"Inching" With Single Phase Lathe Motor.

Newman109 · Mar 21, 2018

My question is at the end of the post.

Background:
Last summer, The control transformer burned up in my lathe. It damaged all of the contactors so that I had to replace all of the parts and most of the wiring in the electrical control box. After the repairs, the motor operates perfectly in both forward and reverse modes, although it runs too fast in jog mode. There are three contactors: forward, reverse and jog. Forward and reverse are operated with the forward and reverse lever into a rotary switch on the face of the lathe. Jog is operated by a button on the face of the control panel.

I've read that jogging uses full voltage (in this case 240 V, single phase). When I now jog, I have to engage the lowest of 8 speeds and I still get three or four turns of the chuck feeding full 240 V to the motor. Formerly, jog was much slower. I had to redesign the panel wiring myself since I had no schematic.

To address this issue, I've done some research and the term "Inching" is defined as providing lower power, say to a lathe motor, to provide small increments of turning. It would be a very simple matter with my present design to change the jog contactor so that it provides only 120 volts using line and neutral from the Acme control transformer output to the single phase motor.

I presume that the motor would run slower and in Inch (or Jog) mode the jog button would only be activated for an instant, less than say a half second.

My question is: would this damage the motor, given the very short duration of the power input for inching/jogging at the lower half-voltage?

Thanks in advance.

rons · Mar 22, 2018

My answer is below. First, I have a DC variable speed drive lathe which I could do jogging/inching with. Have never used it. The hand wheel is right there and that is what I use.

The number and durations of starts at whatever voltage is going to create heat. I do not have a formula but I remember that when I bought my Hardinge lathe the seller said that I should not start the motor more than 5 times in a minute. That advice is considering a full 3 phase 230 voltage start. I considered that advice as reasonable considering that the shop has probably seen their far share of damaged motors.

Notice the 3rd paragraph in the link regarding the starter.

Jogging and Inching in Motors

Newman109 · Mar 22, 2018

Of course the number of starts should be limited for the reasons mentioned, possible overheating and subsequent damage to the motor. In my case, the jog or inch function will probably be used very seldom, and then only once or twice to examine a setup in the lathe.

As it is now, the jog function is much too active. I've developed a design to slow it down and it involves reducing the output voltage of the jog relay to 120 volts from my isolated control transformer. I plan to test it in the next week or so.

I just wondered what others might have to add. Thanks for the response.

johansen · Mar 22, 2018

Any possibility you have a 2 speed motor and the jog relay is not wired right,

Newman109 · Mar 22, 2018

johansen said:
Any possibility you have a 2 speed motor and the jog relay is not wired right,

The motor is a single speed, 2 hp 240 volt single phase TECF. My question really has to do with the effects of low voltage on the motor in jog/inch mode. I'm getting the idea that very short increments of use should not cause any harm. In any case, I'm the only one using the lathe so I can control usage.

The innards of the control panel were burned out. I believe that the original design of the jog function fed 120 volts into the motor to obtain a slow inch/jog mode. There's no way to tell since the original schematic has been lost and the wiring was burnt to a crisp.

I'm going to test the jog function on the low voltage (120V) briefly. As mentioned, the use of jog is very limited. usually to indicate on a four jaw chuck so I don't anticipate heat buildup.

The reason that the panel burned up was that the original control transformer had no shutoff so 240V was fed into it all of the time. I didn't realize that. That has now been corrected with an in-line shutoff switch. For the record, it lasted 25 years. LOL.

This is the panel after the rebuild. I can rewire the jog to provide the lower voltage, but I have to devise an interlock to remove the jog circuit when running the lathe to do cutting in operation.

sfriedberg · Mar 22, 2018

The only time I've ever had my 10HP PhasePerfect shut itself down was when I was repeatedly jogging a lathe with a 7.5HP (three-phase) motor. Unless you've got some kind of soft-start technology involved, jogging basically calls for locked-rotor current, way above the rating for continuous load. A couple of short bursts is not a big deal. 10 or 12 such bursts in rapid succession is a problem.

wheels17 · Mar 22, 2018

The single phase inching question is very different than the 3 phase inching. You will be applying voltage to both the regular and start winding of the motor, as you will probably never get up to the speed where the start winding drops out with the centrifugal switch.

