I have a conventional Bridgeport knee mill with an 8-position, 2 speed switch and a 3 phase 2 hp motor.
The switch positions (clockwise from the top) are
OFF, Fwd low, OFF, Fwd high, OFF, Rev high, OFF, Rev low, OFF
Depending on the speed and pulley configuration, the mill can take up to ~8 seconds to coast down to a stop when switched from Fwd high to OFF. I can speed this up by flipping through from Fwd high, to OFF, to Fwd low, to OFF, to Rev low, to OFF. There's a bit of a bump when quick reversing like that, and with some practice I can get pretty close to stopped before going to the final OFF position, but there's still a second or two of coast down time and I don't always get it right. Plus, it just seems like a hokey way to operate the machine.
What I'd like is for the OFF positions, or maybe just the bottom OFF position, to engage a dynamic braking resistor (or something similar) that would cause the motor to dump its kinetic energy into the resistor stopping quickly and automatically. I imagine something like this could stop the motor almost as quickly as switching it into reverse.
Can this work with an AC induction motor? I think maybe it's more complicated than just shorting some windings. I suspect you might need to apply a rotating field to the stator to develop reverse torque. Maybe that's why I only hear of this being done with a VFD. I don't like VFD as a solution as it is much more complicated than my current switch, and VFDs always seem to take seconds to spin up the motor. I don't want to trade a quick slow-down for a slow spin-up. I like how fast my motor reaches full speed when I switch it to Fwd high.
Has anyone seen a simple, single speed or two speed, 3-phase motor with dynamic braking?
Any pointers on how to do this?
Thanks,
Chris
My switch looks vaguely like this one
The switch positions (clockwise from the top) are
OFF, Fwd low, OFF, Fwd high, OFF, Rev high, OFF, Rev low, OFF
Depending on the speed and pulley configuration, the mill can take up to ~8 seconds to coast down to a stop when switched from Fwd high to OFF. I can speed this up by flipping through from Fwd high, to OFF, to Fwd low, to OFF, to Rev low, to OFF. There's a bit of a bump when quick reversing like that, and with some practice I can get pretty close to stopped before going to the final OFF position, but there's still a second or two of coast down time and I don't always get it right. Plus, it just seems like a hokey way to operate the machine.
What I'd like is for the OFF positions, or maybe just the bottom OFF position, to engage a dynamic braking resistor (or something similar) that would cause the motor to dump its kinetic energy into the resistor stopping quickly and automatically. I imagine something like this could stop the motor almost as quickly as switching it into reverse.
Can this work with an AC induction motor? I think maybe it's more complicated than just shorting some windings. I suspect you might need to apply a rotating field to the stator to develop reverse torque. Maybe that's why I only hear of this being done with a VFD. I don't like VFD as a solution as it is much more complicated than my current switch, and VFDs always seem to take seconds to spin up the motor. I don't want to trade a quick slow-down for a slow spin-up. I like how fast my motor reaches full speed when I switch it to Fwd high.
Has anyone seen a simple, single speed or two speed, 3-phase motor with dynamic braking?
Any pointers on how to do this?
Thanks,
Chris
My switch looks vaguely like this one