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Electric Motors/surface grinder


hello Will. any help is appreciated. I have a 1940's era Brown & Sharpe #2 surface grinder, that is belt driven ( flat belt). the single motor drives the spindle, as well as having, at one time, driven the oscillation drive for the table. the original motor was 1.5 hp 3450 RPM, 3-phase. it has since been retrofitted with a 1.5hp 1725 rpm. 3ph. i want to take it back to the proper rpm, as the grinding wheels & process work best around the higher. i can use a modern 3600 rpm motor, and i can either go 3 phase ( through a Rotophase phase convertor, or, as a last result, go with a VFD, to get a better "balanced" 3 ph)or go single phase 240v. my question is: can i got with a industrial machine tool motor, or is there a specialized "grinder" motor ( not just a spindle motor) that i need to go with. Not having much luck getting feedback from anyone on this... i want to have the best grinding finish i can and not have motor induced vibrations that mar the finish.


Very well written and described. Nice, very nice job.  A bit off the subject, but it seems these days that once most humans reach 140 Characters, their ability to hold attention or concentrate shuts down.

I was " admonished" for providing not only a yes or no answer,   but for providing options. It apparently hurt?

More than two paragraphs, even if the content provides detailed information, advantages, and or options for cost, time, cost versus, so on, still too much.

I actually gave up on a pay site, my tv has no picture what could the problem be?

The questioner does not want to hear the back of the tv is toward the viewing area, no model type history, just no picture.

If the answer is not some combination of key strokes from the remote,   forget it.

Anyway, apparently you are somewhat of a true craftsman, I appreciate that. It is encouraging.

Maybe you have seen this but this is a patent for an old machine not the NEW version you have.

Well you have several questions in here, first if you can go three phase to about anything do it.

A three phase motor [standard induction fixed speed, single winding two bearing] is about as simple as simple gets.

A bit of an overstatement, I am assuming the end housings are bored in the middle and the bores from end to end are in line,  no warping of the frame,  no soft foot,  the housing bores are within C3 motor bearing tolerances,  the bearings are not gray market,  not the cheapest off the shelf,  the rotor is dynamically balanced with as much precision that the rotating element will adjust or react to.

At some point, dependent on the balance machine used,  and the accuracy and calibration, let's say we have the best machine available,  most likely you will find a human will get you the last few .001 ounces of correction by using both experience and having a visual of the rotating element as to where mass can be added or subtracted.

The double edged sword you already understand,  more RPM better productivity, more maintenance issues.

It goes without saying that the base, the entire structure, from floor to rigid conduit, to cooling mist, to anything applying some sort of restriction or force to the machine can wack out the lack of unwanted vibrations, harmonics etc.

True balance which is often blamed for shaking things, is more often than not some sort of mechanical looseness, binding, so on. The actual rotating element, in this case the rotor, once balanced is not going to change,  to change the balance and then effect the performance on the working end, would require the rotating element to loose mass or gain mass, if something is not added or removed from the actual rotor,  the balance once balanced correctly is not going to change.

Now that old of a motor could and might likely be constructed with a true caged rotor, versus a cast rotor.

In the balance stand any issues with looseness in the spider that holds the laminates, or cracks in the barred conductors will show up.

So whatever motor,  if you want the best physical condition that will not impair the working end,  the motor will need to be gone through, line bored might be extreme but not impossible.

Three phase is simple because there are no switches capacitors, devices to put the windings out of phase to create rotation, just a pile of steel with a caged winding in the rotor plus what is called resistance rings or ends, that tie the rotor conductors together   plus the make up of the material   is engineered into the motor,  and a fixed value is needed to end up with a designed resistance and impedance,   that I believe is what you are asking by mentioning an "industrial motor".

You did not mention the frame,  of the motor, it might or might not be apparent on the name plate if there is one,  because along the same lines,  we have more choices     go for an old cast rotor or they do make new cast body motors,  versus some kind of junk steel.

1940 the original motor was probably a 203 pre nema 2 pole possibly a c or d design, which indicates the motor is built for periods of high torque,  maybe on start, maybe during process.

