALF not double!!

          Now you can sleep tonight. Lou

          Comment


          • #6
            Re: stuck on theory

            I want to clarify my last posting, the reason why in the case of a motor is that the windings of a dual primary motor. Dual 120v windings, are put in series when hooked to 240v so now the impedance (resistance) is not the same as it was with 120v . It is now 2 times the resistance as before. So in this case the resistance actually changes it is not fixed. It is confusing becasue of the change in resistance. But yes, the current will double is you double a voltage over a fixed resistance. With the electric heaters, the design resistance is different for a 240v heater from a 120v heater to make the same wattage. So here again the resistance changes, so I think that is where the confusion is. Even I get confused.
            Lou

            Comment


            • #7
              Re: stuck on theory

              Anyone looking for the question and circuit posted by WBrooks here is a link to it. http://www.ridgidforum.com/forum/showthread.php?t=19509

              Comment


              • #8
                Re: stuck on theory

                Originally posted by QROKING View Post
                WHOO!! hold on there. You guys are WRONG. The problem is when you double the voltage you half the current NOT DOUBLE IT!!! When you half the voltage you double the current. You must have read the book wrong. ANYTIME you have a fixed resistance or impedance like a heating element or a motor winding. If you go from 120v to 240v the current draw will be HALF not double!!

                Now you can sleep tonight. Lou
                Thanks alot for the reply, In trying to understand this, isnt a light bulb considered a fixed resistance? Because my book shows a light bulb, when I double the voltage the current doubles and the wattage goes times 4, or do I need to buy some new books?....lol
                Thanks again for the reply.

                Comment


                • #9
                  Re: stuck on theory

                  Originally posted by QROKING View Post
                  I want to clarify my last posting, the reason why in the case of a motor is that the windings of a dual primary motor. Dual 120v windings, are put in series when hooked to 240v so now the impedance (resistance) is not the same as it was with 120v . It is now 2 times the resistance as before. So in this case the resistance actually changes it is not fixed. It is confusing becasue of the change in resistance. But yes, the current will double is you double a voltage over a fixed resistance. With the electric heaters, the design resistance is different for a 240v heater from a 120v heater to make the same wattage. So here again the resistance changes, so I think that is where the confusion is. Even I get confused.
                  Lou
                  Thanks alot for the reply, this information helped a bunch, I think its finally starting to make some sense.
                  Thanks

                  Comment

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