Old 3-phase motors and Variable Frequency Drives

[TDKPE]

. . . wound rotor motor drives . . .

How about replacing the old ‘magnetic’ controls those motors were originally controlled by with variable frequency vector drives with sensors mounted off the outboard end of the eddy current brakes?  


I’ve been tasked with modifying some old bridge cranes for in-house use (up to 50 ton), mostly to lengthen the beam span, and to replace the old AC controls with radio controlled VFD systems, which also eliminated the operator’s cab.  These cranes get a whole second life, but I have my doubts as to how long those almost antique P&H wound-rotor motors will last.  Those vector drives will suspend a load without the brake being set, but the motors weren’t made for that, and don’t have an independent fan.  

Just thought you’d get a kick out of those junkyard mentality projects.  And I actually used to work in a junkyard, many moons ago.  

[MstrCarpenter]

Actually it tightens on many tablesaws when stopped. Table saw arbors that come through the blade from the right are left-handed threaded and arbors that come through the blade from the left are regular threaded.

The VFD has DC braking terminals if it can use an external braking resistor and they are also NOT in between the motor out of the VFD and the motor.

Also do any duirection reverse switching of the lathe through the VFD terminals as well.

Also pay attention to your gearing (on gear head lathes) or pulleys used for speed and the frequency settings you need to obtain full torque at speed.

All this being said, VFDs on lathes are very awesome.

I don't claim to be very knowledgeable with VFD's; in fact the only one I've used, I managed to let the magic smoke out. I prefer a rotary convertor for my wood shop that runs a Unisaw, a 36" band saw, a big grinder, and/or my largest compressor. However, I do know quite a bit about woodworking machines. On almost every woodworking machine  the threads are designed so the nut, bolt, collet, or chuck will tend to be tightened in use. If you stop the arbor too fast, the momentum of the cutter will tend to loosen the nut or bolt. This happens on my 15" Hitachi miter saws if the bolt isn't really cranked tight. Granted it's a big blade and an aggressive brake but it's also happened on both my T.S. and RAS with a dado set installed.

So this post might not be helpful to those without a VFD so here's a tip worth remembering; The nut (or bolt) on woodworking tools comes off the same way the cutter turns.

[Tapper]

I can only attest to the need for some sort of braking system on Dewalt Radial Arm saws. The first one I bought years ago, a GE, was swinging a 16" blade. The seller had three phase power and demonstrated it to me. When he shut it off, we wandered over to another part of his large commercial shop to look at another piece of equipment. Several minutes later I could still hear that 16" blade spinning down.

Using common sense and caution, AND selecting the proper VFD, this problem can be solved easily.

Doug

[Bobby Thompson]

How about replacing the old ‘magnetic’ controls those motors were originally controlled by with variable frequency vector drives with sensors mounted off the outboard end of the eddy current brakes?  


I’ve been tasked with modifying some old bridge cranes for in-house use (up to 50 ton), mostly to lengthen the beam span, and to replace the old AC controls with radio controlled VFD systems, which also eliminated the operator’s cab.  These cranes get a whole second life, but I have my doubts as to how long those almost antique P&H wound-rotor motors will last.  Those vector drives will suspend a load without the brake being set, but the motors weren’t made for that, and don’t have an independent fan.  

Just thought you’d get a kick out of those junkyard mentality projects.  And I actually used to work in a junkyard, many moons ago.  

Wound rotor motors do not require a drive. The rotor is wound and has resistors on the drive. On startup the starter controls the amount of resistance for starting to allow for the heavy load to start. With the wound rotor it is a constant rpm even as the load changes. I worked on Motor/Generator sets use to power aircraft on the ground. The wound rotor allowed the generator to maintain a constant frequency which is critical for the aircraft. The frequency is 400 hertz. I also worked on generators on a radar site about a mile south of the DMZ in South Korea. Radar equipment would kick out at anything below 398 hz and anything over 402 hz. I also installed starters on 1000 hp wound rotor motors on bandsaws in a sawmill.