Motor Speed

It might not turn at all.

The purpose of start capacitors on a single phase motor is to introduce a phase-shift into the motor's windings so that, when the power is applied, it will turn in the desired direction; there may be a speed-operated switch present to disconnect the capacitors as the motor runs up to speed, as they are not needed after speed has been reached. Reduce the capacitance and there may not be enough shifted phase left to induce rotation at power-up. Try it and see, though be prepared in the trial to switch the motor off immediately if it fails to turn to save cooking it. Take care that the supply wiring and circuit protective devices can withstand the stall current for a bit longer than would otherwise be normal.

One option to consider is to add some inertia to the motor shaft in the form of a flywheel, and the remark regarding the wiring and protective devices would still apply.

A motor with two capacitors uses one or both to shift the phase of the current to get proper direction, but I've never seen a motor with two capacitors (and I've seen and worked on a lot of them) where at least one was not used as a "run" capacitor. Typically, one is a "start" capacitor and the other is a "run" capacitor.

Don't screw with removing or changing the values of the capacitors. Your problem is mechanical in nature.

I would use a soft-start device of some sort. Could be mechanical like a liquid clutch attached to the drive system or electronic attached to the motor starter.

I agree with the others - don't mess with the capacitors. Try a centrifugal clutch or have a handle to tighten up the belts slowly.

Put a 3 phase motor on your lathe and a VFD.

You get soft start and speeds from very slow to plus 200% with out changing a gear.

Also you can have a brake, and reverse.

It hammers the gears into rotation

Please define?

do you have multiple speeds @ the gearbox?

safety/foot brake? { should cut the power & may have a band brake }

the chuck [part holder] is very useful for mild steel @ 300-500 rpm

I suspect your set for 1000 rpm +

Forgive my slang.

First of all you twit, of course it has multiple speeds, that is it's function as a "gearbox". Secondly, there is no brake.

300-500 rpm? Is that a .500" dia. part or a 8.00" part.? Will we be using HSS, Carbide or Ceramic?

If your going to throw numbers around so loosely and comment on what you have limited knowledge of, please stay off your computer.

Forgiven

But if you're going to ask ?'s, include enough info [ mtr brand, working speed ] to have some idea of the actual issue!

Carbide, 1/2 - 2", mist cooling

& of course have the courtesy to identify your self before you insult

Sir,

I apologize for my comment, truly. I have no xecuse. Thank you for your input and suggestions.

awl2@capay.com

No sweat

but you know how it is

you can never quite tell if

you're replying to a

student trying to get you to do his homework

or someone just a bit out of their realm

& needing a point in the right direction

here's a link to cheap hz drives

http://web4.automationdirect.com/adc/Search/Search

Sounds like the off-shore lathe I bought .

14x40 gap bed.

It is a metric sized motor. I had a capacitor fail and it took out a winding. Then I discovered how expensive a metric frame motor is in Canada.

I re-wound the motor by hand. The original insulation system in the motor is very poor. I had to replace about half the slot insulators because the original winding had cut through them. Only tricky part was the potting of the coils and bake in the oven. I have a very tolerant spouse, but that really pushed the limit.

The starting switch in the motor is really shabby and occasionally the motor will growl and barely turn. FAST stop and restart and it is back to normal. The motor is a very weak link in the system.

I understand the comment with a fast start. However, the acceleration torque should not harm anything considering you can absorb 2 HP in cutting. That is one heavy cut!

I suspect the band break on the foot operated brake could be more violent if you really trod on it, like when you are cutting a thread towards a shoulder. I had the nut come loose on the band brake and that made a racket also.

I had some rather loud gear noise, but it was a loose clearance in the drive train under the cover. Re-adjust mesh and it is much quieter.

Great comments, I wonder if I can change the value of the start capacitior? So it starts just a little slower. I know the gears can probably take it the way it is but honestly it's like dumping the clutch in your brand new KIA @ RPM. Ouch

As mostly said above . it seems there is something wrong in selection of capacitors. The capacitor to b connected after reaching full speed , is connected initially and so the high speed. You can check following options:-

1) Check the capacitor connections and their capacity. The current should reduce after immediate start and it is not doing. Check without the capacitors also with Automatic small voltage transformer to see the effect. IF it raves up you can switch off .

