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:
Firstly I'll stop you there, back EMF means two dinstinct things in (DC) motors - the back EMF generated by the spinning windings
in the motor's magnetic field - this is proportional to the rotational speed.
Secondly the back-EMF of a winding when the current is cut off due to its self-inductance - you seem to be talking about this primarily.
Both forms of back-EMF are voltages induced due to a change in the magnetic flux linking a winding, but the mechanisms are
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 switches
To prevent this, there are three options:
- Disconnect the motor's leads completely, and let it freewheel
Surely there is a smiley missing - disconnecting the leads is what generates the back EMF spike!
- 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:
Only works for unidirectional motor - if you are driving bi-driectionally from an H-bridge each arm of the bridge needs diodes - 4 in total.
There is minimal (no?) braking effect from this as the current reduces rapidly and then there is no current (unless PWM is involved).
- 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:
You mean buck converter, not boost converter - the point is this is PWM via half an H-bridge and has diodes built-in (all power MOSFETs have
diodes built-in due to the vertical current flow). The PWM rate has to match the inductance of the windings (be high enough to reduce
current oscillations to a low enough level).
The important thing with any circuit connected to an inductor is that there is an alternate path for current to continue flowing when
any switch (semiconductor or otherwise) opens. Without an alternative the rate of change of current will be extremely high, thus the
rate of flux-linkage changes very rapidly, hence the induced EMF is very high (and something is damaged).
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.
I think you are confusing the two types of back-EMF - the switching transients are typically in the realm of microseconds or at
worst a few milliseconds.
When you switch off all the MOSFETs in the Hbridge the last transient spike will dissipate very quickly and the motor is no longer carrying
any current. Voltage, yes, the rotational back-EMF will be present.
Also when the switch opens the current during the transient is still flowing in the same direction, so it is still trying to push the rotor
round normally, so rather than braking the transient is actually powering the motor (albeit very briefly).
B) Is there a way to maintain the option to use regenerative braking and still freewheel?
No, that would imply generating energy from nowhere.
My primary sources are: