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David Powell
05-04-2010, 08:52 AM
I have an occasional job involving grinding a point on small pins. I do it with a toolpost grinder in a 13/36 lathe driven by a 3/4 hp Tamper motor running on 220 I phase. Each pin takes about 1 minute or less, and involves 3 starts, I to screw the pin in the fixture, I to grind and 1 in reverse to unscrew it.( fitting and removing by hand is tedious and hard on arthritis hands) I read somewhere that single phase motors are only supposed to be stopped and started about 20 times an hour. Am I risking my motor.s health by working this fast? It does not seem to get very hot, even after an hour or so it is barely ' hand warm" Regards David Powell.

Paul Alciatore
05-04-2010, 09:52 AM
Frankly, I have never heard of that limitation. It probably depends on the quality of the motor. A cheap import may have more trouble than a quality model. The lack of heat buildup is probably a good indication. And it souds like your load is fairly light. Startup under a heavy load would be much worse.

In your case, there appears to be little you can do about it. I would just do the job and perhaps keep in mind a source for a better motor if it is needed. If the number of such parts is limited, you probably will not have any trouble in any case. If you are going to do 100K or more, consider getting the replacement motor now.

JoeFin
05-04-2010, 11:03 AM
I read somewhere that single phase motors are only supposed to be stopped and started about 20 times an hour. Am I risking my motor.s health by working this fast?

I've only seen that spec on larger motors (+25Hp), and it has every thing to do with the "Lock Rotor Current" experienced at start up.

I once specified a Baldor 5Hp motor that was stop / started probably 200 - 300 cycles per hr. The motor load pully had to be sized properly as to allow the motor to come up to full speed almost instantly as to experience a shorter duration of "Starting Current".

That would be difficult at best in your case because your load is the lathe spindal and for an occasional job, you wouldn't want to go resizing your pully/gear ratio.

I don't know how to advise you since most "Soft Start" - VFD technology relies on reduced voltage/High current to start the motor, and will generate a LOT of Heat in the motor by inself.

"Down and Dirty" - I would think of a 110v cabinet fan placed directly over the motor to provide some extra cooling. That and periodic "Hand Test" temperature checks to see just how much of a problem it really is

fasto
05-04-2010, 12:03 PM
I read somewhere that single phase motors are only supposed to be stopped and started about 20 times an hour.
This is because the starting capacitor is only rated for intermittent duty. You might want to have a spare on hand, in case it dies in the middle of a job.

Oldbrock
05-04-2010, 04:06 PM
With a leaver actuated collet setup you could get by without stopping anything couldn't you? Peter

Black_Moons
05-04-2010, 04:19 PM
VFD's softstart is *WAYYYY* more gental on a motor then regular startup.
Infact a VFD is designed to limit continious current to below the motors limits, so a VFD can softstart a motor as slow as you want or as quickly as it can without damage, reguardless of number per hour.

As far as single phase motors, starting them is stressful on the caps/starter/coils etc, but this can be compensated for by using it at less then rated torque/hp once it starts (Lower torque = lower current = less heat = it gets time to cool off from its last startup)

David Powell
05-04-2010, 05:00 PM
Thankyou all for your interesting and insightful comments. I always start with the belts slackened and engage drive once the motor is up to speed, so there is minimal starting load. The parts are screwed into a fixture and then a locknut is tightened to make sure they do not move.As there is no definite spindle lock I always stop the motor while using the wrench on the locknut, just in case the belts ' catch up' and give the spindle a turn or two and catch me in the process,I screw the parts in and out under power, holding them lightly in pliers( NOT clamped on vicegrips). At the moment theres no money to spare for anything not absolutely essential to keep the shop going so a VFD is out of the question, much though I would enjoy and use one on this lathe. The job is done for now and it wont be until fall that it returns. Regards David Powell.

JoeFin
05-04-2010, 06:53 PM
VFD's softstart is *WAYYYY* more gental on a motor then regular startup.
Infact a VFD is designed to limit continious current to below the motors limits, so a VFD can softstart a motor as slow as you want or as quickly as it can without damage, reguardless of number per hour.


Sorry - That would not be accurate

First Generation Solid State Soft Starts used voltage /frequency limiting and just pumped enormous amounts of current into the motor. While the New VFDs can be programmed "Current Limiting" the reduced frequency just looks a lot more like DC then AC at start up and produces quite a bit more heat.