2) Fly wheel is also a better option to have inertia.

3) Soft starter/ VFD definitely going to solve your problem

4) Chck if the belt are loose.

Shivaganti

shivaganti@yahoo.com

Single phase and you are basically hooped. Three phase and there are lots of options.

In my shop I only have single phase, and replacing a "brand new" 2 HP motor is costly.

Capacitors have nothing to do with the start being hard or soft. They are only necessary to make the motor turn at all on a 1 phase source. Changing the capacitance will only serve to make it not start at all.

Soft starters will not work on a capacitor start motor for very long. The starting caps look like a short circuit to the SCRs in the soft starter, the fast rise time of the SCR switching and the harmonics created by it will cause the capacitors to overheat. So it becomes a race to see if the caps or the SCRs will fail first, either way you are screwed.

Your choices are; 1) replace the motor with a 3 phase version and a VFD that will take your 1 phase source and give you a 3 phase output (very common nowadays); or 2) a purely mechanical solution such as a clutch, fluid coupling or a "torque tamer" device of some sort.

I've never dealt with three phase at home. What type of phase converter do you recomend and why. Rotary or Dry.

Thanks for your input, you were right on.

I did some catalogue pricing.

A 2 hp AC variable speed drive can be fed from single phase. Use a 2 hp 3 phase AC motor. Then you have variable speed, ramped starting, torque limit, and all the bells and whistles. This can all be had for about $2000. Before motor mount changes, and electrician wiring it up etc.

Hmmm, that's about 25% of the cost of my lathe.

Oh well, I can dream.

The motor has a centrifugal clutch. I removed it and shortened the springs that hold it closed causing it to engage with minimal rotation. I figured the starting circuit needs to be live just long enough to give direction and movenent, then the running circuit takes over and drives it. It's like a little bump-start. To my suprise it actually works.

I only discovered this by much research, trial and error and foolishly disregarding all of your advise. I disconnected the starting capacator wire going to the clutch switch.

Suprise! I'm not an engineer. It hummed a little and would not start on it's own in any low gear. If I spun it just before start it would purr. A pull start doesn't appeal to me. I was going to try and find a lower valued capacator but after doing some reading discovered, as several of you pointed out, it's not the power of the capacator it's has something to do with creating a field out of phase? I don't exactly remember but it made me realize that I just needed to disconnect it quicker. I can do the mechanical.

I would like to thank you all for your help, I have learned a great deal.

"MACHINISTS MAKE BETTER SCREWS"

If it stalls and needs a "pull start", be careful.

It is never safe to grab motors to get them going. It is only a matter of time before fingers get abused.

You only have a few seconds at stall before permanent damage occurs. With a stalled motor the current can be 5 times or more of rated and the power burnt up is the square of that.

Similarly a slow start because the starting circuit is disconnected too early can prolong the overload starting current.

The motors used in knock off lathes from Taiwan, India or China just aren't the same quality as used in a Hardinge lathe or other name brand production lathe.

You are correct the capacitor causes a phase shift in the starting winding so it is matched to the winding. Since the winding is orthogonal to the running winding it introduces a "rotating" field that causes the motor to start in a particular direction. Reversing the connections to the winding will cause the motor to spin in the opposite direction. As the motor comes up to speed the springs and weights open the contacts to the capacitor and take it out of the circuit.

The starting capacitors used are usually an "AC" electrolytic designed for motor starting, meaning they are for intermittent use. Keep them energized and they destroy themselves from overheating. Some motors will have both starting and running capacitors for improved torque capability.

Nema B motors (the usual off the shelf motor) have poor "across the line" starting torque, and will "pull out" of synchronization. You may have noticed it with a table saw with an induction motor. You load it up and suddenly it stalls, you can't start it with material in the blade.

Three phase motors always have the rotating field due to the 3 phases and don't use starting capacitors.

EE, part time hobby machinist.