Centroid Community CNC Support Forum

Hey all I’ve been talking to a good friend who has build a couple of CNC systems. I showed him my build and he noticed right away my system had no capacitors. So he questioned if know about back EMF told him I have no idea what he is talking about. Also that I have no fuses.

So I ask how the Acron board handles this back EMF and IF I need capacitors, fuses?
Look forward to your guys input.

Acorn does not need to handle back EMF because it only generates step and direction pulses and is not connect to the motor windings. You motor drives will be what sees back EMF from the motors and therefore should be protected as the motor driver manufacturer deems fit.

All wiring should be protected by the appropriate sized fuses. Fuses are sized to protect the wire between the fuse and either the next fuse in line or the end device. Wire is selected by the current draw of the items downstream of the wire origin point.

The very same physics principles that make a motor turn dictate that once it turns it will work as a generator. The voltage produced by a spinning motor is referred to as the back EMF. The back EMF is what will make your small DC motor connected to a battery turn at a particular speed -- say you connect a small DC motor to 12V battery. When you connect the battery the motor is not spinning and the full 12 volts produces maximum current and thus maximum torque which will make the motor spin. As the motor spins up it produces back EMF proportional to the speed, with the same polarity as the voltage applied from the battery so it is effectively canceling the battery voltage -- when the back EMF is 5 volts, you only have 7 volts to produce torque. Eventually the back EMF is so high, say 10V , leaving only 2V to produce torque. That toque is minimal, only enough to overcome the motor friction. At that point the motor turns at more or less constant speed -- the system is in equilibrium. Now if you grab the motor shaft with your fingers, you will overcome the minimal torque available from the 2V and the motor will start slowing down, but by the time is slows down to say half the speed, the back EMF will drop to 5V and now you have 7V producing enough torque to fight your fingers. At this point the motor is turning half the speed and again it is more or less in equilibrium (as long as the finger friction is more or less constant).

The back EMF comes in the picture when selecting voltage for a motor controller. Basically, the maximum rotational velocity will determine the maximum back EMF, and the controller supply voltage must be much higher than that -- the difference being what produces the required torque. If the voltage is not high enough a motor will run rough at high speeds.

What about the capacitors? Well, first of all we are not talking about Acorn here. As @tblough pointed out above, Acorn is not connected to motors at all so it will never see any back EMF. Secondly, back EMF has really nothing to do with capacitors -- even if you were designing the motor controller itself. So I'd say take your friend's advice with a grain of salt. I assume you are using off the shelf building blocks -- motors, motor controllers, and power supplies. As such, you should not see any discrete components (e.g., capacitors) anywhere.