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DC Motor Speed Modulation

Guest Yevgenip

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If we consider your first question, you should know that you cant change the current without changing the resistance. A guy named Ohm once decided this and we have to respect it. Then there is only the voltage left, but if you change the voltage you get poor performance. And then to your last comment

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It seems to me you could use any waveform to drive a motor efficiently. You don't have to PWM. In PWM you change the voltage to zero which is sort of like a signal. All you have to do is change the voltage. You don't need to run at full speed and then drop it to zero. You can use any voltage. Wouldn't you say a constant 5 volts on a 12v motor is very reasonable. It should work just fine.

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No Kevin,
Sorry, you don't understand PWM. The motor's speed doesn't change with each pulse. The motor and its load have inertia, which needs a high torque to overcome. The inertia also causes the motor and its load to take time to change speed. The pulses occur much quicker than the delay caused by inertia. A fixed 5V on a 12V motor will result in low torque and speed variations with load changes. A loaded motor probably won't even start turning.

With PWM, the full-voltage pulses are applied very often and the motor's inertia smooths any quick speed changes, but since full-voltage pulses are applied, torque is maximum. When a motor's speed is reduced by PWM, its torque is stll high and therefore it resists speed changes with load. It will start to turn even if the PWM speed control is set to a low speed at startup.

Of course another benefit of PWM is the efficiency of controlling the "average" voltage across the motor without heating loss wasted by linear voltage dropping. The PWM "switch" turns on with a low voltage across it, then turns off without current, therefore has very low power loss.

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Attached are voltage waveforms for Pulse Width Modulation.

(1) Nearly full power, the motor is only being turned off for short intervals, it is on more often than off.

(2) 50% on/off, half power.

(3) low power the motor is off for long periods and only on for short periods.

Also audioguru, there is some (though not much) loss in torque at very low power levels because the motor will have a certain inductance and if the motor is not turned on enough to allow it to saturate properly the current will be lower thus a lower torque.

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