Old Motor Help

Earlier in this thread Jeff Horton mentioned the R/I motor. I did not know what the R/I abbreviation stood for and PM’ed Jeff to ask. He was very kind to explain that it stood for “Repulsion/Induction” motor. This lead me to research how an R/I motor worked and how it compared to a capacitor start motor, since Jeff reported that he perceived his R/I motor was more powerful than a modern capacitor start motor of the same HP rating.

After enough research to understand the theory of operation of the R/I motor, I simply could not find any reason that an R/I motor would perform better than a capacitor start motor. The only difference between the two motors is in the starting mechanism. Once they are running at operating speed they are identical – they are both induction motors. The R/I motor has greater startup torque but this does not play any part once the motor is at operating speed – no more than a couple of seconds after power-on for woodworking applications.

This next led me to think of ways that the motors could be tested to see if the R/I motor did indeed produce more power than an equally rated capacitor start motor but testing HP is not easy and requires specialized equipment.

Then it occurred to me that there’s an indirect way to test the power of a motor. An electric motor is an energy conversion device. If one motor puts out more power than another motor, it must take more power from the lines. Efficiency could play into this, but almost all motors are of equal efficiency, with the modern motor being likely to be more efficient because of improvements in the magnetic properties of the metal and the layout of the electromagnets.

So to test whether an R/I motor is more powerful, you can measure the power into the motor while cutting a certain sized board, then replace the motor with a capacitor start motor of the same HP, and cut the same board while measuring the power into the motor.

My expectation is that the power would be about the same for the two motors but I’d welcome test results from anyone who has two motors and would carry out the tests.

Mike
 
I went back and reread your original posting, Travis. You mention a commutator on the motor. The advice I gave you was for an induction motor. Commutators are usually found on DC motors. If you have a commutator on the motor, and it runs on regular AC, the motor is a universal motor. But this would be unusual in an older 1.5HP motor.

If you have a commutator on the motor, clean the commutator with something like a pencil eraser and check the brushes to see if they are making good contact. They could have worn down.

If there's no commutator on the armature you have an induction motor and my previous advice is appropriate.

Jeff Horton suggested that if the motor won't start when you spin it, the problem could be the centrifugal switch on the shaft. In my experience, this is unlikely. The centrifugal switch closes (makes contact) when the motor stops turning (actually when it's turning slowly). When you get a lot of insulating swarf (like sawdust) in the motor it gets between the contacts and the switch won't make contact. Then, the next time you try to start it, the starter coil and capacitor are not energized, which causes the motor to just humm and not start rotating. If you give the shaft a spin (and the switch is the problem), the motor will begin running in whatever direction you spun it. If the centrifugal switch is the problem (blocked with swarf) you can often fix it by blowing high pressure air into the motor through the holes in the frame.

Regarding old and modern motors, like most electrical things, we've learned to make motors better over time. You'll find that a modern motor has better efficiency, smaller size, will require less maintenance (sealed bearings), much better insulation on the coils, and offer some frame advantages (like TEFC) over old motors. There's a lot of personal satisfaction in getting an old motor to work but beyond that, a modern motor is a better choice for most woodworking applications.

Mike

Oh stupid me. I am sure its an all AC motor. This old Railroader gets confused sometimes and forgets that all the electrical motors on a locomotive are 75 volts DC motors. Even though I was a Machinist, while out on road calls our Union Contract allowed the carrier to "force" us do other crafts. Doing an Electrician's job was quite common.

It was a catch 22. If you dug too deeply into an electrical problem, you got "time-slipped" by the Electrician's, yet often times a little bit of maintenance would get the fuel pump, or generator blower motor up and running. That would get a locomotive running which could pull freight. That would get you home a lot quicker.

One common trick was to"stone the comm" with a special eraser type stick. I can count the number of times that fixed the problem. That was a 5 minute job. Pulling the 3 hp motor, replacing the lovejoy coupling, attaching and detaching all the fuel lines, that took a lot longer to do. Anyway that is where I got the "comm" term from. I know so little about AC and DC motors that I assumed the terms applied to both types. I think I just showed my stupidity. :(

Thanks for all the help anyway. I will look into this in the spring. Its -10 degrees and far to cold to work in my shop right now, but just right for snowmobiling :)
 
Top