View Full Version : "Soft-start" for lathe motor
02-10-2008, 04:54 AM
The motor on my belt-drive lathe is mounted on a platform that's hinged to the motor support. When the motor starts, the torque reaction causes the motor/platform to jump up, then fall back with a loud thump.
I tried several mechanical fixes to no avail, and I got tired of holding the motor down by hand as I flipped the switch. I decided to try an electrical fix. The first approach was to put a resistor in the start circuit, but that had little effect.
What works is a "soft-start" circuit consisting of 75 feet of #16 wire (actually 25 feet of extension cord with the conductors spliced together) connected to a 10-amp microswitch, and wired in parallel with the regular switch. The microswitch is mounted right next to the regular switch inside the same box, so I can hit it for a second before flipping the switch.
The motor comes right up to speed, but doesn't jump around. :)
02-10-2008, 06:55 AM
Why not fit an adjustable limk to the hinged platform ?
Adjust the belt and lock it in place.
If you need to have it hinged for belt changing then fit a spring loaded link, this will give you enough movement to slide belts over but still have it firm enough .
02-10-2008, 08:58 AM
I had a similar problem with a 1.5hp "Farm duty" motor that was on my gear head lathe when I bought it last year. Farm duty motors have enormous starting torque as they are designed to restart when hooked up to a grain auger that is full. And, various other hookups as dreamed up by a farmer (manure escavator?)
The lathe would start with a loud bang which I figured wasn't helping the gear train (it wasn't!) so I replaced the 240mfd start capacitor with a 120mfd one. It helped quite a bit, though I could have gone even lower. The final solution was a 3phase motor and VFD which allows a very "soft" start if needed.
John's solution is cheaper and the primary belt can be set a bit loose to help with "hiccups". I did this on my shaper and it has bailed me out a few times!
02-10-2008, 02:27 PM
This sounds like a somewhat decent work-around, but it could have some perils. What you have is essentially a combination of a resistor and an inductor. I say inductor as the parallel conductors in the extension cord are probably connected at the ends such that the current is flowing in the opposite direction in one of the wires, producing opposing magnetic fields. If it is coiled up, it could enhance the inductive effect. It is a resistor because all wire has resistance and I am sure #16 is undersized for your motor. I'm not sure which effect is the strongest, but I suspect it's the resistance. Resistance produces heat and your cord could overheat if it is in the circuit for any length of time.
You have the twin dangers of both fire and shock. I would be sure to mount it in a well grounded, metal enclosure in case it should overheat and catch fire or melt. That enclosure should also have sufficient ventilation to help keep it cool. And don't coil it too tight as that would prevent cooling.
02-10-2008, 02:31 PM
That is a reduced voltage start circuit, and they work well. Not as well on single-phase as 3 phase, but can still work.
They work on AC compressors, we did one for an HVAC supplier and they are happy as clams in mud. Reduces the start-up current surge also, which makes tham (and the power co ) quite pleased.
You can make it timed and automatic, so no microswitch. Just be sure it doesn't stay "in", unless all parts can tolerate at least full load current.
02-10-2008, 04:21 PM
The microswitch is in parallel with the regular switch, so there's almost no current flowing through the "resistor" once I flip the regular switch on. I only hold the microswitch closed for about a second when starting the motor anyway, and that's not long enough to heat up the resistor. I let it run one time on the resistor with no load for about a minute, and I could barely tell a difference in the temperature of the wire.
The fix also eliminated the occasional tripping of the circuit breaker on that circuit when I started the lathe.
02-10-2008, 06:01 PM
Why not a Screen Door spring fastened to the motor bracket and the floor to provide the tension?