Reversing the motor of a Stihl USG grinder !!!!

[Patrick62]

Yes, Virgina, it can indeed be done.
I will be the first to tell you that this modification is beyond the scope of the normal tinkerer....
The first task is to pry the ends of the case open, drive the plastic fan off the end, and get the windings like this. THen very carefully cut, and undo the lashings... They very carefully buried the connections in the windings. The lighter gauge coil is the starting coil, simply isolate it from the run coil, and those leads come back out to the switch box. I then put the motor back together and tested it. It will now run in either direction. Tomorrow I will obtain the reversing switch and do the wiring in the switch box.

My Oregon 511 does a good job with a reversible motor. I am anxious to see how nice the USG can do with a reversible motor. The accuracy of this grinder is exceptional!

I told the boss on the way out the door today that I am working on the grinder... He rolled his eyes.

 

[Del_]

This is awesome.

I've had a USG for about 30 years now and would love for it to be reversible.

 

[Patrick62]

i think that motor on your grinder is built a different way than what is on my newer machine. Some exploration would be in order to see if there is a capacitor start winding in there, and the leads are accessible for a reversing switch

 

[RED-85-Z51]

On the one i had, i could stop it, spin the wheel backwards and flip it on and it would spin backwards

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[Del_]

On the one i had, i could stop it, spin the wheel backwards and flip it on and it would spin backwards

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I will give it a try.

 

[John Lyngdal]

Have a HOS and a USG and I'm going to see if I can get either of them to spin backwards.

 

[Patrick62]

I got the switch done, works like a charm. I will go put it back on the store grinder tomorrow. Maybe sharpen a chain while I am there!
the readers digest condensed version is you move one wire in the windings to make the starter winding separate from the run winding in the motor itself.
Then, with a DPDT switch crossed up to make it a reversing switch, white to one side, Hot to the capacitor, and then cap to the other side of the switch. SImple.
Easy peasy.

 

[John Lyngdal]

Have a HOS and a USG and I'm going to see if I can get either of them to spin backwards.

No joy with my HOS grinder.
It just louder 60 Hz buzz and spun the wheel in the usual direction when power was applied.

 

[anlrolfe]

The switch configuration would ge called a 4-way.
Reverse the polarity through the start winding will reverse starting torque direction.
Big question, will the arbor nut loosen when you flip direction? On something like a right angle hand grinder the direction of rotation tends to tighten the nut....

 

[Patrick62]

Will the nut loosen? I don't think so... I have reversed my 511 for years now, but it has a little bolt that holds the wheel on.
The USG has that giant nut, but not that much mass.... if it is on fairly snug, it should stay on.

 

[Del_]

Will the nut loosen? I don't think so... I have reversed my 511 for years now, but it has a little bolt that holds the wheel on.
The USG has that giant nut, but not that much mass.... if it is on fairly snug, it should stay on.

So only the startup windings need the polarity reversed and the main drive windings don't?

I was just trying to start my USG in reverse and when I spun it in reverse and hit the start switch it corrected itself and went back to it's designed rotation direction.

But when I wrapped a bit of starter cord around the motor shaft and gave it a light pull and hit the on switch it ran in reverse just fine.

So here's a question:

Do the starter coils stay in the circuit when the motor is running in normal operation aiding somewhat in the final drive horsepower?

If so this may mean that when my usg is running in reverse that the starter windings are fighting the main drive windings.

Or are the starter windings switched out of the circuit once the main drive winding take over?

I'd hate to think that the starting windings may end up being abused when starting in reverse using the string method(by fighting the main windings). BTW, I did it several times and I did not notice any change in the sound of the motor running.

It only takes a few seconds to wrap the string and give it a pull.

 

[anlrolfe]

Do the starter coils stay in the circuit when the motor is running in normal operation aiding somewhat in the final drive horsepower?

It depends,
For a split phase motor or capacitor start motor the start winding is opened by a centrifugal switch or current relay or choked down with a solid-state device. For these motors the start gets disconnected from the line at approx. 75% running speed. This may be why you can spin it really fast and get the desired rotation.
For a permanent capacitor(start/run) the start winding is sometimes called an auxiliary winding and stays in the circuit constantly. These may not be easily reversible.
This motor looks like a 4-pole, not including magnetic slip, RPM=(120xHz)/poles or 1,800 for the stator field and probably 1,725-1,750 for rotor. (Note: 120 is a constant, NOT the volts)

I can't tell in the picture if any of the wire closest to the rotor is thinner, optical illusion, I think they are all the same gauge.
That would indicate Permanent Split Capacitor. Changing which coil is connected to the line with lagging current vs with capacitor determines direction.


If these windings aren't identical, switching the capacitor could degrade, damage or destroy the motor.

 

[Patrick62]

It is a 2 pole motor, and I measured the run windings at 4 ohms, and the start windings at 12 ohms. Cap start/run. The start winding stays energized, no switch. One option is to simply disable the capacitor, and you could just spin the motor the desired direction, and away it would go, there would be some loss of torque if you loaded the motor hard. Probably not a big deal on these grinders. It is possible that the start winding would generate voltage against the line with the capacitor in the middle... possibly over volt the cap if you start it with a pull cord.

 

[nixon]

If you have shop air , give the cooling fan a blast of air from the air nozzle in the direction you want it to turn . Takes about 2 seconds . You don’t even have to remove the fan cover .

 

[SteveSr]

It is a 2 pole motor, and I measured the run windings at 4 ohms, and the start windings at 12 ohms. Cap start/run. The start winding stays energized, no switch. One option is to simply disable the capacitor, and you could just spin the motor the desired direction, and away it would go, there would be some loss of torque if you loaded the motor hard. Probably not a big deal on these grinders. It is possible that the start winding would generate voltage against the line with the capacitor in the middle... possibly over volt the cap if you start it with a pull cord.

This is a split phase capacitor-run motor. If the windings aren't equal you my have an issue reversing it. Check the capacitor voltage both directions. It would also be good to measure motor current both directions. both of these measurements should be the same in both directions. Just because you can get it to reverse doesn't mean that it will live long and prosper.

 

[heimannm]

Not my artwork, if I remember correctly is was supplied by a member from Australia?

I modified both my Carlton and no-name lime green units to run in either direction. The Carlton is used somewhat regularly and I have not had any issues with the motor. Based on the time stamp below, I guess I have been using it with the reversing switch for 9 years.



Mark

 

[Patrick62]

Hee hee hee, that was my artwork from years ago, when I reversed the 511 grinder. It is the same circuit on the USG

 

[SteveSr]

View attachment 871424

Not my artwork, if I remember correctly is was supplied by a member from Australia?

Mark

Makes sense now and will pass my previous test. The smaller (high resistance ) winding isn't really a "start" winding but with the run capacitor does provide the required phase shift to make the motor start and run. Effectively reverse the winding and it will run in the opposite direction!

 

[heimannm]

You must have moved...

Mark

 

[Patrick62]

I have never been "down under"
however!!! I can't believe that when I created that diagram years ago, there is a typo in there... the Ohms values are in wrong with respect to the windings!! OMG !!!
I should cut, paste that into my machine and edit that!!! however, you get the general gist of the idea...