I'm not knowledgeable enough to make a conclusive statement on the full effect. You will be applying half the voltage to the start winding and the start capacitor which doesn't sound bad, but I don't know what the impedance will be at low speed. Repeated jogs may, or may not overheat the start winding and capacitor because they are repeatedly energized in a near locked rotor condition. My guess is the impedance (not resistance) would result in 1/4 the power dissipation of a single 220V start(P=V^2/R) but repeated energizing for jogs could create a lot of heating. There are probably effects that I do not know about.

I'd love to see a motor expert chime in. It's an interesting question.

(I've always liked the Rumsfeld quote:"Reports that say that something hasn't happened are always interesting to me because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know.")

"

Newman109 · Mar 22, 2018

wheels17 said:
The single phase inching question is very different than the 3 phase inching. You will be applying voltage to both the regular and start winding of the motor, as you will probably never get up to the speed where the start winding drops out with the centrifugal switch.

I'm not knowledgeable enough to make a conclusive statement on the full effect. You will be applying half the voltage to the start winding and the start capacitor which doesn't sound bad, but I don't know what the impedance will be at low speed. Repeated jogs may, or may not overheat the start winding and capacitor because they are repeatedly energized in a near locked rotor condition. My guess is the impedance (not resistance) would result in 1/4 the power dissipation of a single 220V start(P=V^2/R) but repeated energizing for jogs could create a lot of heating. There are probably effects that I do not know about.

I'd love to see a motor expert chime in. It's an interesting question.

(I've always liked the Rumsfeld quote:"Reports that say that something hasn't happened are always interesting to me because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know.")

"

That's interesting. As I say, I won't ever be doing any excess repetitive inching or jogging. Perhaps 1 to 3 times and that would be that.

In any case, I will only be rigging a test circuit with a spare two pole contactor from my collection initially. If it appears OK, I'll make the change.

JST · Mar 22, 2018

Inching would be similar to a low voltage start.

The start winding will get low current simply because the capacitor is the same size as used at high voltage, and its impedance will lower the current. That will reduce starting torque, which may not be such a bad thing for an inching function, at least it will not take off as fast (it will probably not go any slower if it is allowed to start fully on low voltage, but if low enough it might be slow with a load on it due to friction)

The main winding may take considerable current, even though voltage is lower and impedance will tend to limit it.

Under these conditions, a jog will not be equivalent to a full start. It might well be equivalent to perhaps 2/3 of a start, more if you run it very long in jog mode, which I doubt you will do. I do not think that an occasional jog operation is going to do any damage.

At least not if the jog mode actually allows it to run. If it is too weak to turn well, then there might be more of a problem.

Technical Ted · Mar 23, 2018

NEMA has guidelines for maximum motor starts per hour. Go here: http://www.landbelectric.com/download-document/78-maximum-nema-inertia-starts.html

If you do a google search for "NEMA motor starts per hour" you'll get all kinds of info. The number is fairly low... Frequent starting breaks down motor installation faster shortening the life of the motor. It can also cause over heating. 3 phase motors can be started more times per hour than single phase too.

This guideline may not directly apply to your "inching", but I think it's good to know this information for general motor usage anyways.

Ted

rons · Mar 23, 2018

I advertised a pendant that is used for positioning on CNC machines. I had no profit in the thing.

HPG-B

Figuring out how to use something like this for positioning would give some fine control. With just a contactor/push button arrangement you will get a burst of phase reversals going to the motor. A pendant would allow a proportional amount of phase reversals. This approach would take some Silicon.

SAF · Mar 23, 2018

Newman,

I think your on the right track. I serviced a bell tower a few years back that used a similar setup for ringing a large bell. A 1Φ gearmotor wired for 230V and powered from 120V. It used a drum switch with a cam for reversing. The output of the motor had reduced torque and speed with the applied load. The main windings were series wired for high voltage, and the start winding was powered at its normal 120V, but separated out for reversing operation in conjunction with the drum /cam switch.

This setup has been in use since 1961. The problem it had was that the centrifugal start switch was worn out, as the bell was used daily for many years.

I think that the reason that your transformer burned up was because it was too small for the 2HP motor for more than occasional use. The 500VA unit you have seems a bit small as well, for regular use. What size was the unit that burned up? Leaving a transformer energized normally causes no problems, overloading it though, will cause the windings to overheat and fail.

I would expect that the loaded motor at reduced voltage would pull somewhere around half of its nameplate value. A 2HP motor @ 230V is around 12A FLA. Your 500VA xformer is rated at 4.1A @ 120V output. So if the reduced voltage motor start draws about half of the 12A, your transformer may still be a bit undersized for longevity.

One thing to note about the torque output and standard 1Φ motor internal connections.
A standard motor when wired for 240V, uses the center point tap of the two run windings, to provide a tap for 120V to supply the start winding circuit with 120V.

If you wire for 240V, and supply with 120V, then your standard center tap for the start winding, will only be receiving 60V. This may be what you want or not. The bell motor didn't have enough start torque wired the standard way, receiving 60V. It needed to be isolated from the center tap and connected directly to the 120V supply along with the run winding, to have enough power to swing the bell weight.

I would suggest that you open your motor and inspect the start switch contacts for wear, to preclude having a problem down the road. The start cap on the bell was still good after many years of starts.

SAF Ω

Newman109 · Mar 23, 2018

SAF said:
Newman,

I think your on the right track. I serviced a bell tower a few years back that used a similar setup for ringing a large bell. A 1Φ gearmotor wired for 230V and powered from 120V. It used a drum switch with a cam for reversing. The output of the motor had reduced torque and speed with the applied load. The main windings were series wired for high voltage, and the start winding was powered at its normal 120V, but separated out for reversing operation in conjunction with the drum /cam switch.

This setup has been in use since 1961. The problem it had was that the centrifugal start switch was worn out, as the bell was used daily for many years.

I think that the reason that your transformer burned up was because it was too small for the 2HP motor for more than occasional use. The 500VA unit you have seems a bit small as well, for regular use. What size was the unit that burned up? Leaving a transformer energized normally causes no problems, overloading it though, will cause the windings to overheat and fail.

I would expect that the loaded motor at reduced voltage would pull somewhere around half of its nameplate value. A 2HP motor @ 230V is around 12A FLA. Your 500VA xformer is rated at 4.1A @ 120V output. So if the reduced voltage motor start draws about half of the 12A, your transformer may still be a bit undersized for longevity.

One thing to note about the torque output and standard 1Φ motor internal connections.
A standard motor when wired for 240V, uses the center point tap of the two run windings, to provide a tap for 120V to supply the start winding circuit with 120V.

If you wire for 240V, and supply with 120V, then your standard center tap for the start winding, will only be receiving 60V. This may be what you want or not. The bell motor didn't have enough start torque wired the standard way, receiving 60V. It needed to be isolated from the center tap and connected directly to the 120V supply along with the run winding, to have enough power to swing the bell weight.

I would suggest that you open your motor and inspect the start switch contacts for wear, to preclude having a problem down the road. The start cap on the bell was still good after many years of starts.

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SAF Ω

Very interesting points you make.

The control transformer that burned out was physically not very big. As I mentioned, it was connected to 240 V input for some 25 years before it failed. When it went, it permitted 240 volts in invade the 120 V coils in the existing forward, reverse and jog contactors which destroyed them as well as their wiring. I added a fan to the panel and the new transformer is isolated from the mains by a separate 240 V switch that is off when the lathe is not in use.

The control transformer in my lathe does not drive the motor but rather only the 120 V contactor coils. It is a very high quality unit built by Acme and is under a very light load. I happened to have two of those that I bought from a member here some years ago. He had a pallet full of them at great prices. I used the other one to provide low voltage in my home brew, 7.5 hp self-starting RPC. It controls the contactors, the red power light and a Hobbs meter.

The lathe motor gets its power directly from the mains through a pair of Swiss reversing contactors controlled by a rotary switch and using 120 V from the Control transformer.

I'm quite certain now that the motor will run slower at 120 V for inch or jog. My only concern is how to isolate the jog contactor that would provide the lower voltage to the motor from getting 240 V by accident when the lathe is in operation. That would take out the whole panel again and I'm in no hurry to do that! LOL.

