The interesting bit comes when the controller is asked to stop the motor. This occurs when it receives a signal indicating zero speed.
When the controller is commanded to stop, it first slews back from what it is currently doing to 0%, then it slews from 0% to 100% "braking." Braking is accomplished by turning on both lower switches. (sw2,sw4)
This creates a current path between the terminals of the motor, and if the motor is turning the motor will generate a voltage resists the motors turning direction. (This is called back EMF in the motor tutorial). This back EMF then slows the motor down until the motor stops.
Braking, like going forward and backward, starts out with a 0% duty cycle PWM wave and then slews up to 100% duty cycle. This starts braking softly and then increases up to a full brake condition. The current flow is shown in the diagram below.
The hypothesis was that by shorting the motor, I induced a change in the current flowing through the motor, this change in current was reflected as a voltage potential across the motor terminals and was governed by the equation
The equation is the classic description of voltage behavior across a coil due to a change in current.
The perplexing part to me was how could the voltage be greater than the input voltage? The theory is that by rapidly changing the current I induced a higher voltage spike than simply the back EMF voltage.
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