"Soft Start" box on 17" lathe, what can you tell me?

My new (to me) 1760 came out of a CNC production shop where it's main purpose was to make fixtures for the big VMCs and CNC Lathes. This is a big shop with full commercial 3ph power. However, on the back is a box labeled "Soft Start" with timing and torque pots. The lathe is 7.5/3.75 hp, and the box is labeled/rated at 5hp. The machine was unhooked and moved for the auction, and this box wasn't connected to anything. So far as I can see, it wasn't hooked up when in use, but not sure. Below are pics of the external data/spec plate, and a pic of the internal adjustment panel. Can anyone explain what this does, and if it might be of use to me in minimizing start up in-rush current in my piddly residential shop (borderline for a lathe this size).

Click for larger size, click again for full size...

A soft start slowly increases the voltage going to the motor. Motor torque varies by the square of the applied voltage, so by ramping the voltage, you are ramping the torque. Hence the name "soft start". Indirectly, you are also reducing the current drawn by the motor during this process because current and torque pretty much go hand-in-hand.

If your load did not need all of the available motor torque to accelerate, it works. If your load needs all of the available motor torque to accelerate, then all you do is heat up the motor until the soft start is done, then it goes across the line anyway.

On a lathe, chances are pretty good that you do not need all of the available torque to accelerate it, so it should work fine, probably did. The only caveat I see here is that the unit is NOT rated to be used on the high speed windings of that motor. So you will ALWAYS need to start out with the low speed, then switch to high speed. the wiring is going to be critical, you will need a lot more information about the motor winding connections, the type of starter you have etc.

Ok, thanks.

Pretty much as I suspected, this sounds like something I probably won't be using. But I can't imagine why it would be on there in that shop. <shrug> When I first got it, I was unable to access the motor compartment (stuff in the way) and we guessed that it was a 7.5 hp lathe. Then we spotted that soft start box and amended our guess that it must be just 5 hp. Finally got in there and found a 7.5hp. Combine that with the fact that it was in a full commercial shop with a HUGE 3ph power grid (running several 20-40hp VMCs and CNC lathes), and it just doesn't add up. Oh well...

Anyway, I'll ponder over research it further if it looks like I will need it.

Reason for reason...

One of the biggest reasons for using soft-start circuitry on a machine is to minimize the 'hurt' on your building's power system.

Another big reason... relates to what JRAEF noted- is to reduce the TORQUE applied during startup. Example- if you have a high inertia load, and want to cut down on wear-and-tear on the machine... run a soft-starter, and bring it up 'easy'.

if there is a clutch to start the spindle turning on your lathe i can imagine you will need this device

unless your planning on spinning 15" diameter 300lb billets at 400 rpm

As stated, the building it spent (most? all?) of it's life in needed no help with power. At least not that I can imagine. It could be that this is left over from some previous life, but the business I got it from owned it for most (if not all) of it's life.

Maybe they were spinning some heavy stuff and needed/wanted the soft start for that reason? But why 5hp rated rather than 7.5 hp rated?

And it (sadly) doesn't have a clutch, so that may make this an important feature for my shop. But I'm going to try things without it. I have been told that some (in a similar HSM type shop and with small RPCs) do start on 3.75hp, then hot switch up to 7.5hp, so that may be what I have to do. Hopefully not...

BadDog said:

... But why 5hp rated rather than 7.5 hp rated? ...

Click to expand...

You have a 2 speed motor there. In the lower speed, the HP is lower, probably 1/2 of the higher speed HP, or less depending on the configuration. So if, in the original control scheme, it was designed to force one to ALWAYS start the motor in low speed, then this soft start would have been sufficient. Going from Low to High speed, the load is already spinning and there would not be much need to have the soft start used for that transition. So when you would switch to High speed, the motor would be Across-the-Line at that point. This is actually a fairly common scheme with 2 speed motors and soft starters.