Motor

I have a question about motor. Theoritically said that torsion of motor is equivalent with the current. but when i try to do so, i saw
something strange, that when the motor is loaded, the current is increase too. it just like the motor control the torsion itself.
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noviandi snipped-for-privacy@yahoo.com (noviandi_saputradewa) wrote in message

No..no, i have no deal with programming. but please try to do this: first use DC motor and then connect the DC motor with regulated dc power supply. if u add the current the motor speed will be increase,this case is i add the current. but when i loaded the motor shaft with gear the current is increase, note that i didn't add the current but the current add by itself. my question is why it was happend?
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On 30 Jan 2004 05:22:31 -0800, noviandi snipped-for-privacy@yahoo.com (noviandi_saputradewa) wrote:

It happened because you were providing a constant voltge to the motor. The current in a DC motor is the applied voltage minus the back EMF from the motor speed divided by the coil resistance. When a motor is not loaded, the motor speed rises to a level so that the back EMF is almost equal to the applied voltage, so there is very little current and very little torque. But when you begin to load a motor, the speed goes down a little, which reduces the back EMF, which raises the current.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
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Which will end up with the current in the motor being proportional to the torque exerted by the armature less electrical losses. If you use a constant-current supply on the motor then at low motor speeds the motor torque will be more or less constant. If you use a constant-voltage supply then the motor will attempt to turn at a constant speed; putting a load on the motor will slow it down and increase it's current as described below, until the armature is generating sufficient torque (from the increased current) to balance the load.
Keep in mind that the "torque vs. current" rule is only exact for an ideal motor with no mechanical or electrical losses, and with no nonlinearities. One very good way of modelling a real motor is to model an ideal motor that's wrapped with electrical and mechanical losses. This makes for a nice modular motor model that's easy to understand.

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Thanks for your answer, but i still have difficulty to understand the mechanism of back EMF and the connection with the motor speed.

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On 30 Jan 2004 22:07:18 -0800, noviandi snipped-for-privacy@yahoo.com (noviandi_saputradewa) wrote:

A DC motor is a rotating coil inside a fixed permanent magnetic field. The same mechanism that makes this a motor also makes is a generator. If the shaft is rotated, a voltage is generated in the coil. When a motor is used as a motor, it still acts as a generator. When the speed increases, the generated voltage opposes the applied voltage.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
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