Lathe motor soft start

[Mr Fixit]

I have a friend that has an under feed SB 9 and when he got it the same thing would happen. The clunk was the motor bouncing. What we found was 3 things, the pivot points were worn and the tension on them was not there, and the wrong size belt was in use which caused the cam lever not to lock in place because it would not get to that point.

So, check the length of the belt too.

TX
Mr fixit for the family
Chris

[garyhlucas]

why are soft starters so expensive? i wanted one for the lathe and bought a vfd instead.

They are expensive because they are partly mechanical. The motor gets started by switching electronics that reduce the voltage and when the motor is at full speed a contactor picks up and carries the load. Shutting down the electronics come on and the contactor opens without arcing as a regular contactor does. I have taken a few apart after a failure and they are an interesting piece of kit.

[darryl]

Couple things- if the weight of the motor is used to control belt tension, then a shock absorber of some type to restrain the motor from any fast pivoting might be a good idea.

You could also investigate variable inductors in series with motor current.

[bborr01]

I've been looking at putting a soft start on the A/C unit on our 5th wheel so I can run it on a smallish generator. Pricey at $250 to $300 though.

[Paul Alciatore]

Probably the old supply and demand thing.

why are soft starters so expensive? i wanted one for the lathe and bought a vfd instead.

[dian]

yes, but i thought it was just a coil.

[J Tiers]

I've been looking at putting a soft start on the A/C unit on our 5th wheel so I can run it on a smallish generator. Pricey at $250 to $300 though.

It might not work well anyway, those are usually designed as a unit.......some parts need full volts, just the motor might want a soft start.

[I make chips]

So here's what I did since soft starters are so spendy
The tensioning assembly works as it should. On that hanging contrivance is the motor and the first reduction 5/8 vee belt with a two step pulley from the motor. This swinging contrivance tensions the vee belt that goes up to the spindle.
I raised the motor a bit which loosened the motor vee belt so it now slips for a second when the motor starts. No more starting torque bump. It doesn't seemingly slip under load so it's all a free fix. The vee to the spindle is the one that slips under load and nothing I can do about that as it does get some oil on it.


*bbor, get a hard start kit for your AC unit and your generator will thank you. They're pretty cheap on ebay and are essentially one or two capacitors.

[J Tiers]

The hard start kit usually boosts the start current to get it going........ he may want a low current starter, but no idea if that would fit in/work with an already made compact A/C for a camper

[bborr01]

It might not work well anyway, those are usually designed as a unit.......some parts need full volts, just the motor might want a soft start.

From what I've heard they are wired into the electric circuit of the A/C, not something that goes between the power to the A/C unit. Probably gets wired into the starting circuit of the motor.

[Ian B]

Just a suggestion - would a flywheel on the motor pulley help you? It'll retard the acceleration of the rotor down a bit. It'll make stopping the spindle a pain though. But cheap to do.

Ian

[J Tiers]

From what I've heard they are wired into the electric circuit of the A/C, not something that goes between the power to the A/C unit. Probably gets wired into the starting circuit of the motor.

Yes indeed. Controls will not want soft-started......😉

[nickel-city-fab]

Huh, I wonder what's up? Mine is the horizontal model, and it never chirps the belt until I stall it out in back gears. My motor hangs by its own weight on the countershaft assembly, using a vee belt. The flat belt is tensioned by the lever. No chirps. My flat belt is an automotive poly-vee type with the ribs facing the pulleys.

[darryl]

I'm late to this discussion, but my thought here is if this is a start cap motor, you could reduce the value of the start cap. That would give a slower start. One way to do this is to wire a second start cap of the same value in series with the original. If you have a spare on hand, this would be an easy try. I'd be looking at both ways- something mechanical to control the motor, and the reduced value start cap. Don't reduce the voltage rating, just the capacity value.

We have two supposedly identical motors for our table saw- one as a spare- but they have differing values for the start cap. One is 1000 uf, and the other is 280 uf. I don't know if these are the correct values for the motors, but the one with the smaller cap takes 3 times longer to come up to speed.

[I make chips]

Captain and Bob,

The motor's weight doesn't tension a belt. There are two belts on a small South Bend.The one on the motor is a 5/8 wide vee belt IIRC 39" long that runs a two step pulley system that can move the belt for low or high range. Then the long belt either a vee or flat that runs step pulleys up to the spindle. There's a turnbuckle that is used to adjust tension on the spindle belt.

The motor is bolted to a swinging casting that tensions the belt to the spindle. The belt from the motor to the jackshaft is fixed as the motor is bolted down.
When you switch it on, the mass of the motor jerks the whole swinging casting frame and it makes the linkage go clunk. It's the way it is and nothing is out of order.
The 5/8 vee belt is the one that chirps on the motor from the inrush starting torque of the motor.

If the primary belt is tightened a lot by moving the motor yeah it won't chirp. But, it becomes difficult to move it to change speeds.