I've used a few DC motors before with a flyback diode, and haven't thought much of it. I've recently been thinking about building a larger DC motor driver - 36V, 100A for a golf cart. In doing so. I've been reading about motor controller design. I'm a bit confused about how back EMF is treated, this is how I understand it:
When an inductive load, like a motor, is rapidly disconnected from it's voltage source, the inductor will try and maintain constant current. If it is left with nowhere to go, voltage will spike over across the motor's leads. This can burn out components, mostly solid state, or cause arcing in switchesTo prevent this, there are three options:1. Disconnect the motor's leads completely, and let it freewheel
2. Connect a flyback diode to regulate the voltage across the leads, but also dissipate current in the winding \ diodes. This has a motor braking affect. See the flyback diodes below:
3. Using an additional MOSFET, PWM the motor between shorting (to gain current momentum) and connected to the battery regularly (using the current momentum to charge the battery). This is similar to a boost converter. Switching must be done very quickly to avoid braking the motor with great force (shorted) or driving the motor (connected regularly). Times are determined by the resistance and inductance of the motor. See below for sample circuit:
A) This makes sense, but how to electronic PWM motor controllers let motors freewheel? Without disconnecting the controller, some sort of current is going to have to be disappated (either through a flyback diode or the battery), resulting in motor braking.
B) Is there a way to maintain the option to use regenerative braking and still freewheel?
My primary sources are:http://electronics.stackexchange.com/questions/56186/how-can-i-implement-regenerative-braking-of-a-dc-motorhttp://zeva.com.au/Projects/Speedy/
3. Flyback diode will break down at a certain voltage, and short the motor to prevent a large voltage spike. Once the spike dissipates, it goes back to essentially being open, leaving only the EMF from point 1.