When we go to look for a motor, we will probably want to try and find a c of d design,  [B] design is the most common for vanilla type general purpose motors.

A barred type rotor is preferable to a cast rotor, for obvious and not so obvious reasons.

While a cast rotor  [where the laminations of steel make up the rotor body, are "cast" with an engineered compound of copper, and other metals and by adjusting the end or resistance ends or rings,  the total form factor/resistance/impedance of the rotor is hopefully the result.

It is adequate for most general applications, but because we want the best we can get at the most cost effective method of getting there,  we need to shop for a used pre nema, same vintage type motor,   which when we add the fairly hard to find,  d or c design type, we should have a barred or cage type rotor, where actual bars comprised of engineered materials are used to form the bars, and the resistance rings are used for the fine tuning [if you will]

We should be able to find a motor,  still intact, the cost downside may be in the location and the shipping costs. Found local to you better,  but if shipped the freight charges could easily exceed the cost of the motor.

We have a lot more to talk about,  one thing real quick,  when you mention ROTOPHASE that is a device ripe for misleading characteristics.

The lingo is confusing.  There are true roto phase machines which create a temporary third leg, and is used to start the motor, or create rotation, and once to speed, the false third leg is dropped from the secondary load supply and the three phase load is now running on two actual legs with a partial or induced third.

Then there are rotary convertors,  and static,  [included in static is a vfd the more complex and true three phase supply]

If there is no real need for motor speed change  then a rotary type convertor will produce three phases constantly and some very balanced as to the output voltage measured line to line to line.

The much cheaper roto phase,  actually rotophase is a specific manufacturers name of their product, but used as general term for a device that produces three phase.

A VFD, I would go with a pulse width type VFD<  where the name says it all,  the voltage when viewed on a scope is manipulated,  for example  on the AC positive side of the time line, the "pulse or width of the voltage is shortened or extended, producing more or less 'pulses" per second, and by thinking of it as a physical length, you can kind of envision smaller pulses having more per time period    [normally in cycles per second] the longer pulses are less per time period,    results are longer pulses less cycles an lower speed, shorter pulses more cycles and higher speeds.

Now instead of a utility at 59.9 standard hertz, the vfd or even the rotary device now applies a voltage and cycle combination which controls the motor from it's US common 60 hertz base speed bounced off the number of poles built or designed into the motor.

To be more precise, 3600 is two poles at 60 cycles,  4 poles is 1800 with four poles called the synchronous speeds.

In other words without friction and windage,  a two pole motor would spin at 3600 RPM, they don't because of friction and windage   The friction comes from the anti friction bearings,  and although a good bearing and good lube will create very little friction, but still some.

Then add the windage, which is simply the mass overcoming the forces needed to spin through the ambient "air"  just the body has to waste power and speed, then add fins built on the rotor, or fans attached to the shaft, and more windage    so you will not see motors rated at 3600 but more likely 3590 or some speed a bit less than synchronous speed,  a physics law.

Other advantages of vfd drives is overload protection, ramping adjustments, braking, decel, on and on,  most inexpensive drives have around 40 user adjustable parameters based on the load or application.

Probably the best way to drill down all the options is to contact me directly    my home address is wbwill@,   one of my shops is MEAR Services INC in KANSAS CITY MISSOURI  then   EMR Repair Inc   is

Phone numbers  MEAR  816-650-4030     EMR   816-587-3930   

Just start with my personal address,  put Brown and Sharpe grinder allexperts    and we go from there,  right now I have nine phones ringing,  and a ton of options to go,   

You might measure the original bolt holes    if still there   and you should find  2f the cc hole to hole parallel to the shaft  at 5.5 inches   and then 8 inches across the motor body.

That will tell us the original motor was probably a 203 frame and we can go from there.

I will look for your email and get back asap,  list your questions as they come up, and we will deal with all that.  

William P Babbitt

President EMR Repair Inc

President MEAR Services Inc  

Electric Motors

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Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.

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