Motor

Last Updated: 3 January 2008

The identification plate on the motor indicates the following specifications:

Type NEMA Frame Model Power Rotation Electrical Current @ 240v Current @ 440v Contact rating
RR 75U B1261K1561 3/4 hp 1725 rpm 220v/440v 3φ 60 Hz 2.4a 1.2a 40°C




It's remarkable that this motor is only 3/4 horsepower, considering it's the single largest AC motor I have ever seen in a tool.  It weighs around 40 lb and has a diameter of just under 9 inches.

I believe this mismatch between size and rating is due to the 40°C contact rating.  Modern motors have contact ratings of 130°C, which means a lot more current is permitted.  More current means more power in a smaller package.

Wagner Electric Corporation doesn't exist anymore, so I can't contact them for help.


What to do about 3 phase: Phase Conversion vs. Motor Conversion
One way of running this saw using single-phase residential power is to use a rotary or static phase converter.

Static phase converters are very cheap (I could build one with a couple capacitors), but only permit 2/3 rated power at the motor (because one of the motor windings is used to produce the third phase).

Rotary phase converters are more complex (more expensive), but permit 100% power in the machine because an idler motor is used to produce the third phase.  I could build a rotary phase converter, but I'd have to acquire a 3-phase motor with 3/4 horsepower to use as an idler.  Getting the necessary parts could be pricey.

More than likely, the simplest and cheapest way to power this saw with my 220v single-phase power in the shop is by replacing this motor with a modern 220v single-phase unit.  This presents 2 main problems: shaft diameter and NEMA frame size.

Problem 1: NEMA Frame Size
The frame specifier of "75U" implies that this motor was manufactured between 1952 and 1964, obviously suggesting it's not the saw's original drive (refer to this brief explanation of NEMA frames).

Vertical dimension from the bottom of the motor base to the center of the shaft is 4 1/2".  The mounting assembly in the saw has lots of adjustments, so I think anything within about 1/2" of that dimension should work fine.

As evident from the photo, the motor sits on a simple flat plate with 4 threaded mounting holes.  This will permit maximum flexibility in changing to a different motor frame.  Making an adaptor will be almost trivial.

To permit proper operation of the variable speed drive, the motor mount must permit movement of the motor itself along an arc that defines its standoff distance from the variable speed pulley.  This is accomplished via hinged attachments (as shown) and a strong spring tensioner.  The tensioner is a simple compression spring that depresses the tab upon which it acts (as shown) with sufficient force to cause the motor plate to pivot into a level position on the hinges.  In effect, the spring tensioner takes the weight of the motor and allows some adjustment in its standoff distance from the variable speed pulley.  This sounds complex, but it's not.  It'll be clearer when I show pictures of the unit during reassembly.
I got some good advice that I will probably never find specifications for the 75U frame size.  A check with McMaster's offerings indicates 2 candidate replacements: a 1.5 hp TEFC unit on a 56H frame, and a 2hp ODP unit on a 182T frame. [TEFC: Totally Enclosed Fan Cooled; ODP: Open Drip-Proof].  The dimensions on these motors differ significantly from each other.  The 2hp motor is closest in dimension to the Wagner, but the RPM is a little higher (1740).  Price is $295.

The 1.5hp motor will require more fabrication to get it to work, but runs at 1725 RPM and is capacitor-run and capacitor-start (which is more efficient and offers greater torque).  Price is $259.  Other than the power rating, this motor is superior to the 2hp unit.

Which motor is the better choice will likely come down to price.  The 1.5hp motor (the better motor) is cheaper, but I will need to buy some aluminum to make mounting adaptors.  The 2hp will almost bolt right in place of the old one (I'd only need to cut 4 new mounting holes in the plate).  More to come...

Problem 2: Shaft Diameter
The shaft diameter is 3/4", which seems to be non-standard today.  McMaster doesn't carry any motors with a 3/4" shaft, so whatever motor I get I will need to buy a new pulley as well.  The 1.5hp motor uses a 5/8" shaft whereas the 2hp motor uses a 1-1/8" shaft.  The outer pulley diameter is 2.975".  The two closest pulleys available from McMaster are 2.85" and 2.95".  The 2.85" is available in 5/8" and 1-1/8" mountings; the 2.95" is only available in 5/8" mounting.  More to come...

Reassembly

Considerable effort went into chosing the replacement motor, and in the end I didn't buy the motor from McMaster-Carr.  I found nearly the identical motor at my local Tractor Supply store.  It's a GE commercial unit manufactured by Regal-Beloit.  It's "farm duty" (no idea what that means), and came wired with a plug for 115v, a switch, and internal thermal overload protection.  The thermal protection makes it more desireable than McMaster's offering.  It is a capacitor-start / capacitor-run motor, which means it offers a great deal of torque.

Note in this photo I've already rewired it for 240v and provided an Amphenol circular connector for the machine wiring harness.





This motor sits on a NEMA 56H plate, which means the shaft sits 1" lower than the factory motor and the frame is slightly narrower.  To remedy this, two simple adaptor plates were constructed from 1"x2"x0.120" square A36 tube and 1/4" steel bar.

In this photo the adaptors are ready for the motor to be mocked into position for alignment before the motor mount holes are bored and tapped.

Here's the motor mocked into position on that plate, and then installed in the drivetrain housing of the saw.  I secured it to the base using four 1/4-20 stainless screws, which I had hoped would be enough to cope with the torque.  When the motor starts up with all the belts connected it tends to bounce a couple times as the sudden onset of torque causes a reaction at the pivot hinge of the motor plate.

So far the screws are holding up fine.  Why 1/4-20 rather than 3/8-16 (which I'd have preferred)?  The 1/4-20's are smell enough diamter to permit lots of adjustment in the position of the motor so the pulleys can be properly aligned.






Belts
There are 4 belts required for the unit:
1. Motor to air pump (my saw has no air pump, so I don't need this particular belt)
2. Motor to Variable Speed Drive (VSD)
3. VSD to gearbox
4. Gearbox to speed indicator driver

A member at Practical Machinist contacted DoAll and they provided the following Gates belts:
Application Gates Number Standard Size Length (in) Circumference (in) Max. Width (in) Height (in)
Motor to air pump 2260 4L260 1/2 5/16
Motor to VSD 3370 5L370 36.92 37 21/32 3/8
VSD to gearbox 3510 5L510 50.94 51 21/32 3/8
Gearbox to speed indicator 1290 3L290 28.68 29 3/8 7/32

But I found some of these numbers are not accurate for my machine!  Specifically, the main drive belts - motor to VSD and VSD to gearbox - were wrong.  After some careful measurements, I'm using the following:

Application Gates Number Standard Size Length (in) Circumference (in) Max. Width (in) Height (in)
Motor to VSD 3340 5L540 53.94 54 21/32 3/8
VSD to gearbox 3540 5L340 33.91 34 21/32 3/8

I'm using only Gates Truflex belts.