*Originally posted by jonnywalk *
**If my understanding of the defination of torque is correct and i think it is then the output torqe of the motor is related to the voltage and no matter how you gear it you still get the same torque but by gearing you are essentially (and I say this for simplicity’s sake) spreading the torque out over a larger time or concentrating it. The real advantage to gearing is you keep the motor turning so you don’t stall it out. The same principle with a car, you have the same torque and horsepower in every gear but unless you car is really REALLY powerful you have to reduce it so you don’t stall out until you get into higher into the power band. Hope this helps some. **
In a car, which you mentioned, the [horse]power depends on the speed of the motor, and the torque is different function of motor speed. The power available at a given engine speed is constant through the gears, but the torque changes with the gears - you try never to start in third gear because, while it gives speed, it doesn’t give enough torque to push the car from a stop. Once we’ve passed throught the torque curve a few times, we can get into top gear, and feel the wind.
Matt is correct: the product of the torque and the speed remain constant (power). If the second gear goes half as fast, the torque available doubles, less a bit for friction.
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But, with electric motors, torque is related to current, and speed is related to voltage. And they are locked in a struggle with the internal ‘resistance’ of the motor.
The voltage applied to the terminals has to drive current into the armature, which if turning, generates a voltage to oppose the current coming in. Imagine a square, with a battery at the right side, + terminal on top, a resistor across the top, a generator with + terminal on top on the left, and the bottom of the square is a wire joining the - terminals of the battery and generator. The generator is the spinning armature of your motor; the resistor, the combined effect of armature resistance and the brushes.
The current that can enter the motor is given by Ohm’s Law: current equals volts divided by resistance. The voltage here is the Applied voltage minus the Generated voltage. The torque is in proportion to the current, so at start, with the armature not spinning (just as when the armature is locked by a heavy load), the current is limited by resistance only, and the torque is tremendous.
As the torque causes the armature to spin, the voltage generated increases, causing the amout of current to drop, since the voltage difference the resistance feels is less. The torque drops off, the armature’s acceleration decreases. If there is no load, the armature will speed up with decreasing acceleration until the current is all but cut off. At this free speed, the generated voltage is almost equal to the battery voltage. At high speed (free running), there is virtually no torque (just a little for friction).
When you apply a reasonable load, the motor will speed up to where the generated voltage limits the current in the resistance to the current which will generate exactly enough torque to overcome friction internally and drive the load at the speed. Increase the load (ram into the end wall, pull a goal) and the motor must slow down, to let the current increase the torque. Too much load, means no speed, and the rotor locks, and you get (in the Chiaphuas) 170 A, briefly, we hope.
So much for electric motors 101. It’s a long course,:eek: