AC Motors and Backgear or No Backgear

I've seen a number of threads about converting 10 EE lathes to an AC motor with VFD drives of various types. Some have incorporated the existing backgear arrangement, others have gone with a larger HP motor (typically 10 HP) with no backgear. Monarch appears to have gone the no backgear approach on some of their later retrofit offerings and maybe on some of their last new lathes.

I would like to get some discussion and feedback from those who have done the conversions regarding the relative benefits and tradeoffs of motor size and the merits of keeping the backgear arrangement. Obviously keeping the backgear adds complexity.

With a 10 HP motor and a reasonable drive package, are there really any slow speed operations that can't be performed or can't be performed as smoothly or easily with a no-backgear approach?

How slow is slow and how slow is slow enough?

Talking to Monarch several years ago about their AC conversions. They said their 5hp with back gear is a better choice, if you plan on doing alot of threading.

Back gear, not much oomph out of a VFD motor at low RPM without the encoder option.

Mine came with a 5 HP three-phase AC motor and no back gear, so I had no option. Running it with an AC Tech VFD, performance is OK, but not great. I think a constant-torque vector drive would work better, but that's not what I have.

I've also considered adding a back gear. The original ratio was 5:1, which seems about right.

- Leigh

At the time VFDs were first offered, the choices were 5 HP with backgear and open belt, or 7.5 HP with open belt only.

Possibly these were primitive V/Hz drives, as the open belt only option proved to be quite unpopular. Probably not much torque with a couple of Hz going into that physically large motor. And, who knows what the attendant "cogging" might have done to the surface finish.

A backgear is a must for any serious toolroom work.

And, with a backgear, 5 HP is quite sufficient, as the original machine had a 3 HP drive, which was upgraded to 5 HP in the mid-1950s, after the basic machine was already nearly 20 years old.

With a 5 HP sensorless vector drive, such as the Polyspede, and the backgear, you really have a quite up-to-date drive system.

The remainder of the machine is essentially timeless, as C. Bickel designed the machine right the first time.

At least one owner has installed a 3 HP motor and a sensorless vector drive, and he reports good results.

What has happened in the mean time is the limit for single-phase drives has been raised from 3 HP to at least 5 HP, or more, with no derating required.

There is another thread in this Forum on Polyspede drives.

Interested parties should seriously consider that drive.

For, even with a 13EE or EE1000, single-phase is a real possibility.

DC & backgear

I frequently use lathes for winding transformers and saturable reactors. A while back, I had to wind rectangular copper ribbon at 3 1/4 TPI. My South Bend and Sheldon R15 only go down to 4 TPI, so I had to use my 3 HP MG 10 EE, even though it is in the middle of rebuild. For that sort of work, DC and backgear is the only way to go. I found a 5 HP 10 EE motor in perfect condition and have made a saturable reactor to control it. I probably will include a feedback tachometer, making it essentially a servo system, and the saturable reactor is intrinsically current limiting, so I will be able to develop full torque at very low speeds without worrying about cooking something. The great thing about that setup is that it limits starting surges, good for the commutator, and if you lock the spindle, it just sits there drawing rated current. Since the current is mostly flowing through one armature winding instead of being divided between all of them, that one will eventually overheat, but there is plenty of time to shut it off. Last summer I repaired a 10 EE thyratron control for a local shop, and I finished with the same low opinion of thyratrons that I developed in the pre-solid state 1950s. I would rather have the MG, even with the reduced horsepower.

Bill Plumpe

"Last summer I repaired a 10 EE thyratron control for a local shop, and I finished with the same low opinion of thyratrons that I developed in the pre-solid state 1950s. I would rather have the MG, even with the reduced horsepower."

The 13EE combines the best of the Ward-Leonard System (M-G) and the best of the early phase-controlled system (thyratron).

The large M-G does the heavy lifting of providing armature power, which is considerable in a 13EE, whereas the thyratron controls just the field of the spindle motor, with the combination of the two becoming a part electro-mechanical, part electronic drive, with the thyratron part being there mainly to implement constant surface speed cutting.

Placing the high-powered circuitry in the M-G and the low-powered circuitry in the thyratron made sense.

The follow-on, the EE1000, was all-electronic, and it required a large, internal ∆-Y anode transformer and six ... count 'em ... C16J thyratrons just for the armature, plus two additional thyratrons for the field.

peterh5322 said:

And, with a backgear, 5 HP is quite sufficient.

Click to expand...

I would prefer to say "the workable minimum" and the backgear is a must if you want to keep it an allround lathe.

Which is what I stated ...

http://www.practicalmachinist.com/vb/showpost.php?p=913783&postcount=5

... above.

For a 13EE or an EE1000, obviously the HP will be much, much higher.

Variacs

Another possibility that I have not seen mentioned on this forum is that any variable DC supplies that will furnish the rated voltage and current could be used. Variacs ( also called Powerstats, variable autotransformers ) can be used. For the armature, a typical Variac rated at 240 VAC, 22 amps with a fullwave bridge rectifier would do nicely and one that will supply 1.2 amps at 90 VAC, would do for the field. The large variacs are about 13 inches in diameter and cost more than anyone is likely to want to spend for a new one, but I see them in surplus stores for much less. Even the tiny Superior type 10s or their many clones will handle the field. The drill would be to turn the field up to 92 volts and bring up the armature to the desired speed. If the speed is not high enough when it reaches full voltage, then turn the field down until it is reached. A little complex, but anyone who can operate a 10 EE is smart enough to handle it. The only real downside is that there is no compensation for load. It would probably be a good idea to put some stops on the field Variac to avoid over driving it or going so low that the motor overspeeds. My Sheldon lathe uses a Variac control, albeit three phase, now. Forward and reverse are selected by size 3 contactors with lockout contacts to prevent closing both at the same time. You do have to anticipate the speed to set initially to wind up at the desired cutting speed, but it isn't a great hardship. The point is that this is a way for people with good motors and disfunctional drives to get running with a minimum of labor and electrical knowledge.

Bill Plumpe