9100
Diamond
- Joined
- Nov 1, 2004
- Location
- Webster Groves, MO
A customer asked me to modify a magnetic base drill. This is the type you can mount on a steel plate and drill like it was a drill press. They were using it to drill and tap 1/2-13 holes. The problem was that it did not reverse so they could tap, but then had to release the tap from the chuck and unwind it by hand. The motor is a series wound so called universal motor with a commutator and brushes. Reversing was only a matter of running the brush leads to a DPDT switch with a center off position. The problem is that an operator can throw the switch through off, to the reverse position, meaning that the back EMF is now in series with the supply, causing a large flash from the brushes. You can't trust employees to let the motor stop before reversing, so I need either a timer to delay or a surge limiter that will not allow the large current. I have solid state timing modules I can put in series with the brushes, but running on AC, it would be possible to switch at a zero crossing and not giving the module time to react. I also have a size limitation. They would like to have the device mounted on the drill. A cursory search hasn't turned up anything. I can make an inductor that will limit current, but there would always be some voltage drop.
Bill
Bill