From SEB, 1-29- 2009
"Jon Slaughter"
" Thesis on DC Motors"
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A motor has maximum torque at stall. Lowering the voltage reduces speed and increases torque.
The reason has to do with the back emf. When the motor is stalled there is no back-emf to resist current and hence large current can flow and hence a large torque(since it is proportional to the current). As current flows a back-emf counters it reducing the overall current.
If say, you lower speed by loading the motor so it can't turn then it will overhead it. (this is easily demonstrated by jaming a fan and watching it burn up)
But PWM is different!!! It doesn't load the motor to lower the speed but reduces the current!! Hence at low speeds there is low average current but high peak current since the rpms are low.
e.g., suppose the motor draws 1A at stall.
If we PWM at a duty cycle of d then d will control the speed(it will be approximately proportional assuming no loading effect).
At, say, d of 1/100 which the motor turns slowly it will draw 1A but only for 1/100 of the cycle. The average current is 10mA. This is definitely not enough to get the motor to speed up.
What happens is you are "pulsing" the motor with high peak currents but low average currents. An example is turning a bicycle wheel by your hand. To keep it going fast you have to "pulse" and keep it up.. you can only get it to go so fast though. Eventually it's inertia and your hand speed keep it from going any faster.
If you grabed the wheel for only 1us and turned it with a huge force it would be the same as some weak kid turning it continuously with a small force. You might cause it to go fast quickly but only for that small time frame.. for the rest of the time it is not getting any force(unlike with the kid).
So even at stall speeds, while we are drawing a large current, because it is using PWM the average current is low. (the peak current is still important for practical matters though)
You have to realize that the "impedence" of the motor depends inversely on the angular velocity. It is independent of the duty cycle. The PWM basically prevents enough average current to cause it to spin up to speed(again, even though high peak currents occur).
About the only thing you can say is that at low speeds you have high peak currents and vice versa. PWM is simply controlling the average current.
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