Thanks. Got to get my pencil out and do some thinking.

johansen · Mar 23, 2018

At half voltage I would expect 3 times the full load current to immediately flow in the windings.

This is simply half the rule of thumb of locked rotor current is 6 times full load current.

the reduced voltage also means no saturation, so i would expect the ratio of current between the start and the run windings to be different at locked rotor current for half and full voltage. its also possible the current will be less than half the normal locked rotor current due to the lack of saturation.

another possibility for reduced voltage jogging is wiring the start and the run windings in series, with the capacitor connected in parallel with the run winding. i have not tried this myself.

Newman109 · Mar 23, 2018

johansen said:
At half voltage I would expect 3 times the full load current to immediately flow in the windings.

This is simply half the rule of thumb of locked rotor current is 6 times full load current.

the reduced voltage also means no saturation, so i would expect the ratio of current between the start and the run windings to be different at locked rotor current for half and full voltage. its also possible the current will be less than half the normal locked rotor current due to the lack of saturation.

another possibility for reduced voltage jogging is wiring the start and the run windings in series, with the capacitor connected in parallel with the run winding. i have not tried this myself.

At lower voltage you are saying that I might see something like 36 amps.That might be the case at constant operation at 120V. I'm only contemplating short momentary pulsing for inching or jogging.

In any case, I'm fairly certain that the original wiring fed lower voltage to the moor for jogging. I just don't have the original diagram.

atomarc · Mar 23, 2018

The horse has long since left the barn but it sure would have been a great move after the electrical fire to gut the shebang and replace it with a nice frequency drive. All this 'jogging' exercise would be a moot point.

Stuart

Newman109 · Mar 23, 2018

atomarc said:
The horse has long since left the barn but it sure would have been a great move after the electrical fire to gut the shebang and replace it with a nice frequency drive. All this 'jogging' exercise would be a moot point.

Stuart

Hmm, doesn't that require a 3 phase motor? If I can vary the speed on a single phase motor, then I would certainly do it.

Right now, I have only one problem, jogging speed. If a single phase motor can be speed controlled, that would be the way to go and I wouldn't hesitate.

JST · Mar 23, 2018

Newman109 said:
Hmm, doesn't that require a 3 phase motor? If I can vary the speed on a single phase motor, then I would certainly do it.

Right now, I have only one problem, jogging speed. If a single phase motor can be speed controlled, that would be the way to go and I wouldn't hesitate.

Not usually with standard motors... the pesky start switch is the big issue. Motors with a single start/run capacitor like gearmotors often have, are definitely controllable in speed. Trying to do that with a standard start switch motor will just let the switch engage and draw beaucoup current, shutting down the drive.

The other issue is that the standard motor with no run cap is true single phase, and does not respond well to slowing. With run cap, the motor has two phases, and a more even torque, so it is more effectively controlled by frequency.

Newman109 · Mar 24, 2018

JST said:
Not usually with standard motors... the pesky start switch is the big issue. Motors with a single start/run capacitor like gearmotors often have, are definitely controllable in speed. Trying to do that with a standard start switch motor will just let the switch engage and draw beaucoup current, shutting down the drive.

The other issue is that the standard motor with no run cap is true single phase, and does not respond well to slowing. With run cap, the motor has two phases, and a more even torque, so it is more effectively controlled by frequency.

The motor on my lathe has two caps. I presume that one is start and the other run. It's large physically and it would be difficult to source one that could be mounted in the same manner.

Looks like I'll just have to soldier on. When the weather picks up, I'll start working on the issue.

Thanks.

atomarc · Mar 24, 2018

Newman109 said:
Hmm, doesn't that require a 3 phase motor? If I can vary the speed on a single phase motor, then I would certainly do it.

Right now, I have only one problem, jogging speed. If a single phase motor can be speed controlled, that would be the way to go and I wouldn't hesitate.

Yes..it would have required the installation of a 3 phase motor. They are plentiful and cheap and coupled with the inverter would have opened up all kinds of possibilities.

You chose a different route so it's water under the proverbial bridge.

Stuart

"Inching" With Single Phase Lathe Motor.

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